Itinerant fermions on a triangular lattice: Unconventional magnetism and other ordered states

NASA Astrophysics Data System (ADS)

Ye, Mengxing; Chubukov, Andrey V.

2018-06-01

We consider a system of 2D fermions on a triangular lattice with well separated electron and hole pockets of similar sizes, centered at certain high-symmetry points in the Brillouin zone. We first analyze Stoner-type spin-density-wave (SDW) magnetism. We show that SDW order is degenerate at the mean-field level. Beyond mean-field, the degeneracy is lifted and is either 120∘ "triangular" order (same as for localized spins), or a collinear order with antiferromagnetic spin arrangement on two-thirds of sites, and nonmagnetic on the rest of sites. We also study a time-reversal symmetric directional spin bond order, which emerges when some interactions are repulsive and some are attractive. We show that this order is also degenerate at a mean-field level, but beyond mean-field the degeneracy is again lifted. We next consider the evolution of a magnetic order in a magnetic field starting from an SDW state in zero field. We show that a field gives rise to a canting of an SDW spin configuration. In addition, it necessarily triggers the directional bond order, which, we argue, is linearly coupled to the SDW order in a finite field. We derive the corresponding term in the free energy. Finally, we consider the interplay between an SDW order and superconductivity and charge order. For this, we analyze the flow of the couplings within parquet renormalization group (pRG) scheme. We show that magnetism wins if all interactions are repulsive and there is little energy space for pRG to develop. However, if system parameters are such that pRG runs over a wide range of energies, the system may develop either superconductivity or an unconventional charge order, which breaks time-reversal symmetry.

Field-induced spin-density wave beyond hidden order in URu2Si2

NASA Astrophysics Data System (ADS)

Knafo, W.; Duc, F.; Bourdarot, F.; Kuwahara, K.; Nojiri, H.; Aoki, D.; Billette, J.; Frings, P.; Tonon, X.; Lelièvre-Berna, E.; Flouquet, J.; Regnault, L.-P.

2016-10-01

URu2Si2 is one of the most enigmatic strongly correlated electron systems and offers a fertile testing ground for new concepts in condensed matter science. In spite of >30 years of intense research, no consensus on the order parameter of its low-temperature hidden-order phase exists. A strong magnetic field transforms the hidden order into magnetically ordered phases, whose order parameter has also been defying experimental observation. Here, thanks to neutron diffraction under pulsed magnetic fields up to 40 T, we identify the field-induced phases of URu2Si2 as a spin-density-wave state. The transition to the spin-density wave represents a unique touchstone for understanding the hidden-order phase. An intimate relationship between this magnetic structure, the magnetic fluctuations and the Fermi surface is emphasized, calling for dedicated band-structure calculations.

Identifying the critical point of the weakly first-order itinerant magnet DyCo2 with complementary magnetization and calorimetric measurements

NASA Astrophysics Data System (ADS)

Morrison, K.; Dupas, A.; Mudryk, Y.; Pecharsky, V. K.; Gschneidner, K. A.; Caplin, A. D.; Cohen, L. F.

2013-04-01

We examine the character of the itinerant magnetic transition of DyCo2 by different calorimetric methods, thereby separating the heat capacity and latent heat contributions to the entropy—allowing direct comparison to other itinerant electron metamagnetic systems. The heat capacity exhibits a large λ-like peak at the ferrimagnetic ordering phase transition, a signature that is remarkably similar to La(Fe,Si)13, where it is attributed to giant spin fluctuations. Using calorimetric measurements, we also determine the point at which the phase transition ceases to be first order: the critical magnetic field, μ0Hcrit = 0.4 ± 0.1 T and temperature Tcrit = 138.5 ± 0.5 K, and we compare these values to those obtained from analysis of magnetization by application of the Shimizu inequality for itinerant electron metamagnetism. Good agreement is found between these independent measurements, thus establishing the phase diagram and critical point with some confidence. In addition, we find that the often-used Banerjee criterion may not be suitable for determination of first order behavior in itinerant magnet systems.

Emergent Rotational Symmetries in Disordered Magnetic Domain Patterns

NASA Astrophysics Data System (ADS)

Su, Run; Seu, Keoki A.; Parks, Daniel; Kan, Jimmy J.; Fullerton, Eric E.; Roy, Sujoy; Kevan, Stephen D.

2011-12-01

Uniaxial systems often form labyrinthine domains that exhibit short-range order but are macroscopically isotropic and would not be expected to exhibit precise symmetries. However, their underlying frustration results in a multitude of metastable configurations of comparable energy, and driving such a system externally might lead to pattern formation. We find that soft x-ray speckle diffraction patterns of the labyrinthine domains in CoPd/IrMn heterostructures reveal a diverse array of hidden rotational symmetries about the magnetization axis, thereby suggesting an unusual form of emergent order in an otherwise disordered system. These symmetries depend on applied magnetic field, magnetization history, and scattering wave vector. Maps of rotational symmetry exhibit intriguing structures that can be controlled by manipulating the applied magnetic field in concert with the exchange bias condition.

Emergent rotational symmetries in disordered magnetic domain patterns.

PubMed

Su, Run; Seu, Keoki A; Parks, Daniel; Kan, Jimmy J; Fullerton, Eric E; Roy, Sujoy; Kevan, Stephen D

2011-12-16

Uniaxial systems often form labyrinthine domains that exhibit short-range order but are macroscopically isotropic and would not be expected to exhibit precise symmetries. However, their underlying frustration results in a multitude of metastable configurations of comparable energy, and driving such a system externally might lead to pattern formation. We find that soft x-ray speckle diffraction patterns of the labyrinthine domains in CoPd/IrMn heterostructures reveal a diverse array of hidden rotational symmetries about the magnetization axis, thereby suggesting an unusual form of emergent order in an otherwise disordered system. These symmetries depend on applied magnetic field, magnetization history, and scattering wave vector. Maps of rotational symmetry exhibit intriguing structures that can be controlled by manipulating the applied magnetic field in concert with the exchange bias condition. © 2011 American Physical Society

Successive magnetic transitions of the pseudo-ternary compounds Ho1-xRxRh2Si2 (R=Y, La)

NASA Astrophysics Data System (ADS)

Shigeoka, Toru; Uchima, Kiyoharu; Uwatoko, Yoshiya

2018-05-01

Magnetic measurements on the pseudo-ternary compounds Ho1-xRxRh2Si2 (R = Y or La = Y or La) were performed in order to get information on the origin of "the successive component-separated magnetic transitions" which appear in HoRh2Si2. The lattice parameters a and c remain almost constant during changes to Y composition x, while they increase with increasing La composition x. The c/a ratios are also constant in the Y-system, and they increase with increasing x in the La-system especially for above around x = 0.4. The transition temperatures, TN1 = 29.1 K, Tt = 27.3 K and TN2 =12.1 K at x = 0, decrease with increasing x. The rates of decrease for TN1 and TN2 in the Y-system accord with those in the La system below x = 0.4. The critical compositions for TN1 and TN2 are determined to be xN1 = 0.88 and xN2 = 0.59 in the La-system, respectively, and xN1 = 0.98 and xN2 = 0.75 in the Y-system. In both the systems, "the successive component-separated magnetic transitions" appear for the wide x regions. The magnetic ordered state persists in very dilute Ho-compounds in spite of much weak magnetic interactions. The effective magnetic moments are almost constant for the ordered compounds; μeff = 10.6 ± 0.30 μB/Ho. These behaviors are strange, indicating a strong correlation exists in these systems.

Fast, purely growing collisionless reconnection as an eigenfunction problem related to but not involving linear whistler waves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bellan, Paul M.

If either finite electron inertia or finite resistivity is included in 2D magnetic reconnection, the two-fluid equations become a pair of second-order differential equations coupling the out-of-plane magnetic field and vector potential to each other to form a fourth-order system. The coupling at an X-point is such that out-of-plane even-parity electric and odd-parity magnetic fields feed off each other to produce instability if the scale length on which the equilibrium magnetic field changes is less than the ion skin depth. The instability growth rate is given by an eigenvalue of the fourth-order system determined by boundary and symmetry conditions. Themore » instability is a purely growing mode, not a wave, and has growth rate of the order of the whistler frequency. The spatial profile of both the out-of-plane electric and magnetic eigenfunctions consists of an inner concave region having extent of the order of the electron skin depth, an intermediate convex region having extent of the order of the equilibrium magnetic field scale length, and a concave outer exponentially decaying region. If finite electron inertia and resistivity are not included, the inner concave region does not exist and the coupled pair of equations reduces to a second-order differential equation having non-physical solutions at an X-point.« less

Nonequilibrium critical behavior of model statistical systems and methods for the description of its features

NASA Astrophysics Data System (ADS)

Prudnikov, V. V.; Prudnikov, P. V.; Mamonova, M. V.

2017-11-01

This paper reviews features in critical behavior of far-from-equilibrium macroscopic systems and presents current methods of describing them by referring to some model statistical systems such as the three-dimensional Ising model and the two-dimensional XY model. The paper examines the critical relaxation of homogeneous and structurally disordered systems subjected to abnormally strong fluctuation effects involved in ordering processes in solids at second-order phase transitions. Interest in such systems is due to the aging properties and fluctuation-dissipation theorem violations predicted for and observed in systems slowly evolving from a nonequilibrium initial state. It is shown that these features of nonequilibrium behavior show up in the magnetic properties of magnetic superstructures consisting of alternating nanoscale-thick magnetic and nonmagnetic layers and can be observed not only near the film’s critical ferromagnetic ordering temperature Tc, but also over the wide temperature range T ⩽ Tc.

High-pressure electronic phase diagrams in FeSe1-xSx superconductors

NASA Astrophysics Data System (ADS)

Matsuura, Kohei; Arai, Yuki; Hosoi, Suguru; Ishida, Kousuke; Mizukami, Yuta; Watashige, Tatsuya; Kasahara, Shigeru; Matsuda, Yuji; Maejima, Naoyuki; Machida, Akihiko; Watanuki, Tetsu; Fukuda, Tatsuo; Uwatoko, Yoshiya; Shibauchi, Takasada

The spin fluctuations are believed to be related to the mechanism of the unconventional superconductors. On the other hand, many recent studies suggest that the nematic order that spontaneously breaks rotational symmetry of the system exists in the Fe-based superconductors and its quantum fluctuations may play an essential role for the superconductivity. However, this remains unclear because the nematic order usually coexists with the magnetic order. To solve this issue, FeSe exhibiting a nonmagnetic nematic order is a key system. Under pressure, this order is suppressed and concurrently magnetic order appears, which competes with high-Tc superconducting phase. In isovalent substitution system FeSe1-xSx, we found a nonmagnetic nematic quantum critical point. Here we report our recent high-pressure studies in high-quality single-crystalline FeSe1-xSx up to 8 GPa. We find a systematic change of the pressure phase diagram in FeSe by the S-substitution. Our results imply that the respective role of nematic and magnetic fluctuations can be elucidated from the precise control of pressure and substitution in this system.

Absence of magnetic order in low-dimensional (RKKY) systems

NASA Astrophysics Data System (ADS)

Pedrocchi, Fabio; Leggett, Anthony; Loss, Daniel

2012-02-01

We extend the Mermin-Wagner theorem to a system of lattice spins which are spin-coupled to itinerant and interacting charge carriers. We use the Bogoliubov inequality to rigorously prove that neither (anti-) ferromagnetic nor helical long-range order is possible in one and two dimensions at any finite temperature. Our proof applies to a wide class of models including any form of electron-electron and single-electron interactions that are independent of spin. In the presence of Rashba or Dresselhaus spin-orbit interactions (SOI) magnetic order is not excluded and intimately connected to equilibrium spin currents. However, in the special case when Rashba and Dresselhaus SOIs are tuned to be equal, magnetic order is excluded again. This opens up a new possibility to control magnetism electrically. [4pt] References: D. Loss, F. L. Pedrocchi, and A. J. Leggett, Phys. Rev. Lett. 107, 107201 (2011).

Design and investigations of the superconducting magnet system for the multipurpose superconducting electron cyclotron resonance ion source.

PubMed

Tinschert, K; Lang, R; Mäder, J; Rossbach, J; Spädtke, P; Komorowski, P; Meyer-Reumers, M; Krischel, D; Fischer, B; Ciavola, G; Gammino, S; Celona, L

2012-02-01

The production of intense beams of heavy ions with electron cyclotron resonance ion sources (ECRIS) is an important request at many accelerators. According to the ECR condition and considering semi-empirical scaling laws, it is essential to increase the microwave frequency together with the magnetic flux density of the ECRIS magnet system. A useful frequency of 28 GHz, therefore, requires magnetic flux densities above 2.2 T implying the use of superconducting magnets. A cooperation of European institutions initiated a project to build a multipurpose superconducting ECRIS (MS-ECRIS) in order to achieve an increase of the performances in the order of a factor of ten. After a first design of the superconducting magnet system for the MS-ECRIS, the respective cold testing of the built magnet system reveals a lack of mechanical performance due to the strong interaction of the magnetic field of the three solenoids with the sextupole field and the magnetization of the magnetic iron collar. Comprehensive structural analysis, magnetic field calculations, and calculations of the force pattern confirm thereafter these strong interactions, especially of the iron collar with the solenoidal fields. The investigations on the structural analysis as well as suggestions for a possible mechanical design solution are given.

International Symposium on Magnetic Suspension Technology, Part 2

NASA Technical Reports Server (NTRS)

Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1992-01-01

In order to examine the state of technology of all areas of magnetic suspension and to review related recent developments in sensors and controls approaches, superconducting magnet technology, and design/implementation practices, a symposium was held. The proceedings are presented. The sessions covered the areas of bearings, sensors and controls, microgravity and vibration isolation, superconductivity, manufacturing applications, wind tunnel magnetic suspension systems, magnetically levitated trains (MAGLEV), space applications, and large gap magnetic suspension systems.

Externally driven magnetic granular layers at a liquid/air interface: self-organization, flows and magnetic order

NASA Astrophysics Data System (ADS)

Snezhko, Alexey

2007-03-01

Collective dynamics and pattern formation in ensembles of magnetic microparticles suspended at the liquid/air interface and subjected to an alternating magnetic field are studied. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (``snakes'') emerging in such systems in a certain range of field magnitudes and frequencies. These remarkable structures are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. In addition, a large-scale vortex flows are induced in the vicinity of the dynamic structures. Some features of the self-localized snake structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density. Self-assembled snakes have a complex magnetic order: the segments of the snake exhibit long-range antiferromagnetic ordering mediated by the surface wave, while each segment is composed of ferromagnetically aligned chains of microparticles. A phenomenological model describing magnetic behavior of the magnetic snakes in external magnetic fields is proposed.

Long-range dynamical magnetic order and spin tunneling in the cooperative paramagnetic states of the pyrochlore analogous spinel antiferromagnets CdYb2X4 (X =S or Se)

NASA Astrophysics Data System (ADS)

Dalmas de Réotier, P.; Marin, C.; Yaouanc, A.; Ritter, C.; Maisuradze, A.; Roessli, B.; Bertin, A.; Baker, P. J.; Amato, A.

2017-10-01

Magnetic systems with spins sitting on a lattice of corner sharing regular tetrahedra have been particularly prolific for the discovery of new magnetic states for the last two decades. The pyrochlore compounds have offered the playground for these studies, while little attention has been comparatively devoted to other compounds where the rare earth R occupies the same sublattice, e.g., the spinel chalcogenides Cd R2X4 (X =S or Se ). Here, we report measurements performed on powder samples of this series with R =Yb using specific heat, magnetic susceptibility, neutron diffraction, and muon-spin-relaxation measurements. The two compounds are found to be magnetically similar. They long-range order into structures described by the Γ5 irreducible representation. The magnitude of the magnetic moment at low temperature is 0.77 (1) and 0.62 (1) μB for X =S and Se , respectively. Persistent spin dynamics is present in the ordered states. The spontaneous field at the muon site is anomalously small, suggesting magnetic moment fragmentation. A double spin-flip tunneling relaxation mechanism is suggested in the cooperative paramagnetic state up to 10 K. The magnetic space groups into which magnetic moments of systems of corner-sharing regular tetrahedra order are provided for a number of insulating compounds characterized by null propagation wave vectors.

Ising-like chain magnetism, Arrhenius magnetic relaxation, and case against 3D magnetic ordering in β-manganese phthalocyanine (C₃₂H₁₆MnN₈).

PubMed

Wang, Zhengjun; Seehra, Mohindar S

2016-04-06

Previous magnetic studies in the organic semiconductor β-manganese phthalocyanine (β-MnPc) have reported it to be a canted ferromagnet below T(C)  ≈  8.6 K. However, the recent result of the lack of a λ-type anomaly in the specific heat versus temperature data near the quoted T(C) has questioned the presence of long-range 3-dimensional (3D) magnetic ordering in this system. In this paper, detailed measurements and analysis of the temperature (2 K-300 K) and magnetic field (up to 90 kOe) dependence of the dc and ac magnetic susceptibilities in a powder sample of β-MnPc leads us to conclude that 3D long-range magnetic ordering is absent in this material. This is supported by the Arrott plots and the lack of a peak in the ac susceptibilities, χ' and χ″, near the quoted T(C). Instead, the system can be best described as an Ising-like chain magnet with Arrhenius relaxation of the magnetization governed by an intra-layer ferromagnetic exchange constant J/k(B)  =  2.6 K and the single ion anisotropy energy parameter |D|/k(B)  =  8.3 K. The absence of 3D long range order is consistent with the measured |D|/  >  J.

Expanded Equations for Torque and Force on a Cylindrical Permanent Magnet Core in a Large-Gap Magnetic Suspension System

NASA Technical Reports Server (NTRS)

Groom, Nelson J.

1997-01-01

The expanded equations for torque and force on a cylindrical permanent magnet core in a large-gap magnetic suspension system are presented. The core is assumed to be uniformly magnetized, and equations are developed for two orientations of the magnetization vector. One orientation is parallel to the axis of symmetry, and the other is perpendicular to this axis. Fields and gradients produced by suspension system electromagnets are assumed to be calculated at a point in inertial space which coincides with the origin of the core axis system in its initial alignment. Fields at a given point in the core are defined by expanding the fields produced at the origin as a Taylor series. The assumption is made that the fields can be adequately defined by expansion up to second-order terms. Examination of the expanded equations for the case where the magnetization vector is perpendicular to the axis of symmetry reveals that some of the second-order gradient terms provide a method of generating torque about the axis of magnetization and therefore provide the ability to produce six-degree-of-freedom control.

Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems

NASA Astrophysics Data System (ADS)

de La Cruz, Clarina; Huang, Q.; Lynn, J. W.; Li, Jiying; , W. Ratcliff, II; Zarestky, J. L.; Mook, H. A.; Chen, G. F.; Luo, J. L.; Wang, N. L.; Dai, Pengcheng

2008-06-01

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-Tc) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile `electrons' or `holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at ~137K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-Tc copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Magnetic Properties of nickel hydroxides layers 30A apart obtained by intercalation with dodecyl sulfate ion

NASA Astrophysics Data System (ADS)

Shmavonyan, Gagik; Zadoyan, Ovsanna

2013-03-01

Magnetic systems with reduced dimensionality make good test beds for checks on theoretical models. Here, changes in the nature of magnetic ordering in quasi-2d system of layered Ni hydroxides (LH-Ni-) with variations in the interlayer spacing c are investigated. Magnetic properties of LH-Ni-DS with c ~ 30 A° synthesized by intercalating dodecyl sulfate ion, (C12H25OSO3)- between the layers are compared with those of LH-Ni-Ac (c ~ 8.5 A°) containing the acetate (Ac) ligand. Measurements included those of magnetization M vs. T and H, ac susceptibilities (f = 0.1 Hz - 1000 Hz) and EMR (Electron Magnetic Resonance) spectra at 9.28 GHz. Results show that just like LH-Ni-Ac, LH-Ni-DS also orders ferromagnetically but with Tc ~ 23 Kabout 45 % largerthanT c 16 Kreportedfor LH-Ni-Ac.. In EMR studies, linewidth is strongly temperature-dependent, decreasing with decreasing T from 300 K, reaching a minimum near 45 K and then increasing sharply for T < 45 K, the latter due to short range magnetic ordering. These results differ with the model of Drillon et al in which interlayer dipolar interaction between clusters of correlated spins in the layers yields TC nearly independent of c. Roles of magnetic anisotropy and exchange constants in determining TC in the LH-Ni systems is discussed.

MTL distributed magnet measurement system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nogiec, J.M.; Craker, P.A.; Garbarini, J.P.

1993-04-01

The Magnet Test Laboratory (MTL) at the Superconducting Super collider Laboratory will be required to precisely and reliably measure properties of magnets in a production environment. The extensive testing of the superconducting magnets comprises several types of measurements whose main purpose is to evaluate some basic parameters characterizing magnetic, mechanic and cryogenic properties of magnets. The measurement process will produce a significant amount of data which will be subjected to complex analysis. Such massive measurements require a careful design of both the hardware and software of computer systems, having in mind a reliable, maximally automated system. In order to fulfillmore » this requirement a dedicated Distributed Magnet Measurement System (DMMS) is being developed.« less

Synthesis, crystal structure and magnetic properties of the two polymorphs of novel S=1 osmate; Li{sub 4}MgOsO{sub 6}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nguyen, Phoung-Hieu T.; Kemei, Moureen C.; Tan, Malinda S.

2016-10-15

Li{sub 4}MgOsO{sub 6} was synthesized by two different solid-state reaction procedures. The crystal structures were determined by X-ray powder diffraction technique and it was revealed that Li{sub 4}MgOsO{sub 6} crystallizes in two different crystal symmetries in ordered rock salt structure type, namely monoclinic C2/m and orthorhombic Fddd. The unit cell constants for the monoclinic system are a=5.1074(4) Å, b=8.8182(4) Å, c=5.0902(2) Å, and β=109.845(4)° and those of the orthorhombic structure are a=5.8485(1) Å, b=8.3821(1) Å, and c=17.6212(3) Å. In both systems, Os{sup 6+} ions reside exclusively in a specific crystallographic position while Li{sup +} and Mg{sup 2+} ions exhibit mixmore » occupancy. The temperature dependent magnetic susceptibility data for both S=1 osmate systems do not support the occurrence of any magnetic transition down to 2 K. The Curie–Weiss fit to the paramagnetic regime of the magnetic susceptibility data reveal highly negative θ value (−114.81 K and −121.87 K for C2/m and for Fddd systems, respectively), which are indicative of predominant antiferromagnetic (AFM) interactions in both systems. The experimental effective magnetic moment (μ{sub eff}) value for the monoclinic phase is 2.13 μB and that of the orthorhombic system is 2.34 μB. Due to the rather strong AFM interactions and lack of magnetic transition down to 2 K, both of these novel osmates are placed in the class of highly frustrated magnets. Low temperature magnetic susceptibility (below 2 K) and dynamic magnetic properties studies (μsr studies) are in order to better understand the magnetic ground states of these two polymorphs of Li{sub 4}MgOsO{sub 6}. - Graphical abstract: The structural transformation between two modifications of highly frustrated Li{sub 4}MgOsO{sub 6}. - Highlights: • Li4MgOsO{sub 6} was synthesized in two different crystal systems. • The monoclinic variant crystallizes in C2/m space group, while the orthorhombic version forms in Fddd space group. • The Os{sup 6+} ions are fully ordered while Li{sup +} and Mg{sup 2+} are mixed occupied. • These systems are the first Os{sup 6+} compounds in ordered NaCl structure type. • Both compounds exhibit high degree of geometric magnetic frustration.« less

Microstructural, Magnetic Anisotropy, and Magnetic Domain Structure Correlations in Epitaxial FePd Thin Films with Perpendicular Magnetic Anisotropy

NASA Technical Reports Server (NTRS)

Skuza, J. R.; Clavero, C.; Yang, K.; Wincheski, B.; Lukaszew, R. A.

2009-01-01

L1(sub 0)-ordered FePd epitaxial thin films were prepared using dc magnetron sputter deposition on MgO (001) substrates. The films were grown with varying thickness and degree of chemical order to investigate the interplay between the microstructure, magnetic anisotropy, and magnetic domain structure. The experimentally measured domain size/period and magnetic anisotropy in this high perpendicular anisotropy system were found to be correlated following the analytical energy model proposed by Kooy and Enz that considers a delicate balance between the domain wall energy and the demagnetizing stray field energy.

Quantum criticality among entangled spin chains

DOE PAGES

Blanc, N.; Trinh, J.; Dong, L.; ...

2017-12-11

Here, an important challenge in magnetism is the unambiguous identification of a quantum spin liquid, of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems wherein classical order is suppressed by a frustrating lattice, an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point—a region of highly fluctuating spins—by an applied magnetic field. Such systems suffer from a weak specific-heat peak at themore » quantum critical point, with little entropy available for quantum fluctuations. Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K 2PbCu(NO 2) 6. Across the temperature–magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point.« less

Quantum criticality among entangled spin chains

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blanc, N.; Trinh, J.; Dong, L.

Here, an important challenge in magnetism is the unambiguous identification of a quantum spin liquid, of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems wherein classical order is suppressed by a frustrating lattice, an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point—a region of highly fluctuating spins—by an applied magnetic field. Such systems suffer from a weak specific-heat peak at themore » quantum critical point, with little entropy available for quantum fluctuations. Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K 2PbCu(NO 2) 6. Across the temperature–magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point.« less

Quantum criticality among entangled spin chains

NASA Astrophysics Data System (ADS)

Blanc, N.; Trinh, J.; Dong, L.; Bai, X.; Aczel, A. A.; Mourigal, M.; Balents, L.; Siegrist, T.; Ramirez, A. P.

2018-03-01

An important challenge in magnetism is the unambiguous identification of a quantum spin liquid1,2, of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems3,4 wherein classical order is suppressed by a frustrating lattice5, an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point—a region of highly fluctuating spins—by an applied magnetic field. Such systems suffer from a weak specific-heat peak at the quantum critical point, with little entropy available for quantum fluctuations6. Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K2PbCu(NO2)6. Across the temperature-magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point.

Effect of zero magnetic field on cardiovascular system and microcirculation

NASA Astrophysics Data System (ADS)

Gurfinkel, Yu. I.; At'kov, O. Yu.; Vasin, A. L.; Breus, T. K.; Sasonko, M. L.; Pishchalnikov, R. Yu.

2016-02-01

The effects of zero magnetic field conditions on cardiovascular system of healthy adults have been studied. In order to generate zero magnetic field, the facility for magnetic fields modeling ;ARFA; has been used. Parameters of the capillary blood flow, blood pressure, and the electrocardiogram (ECG) monitoring were measured during the study. All subjects were tested twice: in zero magnetic field and, for comparison, in sham condition. The obtained results during 60 minutes of zero magnetic field exposure demonstrate a clear effect on cardiovascular system and microcirculation. The results of our experiments can be used in studies of long-term stay in hypo-magnetic conditions during interplanetary missions.

Magnetic structure in Mn1 -xCoxGe compounds

NASA Astrophysics Data System (ADS)

Altynbaev, E.; Siegfried, S.-A.; Strauß, P.; Menzel, D.; Heinemann, A.; Fomicheva, L.; Tsvyashchenko, A.; Grigoriev, S.

2018-04-01

The magnetic system of the pseudobinary compound Mn1 -xCoxGe has been studied using small-angle neutron scattering and susceptibility measurements. It is found that Mn1 -xCoxGe orders magnetically at low temperatures in the whole concentration range of x ∈[0 /0.9 ] . Four different states of the magnetic structure have been found at low temperatures: the long-range-ordered (LRO) short-period helical magnetic structure at x

Ferromagnetism and spin glass ordering in transition metal alloys (invited)

NASA Astrophysics Data System (ADS)

Crane, S.; Carnegie, D. W., Jr.; Claus, H.

1982-03-01

Magnetic properties of transition metal alloys near the percolation threshold are often complicated by metallurgical effects. Alloys like AuFe, VFe, CuNi, RhNi, and PdNi are in general not random solid solutions but have various degrees of atomic clustering or short-range order (SRO), depending on the heat treatment. First, it is shown how the magnetic ordering temperature of these alloys varies with the degree of clustering or SRO. Second, by systematically changing this degree of clustering or SRO, important information can be obtained about the magnetic phase diagram. In all these alloys below the percolation limit, the onset of ferromagnetic order is probably preceded by a spin glass-type ordering. However, details of the magnetic phase diagram near the critical point can be quite different alloy systems.

Magnetization reversal modes in fourfold Co nano-wire systems

NASA Astrophysics Data System (ADS)

Blachowicz, T.; Ehrmann, A.

2015-09-01

Magnetic nano-wire systems are, as well as other patterned magnetic structures, of special interest for novel applications, such as magnetic storage media. In these systems, the coupling between neighbouring magnetic units is most important for the magnetization reversal process of the complete system, leading to a variety of magnetization reversal mechanisms. This article examines the influence of the magnetic material on hysteresis loop shape, coercive field, and magnetization reversal modes. While iron nano-wire systems exhibit flat or one-step hysteresis loops, systems consisting of cobalt nano-wires show hysteresis loops with several longitudinal steps and transverse peaks, correlated to a rich spectrum of magnetization reversal mechanisms. We show that changing the material parameters while the system geometry stays identical can lead to completely different hysteresis loops and reversal modes. Thus, especially for finding magnetic nano-systems which can be used as quaternary or even higher-order storage devices, it is rational to test several materials for the planned systems. Apparently, new materials may lead to novel and unexpected behaviour - and can thus result in novel functionalities.

Debye temperatures and magnetic structures of UFe xAl 12- x (3.6⩽ x⩽5) intermetallic alloys

NASA Astrophysics Data System (ADS)

Rećko, K.; Dobrzyński, L.; Szymański, K.; Hoser, A.

2000-03-01

Uranium ternary compounds UFe xAl 12- x crystallize in a body-centred tetragonal structure ThMn 12 (I 4/mmm No.139). The neutron powder diffraction, magnetization measurements as well as Mössbauer investigations clearly indicate the magnetic ordering within the iron sites. The rearrangement of iron magnetic moments from uncompensated antiferromagnetic system in UFe xAl 12- x with x<4, through coexistence of antiferro- and ferromagnetic iron components (4⩽ x<5) to pure ferromagnetic ordering for alloy with x=5 is observed. The neutron diffraction studies of magnetic structures of the aforementioned powder samples show a very rich world of possible uranium-iron magnetic interactions. For all these alloys the magnetic neutron scattering is generally weak in comparison to the nuclear one. Because of identical chemical and magnetic unit cells there are no pure magnetic reflections. Therefore, in order to extract magnetic part of the scattering one should be particularly careful in taking proper account of the thermal vibration effects.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gorai, Prashun; Toberer, Eric S.; Stevanović, Vladan

Here, at room temperature and above, most magnetic materials adopt a spin-disordered (paramagnetic) state whose electronic properties can differ significantly from their low-temperature, spin-ordered counterparts. Yet computational searches for new functional materials usually assume some type of magnetic order. In the present work, we demonstrate a methodology to incorporate spin disorder in computational searches and predict the electronic properties of the paramagnetic phase. We implement this method in a high-throughput framework to assess the potential for thermoelectric performance of 1350 transition-metal sulfides and find that all magnetic systems we identify as promising in the spin-ordered ground state cease to bemore » promising in the paramagnetic phase due to disorder-induced deterioration of the charge carrier transport properties. We also identify promising non-magnetic candidates that do not suffer from these spin disorder effects. In addition to identifying promising materials, our results offer insights into the apparent scarcity of magnetic systems among known thermoelectrics and highlight the importance of including spin disorder in computational searches.« less

Transfer of Magnetic Order and Anisotropy through Epitaxial Integration of 3d and 4f Spin Systems.

PubMed

Bluschke, M; Frano, A; Schierle, E; Minola, M; Hepting, M; Christiani, G; Logvenov, G; Weschke, E; Benckiser, E; Keimer, B

2017-05-19

Resonant x-ray scattering at the Dy M_{5} and Ni L_{3} absorption edges was used to probe the temperature and magnetic field dependence of magnetic order in epitaxial LaNiO_{3}-DyScO_{3} superlattices. For superlattices with 2 unit cell thick LaNiO_{3} layers, a commensurate spiral state develops in the Ni spin system below 100 K. Upon cooling below T_{ind}=18  K, Dy-Ni exchange interactions across the LaNiO_{3}-DyScO_{3} interfaces induce collinear magnetic order of interfacial Dy moments as well as a reorientation of the Ni spins to a direction dictated by the strong magnetocrystalline anisotropy of Dy. This transition is reversible by an external magnetic field of 3 T. Tailored exchange interactions between rare-earth and transition-metal ions thus open up new perspectives for the manipulation of spin structures in metal-oxide heterostructures and devices.

Novel diluted magnetic semiconductor materials based on zinc oxide

NASA Astrophysics Data System (ADS)

Chakraborti, Deepayan

The primary aim of this work was to develop a ZnO based diluted magnetic semiconductor (DMS) materials system which displays ferromagnetism above room temperature and to understand the origin of long-range ferromagnetic ordering in these systems. Recent developments in the field of spintronics (spin based electronics) have led to an extensive search for materials in which semiconducting properties can be integrated with magnetic properties to realize the objective of successful fabrication of spin-based devices. For these devices we require a high efficiency of spin current injection at room temperature. Diluted magnetic semiconductors (DMS) can serve this role, but they should not only display room temperature ferromagnetism (RTFM) but also be capable of generating spin polarized carriers. Transition metal doped ZnO has proved to be a potential candidate as a DMS showing RTFM. The origin of ferromagnetic ordering in ZnO is still under debate. However, the presence of magnetic secondary phases, composition fluctuations and nanoclusters could also explain the observation of ferromagnetism in the DMS samples. This encouraged us to investigate Cu-doped(+ spin in the 2+ valence state) ZnO system as a probable candidate exhibiting RTFM because neither metallic Cu nor its oxides (Cu2O or CuO) are ferromagnetic. The role of defects and free carriers on the ferromagnetic ordering of Cu-doped ZnO thin films was studied to ascertain the origin of ferromagnetism in this system. A novel non-equilibrium Pulsed Laser Deposition technique has been used to grow high quality epitaxial thin films of Cu:ZnO and (Co,Cu):ZnO on c-plane Sapphire by domain matching epitxay. Both the systems showed ferromagnetic ordering above 300K but Cu ions showed a much stronger ferromagnetic ordering than Co, especially at low concentrations (1-2%) of Cu where we realized near 100% polarization. But, the incorporation of Cu resulted in a 2-order of magnitude rise in the resistivity from 10-1 to 101 Ohm cm which can prove to be detrimental to the injection of polarized electrons. In order to decrease the resistivity and to understand the role of free carriers in mediating the ferromagnetic ordering, the Cu-doped ZnO films were co-doped with an n-type dopant like Al which increased the free carriers concentration by 3 orders of magnitude from 1017 to 1020 cm -3 without significantly altering the near 100% spin polarization in the Cu:ZnO system. This lack of correlation between free carrier concentration and the magnetic moment implied that a free carrier mediated exchange does not stabilize the long range ferromagnetic ordering. A reduction in the number of oxygen vacancies brought about by high temperature oxygen annealing had a large degrading effect on the ferromagnetism by reducing the total saturation magnetization by almost an order of magnitude. This strong dependence of magnetization on vacancy concentration and the corresponding weak relationship with free carriers pointed towards a defect mediated mechanism, such as a bound magnetic polaron mediated exchange as being responsible for stabilizing the ferromagnetic ordering in these systems. However, a BMP mechanism would not guarantee a strong coupling between the free carriers and the localized spins to produce spin-polarized current. To investigate this we have fabricated spin valve type device structures where a nonmagnetic ZnO layer was sandwiched between two ferromagnetic (Cu,Al):ZnO layers allowing us to study spin polarized carrier injection across the nonmagnetic semiconductor gap. Initial results have shown evidence of spin polarized carrier injection across the nonmagnetic semiconductor layer even at 300K. Hence, this work demonstrates that the (Cu,Al):ZnO system may become a viable solution for spin injection into spintronic devices.

Mechanism for the occurrence of paramagnetic planes within magnetically ordered cerium systems

NASA Astrophysics Data System (ADS)

Kioussis, Nicholas; Cooper, Bernard R.; Banerjea, Amitava

1988-11-01

Hybridization of moderately delocalized f electrons with band electrons gives rise to a highly anisotropic two-ion interaction. Previously it has been shown that such an interaction explains the experimentally observed unusual magnetic behavior of CeBi, yielding a phase transition from a higher-temperature type-I (↑↓) to a lower-temperature type-IA (↑↑↓↓) antiferromagnetic structure. If the hybridization-mediated interaction is the key to understanding the magnetic behavior of such moderately delocalized f-electron systems, we should expect to be able to understand on this basis the even more unusual magnetic behavior of CeSb. In CeSb, there is a sequence of magnetic structures in which the higher-temperature structures involve a periodic stacking of paramagnetic \\{001\\} planes alternating with magnetically ordered \\{001\\} planes of [001]-moment alignment. In this paper we show that such a coexistence of paramagnetic and magnetically ordered Ce3+ sites can be understood on the basis of the hybridization-mediated interionic interaction when there are cubic crystal-field (CF) interactions of comparable strength. The tendency to form paramagnetic planes is found to increase with increasing CF strength (Γ7 ground state); and the stability of the up-down paramagnetic plane arrangement at high temperatures is shown to arise from the reconciliation of the magnetic ordering with the CF interactions. We also find that for a certain range of parameters a different novel situation occurs, with a fully nonmagnetic (singlet) ground state for the Ce3+ ion. This singlet state is not Kondo-like, and occurs in such a way that the system would be expected to fluctuate between two differently polarized states, one of which is the singlet state.

Non-equilibrium magnetic colloidal dispersions at liquid-air interfaces: dynamic patterns, magnetic order and self-assembled swimmers.

PubMed

Snezhko, Alexey

2011-04-20

Colloidal dispersions of interacting particles subjected to an external periodic forcing often develop nontrivial self-assembled patterns and complex collective behavior. A fundamental issue is how collective ordering in such non-equilibrium systems arises from the dynamics of discrete interacting components. In addition, from a practical viewpoint, by working in regimes far from equilibrium new self-organized structures which are generally not available through equilibrium thermodynamics can be created. In this review spontaneous self-assembly phenomena in magnetic colloidal dispersions suspended at liquid-air interfaces and driven out of equilibrium by an alternating magnetic field are presented. Experiments reveal a new type of nontrivially ordered self-assembled structures emerging in such systems in a certain range of excitation parameters. These dynamic structures emerge as a result of the competition between magnetic and hydrodynamic forces and have complex unconventional magnetic ordering. Nontrivial self-induced hydrodynamic fields accompany each out-of-equilibrium pattern. Spontaneous symmetry breaking of the self-induced surface flows leading to a formation of self-propelled microstructures has been discovered. Some features of the self-localized structures can be understood in the framework of the amplitude equation (Ginzburg-Landau type equation) for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density and the Navier-Stokes equation for hydrodynamic flows. To understand the fundamental microscopic mechanisms governing self-assembly processes in magnetic colloidal dispersions at liquid-air interfaces a first-principle model for a non-equilibrium self-assembly is presented. The latter model allows us to capture in detail the entire process of out-of-equilibrium self-assembly in the system and reproduces most of the observed phenomenology.

Magnetic Fluctuations, Precursor Phenomena, and Phase Transition in MnSi under a Magnetic Field

NASA Astrophysics Data System (ADS)

Pappas, C.; Bannenberg, L. J.; Lelièvre-Berna, E.; Qian, F.; Dewhurst, C. D.; Dalgliesh, R. M.; Schlagel, D. L.; Lograsso, T. A.; Falus, P.

2017-07-01

The reference chiral helimagnet MnSi is the first system where Skyrmion lattice correlations have been reported. At a zero magnetic field the transition at TC to the helimagnetic state is of first order. Above TC, in a region dominated by precursor phenomena, neutron scattering shows the buildup of strong chiral fluctuating correlations over the surface of a sphere with radius 2 π /ℓ, where ℓ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above TC, both of general relevance to chiral magnetism, remain an open question.

Chaos Suppression in Fractional order Permanent Magnet Synchronous Generator in Wind Turbine Systems

NASA Astrophysics Data System (ADS)

Rajagopal, Karthikeyan; Karthikeyan, Anitha; Duraisamy, Prakash

2017-06-01

In this paper we investigate the control of three-dimensional non-autonomous fractional-order uncertain model of a permanent magnet synchronous generator (PMSG) via a adaptive control technique. We derive a dimensionless fractional order model of the PMSM from the integer order presented in the literatures. Various dynamic properties of the fractional order model like eigen values, Lyapunov exponents, bifurcation and bicoherence are investigated. The system chaotic behavior for various orders of fractional calculus are presented. An adaptive controller is derived to suppress the chaotic oscillations of the fractional order model. As the direct Lyapunov stability analysis of the robust controller is difficult for a fractional order first derivative, we have derived a new lemma to analyze the stability of the system. Numerical simulations of the proposed chaos suppression methodology are given to prove the analytical results derived through which we show that for the derived adaptive controller and the parameter update law, the origin of the system for any bounded initial conditions is asymptotically stable.

Tunable Majorana corner states in a two-dimensional second-order topological superconductor induced by magnetic fields

NASA Astrophysics Data System (ADS)

Zhu, Xiaoyu

2018-05-01

A two-dimensional second-order topological superconductor exhibits a finite gap in both bulk and edges, with the nontrivial topology manifesting itself through Majorana zero modes localized at the corners, i.e., Majorana corner states. We investigate a time-reversal-invariant topological superconductor in two dimensions and demonstrate that an in-plane magnetic field could transform it into a second-order topological superconductor. A detailed analysis reveals that the magnetic field gives rise to mass terms which take distinct values among the edges, and Majorana corner states naturally emerge at the intersection of two adjacent edges with opposite masses. With the rotation of the magnetic field, Majorana corner states localized around the boundary may hop from one corner to a neighboring one and eventually make a full circle around the system when the field rotates by 2 π . In the end, we briefly discuss physical realizations of this system.

Topology optimized and 3D printed polymer-bonded permanent magnets for a predefined external field

NASA Astrophysics Data System (ADS)

Huber, C.; Abert, C.; Bruckner, F.; Pfaff, C.; Kriwet, J.; Groenefeld, M.; Teliban, I.; Vogler, C.; Suess, D.

2017-08-01

Topology optimization offers great opportunities to design permanent magnetic systems that have specific external field characteristics. Additive manufacturing of polymer-bonded magnets with an end-user 3D printer can be used to manufacture permanent magnets with structures that had been difficult or impossible to manufacture previously. This work combines these two powerful methods to design and manufacture permanent magnetic systems with specific properties. The topology optimization framework is simple, fast, and accurate. It can also be used for the reverse engineering of permanent magnets in order to find the topology from field measurements. Furthermore, a magnetic system that generates a linear external field above the magnet is presented. With a volume constraint, the amount of magnetic material can be minimized without losing performance. Simulations and measurements of the printed systems show very good agreement.

Field-controlled structures in ferromagnetic cholesteric liquid crystals.

PubMed

Medle Rupnik, Peter; Lisjak, Darja; Čopič, Martin; Čopar, Simon; Mertelj, Alenka

2017-10-01

One of the advantages of anisotropic soft materials is that their structures and, consequently, their properties can be controlled by moderate external fields. Whereas the control of materials with uniform orientational order is straightforward, manipulation of systems with complex orientational order is challenging. We show that a variety of structures of an interesting liquid material, which combine chiral orientational order with ferromagnetic one, can be controlled by a combination of small magnetic and electric fields. In the suspensions of magnetic nanoplatelets in chiral nematic liquid crystals, the platelet's magnetic moments orient along the orientation of the liquid crystal and, consequently, the material exhibits linear response to small magnetic fields. In the absence of external fields, orientations of the liquid crystal and magnetization have wound structure, which can be either homogeneously helical, disordered, or ordered in complex patterns, depending on the boundary condition at the surfaces and the history of the sample. We demonstrate that by using different combinations of small magnetic and electric fields, it is possible to control reversibly the formation of the structures in a layer of the material. In such a way, different periodic structures can be explored and some of them may be suitable for photonic applications. The material is also a convenient model system to study chiral magnetic structures, because it is a unique liquid analog of a solid helimagnet.

Magnetic properties of four Cu(ii)-amino acid salts

NASA Astrophysics Data System (ADS)

Calvo, Rafael

1984-03-01

We report a comparative study of magnetic properties of the Cu(II) salts of the amino acids l-alanine, dl-α-amino-n-butyric acid, α-amino isobutyric acid, and l-isoleucine. The position of the EPR lines of these quasi-two-dimensional magnetic systems was measured as a function of temperature T between 293 and 1.5 K, at 9.3 GHz and for magnetic fields applied along three axes of single crystal samples. Large changes of the gyromagnetic factor with T have been observed. They are attributed to an internal mean field, proportional to the applied field, which appears when the temperature is lowered due to short range magnetic order in the paramagnetic phase of the salts. The problem of short range magnetic order and g shifts in Cu-amino acid salts is discussed and compared with previous observations in Mn one-dimensional systems.

Read-out electronics for DC squid magnetic measurements

DOEpatents

Ganther, Jr., Kenneth R.; Snapp, Lowell D.

2002-01-01

Read-out electronics for DC SQUID sensor systems, the read-out electronics incorporating low Johnson noise radio-frequency flux-locked loop circuitry and digital signal processing algorithms in order to improve upon the prior art by a factor of at least ten, thereby alleviating problems caused by magnetic interference when operating DC SQUID sensor systems in magnetically unshielded environments.

Pressure-induced magnetic order in FeSe: A muon spin rotation study

NASA Astrophysics Data System (ADS)

Khasanov, Rustem; Guguchia, Zurab; Amato, Alex; Morenzoni, Elvezio; Dong, Xiaoli; Zhou, Fang; Zhao, Zhongxian

2017-05-01

The magnetic order induced by the pressure was studied in FeSe by means of muon spin rotation (μ SR ) technique. By following the evolution of the oscillatory part of the μ SR signal as a function of angle between the initial muon spin polarization and 101 axis of the studied FeSe sample, it was found that the pressure-induced magnetic order in FeSe corresponds either to the collinear (single-stripe) antiferromagnetic order as observed in parent compounds of various FeAs-based superconductors or to the bi-collinear order as obtained in the FeTe system, but with the Fe spins turned by 45o within the a b plane. The value of the magnetic moment per Fe atom was estimated to be ≃0.13 -0.14 μB at p ≃1.9 GPa.

A new magnet design for future Kibble balances

NASA Astrophysics Data System (ADS)

Li, Shisong; Stock, Michael; Schlamminger, Stephan

2018-06-01

We propose a new permanent magnet system for Kibble balance experiments, which combines advantages of the magnet designs invented by the National Physical Laboratory (NPL) and by the Bureau International des Poids et Mesures (BIPM). The goal of the proposed magnet system is to minimize the coil-current effect and to optimize the shielding at the same time. In the proposed design, a permanent magnet system with two gaps, each housing a coil, is employed to minimize the coil current effect, by reducing the linear coil-current dependence reported for the single air gap design by at least one order of magnitude. Both air gaps of the magnet are completely surrounded by high-permeability material, and hence the coils are shielded from outside magnetic fields and no magnetic field leaks outside of the magnet system. An example of the new magnet system is given and the analysis shows that the magnetic field in the air gap can be optimized to meet the requirement to be used in Kibble balances.

Neutron Scattering Studies on Correlated Transition-Metal Oxides

NASA Astrophysics Data System (ADS)

Zhu, Mengze

We have explored the collective phenomena of correlated electrons in two different transition-metal oxides, Ruddlesden-Popper type ruthenates (Sr,Ca) n+1RunO3n+1 and inverse-trirutile chromates Cr2MO6 (M = Te, Mo and W), using neutron scattering in combination with various material characterization methods. (Sr,Ca)n+1RunO 3n+1 are 4d transition-metal oxides exhibiting competing magnetic and electronic tendencies. The delicate balance among the competing states can be readily tuned by perturbations, such as chemical doping and magnetic field, which gives rise to emergent phenomena. We have investigated the effects of 3d transition-metal doping on the magnetic and electronic properties of layered ruthenates. For instance, the single-layer (n = 1) Sr2RuO4 is an unconventional superconductor possessing an incommensurate spin density wave instability with a wave vector qic= (0.3 0.3 L) driven by Fermi surface nesting. Upon Fe substitution, we have unveiled an unexpected commensurate spin density wave order with a propagation vector qc= (0.25 0.25 0) in Sr2Ru1-xFexO 4 (x = 0.03 and 0.05), despite the magnetic fluctuations persisting at qic. The latter feature is corroborated by the first principles calculations, which show that Fe doping barely changes the nesting vector of the Fermi surface. These results suggest that in addition to the known incommensurate magnetic instability, Sr2RuO4 is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping. We have also studied the effects of a magnetic field. For example, the bilayer (n = 2) Ca3(Ru1-xTi x)2O7 (x = 0.03) is a G-type antiferromagnetic Mott insulator. We have revealed that a modest magnetic field can lead to colossal magnetoresistance arising from an anomalous collapse of the Mott insulating state. Such an insulator-to-metal transition is accompanied by magnetic and structural transitions. These findings call for deeper theoretical studies to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities, as a magnetic field usually stabilizes the insulating ground state in Mott-Hubbard systems. Cr2MO6 (M = Te, W and Mo) are spin dimer systems with the magnetic ions Cr3+ structurally dimerized favoring a singlet ground state. However, all three compounds investigated exhibit long-range antiferromagnetic orders at low temperature owing to the inter-dimer interactions. We have shown that the inter-dimer exchange coupling can be tuned from antiferromagnetic in Cr2TeO6 to ferromagnetic in Cr2WO6 and Cr2MoO6, by altering the degree of d-p orbital hybridization between W(Mo) and O atoms. The tunability of the inter-dimer interactions without introducing additional complexities such as structural distortions and carrier doping offers a rare opportunity to drive the system toward the quantum critical point (QCP) separating the dimer-based quantum disordered state and the classical long-range antiferromagnetic order. Moreover, we have unraveled Higgs amplitude modes in the magnetic excitation spectra of Cr2TeO6 and Cr2WO6, which are generally believed to survive only in systems close to the QCP where the ordered moment is suppressed significantly from its fully saturated value by quantum fluctuations. However, these two compounds are away from the QCP with the ordered moment reduced only by 24%. This study suggests that Higgs amplitude modes are not the privilege of ordered systems in the vicinity of the QCP, but may be common excitation modes in ordered spin dimer systems.

Revealing hidden antiferromagnetic correlations in doped Hubbard chains via string correlators

NASA Astrophysics Data System (ADS)

Hilker, Timon A.; Salomon, Guillaume; Grusdt, Fabian; Omran, Ahmed; Boll, Martin; Demler, Eugene; Bloch, Immanuel; Gross, Christian

2017-08-01

Topological phases, like the Haldane phase in spin-1 chains, defy characterization through local order parameters. Instead, nonlocal string order parameters can be employed to reveal their hidden order. Similar diluted magnetic correlations appear in doped one-dimensional lattice systems owing to the phenomenon of spin-charge separation. Here we report on the direct observation of such hidden magnetic correlations via quantum gas microscopy of hole-doped ultracold Fermi-Hubbard chains. The measurement of nonlocal spin-density correlation functions reveals a hidden finite-range antiferromagnetic order, a direct consequence of spin-charge separation. Our technique, which measures nonlocal order directly, can be readily extended to higher dimensions to study the complex interplay between magnetic order and density fluctuations.

Effects of uniaxial pressure on the quantum tunneling of magnetization in a high-symmetry Mn12 single-molecule magnet

NASA Astrophysics Data System (ADS)

Atkinson, James H.; Fournet, Adeline D.; Bhaskaran, Lakshmi; Myasoedov, Yuri; Zeldov, Eli; del Barco, Enrique; Hill, Stephen; Christou, George; Friedman, Jonathan R.

2017-05-01

The symmetry of single-molecule magnets dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry [M n12O12(O2CMe) 16(Me OH ) 4].M e OH single-molecule magnet as a function of uniaxial pressure applied either parallel or perpendicular to the sample's "easy" magnetization axis. At temperatures between 1.8 and 3.3 K, magnetic hysteresis loops exhibit the characteristic steplike features that signal the occurrence of QTM. After applying uniaxial pressure to the sample in situ, both the magnitude and field position of the QTM steps changed. The step magnitudes were observed to grow as a function of pressure in both arrangements of pressure, while pressure applied along (perpendicular to) the sample's easy axis caused the resonant-tunneling fields to increase (decrease). These observations were compared with simulations in which the system's Hamiltonian parameters were changed. From these comparisons, we determined that parallel pressure induces changes to the second-order axial anisotropy parameter as well as either the fourth-order axial or fourth-order transverse parameter, or to both. In addition, we find that pressure applied perpendicular to the easy axis induces a rhombic anisotropy E ≈D /2000 per kbar that can be understood as deriving from a symmetry-breaking distortion of the molecule.

Spin density wave instability in a ferromagnet.

PubMed

Wu, Yan; Ning, Zhenhua; Cao, Huibo; Cao, Guixin; Benavides, Katherine A; Karna, S; McCandless, Gregory T; Jin, R; Chan, Julia Y; Shelton, W A; DiTusa, J F

2018-03-27

Due to its cooperative nature, magnetic ordering involves a complex interplay between spin, charge, and lattice degrees of freedom, which can lead to strong competition between magnetic states. Binary Fe 3 Ga 4 is one such material that exhibits competing orders having a ferromagnetic (FM) ground state, an antiferromagnetic (AFM) behavior at intermediate temperatures, and a conspicuous re-entrance of the FM state at high temperature. Through a combination of neutron diffraction experiments and simulations, we have discovered that the AFM state is an incommensurate spin-density wave (ISDW) ordering generated by nesting in the spin polarized Fermi surface. These two magnetic states, FM and ISDW, are seldom observed in the same material without application of a polarizing magnetic field. To date, this unusual mechanism has never been observed and its elemental origins could have far reaching implications in many other magnetic systems that contain strong competition between these types of magnetic order. Furthermore, the competition between magnetic states results in a susceptibility to external perturbations allowing the magnetic transitions in Fe 3 Ga 4 to be controlled via temperature, magnetic field, disorder, and pressure. Thus, Fe 3 Ga 4 has potential for application in novel magnetic memory devices, such as the magnetic components of tunneling magnetoresistance spintronics devices.

Generation of 24 T at 4.2 K using a layer-wound GdBCO insert coil with Nb3Sn and Nb-Ti external magnetic field coils

NASA Astrophysics Data System (ADS)

Matsumoto, S.; Kiyoshi, T.; Otsuka, A.; Hamada, M.; Maeda, H.; Yanagisawa, Y.; Nakagome, H.; Suematsu, H.

2012-02-01

High-temperature superconducting (HTS) magnets are believed to be a practical option in the development of high field nuclear magnetic resonance (NMR) systems. The development of a 600 MHz NMR system that uses an HTS magnet and a probe with an HTS radio frequency coil is underway. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. The tolerance to high tensile stress is expected for HTS conductors in order to reduce the magnet in volume. A layer-wound Gd-Ba-Cu-O (GdBCO) insert coil was fabricated in order to investigate its properties under a high electromagnetic force in a high magnetic field. The GdBCO insert coil was successfully operated at a current of up to 321 A and an electromagnetic force BJR of 408 MPa in an external magnetic field generated by Nb3Sn and Nb-Ti low-temperature superconducting coils. The GdBCO insert coil also managed to generate a magnetic field of 6.8 T at the center of the coil in an external magnetic field of 17.2 T. The superconducting magnet consisting of GdBCO, Nb3Sn and Nb-Ti coils successfully generated a magnetic field of 24.0 T at 4.2 K, which represents a new record for a superconducting magnet.

Higher-order correlations for fluctuations in the presence of fields.

PubMed

Boer, A; Dumitru, S

2002-10-01

The higher-order moments of the fluctuations for thermodynamic systems in the presence of fields are investigated in the framework of a theoretical method. The method uses a generalized statistical ensemble consistent with an adequate expression for the internal energy. The applications refer to the case of a system in a magnetoquasistatic field. In the case of linear magnetic media, one finds that, for the description of the magnetic induction fluctuations, the Gaussian approximation is satisfactory. For nonlinear media, the corresponding fluctuations are non-Gaussian, having a non-null asymmetry. Furthermore, the respective fluctuations have characteristics of leptokurtic, mesokurtic and platykurtic type, depending on the value of the magnetic field strength as compared with a scaling factor of the magnetization curve.

MAGNETIC GAMES BETWEEN A PLANET AND ITS HOST STAR: THE KEY ROLE OF TOPOLOGY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Strugarek, A.; Brun, A. S.; Réville, V.

Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfvénic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star–planet systems with three-dimensional, global, compressible magnetohydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. Themore » Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star–planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of 10{sup 19} W. Close-in planets are also shown to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.« less

Magnetic Games between a Planet and Its Host Star: The Key Role of Topology

NASA Astrophysics Data System (ADS)

Strugarek, A.; Brun, A. S.; Matt, S. P.; Réville, V.

2015-12-01

Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfvénic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star-planet systems with three-dimensional, global, compressible magnetohydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. The Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star-planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of 1019 W. Close-in planets are also shown to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.

Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu 2 ( Si x Ge 1 - x ) 2

DOE PAGES

Nemkovski, Krill S.; Kozlenko, D. P.; Alekseev, Pavel A.; ...

2016-11-01

In mixed-valence or heavy-fermion systems, the hybridization between local f orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin uctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic eld, chemical composition). Recently, similar effects (mixed-valence, Kondo uctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its 4f shell,more » and the magnetic Eu 2+ state (4f 7) has no orbital component in the usual LS coupling scheme, which can lead to a quite different and interesting physics. In the EuCu 2(Si xGe 1-x) 2 series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence uctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, Mossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the change in the Eu valence state. In particular, the results support the persistence of valence uctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration x c ≈ 0:65. In conclusion, the sequence of magnetic ground states in the series is shown to re ect the evolution of the magnetic spectral response.« less

Evidence of a field-induced Berezinskii-Kosterlitz-Thouless scenario in a two-dimensional spin-dimer system.

PubMed

Tutsch, U; Wolf, B; Wessel, S; Postulka, L; Tsui, Y; Jeschke, H O; Opahle, I; Saha-Dasgupta, T; Valentí, R; Brühl, A; Remović-Langer, K; Kretz, T; Lerner, H-W; Wagner, M; Lang, M

2014-10-27

Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

Design of set-point weighting PIλ + Dμ controller for vertical magnetic flux controller in Damavand tokamak.

PubMed

Rasouli, H; Fatehi, A

2014-12-01

In this paper, a simple method is presented for tuning weighted PI(λ) + D(μ) controller parameters based on the pole placement controller of pseudo-second-order fractional systems. One of the advantages of this controller is capability of reducing the disturbance effects and improving response to input, simultaneously. In the following sections, the performance of this controller is evaluated experimentally to control the vertical magnetic flux in Damavand tokamak. For this work, at first a fractional order model is identified using output-error technique in time domain. For various practical experiments, having desired time responses for magnetic flux in Damavand tokamak, is vital. To approach this, at first the desired closed loop reference models are obtained based on generalized characteristic ratio assignment method in fractional order systems. After that, for the identified model, a set-point weighting PI(λ) + D(μ) controller is designed and simulated. Finally, this controller is implemented on digital signal processor control system of the plant to fast/slow control of magnetic flux. The practical results show appropriate performance of this controller.

Magnetic quasi-long-range ordering in nematic systems due to competition between higher-order couplings

NASA Astrophysics Data System (ADS)

Žukovič, Milan; Kalagov, Georgii

2018-05-01

Critical properties of the two-dimensional X Y model involving solely nematic-like terms of the second and third orders are investigated by spin-wave analysis and Monte Carlo simulation. It is found that, even though neither of the nematic-like terms alone can induce magnetic ordering, their coexistence and competition leads to an extended phase of the magnetic quasi-long-range-order phase, wedged between the two nematic-like phases induced by the respective couplings. Thus, except for the multicritical point, at which all the phases meet, for any finite value of the coupling parameters ratio there are two phase transition: one from the paramagnetic phase to one of the two nematic-like phases followed by another one at lower temperatures to the magnetic phase. The finite-size scaling analysis indicates that the phase transitions between the magnetic and nematic-like phases belong to the Ising and three-state Potts universality classes. Inside the competition-induced algebraic magnetic phase, the spin-pair correlation function is found to decay even much more slowly than in the standard X Y model with purely magnetic interactions. Such a magnetic phase is characterized by an extremely low vortex-antivortex pair density attaining a minimum close to the point at which the two couplings are of about equal strength.

A review of astrophysical reconnection

NASA Astrophysics Data System (ADS)

Uzdensky, Dmitri

Magnetic reconnection is a basic plasma process involving rapid rearrangement of magnetic field topology. It often leads to violent release of magnetic energy and its conversion to the plasma thermal and kinetic energy as well as nonthermal particle acceleration. It is thus believed to power numerous types of explosive phenomena both inside and outside the Solar system, including various kinds of high-energy flares. In this talk I will first give an overview of astrophysical systems where reconnection is believed to play an important role. Examples include pulsed high-energy emission in pulsar magnetospheres; gamma-ray flares in pulsar wind nebulae and AGN/blazar jets; Gamma-Ray Bursts; and giant flares in magnetar systems. I will also analyze the physical conditions of the plasma in some of these astrophysical systems and will discuss the fundamental physical differences between various astrophysical instances of magnetic reconnection and the more familiar solar and space examples of reconnection. In particular, I will demonstrate the importance of including radiative effects in order to understand astrophysical magnetic reconnection and in order to connect our theoretical models with the observed radiation signatures.

Kinematic analysis and simulation of a substation inspection robot guided by magnetic sensor

NASA Astrophysics Data System (ADS)

Xiao, Peng; Luan, Yiqing; Wang, Haipeng; Li, Li; Li, Jianxiang

2017-01-01

In order to improve the performance of the magnetic navigation system used by substation inspection robot, the kinematic characteristics is analyzed based on a simplified magnetic guiding system model, and then the simulation process is executed to verify the reasonability of the whole analysis procedure. Finally, some suggestions are extracted out, which will be helpful to guide the design of the inspection robot system in the future.

Site-specific magnetic anisotropies in R2Fe14B systems

NASA Astrophysics Data System (ADS)

Yoshioka, T.; Tsuchiura, H.

2018-04-01

The local magnetic anisotropy of R ions in R2Fe14B (R = Dy, Ho) systems is studied based on a microscopic effective spin model constructed from the information obtained by using first-principles calculations. By taking into account up to 6-th order crystal electric field parameters, the model satisfactory describes the observed magnetization curves and the temperature dependence of anisotropy constants. We found that at low temperatures, the noncollinear structure appears in the Ho2Fe14B system reflecting the local magnetic anisotropy.

Creation of Frustrated Systems by d-dot Array

NASA Astrophysics Data System (ADS)

Masahiko, Machida

2004-03-01

When a square shape dot of High-Tc superconductor is embedded in s-wave superconducting matrix, half quantized vortices are spontaneously generated at the corners of the dot. This feature gives the magnetic interactions between neighboring dots in array systems composed of sevaral dots of High-Tc superconductor and allows us to make magnetic interaction systems. We propose that we can create interesting frustrated systems like the spin-ice by setting the dots in various manners. In order to demonstrate which types of frustrated systems are possible, we perform numerical simulations for the time-dependent Ginzburg-Landau equation describing dynamics of the superconducting order parameters with d-wave and s-wave symmetries. The simulations reveal that the proposed system has two parameters originated from the magnetic interaction between emerged half vortices. We tune the parameters and show various patterns of half vortices from the Ising to the ice model.

Honeycombs of triangles and magnetic frustration in SrL{sub 2}O{sub 4} (L=Gd, Dy, Ho, Er, Tm, and Yb)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karunadasa, H.; Regan, K.A.; Cava, R.J.

2005-04-01

The crystal structures, magnetic order, and susceptibility have been investigated for magnetically frustrated SrDy{sub 2}O{sub 4}, SrHo{sub 2}O{sub 4}, SrEr{sub 2}O{sub 4}, SrTm{sub 2}O{sub 4}, and SrYb{sub 2}O{sub 4}. Powder neutron-diffraction structural refinements reveal columns of LO{sub 6} octahedra that run along one crystallographic direction, with Sr-O polyhedra in the interstices. The lanthanide sublattice displays multiple triangular interconnections: one-dimensional strings form the backbones of four types of chains of lanthanide triangles sharing edges arranged in a honeycomb pattern. This crystal structure produces strong geometric frustration for the magnetic system that is evidenced in both magnetic susceptibility and neutron-scattering data atmore » low temperatures. The susceptibility measurements for the series, including SrGd{sub 2}O{sub 4} for which data are also reported, lack the sharp features characteristic of three-dimensional long-range magnetic ordering. Metamagnetic behavior is observed in the magnetization vs applied field data at 1.8 K for the cases of L=Dy, Er, and Ho. Magnetic neutron-scattering studies for the Dy and Er materials show only very broad magnetic scattering at low temperatures, while the Ho system exhibits long-range two-dimensional order. Any magnetic scattering in the Tm and Yb compounds, if present, was too weak to be detected in these measurements.« less

SU-E-T-590: Optimizing Magnetic Field Strengths with Matlab for An Ion-Optic System in Particle Therapy Consisting of Two Quadrupole Magnets for Subsequent Simulations with the Monte-Carlo Code FLUKA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baumann, K; Weber, U; Simeonov, Y

Purpose: Aim of this study was to optimize the magnetic field strengths of two quadrupole magnets in a particle therapy facility in order to obtain a beam quality suitable for spot beam scanning. Methods: The particle transport through an ion-optic system of a particle therapy facility consisting of the beam tube, two quadrupole magnets and a beam monitor system was calculated with the help of Matlab by using matrices that solve the equation of motion of a charged particle in a magnetic field and field-free region, respectively. The magnetic field strengths were optimized in order to obtain a circular andmore » thin beam spot at the iso-center of the therapy facility. These optimized field strengths were subsequently transferred to the Monte-Carlo code FLUKA and the transport of 80 MeV/u C12-ions through this ion-optic system was calculated by using a user-routine to implement magnetic fields. The fluence along the beam-axis and at the iso-center was evaluated. Results: The magnetic field strengths could be optimized by using Matlab and transferred to the Monte-Carlo code FLUKA. The implementation via a user-routine was successful. Analyzing the fluence-pattern along the beam-axis the characteristic focusing and de-focusing effects of the quadrupole magnets could be reproduced. Furthermore the beam spot at the iso-center was circular and significantly thinner compared to an unfocused beam. Conclusion: In this study a Matlab tool was developed to optimize magnetic field strengths for an ion-optic system consisting of two quadrupole magnets as part of a particle therapy facility. These magnetic field strengths could subsequently be transferred to and implemented in the Monte-Carlo code FLUKA to simulate the particle transport through this optimized ion-optic system.« less

Application of Magnetic Nanoparticles in Pretreatment Device for POPs Analysis in Water

NASA Astrophysics Data System (ADS)

Chu, Dongzhi; Kong, Xiangfeng; Wu, Bingwei; Fan, Pingping; Cao, Xuan; Zhang, Ting

2018-01-01

In order to reduce process time and labour force of POPs pretreatment, and solve the problem that extraction column was easily clogged, the paper proposed a new technology of extraction and enrichment which used magnetic nanoparticles. Automatic pretreatment system had automatic sampling unit, extraction enrichment unit and elution enrichment unit. The paper briefly introduced the preparation technology of magnetic nanoparticles, and detailly introduced the structure and control system of automatic pretreatment system. The result of magnetic nanoparticles mass recovery experiments showed that the system had POPs analysis preprocessing capability, and the recovery rate of magnetic nanoparticles were over 70%. In conclusion, the author proposed three points optimization recommendation.

Analysis of Document Authentication Technique using Soft Magnetic Fibers

NASA Astrophysics Data System (ADS)

Aoki, Ayumi; Ikeda, Takashi; Yamada, Tsutomu; Takemura, Yasushi; Matsumoto, Tsutomu

An artifact-metric system using magnetic fibers can be applied for authentications of stock certificate, bill, passport, plastic cards and other documents. Security of the system is guaranteed by its feature of difficulty in copy. This authentication system is based on randomly dispersed magnetic fibers embedded in documents. In this paper, a theoretical analysis was performed in order to evaluate this system. The position of the magnetic fibers was determined by a conventional function of random number generator. By measuring output waveforms by a magnetoresistance (MR) sensor, a false match rate (FMR) could be calculated. Optimizations of the density of the magnetic fibers and the dimension of the MR sensor were achieved.

Nonequilibrium Tricritical Point in a System with Long-Range Interactions

NASA Astrophysics Data System (ADS)

Antoniazzi, Andrea; Fanelli, Duccio; Ruffo, Stefano; Yamaguchi, Yoshiyuki Y.

2007-07-01

Systems with long-range interactions display a short-time relaxation towards quasistationary states whose lifetime increases with system size. With reference to the Hamiltonian mean field model, we here show that a maximum entropy principle, based on Lynden-Bell’s pioneering idea of “violent relaxation,” predicts the presence of out-of-equilibrium phase transitions separating the relaxation towards homogeneous (zero magnetization) or inhomogeneous (nonzero magnetization) quasistationary states. When varying the initial condition within a family of “water bags” with different initial magnetization and energy, first- and second-order phase transition lines are found that merge at an out-of-equilibrium tricritical point. Metastability is theoretically predicted and numerically checked around the first-order phase transition line.

Thermoelectricity in transition metal compounds: The role of spin disorder

DOE PAGES

Gorai, Prashun; Toberer, Eric S.; Stevanović, Vladan

2016-11-01

Here, at room temperature and above, most magnetic materials adopt a spin-disordered (paramagnetic) state whose electronic properties can differ significantly from their low-temperature, spin-ordered counterparts. Yet computational searches for new functional materials usually assume some type of magnetic order. In the present work, we demonstrate a methodology to incorporate spin disorder in computational searches and predict the electronic properties of the paramagnetic phase. We implement this method in a high-throughput framework to assess the potential for thermoelectric performance of 1350 transition-metal sulfides and find that all magnetic systems we identify as promising in the spin-ordered ground state cease to bemore » promising in the paramagnetic phase due to disorder-induced deterioration of the charge carrier transport properties. We also identify promising non-magnetic candidates that do not suffer from these spin disorder effects. In addition to identifying promising materials, our results offer insights into the apparent scarcity of magnetic systems among known thermoelectrics and highlight the importance of including spin disorder in computational searches.« less

Weyl Magnon

NASA Astrophysics Data System (ADS)

Li, Fei-Ye; Li, Yao-Dong; Yu, Yue; Kim, Yong Baek; Balents, Leon; Chen, Gang

Conventional magnetic orders in Mott insulators are often believed to be trivial as they are simple product states. In this talk, we argue that this belief is not always right. We study a realistic spin model on the breathing pyrochlore lattice. We find that, although the system has a magnetic ordered ground state, the magnetic excitation is rather nontrivial and supports linear band touchings in its spectrum. This linear band touching is a topological property of the magnon band structure and is thus robust against small perturbation. We thus name this magnon band touching as ``Weyl magnon''. Just like the Weyl fermion, the existence of Weyl magnon suggests the presence of chiral magnon surface states. Unlike the surface Fermi arcs for the Weyl fermions, the chiral surface state for Weyl magnon appears at a finite energy due to the bosonic nature of the magnons. Moreover, the external magnetic field only couples to the spins with a Zeeman term and thus can readily shift the Weyl node position. This provides a way to control the Weyl magnon. Our work will inspire a re-examination of the excitation spectrum of many magnetic ordered systems. Chggst@gmail.com.

Universal phase diagrams with superconducting domes for electronic flat bands

NASA Astrophysics Data System (ADS)

Löthman, Tomas; Black-Schaffer, Annica M.

2017-08-01

Condensed matter systems with flat bands close to the Fermi level generally exhibit, due to their very large density of states, extraordinarily high critical ordering temperatures of symmetry-breaking orders, such as superconductivity and magnetism. Here we show that the critical temperatures follow one of two universal curves with doping away from a flat band depending on the ordering channel, which completely dictates both the general order competition and the phase diagram. Notably, we find that orders in the particle-particle channel (superconducting orders) survive decisively farther than orders in the particle-hole channel (magnetic or charge orders) because the channels have fundamentally different polarizabilities. Thus, even if a magnetic or charge order initially dominates, superconducting domes are still likely to exist on the flanks of flat bands. We apply these general results to both the topological surface flat bands of rhombohedral ABC-stacked graphite and to the Van Hove singularity of graphene.

Geometric Magnetic Frustration in Li3Mg2OsO6 Studied with Muon Spin Relaxation

NASA Astrophysics Data System (ADS)

Carlo, J. P.; Derakhshan, S.; Greedan, J. E.

Geometric frustration manifests when the spatial arrangement of ions inhibits magnetic order. Typically associated with antiferromagnetically (AF)-correlated moments on triangular or tetrahedral lattices, frustration occurs in a variety of structures and systems, resulting in rich phase diagrams and exotic ground states. As a window to exotic physics revealed by the cancellation of normally dominant interactions, the research community has taken great interest in frustrated systems. One family of recent interest are the rock-salt ordered oxides A5BO6, in which the B sites are occupied by magnetic ions comprising a network of interlocked tetrahedra, and nonmagnetic ions on the A sites control the B oxidation state through charge neutrality. Here we will discuss studies of Li3Mg2OsO6 using muon spin relaxation (μSR), a highly sensitive local probe of magnetism. Previous studies of this family included Li5OsO6, which exhibits AF order below 50K with minimal evidence for frustration, and Li4MgReO6, which exhibits glassy magnetism. Li3Mg2RuO6, meanwhile, exhibits long-range AF, with the ordering temperature suppressed by frustration. But its isoelectronic twin, Li3Mg2OsO6 (5d3 vs. 4d3) exhibits very different behavior, revealed by μSR to be a glassy ground state below 12K. Understanding why such similar systems exhibit diverse ground-state behavior is key to understanding the nature of geometric magnetic frustration. Financial support from the Research Corporation for Science Advancement.

A new method for distortion magnetic field compensation of a geomagnetic vector measurement system

NASA Astrophysics Data System (ADS)

Liu, Zhongyan; Pan, Mengchun; Tang, Ying; Zhang, Qi; Geng, Yunling; Wan, Chengbiao; Chen, Dixiang; Tian, Wugang

2016-12-01

The geomagnetic vector measurement system mainly consists of three-axis magnetometer and an INS (inertial navigation system), which have many ferromagnetic parts on them. The magnetometer is always distorted by ferromagnetic parts and other electric equipments such as INS and power circuit module within the system, which can lead to geomagnetic vector measurement error of thousands of nT. Thus, the geomagnetic vector measurement system has to be compensated in order to guarantee the measurement accuracy. In this paper, a new distortion magnetic field compensation method is proposed, in which a permanent magnet with different relative positions is used to change the ambient magnetic field to construct equations of the error model parameters, and the parameters can be accurately estimated by solving linear equations. In order to verify effectiveness of the proposed method, the experiment is conducted, and the results demonstrate that, after compensation, the components errors of measured geomagnetic field are reduced significantly. It demonstrates that the proposed method can effectively improve the accuracy of the geomagnetic vector measurement system.

Size-induced chemical and magnetic ordering in individual Fe-Au nanoparticles.

PubMed

Mukherjee, Pinaki; Manchanda, Priyanka; Kumar, Pankaj; Zhou, Lin; Kramer, Matthew J; Kashyap, Arti; Skomski, Ralph; Sellmyer, David; Shield, Jeffrey E

2014-08-26

Formation of chemically ordered compounds of Fe and Au is inhibited in bulk materials due to their limited mutual solubility. However, here we report the formation of chemically ordered L12-type Fe3Au and FeAu3 compounds in Fe-Au sub-10 nm nanoparticles, suggesting that they are equilibrium structures in size-constrained systems. The stability of these L12-ordered Fe3Au and FeAu3 compounds along with a previously discovered L10-ordered FeAu has been explained by a size-dependent equilibrium thermodynamic model. Furthermore, the spin ordering of these three compounds has been computed using ab initio first-principle calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu3 nanoparticles displayed a low saturation magnetization of about 11 emu/g. This suggests a antiferromagnetic spin structure, with the net magnetization arising from uncompensated surface spins. First-principle calculations using the Vienna ab initio simulation package (VASP) indicate that ferromagnetic ordering is energetically most stable in Fe3Au, while antiferromagnetic order is predicted in FeAu and FeAu3, consistent with the experimental results.

Local atomic and magnetic structure of dilute magnetic semiconductor (Ba ,K ) (Zn,Mn ) 2As2

NASA Astrophysics Data System (ADS)

Frandsen, Benjamin A.; Gong, Zizhou; Terban, Maxwell W.; Banerjee, Soham; Chen, Bijuan; Jin, Changqing; Feygenson, Mikhail; Uemura, Yasutomo J.; Billinge, Simon J. L.

2016-09-01

We have studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba ,K )(Zn ,Mn )2As2 through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. We detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of ≲5 Å , resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment of Mn spins along the crystallographic c axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. We discuss these results in the context of other experiments and theoretical studies on this system.

CaMn 2Al 10: Itinerant Mn magnetism on the verge of magnetic order

DOE PAGES

Steinke, L.; Simonson, J. W.; Yin, W. -G.; ...

2015-07-24

We report the discovery of CaMn 2Al 10, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of 0.83 μB/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only ≈ 9% of Rln2. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility χ is anisotropic below 20 K, with a maximum ratio of χ [010]/χ [001]more » ≈ 3.5. A strong power-law divergence χ(T) ~ T –1.2 below 20 K implies incipient ferromagnetic order, an Arrott plot analysis of the magnetization suggests a vanishing low Curie temperature T C ~ 0. Our experiments indicate that CaMn 2Al 10 is a rare example of a system where the weak and itinerant Mn-based magnetism is poised on the verge of order.« less

Cluster synthesis and direct ordering of rare-earth transition-metal nanomagnets.

PubMed

Balasubramanian, Balamurugan; Skomski, Ralph; Li, Xingzhong; Valloppilly, Shah R; Shield, Jeffrey E; Hadjipanayis, George C; Sellmyer, David J

2011-04-13

Rare-earth transition-metal (R-TM) alloys show superior permanent magnetic properties in the bulk, but the synthesis and application of R-TM nanoparticles remains a challenge due to the requirement of high-temperature annealing above about 800 °C for alloy formation and subsequent crystalline ordering. Here we report a single-step method to produce highly ordered R-TM nanoparticles such as YCo(5) and Y(2)Co(17), without high-temperature thermal annealing by employing a cluster-deposition system and investigate their structural and magnetic properties. The direct ordering is highly desirable to create and assemble R-TM nanoparticle building blocks for future permanent-magnet and other significant applications.

Solitonic Spin-Liquid State Due to the Violation of the Lifshitz Condition in Fe(1+y)Te.

PubMed

Materne, Ph; Koz, C; Rössler, U K; Doerr, M; Goltz, T; Klauss, H H; Schwarz, U; Wirth, S; Rössler, S

2015-10-23

A combination of phenomenological analysis and Mössbauer spectroscopy experiments on the tetragonal Fe(1+y)Te system indicates that the magnetic ordering transition in compounds with higher Fe excess, y≥0.11, is unconventional. Experimentally, a liquidlike magnetic precursor with quasistatic spin order is found from significantly broadened Mössbauer spectra at temperatures above the antiferromagnetic transition. The incommensurate spin-density wave order in Fe(1+y)Te is described by a magnetic free energy that violates the weak Lifshitz condition in the Landau theory of second-order transitions. The presence of multiple Lifshitz invariants provides the mechanism to create multidimensional, twisted, and modulated solitonic phases.

Solitonic Spin-Liquid State Due to the Violation of the Lifshitz Condition in Fe1 +yTe

NASA Astrophysics Data System (ADS)

Materne, Ph.; Koz, C.; Rößler, U. K.; Doerr, M.; Goltz, T.; Klauss, H. H.; Schwarz, U.; Wirth, S.; Rößler, S.

2015-10-01

A combination of phenomenological analysis and Mössbauer spectroscopy experiments on the tetragonal Fe1 +yTe system indicates that the magnetic ordering transition in compounds with higher Fe excess, y ≥0.11 , is unconventional. Experimentally, a liquidlike magnetic precursor with quasistatic spin order is found from significantly broadened Mössbauer spectra at temperatures above the antiferromagnetic transition. The incommensurate spin-density wave order in Fe1 +yTe is described by a magnetic free energy that violates the weak Lifshitz condition in the Landau theory of second-order transitions. The presence of multiple Lifshitz invariants provides the mechanism to create multidimensional, twisted, and modulated solitonic phases.

Absence of spontaneous magnetic order of lattice spins coupled to itinerant interacting electrons in one and two dimensions.

PubMed

Loss, Daniel; Pedrocchi, Fabio L; Leggett, Anthony J

2011-09-02

We extend the Mermin-Wagner theorem to a system of lattice spins which are spin coupled to itinerant and interacting charge carriers. We use the Bogoliubov inequality to rigorously prove that neither (anti-) ferromagnetic nor helical long-range order is possible in one and two dimensions at any finite temperature. Our proof applies to a wide class of models including any form of electron-electron and single-electron interactions that are independent of spin. In the presence of Rashba or Dresselhaus spin-orbit interactions (SOI) magnetic order is not excluded and intimately connected to equilibrium spin currents. However, in the special case when Rashba and Dresselhaus SOIs are tuned to be equal, magnetic order is excluded again. This opens up a new possibility to control magnetism electrically.

Magnetism and High-magnetic Field Magnetization in Alkali Superoxide CsO2

NASA Astrophysics Data System (ADS)

Miyajima, Mizuki; Astuti, Fahmi; Kakuto, Takeshi; Matsuo, Akira; Puspita Sari, Dita; Asih, Retno; Okunishi, Kouichi; Nakano, Takehito; Nozue, Yasuo; Kindo, Koichi; Watanabe, Isao; Kambe, Takashi

2018-06-01

Alkali superoxide CsO2 is one of the candidates for the spin-1/2 one-dimensional (1D) antiferromagnet, which may be sequentially formed by an ordering of the π-orbital of O2 - molecule below TS ˜ 70 K. Here, we report the magnetism and the high-magnetic field magnetization in pulsed-magnetic fields up to 60 T in powder CsO2. We obtained the low temperature phase diagram around the antiferromagnetic ordering temperature TN = 9.6 K under the magnetic field. At T = 1.3 K, we observed a remarkable up-turn curvature in the magnetization around a saturation field of ˜60 T, which indicates the low-dimensional nature of the spin system. The saturated magnetization is also estimated to be ˜1μB, which corresponds to spin-1/2. In this study we compare it with the theoretical calculation.

Field-induced spin density wave and spiral phases in a layered antiferromagnet

DOE PAGES

Stone, Matthew B.; Lumsden, Mark D.; Garlea, Vasile O.; ...

2015-07-28

Here we determine the low-field ordered magnetic phases of the S=1 dimerized antiferromagnet Ba 3Mn 2O 8 using single crystal neutron diffraction. We find that for magnetic fields between μ 0H=8.80 T and 10.56 T applied along themore » $$1\\bar{1}0$$ direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ 0H > 10.56 T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. Finally, the nature of these two transitions is fundamentally different: the low-field transition is a second order transition to a spin-density wave ground state, while the one at higher field, toward the spiral phase, is of first order.« less

Supersolid-like magnetic states in a mixed honeycomb-triangular lattice system.

NASA Astrophysics Data System (ADS)

Garlea, Ovidiu

Field-induced magnetic states that occur in layered triangular antiferromagnets have been of broad interest due to the emergence of new exotic phases, such as topologically ordered states and supersolids. Experimental realization of the supersolid states where spin components break simultaneously the translational and rotational symmetries remains scarce. In this context, the mixed vanadate -carbonate K2Mn3(VO4)2CO3 is a very promising system. This compound contains two types of two-dimensional layers alternately stacked along the crystallographic c-axis: one layer consists of a honeycomb web structure made of edge sharing MnO6 octahedra, while the other consists of MnO5 trigonal bipyramids linked by [CO3] triangles to form a triangular magnetic lattice. Magnetization and heat capacity measurements reveal a complex magnetic phase diagram that includes three phase transition associated with sequential long range magnetic ordering of the different sublattices. The lowest temperature state resembles a supersolid state that was predicted to occur in two-dimensional frustrated magnet with easy axis anisotropy. Such a supersolid phase is defined by a commensurate √3× √3 magnetic superlattice, where two thirds of the spins are canted away from the easy axis direction. Applied magnetic field destabilizes this ordered state and induces a cascade of new exotic magnetic ground states. The nature of these field-induced magnetic states is evaluated by using neutron scattering techniques. Work at the Oak Ridge National Laboratory was sponsored by the US Department of Energy, Office of Science, Basic Energy Sciences, Scientific User Facilities Division and Materials Sciences and Engineering Division.

Magnetocaloric effect: permanent magnet array for generation of high magnetic fields

NASA Astrophysics Data System (ADS)

Lee, Seong-Jae; Kenkel, John; Jiles, David

2002-03-01

The magnetocaloric effect (MCE), the heating or cooling of magnetic materials in a magnetic field, is unusually large in the Gd_5(Si_xGe_1-x)4 alloy system. Normally the maximum in the MCE occurs at the Curie temperature (Tc) because the spin entropy change is a maximum. By suitable selection of the composition of this alloy system the Curie temperature can be changed over the range 25 K for x = 0 to 340 K for x =1, and the composition range around x = 0.5 exhibits the largest magnetocaloric effect. In order to increase the amount of heat exchanged the change in applied magnetic field should be as large as possible, and in this research values above 1.5 Tesla are suggested. We have studied a permanent magnet array based on NdFeB, which with a remanent magnetization of only 1.2 Tesla can still generate a magnetic flux density, or magnetic induction B of 2-3 Tesla. In order to generate the high magnetic induction in the absence of a power supply, a modified hollow cylindrical permanent magnet array (HCPMA) has been designed to produce the required strength of magnetic field. Soft magnetic materials including permalloy (NiFe) were used for focusing the magnetic field in the central region. The magnitude of the magnetic flux density at the center was about 2 Tesla. The magnitude and homogeneity of the magnetic field for this design are comparable with the conventional C-shaped yoke and HCPMA. This can be easily adapted for a low power rotary system in which the magnetocaloric material can be exposed alternately to high and low magnetic fields so that it can accept and reject heat from its surroundings.

Magnetic order in a frustrated two-dimensional atom lattice at a semiconductor surface.

PubMed

Li, Gang; Höpfner, Philipp; Schäfer, Jörg; Blumenstein, Christian; Meyer, Sebastian; Bostwick, Aaron; Rotenberg, Eli; Claessen, Ralph; Hanke, Werner

2013-01-01

Two-dimensional electron systems, as exploited for device applications, can lose their conducting properties because of local Coulomb repulsion, leading to a Mott-insulating state. In triangular geometries, any concomitant antiferromagnetic spin ordering can be prevented by geometric frustration, spurring speculations about 'melted' phases, known as spin liquid. Here we show that for a realization of a triangular electron system by epitaxial atom adsorption on a semiconductor, such spin disorder, however, does not appear. Our study compares the electron excitation spectra obtained from theoretical simulations of the correlated electron lattice with data from high-resolution photoemission. We find that an unusual row-wise antiferromagnetic spin alignment occurs that is reflected in the photoemission spectra as characteristic 'shadow bands' induced by the spin pattern. The magnetic order in a frustrated lattice of otherwise non-magnetic components emerges from longer-range electron hopping between the atoms. This finding can offer new ways of controlling magnetism on surfaces.

Evidence for Coexistence of Bulk Superconductivity and Itinerant Antiferromagnetism in the Heavy Fermion System CeCo(In1−xCdx)5

PubMed Central

Howald, Ludovic; Stilp, Evelyn; de Réotier, Pierre Dalmas; Yaouanc, Alain; Raymond, Stéphane; Piamonteze, Cinthia; Lapertot, Gérard; Baines, Christopher; Keller, Hugo

2015-01-01

In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature—tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magnetic and electronic structure of CeCo(In1−xCdx)5 at different time scales. Different magnetic structures are obtained that indicate a magnetic order of itinerant character, coexisting with bulk superconductivity. The suppression of the antiferromagnetic order appears to be driven by a modification of the bandwidth/carrier concentration, implying that the electronic structure and consequently the interplay of superconductivity and magnetism is strongly affected by hydrostatic and chemical pressure. PMID:26224422

Magnon Hall effect without Dzyaloshinskii-Moriya interaction.

PubMed

Owerre, S A

2017-01-25

Topological magnon bands and magnon Hall effect in insulating collinear ferromagnets are induced by the Dzyaloshinskii-Moriya interaction (DMI) even at zero magnetic field. In the geometrically frustrated star lattice, a coplanar/noncollinear [Formula: see text] magnetic ordering may be present due to spin frustration. This magnetic structure, however, does not exhibit topological magnon effects even with DMI in contrast to collinear ferromagnets. We show that a magnetic field applied perpendicular to the star plane induces a non-coplanar spin configuration with nonzero spin scalar chirality, which provides topological effects without the need of DMI. The non-coplanar spin texture originates from the topology of the spin configurations and does not need the presence of DMI or magnetic ordering, which suggests that this phenomenon may be present in the chiral spin liquid phases of frustrated magnetic systems. We propose that these anomalous topological magnon effects can be accessible in polymeric iron (III) acetate-a star-lattice antiferromagnet with both spin frustration and long-range magnetic ordering.

[A study of magnetic shielding design for a magnetic resonance imaging linac system].

PubMed

Zhang, Zheshun; Chen, Wenjing; Qiu, Yang; Zhu, Jianming

2017-12-01

One of the main technical challenges when integrating magnetic resonance imaging (MRI) systems with medical linear accelerator is the strong interference of fringe magnetic fields from the MRI system with the electron beams of linear accelerator, making the linear accelerator not to work properly. In order to minimize the interference of magnetic fields, a magnetic shielding cylinder with an open structure made of high permeability materials is designed. ANSYS Maxwell was used to simulate Helmholtz coil which generate uniform magnetic field instead of the fringe magnetic fields which affect accelerator gun. The parameters of shielding tube, such as permeability, radius, length, side thickness, bottom thickness and fringe magnetic fields strength are simulated, and the data is processed by MATLAB to compare the shielding performance. This article gives out a list of magnetic shielding effectiveness with different side thickness and bottom thickness under the optimal radius and length, which showes that this design can meet the shielding requirement for the MRI-linear accelerator system.

Weak arrest-like and field-driven first order magnetic phase transitions of itinerant Fe3Ga4 revealed by magnetization and magnetoresistance isotherms

NASA Astrophysics Data System (ADS)

Samatham, S. Shanmukharao; Suresh, K. G.

2017-01-01

The detailed magnetic study of complex 3d-electron based Fe3Ga4 is reported. It undergoes paramagnetic to antiferromagnetic (TN) and antiferromagnetic to ferromagnetic (TC) transitions respectively around 380 and 70 K. The thermal hysteresis of field-cooled cooling (FCC) and field-cooled warming (FCW) hints at first order phase transition below Curie temperature. A weak phase coexistence of ferro and antiferromagnetic phases is suggested by exploring the arrest-like first-order phenomenon. In the intermediate temperature range, field-driven metamagnetic transition from antiferro to ferromagnetic phase is confirmed. Further bringing the system very near to TN, field-induced transitions disappear and above TN predominant paramagnetic contribution is evident. The magnetic H-T phase diagram distinguishing different magnetic phases of Fe3Ga4 is obtained.

The Closed Orbit Measurement System at the DELTA Synchrotron Radiation Facility And First Investigations Concerning the Installation of a Beam Based BPM Calibration System

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jankowiak, A.; Wille, K.; /Dortmund U. /SLAC

2011-08-25

For a synchrotron radiation source it is necessary to operate a monitoring system to determine the beam position with high resolution and accuracy with respect to the axis of the quadrupole magnets. In this paper the present closed orbit measurement system of the DELTA SR-Facility, concerning the hardware setup, data processing and the calibration methods, will be presented. The results of the calibration measurements and the recent operating experience will be discussed. These results show, that the system is close to the design resolution. But the BPM offsets with respect to the magnetic center of the quadrupole magnets turn outmore » to be not acceptable. For some BPMs they are in the order of several 100 micro m. Therefore it was decided to install a beam based BPM calibration system in the near future . This system should allow to determine the BPM offsets relative to the center of the quadrupole magnets for all 40 BPMs. It is planned to install a system in order to change the focussing strength of each quadrupole individually either in a static or dynamic way.« less

Cool Down Experiences with the SST-1 Helium Cryogenics System before and after Current Feeders System Modification

NASA Astrophysics Data System (ADS)

Patel, R.; Panchal, P.; Panchal, R.; Tank, J.; Mahesuriya, G.; Sonara, D.; Srikanth, G. L. N.; Garg, A.; Bairagi, N.; Christian, D.; Patel, K.; Shah, P.; Nimavat, H.; Sharma, R.; Patel, J. C.; Gupta, N. C.; Prasad, U.; Sharma, A. N.; Tanna, V. L.; Pradhan, S.

The SST-1 machine comprises a superconducting magnet system (SCMS), which includes TF and PF magnets. In order to charge the SCMS, we need superconducting current feeders consisting of SC feeders and vapor cooled current leads (VCCLs). We have installed all 10 (+/-) pairs of VCCLs for the TF and PF systems. While conducting initial engineering validation of the SST-1 machine, our prime objective was to produce circular plasma using only the TF system. During the SST-1 campaign I to VI, we have to stop the PF magnets cooling in order to get the cryo- stable conditions for current charging of the TF magnets system. In that case, the cooling of the PF current leads is not essential. It has been also observed that after aborting the PF system cooling, there was a limited experimental window of TF operation. Therefore, in the recent SST-1 campaign-VII, we removed the PF current leads (9 pairs) and kept only single (+/-) pair of the 10,000 A rated VCCLs to realize the charging of the TF system for the extended window of operation. We have observed a better cryogenic stability in the TF magnets after modifications in the CFS. In this paper, we report the comparison of the cool down performance for the SST-1 machine operation before and after modifications of the current feeders system.

Magnetic field induced evolution of intertwined orders in the Kitaev magnet β -Li2IrO3

NASA Astrophysics Data System (ADS)

Rousochatzakis, Ioannis; Perkins, Natalia B.

2018-05-01

Recent scattering experiments in the 3D Kitaev magnet β -Li2IrO3 have shown that a relatively weak magnetic field along the crystallographic b axis drives the system from its incommensurate counter-rotating order to a correlated magnet, with a significant uniform `zigzag' component superimposing the magnetization along the field. Here it is shown that the zigzag order is not emerging from its linear coupling to the field (via a staggered, off-diagonal element of the g tensor) but from its intertwining with the incommensurate order and the longitudinal magnetization. The emerging picture explains all qualitative experimental findings at zero and finite fields, including the rapid decline of the incommensurate order with field and the so-called intensity sum rule. The latter are shown to be independent signatures of the smallness of the Heisenberg exchange J , compared to the Kitaev coupling K and the off-diagonal anisotropy Γ . Remarkably, in the regime of interest, the field H* at which the incommensurate component vanishes, depends essentially only on J , which allows us to extract an estimate of J ≃4 K from reported measurements of H*. We also comment on recent experiments in pressurized β -Li2IrO3 and conclude that J decreases with pressure.

Magnetic monopole condensation transition out of quantum spin ice: application to Pr2 Ir2 O7 and Yb2 Ti2 O7

NASA Astrophysics Data System (ADS)

Chen, Gang

We study the proximate magnetic orders and the related quantum phase transition out of quantum spin ice (QSI). We apply the electromagnetic duality of the compact quantum electrodynamics to analyze the condensation of the magnetic monopoles for QSI. The monopole condensation transition represents a unconventional quantum criticality with unusual scaling laws. The magnetic monopole condensation leads to the magnetic states that belong to the ``2-in 2-out'' spin ice manifold and generically have an enlarged magnetic unit cell. We demonstrate that the antiferromagnetic state with the ordering wavevector Q = 2p(001) is proximate to QSI while the ferromagnetic state with the ordering wavevector Q = (000) is not proximate to QSI. This implies that if there exists a direct transition from QSI to the ferromagnetic state, the transition must be strongly first order. We apply the theory to the puzzling experiments on two pyrochlore systems Pr2Ir2O7 and Yb2Ti2O7. chggst@gmail.com.

Ab initio Studies of Magnetism in the Iron Chalcogenides FeTe and FeSe

NASA Astrophysics Data System (ADS)

Hirayama, Motoaki; Misawa, Takahiro; Miyake, Takashi; Imada, Masatoshi

2015-09-01

The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cable, J.W.

The diffuse scattering of neutrons from magnetic materials provides unique and important information regarding the spatial correlations of the atoms and the spins. Such measurements have been extensively applied to magnetically ordered systems, such as the ferromagnetic binary alloys, for which the observed correlations describe the magnetic moment fluctuations associated with local environment effects. With the advent of polarization analysis, these techniques are increasingly being applied to study disordered paramagnetic systems such as the spin-glasses and the diluted magnetic semiconductors. The spin-pair correlations obtained are essential in understanding the exchange interactions of such systems. In this paper, we describe recentmore » neutron diffuse scattering results on the atom-pair and spin-pair correlations in some of these disordered magnetic systems. 56 refs.« less

Electronic transport on the Shastry-Sutherland lattice in Ising-type rare-earth tetraborides

NASA Astrophysics Data System (ADS)

Ye, Linda; Suzuki, Takehito; Checkelsky, Joseph G.

2017-05-01

In the presence of a magnetic field frustrated spin systems may exhibit plateaus at fractional values of saturation magnetization. Such plateau states are stabilized by classical and quantum mechanisms including order by disorder, triplon crystallization, and various competing order effects. In the case of electrically conducting systems, free electrons represent an incisive probe for the plateau states. Here we study the electrical transport of Ising-type rare-earth tetraborides R B4 (R =Er , Tm), a metallic Shastry-Sutherland lattice showing magnetization plateaus. We find that the longitudinal and transverse resistivities reflect scattering with both the static and the dynamic plateau structure. We model these results consistently with the expected strong uniaxial anisotropy on a quantitative level, providing a framework for the study of plateau states in metallic frustrated systems.

Influence of uniaxial single-ion anisotropy on the magnetic and thermal properties of Heisenberg antiferromagnets within unified molecular field theory

NASA Astrophysics Data System (ADS)

Johnston, David C.

2017-03-01

The influence of uniaxial single-ion anisotropy -D Sz2 on the magnetic and thermal properties of Heisenberg antiferromagnets (AFMs) is investigated. The uniaxial anisotropy is treated exactly and the Heisenberg interactions are treated within unified molecular field theory (MFT) [Phys. Rev. B 91, 064427 (2015), 10.1103/PhysRevB.91.064427], where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the z axis (D >0 ) in applied field Hz=0 are calculated versus D and temperature T , including the ordered moment μ , the Néel temperature TN, the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities χ∥ and χ⊥ in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin μz(Hz,D ,T ) is found, and the critical field Hc(D ,T ) is derived at which the second-order AFM to PM phase transition occurs. The magnetic properties of the spin-flop (SF) phase are calculated, including the zero-field properties TN(D ) and μ (D ,T ) . The high-field μz(Hz,D ,T ) is determined, together with the associated spin-flop field HSF(D ,T ) at which a second-order SF to PM phase transition occurs. The free energies of the AFM, SF, and PM phases are derived from which Hz-T phase diagrams are constructed. For fJ=-1 and -0.75 , where fJ=θp J/TN J and θp J and TN J are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the Hz-T plane similar to previous results are obtained. However, for fJ=0 we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite Hz and T . Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility χ⊥(D ,T ) , the associated effective torque at low fields arising from the -D Sz2 term in the Hamiltonian, the high-field perpendicular magnetization μ⊥, and the perpendicular critical field Hc ⊥ at which the second-order AFM to PM phase transition occurs. In addition to the above results for D >0 , the TN(D ) and ordered moment μ (T ,D ) for collinear AFM ordering along the x axis with D <0 are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with -D Sz2 uniaxial anisotropy with either sign of D , Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.

Magnetic Properties of Strongly Correlated Hubbard Model and Quantum Spin-One Ferromagnets with Arbitrary Crystal-Field Potential: Linked Cluster Series Expansion Approach

NASA Astrophysics Data System (ADS)

Pan, Kok-Kwei

We have generalized the linked cluster expansion method to solve more many-body quantum systems, such as quantum spin systems with crystal-field potentials and the Hubbard model. The technique sums up all connected diagrams to a certain order of the perturbative Hamiltonian. The modified multiple-site Wick reduction theorem and the simple tau dependence of the standard basis operators have been used to facilitate the evaluation of the integration procedures in the perturbation expansion. Computational methods are developed to calculate all terms in the series expansion. As a first example, the perturbation series expansion of thermodynamic quantities of the single-band Hubbard model has been obtained using a linked cluster series expansion technique. We have made corrections to all previous results of several papers (up to fourth order). The behaviors of the three dimensional simple cubic and body-centered cubic systems have been discussed from the qualitative analysis of the perturbation series up to fourth order. We have also calculated the sixth-order perturbation series of this model. As a second example, we present the magnetic properties of spin-one Heisenberg model with arbitrary crystal-field potential using a linked cluster series expansion. The calculation of the thermodynamic properties using this method covers the whole range of temperature, in both magnetically ordered and disordered phases. The series for the susceptibility and magnetization have been obtained up to fourth order for this model. The method sums up all perturbation terms to certain order and estimates the result using a well -developed and highly successful extrapolation method (the standard ratio method). The dependence of critical temperature on the crystal-field potential and the magnetization as a function of temperature and crystal-field potential are shown. The critical behaviors at zero temperature are also shown. The range of the crystal-field potential for Ni(2+) compounds is roughly estimated based on this model using known experimental results.

Aerospace Applications of Magnetic Suspension Technology, part 2

NASA Technical Reports Server (NTRS)

Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1991-01-01

In order to examine the state of technology of all areas of magnetic suspension with potential aerospace applications, and to review related recent developments in sensors and control approaches, superconducting technology, and design/implementation practices, a workshop was held at NASA-Langley. Areas of concern are pointing and isolation systems, microgravity and vibration isolation, bearing applications, wind tunnel model suspension systems, large gap magnetic suspension systems, controls, rotating machinery, science and applications of superconductivity, and sensors. Papers presented are included.

Spin Solid versus Magnetic Charge Ordered State in Artificial Honeycomb Lattice of Connected Elements

PubMed Central

Glavic, Artur; Summers, Brock; Dahal, Ashutosh; Kline, Joseph; Van Herck, Walter; Sukhov, Alexander; Ernst, Arthur

2018-01-01

Abstract The nature of magnetic correlation at low temperature in two‐dimensional artificial magnetic honeycomb lattice is a strongly debated issue. While theoretical researches suggest that the system will develop a novel zero entropy spin solid state as T → 0 K, a confirmation to this effect in artificial honeycomb lattice of connected elements is lacking. This study reports on the investigation of magnetic correlation in newly designed artificial permalloy honeycomb lattice of ultrasmall elements, with a typical length of ≈12 nm, using neutron scattering measurements and temperature‐dependent micromagnetic simulations. Numerical modeling of the polarized neutron reflectometry data elucidates the temperature‐dependent evolution of spin correlation in this system. As temperature reduces to ≈7 K, the system tends to develop novel spin solid state, manifested by the alternating distribution of magnetic vortex loops of opposite chiralities. Experimental results are complemented by temperature‐dependent micromagnetic simulations that confirm the dominance of spin solid state over local magnetic charge ordered state in the artificial honeycomb lattice with connected elements. These results enable a direct investigation of novel spin solid correlation in the connected honeycomb geometry of 2D artificial structure. PMID:29721429

Cryogenic Considerations for Superconducting Magnet Design for the Material Plasma Exposure eXperiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Duckworth, Robert C; Demko, Dr. Jonathan A; Lumsdaine, Arnold

2015-01-01

In order to determine long term performance of plasma facing components such as diverters and first walls for fusion devices, next generation plasma generators are needed. A Material Plasma Exposure eXperiment (MPEX) has been proposed to address this need through the generation of plasmas in front of the target with electron temperatures of 1-15 eV and electron densities of 1020 to 1021 m-3. Heat fluxes on target diverters could reach 20 MW/m2. In order generate this plasma, a unique radio frequency helicon source and heating of electrons and ions through Electron Bernstein Wave (EBW) and Ion Cyclotron Resonance Heating (ICRH)more » has been proposed. MPEX requires a series of magnets with non-uniform central fields up to 2 T over a 5m length in the heating and transport region and 1 T uniform central field over a 1-m length on a diameter of 1.3 m. Given the field requirements, superconducting magnets are under consideration for MPEX. In order to determine the best construction method for the magnets, the cryogenic refrigeration has been analyzed with respect to cooldown and operational performance criteria for open-cycle and closed-cycle systems, capital and operating costs of these system, and maturity of supporting technology such as cryocoolers. These systems will be compared within the context of commercially available magnet constructions to determine the most economical method for MPEX operation. The current state of the MPEX magnet design including details on possible superconducting magnet configurations will be presented.« less

Recognition of exchange striction as the origin of magnetoelectric coupling in multiferroics

NASA Astrophysics Data System (ADS)

Yahia, G.; Damay, F.; Chattopadhyay, S.; Balédent, V.; Peng, W.; Elkaim, E.; Whitaker, M.; Greenblatt, M.; Lepetit, M.-B.; Foury-Leylekian, P.

2017-05-01

The magnetoelectric coupling, a phenomenon inducing magnetic (electric) polarization by application of an external electric (magnetic) field and first conjectured by Curie in 1894, is observed in most of the multiferroics and used for many applications in various fields such as data storage or sensing. However, its microscopic origin is a long-standing controversy in the scientific community. An intense revival of interest developed in the beginning of the 21st century due to the emergence of multiferroic frustrated magnets in which the ferroelectricity is magnetically induced and which present an inherent strong magnetoelectric coupling. The Dzyaloshinskii-Moriya interaction (DMI) well accounts for such ferroelectricity in systems with a noncollinear magnetic order such as the RMnO3 manganites. The DMI effect is, however, inadequate for systems presenting ferroelectricity induced by quasicollinear spin arrangements such as the prominent RMn2O5 manganites. Among different microscopic mechanisms proposed to resolve this incompatibility, the exchange-striction model stands as the most invoked candidate. In this scenario, the polar atomic displacements originate from the release of a frustration caused by the magnetic order. Despite its theoretical description 15 years ago, this mechanism had yet to be unambiguously validated experimentally. The breakthrough finally comes from SmMn2O5 presenting a unique magnetic order revealed by powder neutron diffraction. The unique orientation of its magnetic moment establishes the missing element that definitely validates the exchange striction as the effective mechanism for the spin-induced ferroelectricity in this series. More generally, this is a proof of concept that validates this model on actual systems, facilitating the development of a new generation of multiferroics with unrivaled magnetoelectric properties.

First-principles studiesy of the order-disorder phase transition in FeCo using Wang-Landau Monte-Carlo method

NASA Astrophysics Data System (ADS)

Pei, Zongrui; Eisenbach, Markus; Stocks, G. Malcolm

Simulating order-disorder phase transitions in magnetic materials requires the accurate treatment of both the atomic and magnetic interactions, which span a vast configuration space. Using FeCo as a prototype system, we demonstrate that this can be addressed by combining the Locally Self-consistent Multiple Scattering (LSMS) method with the Wang-Landau (WL) Monte-Carlo algorithm. Fe-Co based materials are interesting magnetic materials but a reliable phase diagram of the binary Fe-Co system is still difficult to obtain. Using the combined WL-LSMS method we clarify the existence of the disordered A2 phase and predict the Curie temperature between it and the ordered B2 phase. The WL-LSMS method is readily applicable to the study of second-order phase transitions in other binary and multi-component alloys, thereby providing a means to the direct simulation of order-disorder phase transitions in complex alloys without need of intervening classical model Hamiltonians. We also demonstrate the capability of our method to guide the design of new magnetic materials. This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division and it used Oak Ridge Leadership Computing Facility resources at Oak Ridge National Laboratory.

Control of drug release through the in situ assembly of stimuli-responsive ordered mesoporous silica with magnetic particles.

PubMed

Zhu, Shenmin; Zhou, Zhengyang; Zhang, Di

2007-12-03

A site-selective controlled delivery system for controlled drug release is fabricated through the in situ assembly of stimuli-responsive ordered SBA-15 and magnetic particles. This approach is based on the formation of ordered mesoporous silica with magnetic particles formed from Fe(CO)5 via the surfactant-template sol-gel method and control of transport through polymerization of N-isopropyl acrylamide inside the pores. Hydrophobic Fe(CO)5 acts as a swelling agent as well as being the source of the magnetic particles. The obtained system demonstrates a high pore diameter (7.1 nm) and pore volume (0.41 cm(3) g(-1)), which improves drug storage for relatively large molecules. Controlled drug release through the porous network is demonstrated by measuring the uptake and release of ibuprofen (IBU). The delivery system displays a high IBU storage capacity of 71.5 wt %, which is almost twice as large as the highest value based on SBA-15 ever reported. In vitro testing of IBU loading and release exhibits a pronounced transition at around 32 degrees C, indicating a typical thermosensitive controlled release.

Design of set-point weighting PI{sup λ} + D{sup μ} controller for vertical magnetic flux controller in Damavand tokamak

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rasouli, H.; Fatehi, A.

2014-12-15

In this paper, a simple method is presented for tuning weighted PI{sup λ} + D{sup μ} controller parameters based on the pole placement controller of pseudo-second-order fractional systems. One of the advantages of this controller is capability of reducing the disturbance effects and improving response to input, simultaneously. In the following sections, the performance of this controller is evaluated experimentally to control the vertical magnetic flux in Damavand tokamak. For this work, at first a fractional order model is identified using output-error technique in time domain. For various practical experiments, having desired time responses for magnetic flux in Damavand tokamak,more » is vital. To approach this, at first the desired closed loop reference models are obtained based on generalized characteristic ratio assignment method in fractional order systems. After that, for the identified model, a set-point weighting PI{sup λ} + D{sup μ} controller is designed and simulated. Finally, this controller is implemented on digital signal processor control system of the plant to fast/slow control of magnetic flux. The practical results show appropriate performance of this controller.« less

Tradeoff between magnet volume and tuning capacitor in a free piston Stirling engine power generation system

NASA Astrophysics Data System (ADS)

Fu, Z. X.; Nasar, S. A.; Rosswurm, Mark

This paper presents the criteria in selecting the size of the tuning capacitor, and the cost tradeoff between magnet volume and tuning capacitor in a free piston Stirling engine power generation system. The permissible range of capacitor size corresponding to different magnet volume, in order to prevent magnet demagnetization and stabilize the operation of the system, is determined. Within the permissible range suitable capacitor size may be selected to compensate the inductive load of the system to improve the overall power factor. If the capacitor size is not in the permissible range, there would exist a danger of losing magnet strength, or unstable operation of the engine that would destroy the engine due to unbounded amplitude of piston oscillations. The theory developed is then applied to a practical system, and the cost tradeoff between magnet volume and capacitor is studied.

Novel phase transitions in coupled dipolar chains.

NASA Astrophysics Data System (ADS)

Mellado, Paula

We study the properties of a classical magnetic system realized by two chains of U(1) rotors coupled via Coulomb interactions in the dumbbell approach. Magnets in chain I and chain II rotate in the x-z and y-z planes respectively. Ground state correlations and the system wave excitation spectrum are found using spin wave theory. The displacement ''d'' of chain II from chain I induces dynamics in the system and yields two first order magnetic phase transitions. The transitions happen at critical displacements, which notably, are independent of the magnetic charge at the tips of the magnets, suggesting a geometrical origin. This work was supported by Fondecyt under Grant No. 1160239.

A model for metastable magnetism in the hidden-order phase of URu2Si2

NASA Astrophysics Data System (ADS)

Boyer, Lance; Yakovenko, Victor M.

2018-01-01

We propose an explanation for the experiment by Schemm et al. (2015) where the polar Kerr effect (PKE), indicating time-reversal symmetry (TRS) breaking, was observed in the hidden-order (HO) phase of URu2Si2. The PKE signal on warmup was seen only if a training magnetic field was present on cool-down. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state producing the PKE, even if the HO ground state respects TRS. We predict that a strong reversed magnetic field should reset the PKE to zero.

Enrichment of magnetic particles using temperature and magnetic field gradients induced by benchtop fabricated micro-electromagnets.

PubMed

Hosseini, A; Philpott, D N; Soleymani, L

2017-11-21

The active transport of analytes inside biosensing systems is important for reducing the response time and enhancing the limit-of-detection of these systems. Due to the ease of functionalization with bio-recognition agents and manipulation with magnetic fields, magnetic particles are widely used for active and directed transport of biological analytes. On-chip active electromagnets are ideally suited for manipulating magnetic particles in an automated and miniaturized fashion inside biosensing systems. Unfortunately, the magnetic force exerted by these devices decays rapidly as we move away from the device edges, and increasing the generated force to the levels necessary for particle manipulation requires a parallel increase in the applied current and the resultant Joule heating. In this paper, we designed a study to understand the combined role of thermal and magnetic forces on the movement of magnetic particles in order to extend the interaction distance of on-chip magnetic devices beyond the device edges. For this purpose, we used a rapid prototyping method to create an active/passive on-chip electromagnet with a micro/nano-structured active layer and a patterned ferromagnetic passive layer. We demonstrated that the measured terminal velocities of particles positioned near the electromagnet edge (∼5.5 μm) closely reflect the values obtained by multi-physics modelling. Interestingly, we observed a two orders of magnitude deviation between the experimental and modelling results for the terminal velocities of particles far from the electromagnet edge (∼55.5 μm). Heat modelling of the system using experimentally-measured thermal gradients indicates that this discrepancy is related to the enhanced fluid movement caused by thermal forces. This study enables the rational design of thermo-magnetic systems for thermally driving and magnetically capturing particles that are positioned at distances tens to hundreds of microns away from the edges of on-chip magnetic devices.

First principles statistical mechanics of alloys and magnetism

NASA Astrophysics Data System (ADS)

Eisenbach, Markus; Khan, Suffian N.; Li, Ying Wai

Modern high performance computing resources are enabling the exploration of the statistical physics of phase spaces with increasing size and higher fidelity of the Hamiltonian of the systems. For selected systems, this now allows the combination of Density Functional based first principles calculations with classical Monte Carlo methods for parameter free, predictive thermodynamics of materials. We combine our locally selfconsistent real space multiple scattering method for solving the Kohn-Sham equation with Wang-Landau Monte-Carlo calculations (WL-LSMS). In the past we have applied this method to the calculation of Curie temperatures in magnetic materials. Here we will present direct calculations of the chemical order - disorder transitions in alloys. We present our calculated transition temperature for the chemical ordering in CuZn and the temperature dependence of the short-range order parameter and specific heat. Finally we will present the extension of the WL-LSMS method to magnetic alloys, thus allowing the investigation of the interplay of magnetism, structure and chemical order in ferrous alloys. This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division and it used Oak Ridge Leadership Computing Facility resources at Oak Ridge National Laboratory.

Magnetic structures and excitations in a multiferroic Y-type hexaferrite BaSrCo 2 Fe 11 AlO 22

DOE PAGES

Nakajima, Taro; Tokunaga, Yusuke; Matsuda, Masaaki; ...

2016-11-30

Here, we have investigated magnetic orders and excitations in a Y-type hexaferrite BaSrCo 2Fe 11AlO 22 (BSCoFAO), which was reported to exhibit spin-driven ferroelectricity at room temperature. By means of magnetization, electric polarization, and neutron-diffraction measurements using single-crystal samples, we establish a H-T magnetic phase diagram for magnetic field perpendicular to the c axis (H ⟂c). This system exhibits an alternating longitudinal conical (ALC) magnetic structure in the ground state, and it turns into a non-co-planar commensurate magnetic order with spin-driven ferroelectricity under H ⟂c. The field-induced ferroelectric phase remains as a metastable state after removing magnetic field below 250more » K. This metastability is the key to understanding of magnetic field reversal of the spin-driven electric polarization in this system. Inelastic polarized neutron-scattering measurements in the ALC phase reveal a magnetic excitation at around 7.5 meV, which is attributed to spin components oscillating in a plane perpendicular to the cone axis. This phasonlike excitation is expected to be an electric-field active magnon, i.e., electromagnon excitation, in terms of the magnetostriction mechanism.« less

Magnetic structures and excitations in a multiferroic Y-type hexaferrite BaSrCo2Fe11AlO22

NASA Astrophysics Data System (ADS)

Nakajima, Taro; Tokunaga, Yusuke; Matsuda, Masaaki; Dissanayake, Sachith; Fernandez-Baca, Jaime; Kakurai, Kazuhisa; Taguchi, Yasujiro; Tokura, Yoshinori; Arima, Taka-hisa

2016-11-01

We have investigated magnetic orders and excitations in a Y-type hexaferrite BaSrCo2Fe11AlO22 (BSCoFAO), which was reported to exhibit spin-driven ferroelectricity at room temperature [S. Hirose, K. Haruki, A. Ando, and T. Kimura, Appl. Phys. Lett. 104, 022907 (2014), 10.1063/1.4862432]. By means of magnetization, electric polarization, and neutron-diffraction measurements using single-crystal samples, we establish a H -T magnetic phase diagram for magnetic field perpendicular to the c axis (H⊥c). This system exhibits an alternating longitudinal conical (ALC) magnetic structure in the ground state, and it turns into a non-co-planar commensurate magnetic order with spin-driven ferroelectricity under H⊥c. The field-induced ferroelectric phase remains as a metastable state after removing magnetic field below ˜250 K. This metastability is the key to understanding of magnetic field reversal of the spin-driven electric polarization in this system. Inelastic polarized neutron-scattering measurements in the ALC phase reveal a magnetic excitation at around 7.5 meV, which is attributed to spin components oscillating in a plane perpendicular to the cone axis. This phasonlike excitation is expected to be an electric-field active magnon, i.e., electromagnon excitation, in terms of the magnetostriction mechanism.

Solitary Magnons in the S=5/2 Antiferromagnet CaFe_{2}O_{4}.

PubMed

Stock, C; Rodriguez, E E; Lee, N; Green, M A; Demmel, F; Ewings, R A; Fouquet, P; Laver, M; Niedermayer, Ch; Su, Y; Nemkovski, K; Rodriguez-Rivera, J A; Cheong, S-W

2016-07-01

CaFe_{2}O_{4} is a S=5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (↑↑↓↓) and B (↑↓↑↓) phases, which differ by the c-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters result in magnetic antiphase boundaries along c which freeze on the time scale of ∼1  ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions ∼1-2 c-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.

Solitary Magnons in the S =5/2 Antiferromagnet CaFe2O4

NASA Astrophysics Data System (ADS)

Stock, C.; Rodriguez, E. E.; Lee, N.; Green, M. A.; Demmel, F.; Ewings, R. A.; Fouquet, P.; Laver, M.; Niedermayer, Ch.; Su, Y.; Nemkovski, K.; Rodriguez-Rivera, J. A.; Cheong, S.-W.

2016-07-01

CaFe2O4 is a S =5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (↑↑↓↓) and B (↑↓↑↓) phases, which differ by the c -axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters result in magnetic antiphase boundaries along c which freeze on the time scale of ˜1 ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions ˜1 - 2 c -axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A +B orders as well as localization of spin excitations in a classical magnet.

Polarized neutron scattering study of the multiple order parameter system NdB 4 [Polarized neutron scattering study on multiple order parameter system NdB 4

DOE Office of Scientific and Technical Information (OSTI.GOV)

Metoki, Naoto; Yamauchi, Hiroki; Matsuda, Masaaki

Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter f-electron system NdB 4. We confirmed the noncollinear “all-in all-out” structure (Γ 4) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the c-axis m c showed diagonally antiferromagnetic structure (Γ 10), inconsistent with previously reported “vortex” structure (Γ 2). The microscopic mixture of these two structures with →q 0=(0,0,0) appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. Themore » unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter Γ 4 coupled with higher-order secondary order parameter Γ 10. The magnetic moments were estimated to be 1.8 ± 0.2 and 0.2 ± 0.05μ B at T = 7.5K for Γ 4 and Γ 10, respectively. We also found a long-period incommensurate modulation of the →q 1=(0,0,1/2) antiferromagnetic structure of mc with the propagation →q s1=(0.14,0.14,0.1) and →q s2=(0.2,0,0.1) in phase III and IV, respectively. The amplitude of sinusoidal modulation was about m c=1.0 ± 0.2μ B at T=1.5 K. The local (0,0,1/2) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of m c, opposite to the coexisting Γ 10. The mc of Γ 10 is significantly enhanced up to 0.6μ B at T=1.5 K, which is accompanied by the incommensurate modulations. As a result, the Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet f-electron state play important roles.« less

Polarized neutron scattering study of the multiple order parameter system NdB 4 [Polarized neutron scattering study on multiple order parameter system NdB 4

DOE PAGES

Metoki, Naoto; Yamauchi, Hiroki; Matsuda, Masaaki; ...

2018-05-17

Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter f-electron system NdB 4. We confirmed the noncollinear “all-in all-out” structure (Γ 4) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the c-axis m c showed diagonally antiferromagnetic structure (Γ 10), inconsistent with previously reported “vortex” structure (Γ 2). The microscopic mixture of these two structures with →q 0=(0,0,0) appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. Themore » unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter Γ 4 coupled with higher-order secondary order parameter Γ 10. The magnetic moments were estimated to be 1.8 ± 0.2 and 0.2 ± 0.05μ B at T = 7.5K for Γ 4 and Γ 10, respectively. We also found a long-period incommensurate modulation of the →q 1=(0,0,1/2) antiferromagnetic structure of mc with the propagation →q s1=(0.14,0.14,0.1) and →q s2=(0.2,0,0.1) in phase III and IV, respectively. The amplitude of sinusoidal modulation was about m c=1.0 ± 0.2μ B at T=1.5 K. The local (0,0,1/2) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of m c, opposite to the coexisting Γ 10. The mc of Γ 10 is significantly enhanced up to 0.6μ B at T=1.5 K, which is accompanied by the incommensurate modulations. As a result, the Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet f-electron state play important roles.« less

First-principles Theory of Magnetic Multipoles in Condensed Matter Systems

NASA Astrophysics Data System (ADS)

Suzuki, Michi-To; Ikeda, Hiroaki; Oppeneer, Peter M.

2018-04-01

The multipole concept, which characterizes the spacial distribution of scalar and vector objects by their angular dependence, has already become widely used in various areas of physics. In recent years it has become employed to systematically classify the anisotropic distribution of electrons and magnetization around atoms in solid state materials. This has been fuelled by the discovery of several physical phenomena that exhibit unusual higher rank multipole moments, beyond that of the conventional degrees of freedom as charge and magnetic dipole moment. Moreover, the higher rank electric/magnetic multipole moments have been suggested as promising order parameters in exotic hidden order phases. While the experimental investigations of such anomalous phases have provided encouraging observations of multipolar order, theoretical approaches have developed at a slower pace. In particular, a materials' specific theory has been missing. The multipole concept has furthermore been recognized as the key quantity which characterizes the resultant configuration of magnetic moments in a cluster of atomic moments. This cluster multipole moment has then been introduced as macroscopic order parameter for a noncollinear antiferromagnetic structure in crystals that can explain unusual physical phenomena whose appearance is determined by the magnetic point group symmetry. It is the purpose of this review to discuss the recent developments in the first-principles theory investigating multipolar degrees of freedom in condensed matter systems. These recent developments exemplify that ab initio electronic structure calculations can unveil detailed insight in the mechanism of physical phenomena caused by the unconventional, multipole degree of freedom.

Magnetic interactions in anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer magnets

NASA Astrophysics Data System (ADS)

Dai, Z. M.; Liu, W.; Zhao, X. T.; Han, Z.; Kim, D.; Choi, C. J.; Zhang, Z. D.

2016-10-01

The magnetic properties and the possible interaction mechanisms of anisotropic soft- and hard-magnetic multilayers have been investigated by altering the thickness of different kinds of spacer layers. The metal Ta and the insulating oxides MgO, Cr2O3 have been chosen as spacer layers to investigate the characteristics of the interactions between soft- and hard-magnetic layers in the anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer system. The dipolar and exchange interaction between hard and soft phases are evaluated with the help of the first order reversal curve method. The onset of the nucleation field and the magnetization reversal by domain wall movement are also evident from the first-order-reversal-curve measurements. Reversible/irreversible distributions reveal the natures of the soft- and hard-magnetic components. Incoherent switching fields are observed and the calculations show the semiquantitative contributions of hard and soft components to the system. An antiferromagnetic spacer layer will weaken the interaction between ferromagnetic layers and the effective interaction length decreases. As a consequence, the dipolar magnetostatic interaction may play an important role in the long-range interaction in anisotropic multilayer magnets.

Microscopic coexistence of magnetism and superconductivity in charge-compensated Ba1-xKx(Fe1-yCoy)2As2

NASA Astrophysics Data System (ADS)

Goltz, Til; Zinth, Veronika; Johrendt, Dirk; Rosner, Helge; Pascua, Gwendolyne; Luetkens, Hubertus; Materne, Philipp; Klauss, Hans-Henning

2014-04-01

We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba1-xKx(Fe1-yCoy)2As2 with x /2≈y. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac-susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of x /2≈y≤0.13, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low-temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for x /2≈y≤0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum Tc of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x /2≈y≥0.25, a tetragonal nonmagnetic ground state is observed. Our DFT calculations yield a significant reduction of the Fe 3d density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density.

Nonlinear compensation techniques for magnetic suspension systems. Ph.D. Thesis - MIT

NASA Technical Reports Server (NTRS)

Trumper, David L.

1991-01-01

In aerospace applications, magnetic suspension systems may be required to operate over large variations in air-gap. Thus the nonlinearities inherent in most types of suspensions have a significant effect. Specifically, large variations in operating point may make it difficult to design a linear controller which gives satisfactory stability and performance over a large range of operating points. One way to address this problem is through the use of nonlinear compensation techniques such as feedback linearization. Nonlinear compensators have received limited attention in the magnetic suspension literature. In recent years, progress has been made in the theory of nonlinear control systems, and in the sub-area of feedback linearization. The idea is demonstrated of feedback linearization using a second order suspension system. In the context of the second order suspension, sampling rate issues in the implementation of feedback linearization are examined through simulation.

Quasi-Periodic Oscillations in AM Herculis - Repeat for HOPR#87/95

NASA Astrophysics Data System (ADS)

Chanmugam, G.

1991-07-01

AM Her variables are close-binary systems in which a white dwarf with a magnetic field of 20--70 MG accretes matter from a companion star. Theoretical studies of magnetically channeled accretion flows in such systems predict that the shock formed near the white dwarf should oscillate with periods of order 0.1--1 s. Optical high-speed photometry has indeed shown the existence of such rapid, quasi-periodic oscillations in some AM Her binaries, but not in others. We will use HST to obtain ultraviolet high-speed photometry of several AM Her systems, in order to explore further the nature of the oscillations, and to extend the search into the UV. HSP observations of two systems (VV Pup and ST LMi, in which the accreting magnetic pole periodically passes behind the limb of the white dwarf) will allow detailed eclipse mapping of the accretion column and the shock oscillations to be carried out.

Strain-sensitive spin-state ordering in thin films of perovskite LaCoO3

NASA Astrophysics Data System (ADS)

Fujioka, J.; Yamasaki, Y.; Doi, A.; Nakao, H.; Kumai, R.; Murakami, Y.; Nakamura, M.; Kawasaki, M.; Arima, T.; Tokura, Y.

2015-11-01

We have investigated the lattice distortion coupled to the Co 3 d -spin-state ordering in thin films of perovskite LaCoO3 with various epitaxial strains by measurements of the magnetization, x-ray diffraction, and optical spectra. In the system with tensile strain about 0.5%, a lattice distortion characterized by the modulation vector q =(1 /6 ,1 /6 ,1 /6 ) emerges at 40 K, followed by a ferromagnetic ordering at 24 K. Alternatively, in systems with tensile strain exceeding 1%, the lattice distortion characterized by q =(1 /4 ,1 /4 ,1 /4 ) emerges at 120 K or higher, and subsequently the ferromagnetic or ferrimagnetic ordering occurs around 90 K. The evolution of infrared phonon spectra and resonant x-ray scattering at the Co K edge suggests that the population change in the Co 3 d spin state causes the strain-induced switching of spin-state ordering as well as of magnetic ordering in this canonical spin-state crossover system.

Non-collinear magnetism with analytic Bond-Order Potentials

NASA Astrophysics Data System (ADS)

Ford, Michael E.; Pettifor, D. G.; Drautz, Ralf

2015-03-01

The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions.

Raman scattering studies of the orbital, magnetic, and conducting phases in double layer ruthenates

NASA Astrophysics Data System (ADS)

Karpus, John Francis

In this dissertation, light scattering techniques are used to probe the exotic orbital, magnetic, and conducting phases of the double layer ruthenate, Ca3Ru2O7, as functions of temperature, applied pressure, and applied magnetic field. These phases result from a rich interplay between the orbital, spin, and electronic degrees of freedom in such a strongly coupled system as Ca3Ru2O7. The Raman-active phonon and magnon excitations in Ca3Ru2O7 convey sufficient information to map out the orbital, magnetic, and conducting (H, T) and (P, T) phase diagrams of this material. This study finds that quasihydrostatic pressure causes a linear suppression of the orbital-ordering temperature (TOO = 48 K at P = 0), up to a T = 0 critical point near P* ˜ 55 kbar, above which the material is in a metallic, orbital-degenerate phase. This pressure-induced collapse of the antiferromagnetic orbital-ordered phase is associated with a suppression of the RuO6 octahedral distortions that are responsible for orbital-ordering. It is also shown that an applied magnetic field at low temperatures induces a change from an orbital-ordered to an orbital-degenerate phase for fields aligned along the in-plane hard-axis, but induces a reentrant orbital-ordered to orbital-disordered to orbital-ordered phase change for fields aligned along the in-plane easy-axis. This complex magnetic field dependence betrays the importance of the spin-orbit coupling in this system, which makes the field-induced phase behavior highly sensitive to both the applied magnetic field magnitude and direction. It is further shown that rapid field-induced changes in the structure and orbital populations are responsible for the highly field-tunable conducting properties of Ca3Ru2O7, and that the most dramatic magneto-conductivities are associated with an "orbital disordered" phase regime in which there is a random mixture of a- and b-axis oriented Ru moments and d-orbital populations on the Ru ions. Dilute La doping in Ca3Ru2O7 changes the lattice parameter along the c-axis and also adds an extra electron, providing bandwidth and band filling control, respectively. This addition of La also lowers the orbital ordering temperature to T ˜ 43 K, and provides a greater sensitivity of the orbital phases to applied magnetic fields, as evidenced by changes in the phases occurring at lower fields and over a greater field range than seen in the undoped system.

Progress in magnet design activities for the material plasma exposure experiment

DOE PAGES

Duckworth, Robert; Lumsdaine, Arnold; Rapp, Juergen; ...

2017-07-01

One of the critical challenges for the development of next generation fusion facilities, such as a Fusion Nuclear Science Facility (FNSF) or DEMO, is the understanding of plasma material interactions (PMI). Making progress in PMI research will require integrated facilities that can provide the types of conditions that will be seen in the first wall and divertor regions of future fusion facilities. In order to meet this need, a new linear plasma facility, the Materials Plasma Exposure Experiment (MPEX), is proposed. In order to generate high ion fluence to simulate fusion divertor conditions, a steady-state plasma will be generated andmore » confined with superconducting magnets. Finally, the on-axis fields will range from 1 to 2.5 T in order to meet the requirements of the various plasma source and heating systems. Details on the pre-conceptual design of the magnets and cryogenic system are presented.« less

Far-from-equilibrium magnetic granular layers: dynamic patterns, magnetic order and self-assembled swimmers

NASA Astrophysics Data System (ADS)

Snezhko, Alexey

2010-03-01

Ensembles of interacting particles subject to an external periodic forcing often develop nontrivial collective behavior and self-assembled dynamic patterns. We study emergent phenomena in magnetic granular ensembles suspended at a liquid-air and liquid-liquid interfaces and subjected to a transversal alternating magnetic field. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (in particular, ``magnetic snakes'', ``asters'', ``clams'') emerging in such systems in a certain range of excitation parameters. These non-equilibrium dynamic structures emerge as a result of the competition between magnetic and hydrodynamic forces and have complex magnetic ordering. Transition between different self-assembled phases with parameters of external driving magnetic field is observed. I will show that above some frequency threshold magnetic snakes spontaneously break the symmetry of the self-induced surface flows (symmetry breaking instability) and turn into swimmers. Self-induced surface flows symmetry can be also broken in a controlled fashion by introduction of a large bead to a magnetic snake (bead-snake hybrid), that transforms it into a robust self-locomoting entity. Some features of the self-localized structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density and the Navier-Stokes equation for hydrodynamic flows.

Exploration of Fermi-Pasta-Ulam Behavior in a Magnetic System

NASA Astrophysics Data System (ADS)

Lewis, Jeramy; Camley, Robert E.; Anderson, Nicholas R.

2018-04-01

We study nonlinear spin motion in one-dimensional magnetic chains. We find significant differences from the classic Fermi-Pasta-Ulam (FPU) problem examining nonlinear elastic motion in a chain. We find that FPU behavior, the transfer of energy among low order eigenmodes, does not occur in magnetic systems with only exchange and external fields, but does exist if a uniaxial anisotropy is also present. The FPU behavior may be altered or turned off through the magnitude and orientation of an external magnetic field. A realistic micromagnetic model shows such behavior could be measurable.

Spin dynamics in pressure-induced magnetically ordered phases in ( C 4 H 12 N 2 ) Cu 2 Cl 6

DOE PAGES

Perren, G.; Möller, J. S.; Hüvonen, D.; ...

2015-08-07

In this paper, we present inelastic neutron-scattering experiments on the S=1/2 frustrated gapped quantum magnet piperazinium hexachlorodicuprate (PHCC) under applied hydrostatic pressure. These results show that at 9 kbar the magnetic triplet excitations in the system are gapless, contrary to what was previously reported. Our results are in agreement with recent muon-spin relaxation experiments which found magnetic order above a quantum-critical point at 4.3 kbar. Finally, we show that the changes in the excitation spectrum can be primarily attributed to the change in a single exchange pathway.

Interplay between superconductivity and magnetism in Fe(1-x)Pd(x)Te.

PubMed

Karki, Amar B; Garlea, V Ovidiu; Custelcean, Radu; Stadler, Shane; Plummer, E W; Jin, Rongying

2013-06-04

The attractive/repulsive relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions, and Fe pnictides, showed superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. However, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds [RRh4B4 (R = Nd, Sm, Tm, Er), R'Mo6X8 (R' = Tb, Dy, Er, Ho, and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn5, EuFe2(As(1-x)P(x))2, etc.], providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe(1-x)Pd(x)Te. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature T(N/S), and turns into short-range AFM correlation with a characteristic peak in magnetic susceptibility at T'(N). Superconductivity sets in when T'(N) reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (short-range) cross-over regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic (FM and AFM) interactions.

Ordering, thermal excitations and phase transitions in dipolar coupled mono-domain magnet arrays

NASA Astrophysics Data System (ADS)

Kapaklis, Vassilios

2015-03-01

Magnetism has provided a fertile test bed for physical models, such as the Heisenberg and Ising models. Most of these investigations have focused on solid materials and relate to their atomic properties such as the atomic magnetic moments and their interactions. Recently, advances in nanotechnology have enabled the controlled patterning of nano-sized magnetic particles, which can be arranged in extended lattices. Tailoring the geometry and the magnetic material of these lattices, the magnetic interactions and magnetization reversal energy barriers can be tuned. This enables interesting interaction schemes to be examined on adjustable length and energy scales. As a result such nano-magnetic systems represent an ideal playground for the study of physical model systems, being facilitated by direct magnetic imaging techniques. One particularly interesting case is that of systems exhibiting frustration, where competing interactions cannot be simultaneously satisfied. This results in a degeneracy of the ground state and intricate thermodynamic properties. An archetypical frustrated physical system is water ice. Similar physics can be mirrored in nano-magnetic arrays, by tuning the arrangement of neighboring magnetic islands, referred to as artificial spin ice. Thermal excitations in such systems resemble magnetic monopoles. In this presentation key concepts related to nano-magnetism and artificial spin ice will be introduced and discussed, along with recent experimental and theoretical developments.

Ferromagnetism in a hexagonal PrRh3 with 4f2 configuration

NASA Astrophysics Data System (ADS)

Park, G. B.; Yamane, Y.; Onimaru, T.; Umeo, K.; Takabatake, T.

2018-05-01

Electrical resistivity ρ , magnetization M and specific heat C are reported for polycrystalline samples of the hexagonal system PrRh3. The magnetic susceptibility M/B obeys the Curie-Weiss law with the effective magnetic moment μeff = 3.88 μB/Pr and the paramagnetic Curie temperature θp = +2.9 K, which indicates ferro-type magnetic interaction between the trivalent Pr ions. A cusp in C(T) at 3.0 K coincides with a bend in ρ (T). Applying magnetic fields, the peak broadens and shifts to higher temperatures. The field dependence indicates a ferro-type magnetic order. The magnetic entropy Sm is (1/3)Rln2 at TC = 3.0 K, suggesting that part of the Pr ions take part in the magnetic order. A broad tail of the magnetic specific heat Cm observed above TC may result from short-range correlations and/or fluctuations of the active magnetic dipole and quadrupoles in the ground state doublet.

Plasma-induced magnetic responses during nonlinear dynamics of magnetic islands due to resonant magnetic perturbations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nishimura, Seiya, E-mail: n-seiya@kobe-kosen.ac.jp

Resonant magnetic perturbations (RMPs) produce magnetic islands in toroidal plasmas. Self-healing (annihilation) of RMP-induced magnetic islands has been observed in helical systems, where a possible mechanism of the self-healing is shielding of RMP penetration by plasma flows, which is well known in tokamaks. Thus, fundamental physics of RMP shielding is commonly investigated in both tokamaks and helical systems. In order to check this mechanism, detailed informations of magnetic island phases are necessary. In experiments, measurement of radial magnetic responses is relatively easy. In this study, based on a theoretical model of rotating magnetic islands, behavior of radial magnetic fields duringmore » the self-healing is investigated. It is confirmed that flips of radial magnetic fields are typically observed during the self-healing. Such behavior of radial magnetic responses is also observed in LHD experiments.« less

A model to study finite-size and magnetic effects on the phase transition of a fermion interacting system

NASA Astrophysics Data System (ADS)

Corrêa, Emerson B. S.; Linhares, César A.; Malbouisson, Adolfo P. C.

2018-03-01

We present a model to study the effects from external magnetic field, chemical potential and finite size on the phase structures of a massive four- and six-fermion interacting systems. These effects are introduced by a method of compactification of coordinates, a generalization of the standard Matsubara prescription. Through the compactification of the z-coordinate and of imaginary time, we describe a heated system with the shape of a film of thickness L, at temperature β-1 undergoing first- or second-order phase transition. We have found a strong dependence of the temperature transition on the coupling constants λ and η. Besides inverse magnetic catalysis and symmetry breaking for both kinds of transition, we have found an inverse symmetry breaking phenomenon with respect to first-order phase transition.

Safety Analysis and Protection Measures of the Control System of the Pulsed High Magnetic Field Facility in WHMFC

NASA Astrophysics Data System (ADS)

Shi, J. T.; Han, X. T.; Xie, J. F.; Yao, L.; Huang, L. T.; Li, L.

2013-03-01

A Pulsed High Magnetic Field Facility (PHMFF) has been established in Wuhan National High Magnetic Field Center (WHMFC) and various protection measures are applied in its control system. In order to improve the reliability and robustness of the control system, the safety analysis of the PHMFF is carried out based on Fault Tree Analysis (FTA) technique. The function and realization of 5 protection systems, which include sequence experiment operation system, safety assistant system, emergency stop system, fault detecting and processing system and accident isolating protection system, are given. The tests and operation indicate that these measures improve the safety of the facility and ensure the safety of people.

Neutron polarization analysis study of the frustrated magnetic ground state of β-Mn1-xAlx

NASA Astrophysics Data System (ADS)

Stewart, J. R.; Andersen, K. H.; Cywinski, R.

2008-07-01

We have performed a neutron polarization analysis study of the short-range nuclear and magnetic correlations present in the dilute alloy, β-Mn1-xAlx with 0.03≤x≤0.16 , in order to study the evolution of the magnetic ground state of this system as it achieves static spin-glass order at concentrations x>0.09 . To this end we have developed a reverse-Monte Carlo algorithm which has enabled us to extract Warren-Cowley nuclear short-range order parameters and magnetic spin correlations. Using conventional neutron powder diffraction, we show that the nonmagnetic Al substituents preferentially occupy the magnetic site II Wyckoff positions in the β-Mn structure—resulting in a reduction of the magnetic topological frustration of the Mn atoms. These Al impurities are found to display strong anticlustering behavior. The magnetic spin correlations are predominantly antiferromagnetic, persisting over a short range which is similar for all the samples studied—above and below the spin-liquid-spin-glass boundary—while the observed static (disordered) moment is shown to increase with increasing Al concentration.

Spin-orbit-driven magnetic structure and excitation in the 5d pyrochlore Cd 2Os 2O 7

DOE PAGES

Calder, Stuart A; Vale, James G.; Bogdanov, Nikolay; ...

2016-06-07

Here, much consideration has been given to the role of spin-orbit coupling (SOC) in 5d oxides, particularly on the formation of novel electronic states and manifested metal-insulator transitions (MITs). SOC plays a dominant role in 5d 5 iridates (Ir 4+), undergoing MITs both concurrent (pyrochlores) and separated (perovskites) from the onset of magnetic order. However, the role of SOC for other 5d configurations is less clear. For example, 5d 3 (Os 5+) systems are expected to have an orbital singlet with reduced effective SOC. The pyrochlore Cd 2Os 2O 7 nonetheless exhibits a MIT entwined with magnetic order phenomenologically similarmore » to pyrochlore iridates. Here, we resolve the magnetic structure in Cd 2Os 2O 7 with neutron diffraction and then via resonant inelastic X-ray scattering determine the salient electronic and magnetic energy scales controlling the MIT. In particular, SOC plays a subtle role in creating the electronic ground state but drives the magnetic order and emergence of a multiple spin-flip magnetic excitation.« less

Tailoring of magnetic orderings in Fe substituted GdMnO3 bulk samples towards room temperature

NASA Astrophysics Data System (ADS)

Pal, A.; Dhana Sekhar, C.; Venimadhav, A.; Murugavel, P.

2017-10-01

The evolution of various magnetic ordering has been studied for the orthorhombic perovskite GdMn1-x Fe x O3 (0  ⩽  x  ⩽  0.7) system to obtain its comprehensive magnetic phase diagram. We observed that the substitution of Fe in GdMnO3 increases the antiferromagnetic Neel temperature (T N) from 40 K to above 400 K and importantly induces a spin-reorientation transition (T SR) for x  ⩾  0.4. These transitions are close to room temperature at x  =  0.5 and then gradually separated at a higher x value. The static orbital ordering induced by the Jahn-Teller distortion seems to play an important role in changing the T N. The variations of spin-reorientation ordering along with the competition between the magnetic orderings as a function of the composition were discussed with respect to antisymmetric exchange interactions and Mn3+ single-ion anisotropy in detail. In addition, the correlation between structural and magnetic properties suggests that the subtle structural change at composition x  =  0.4 may affect the magnetic ordering. The observed tunable T SR and T N in GdMn1-x Fe x O3 could add a practical value for these compositions in fields like spintronics and sensors.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaluarachchi, Udhara S.; Bud’ko, Sergey L.; Canfield, Paul C.

Experimental and theoretical investigations on itinerant ferromagnetic systems under pressure have shown that ferromagnetic quantum criticality is avoided either by a change of the transition order, becoming of the first order at a tricritical point, or by the appearance of modulated magnetic phases. In the first case, the application of a magnetic field reveals a wing-structure phase diagram as seen in itinerant ferromagnets such as ZrZn 2 and UGe 2. Secondly, no tricritical wings have been observed so far. Here, we report on the discovery of wing-structure as well as the appearance of modulated magnetic phases in the temperature-pressure-magnetic fieldmore » phase diagram of LaCrGe 3. Our investigation of LaCrGe 3 reveals a double-wing structure indicating strong similarities with ZrZn 2 and UGe 2. Unlike these simpler systems, LaCrGe 3 also shows modulated magnetic phases similar to CeRuPO. Our finding provides an example of an additional possibility for the phase diagram of metallic quantum ferromagnets.« less

Local atomic and magnetic structure of dilute magnetic semiconductor ( Ba , K ) ( Zn , Mn ) 2 As 2

DOE PAGES

Frandsen, Benjamin A.; Gong, Zizhou; Terban, Maxwell W.; ...

2016-09-06

We studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba,K)(Zn,Mn) 2As 2 through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. Furthermore, we detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of ≲5Å, resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment ofmore » Mn spins along the crystallographic c axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. Finally, we discuss these results in the context of other experiments and theoretical studies on this system.« less

Magnetotransport in Artificial Kagome Spin Ice

NASA Astrophysics Data System (ADS)

Chern, Gia-Wei

2017-12-01

Magnetic nanoarrays with special geometries exhibit nontrivial collective behaviors similar to those observed in spin-ice materials. Here, we present a circuit model to describe the complex magnetotransport phenomena in artificial kagome spin ice. In this picture, the system can be viewed as a resistor network driven by voltage sources that are located at vertices of the honeycomb array. The differential voltages across different terminals of these sources are related to the ice rules that govern the local magnetization ordering. The circuit model relates the transverse Hall voltage of kagome ice to the underlying spin correlations. Treating the magnetic nanoarray as metamaterials, we present a mesoscopic constitutive equation relating the Hall resistance to magnetization components of the system. We further show that the Hall signal is significantly enhanced when the kagome ice undergoes a magnetic-charge-ordering transition. Our analysis can be readily generalized to other lattice geometries, providing a quantitative method for the design of magnetoresistance devices based on artificial spin ice.

Magnetic state of a Zn1 - x Cr x Se bulk crystal

NASA Astrophysics Data System (ADS)

Dubinin, S. F.; Sokolov, V. I.; Korolev, A. V.; Teploukhov, S. G.; Chukalkin, Yu. G.; Parkhomenko, V. D.; Gruzdev, N. B.

2008-06-01

The spin system of a Zn1 - x Cr x Se bulk crystal ( x = 0.045) was studied using thermal-neutron diffraction and magnetic measurements. Previously, it was reported in the literature that thin films (˜200 nm thick) of this type of semiconductors exhibit a ferromagnetic order. In this study, the ferromagnetic order is found to be absent in the bulk crystal.

Tricritical wings and modulated magnetic phases in LaCrGe 3 under pressure

DOE PAGES

Kaluarachchi, Udhara S.; Bud’ko, Sergey L.; Canfield, Paul C.; ...

2017-09-15

Experimental and theoretical investigations on itinerant ferromagnetic systems under pressure have shown that ferromagnetic quantum criticality is avoided either by a change of the transition order, becoming of the first order at a tricritical point, or by the appearance of modulated magnetic phases. In the first case, the application of a magnetic field reveals a wing-structure phase diagram as seen in itinerant ferromagnets such as ZrZn 2 and UGe 2. Secondly, no tricritical wings have been observed so far. Here, we report on the discovery of wing-structure as well as the appearance of modulated magnetic phases in the temperature-pressure-magnetic fieldmore » phase diagram of LaCrGe 3. Our investigation of LaCrGe 3 reveals a double-wing structure indicating strong similarities with ZrZn 2 and UGe 2. Unlike these simpler systems, LaCrGe 3 also shows modulated magnetic phases similar to CeRuPO. Our finding provides an example of an additional possibility for the phase diagram of metallic quantum ferromagnets.« less

Exceptionally large magneto-optical response in dispersions of plate-like nanocrystallites and magnetic nanoparticles

NASA Astrophysics Data System (ADS)

May, Kathrin; Eremin, Alexey; Stannarius, Ralf; Szabó, Balázs; Börzsönyi, Tamás; Appel, Ingo; Behrens, Silke; Klein, Susanne

2017-06-01

We introduce a binary colloidal system with an exceptionally strong magneto-optical response. Its induced optical birefringence at even low magnetic fields (in the mT range) reaches a value with the same order of magnitude as that of nematic liquid crystals. This system is based on a binary mixture of plate-like, non-magnetic pigment nanoparticles and a small volume fraction (< 1 v %) of spherical magnetic nanoparticles. In the field-free state, the suspension is isotropic. Birefringence is caused by an alignment of the pigment platelets, commanded by shape-anisotropic agglomerates of the magnetic nanoparticles in an external magnetic field. We give a semiquantitative discussion about this.

AC magnetic field measurement using a small flip coil system for rapid cycling AC magnets at the China Spallation Neutron Source (CSNS)

NASA Astrophysics Data System (ADS)

Zhou, Jianxin; Kang, Wen; Li, Shuai; Liu, Yudong; Liu, Yiqin; Xu, Shouyan; Guo, Xiaoling; Wu, Xi; Deng, Changdong; Li, Li; Wu, Yuwen; Wang, Sheng

2018-02-01

The China Spallation Neutron Source (CSNS) has two major accelerator systems, a linear accelerator and a rapid cycling synchrotron (RCS). The RCS accelerator is used to accumulate and accelerate protons from the energy of 80 MeV to the design energy of 1.6 GeV at the repetition rate of 25 Hz, and extract the high energy beam to the target. The main magnets of the RCS accelerator are excited by AC current with DC bias. The magnetic field quality is very important for the RCS accelerator operation, since it should guarantee and focus a circulating beam. In order to characterize the AC magnets, a small flip coil measurement system has been developed and one of each type of AC magnets has been studied. The measurement system and selected measurement results are presented in this paper.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnston, David C.

Here, the influence of uniaxial single-ion anisotropy –DS 2 z on the magnetic and thermal properties of Heisenberg antiferromagnets (AFMs) is investigated. The uniaxial anisotropy is treated exactly and the Heisenberg interactions are treated within unified molecular field theory (MFT), where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the z axis (D>0) in applied field H z = 0 are calculated versus D and temperature T, including the ordered moment μ, the Néel temperature T N, the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities χmore » ∥ and χ ⊥ in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin μ z(H z,D,T) is found, and the critical field H c(D,T) is derived at which the second-order AFM to PM phase transition occurs. The magnetic properties of the spin-flop (SF) phase are calculated, including the zero-field properties T N(D) and μ(D,T). The high-field μ z(H z,D,T) is determined, together with the associated spin-flop field H SF(D,T) at which a second-order SF to PM phase transition occurs. The free energies of the AFM, SF, and PM phases are derived from which H z–T phase diagrams are constructed. For f J =–1 and –0.75, where f J = θ pJ/T NJ and θ pJ and T NJ are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the H z–T plane similar to previous results are obtained. However, for f J = 0 we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite H z and T. Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility χ ⊥(D,T), the associated effective torque at low fields arising from the –DS 2 z term in the Hamiltonian, the high-field perpendicular magnetization μ ⊥, and the perpendicular critical field H c⊥ at which the second-order AFM to PM phase transition occurs. In addition to the above results for D > 0, the T N(D) and ordered moment μ(T,D) for collinear AFM ordering along the x axis with D < 0 are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with –DS 2 z uniaxial anisotropy with either sign of D, Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.« less

Magnetic domains and defects in ferromagnetic liquid crystal colloids realized with optical patterning

NASA Astrophysics Data System (ADS)

Hess, Andrew; Liu, Qingkun; Smalyukh, Ivan

A promising approach in designing composite materials with unusual physical behavior combines solid nanostructures and orientationally ordered soft matter at the mesoscale. Such composites not only inherit properties of their constituents but also can exhibit emergent behavior, such as ferromagnetic ordering of colloidal metal nanoparticles forming mesoscopic magnetization domains when dispersed in a nematic liquid crystal. Here we demonstrate the optical patterning of domain structures and topological defects in such ferromagnetic liquid crystal colloids which allows for altering their response to magnetic fields. Our findings reveal the nature of the defects in this soft matter system which is different as compared to non-polar nematic and ferromagnetic systems alike. This research was supported by the NSF Grant DMR-1420736.

Magnetic mapping for structural geology and geothermal exploration in Guadeloupe, Lesser Antilles

NASA Astrophysics Data System (ADS)

Mercier de Lépinay, jeanne; munschy, marc; geraud, yves; diraison, marc; navelot, vivien; verati, christelle; corsini, michel; lardeaux, jean marc; favier, alexiane

2017-04-01

This work is implemented through the GEOTREF program which benefits from the support of both the ADEME and the French public funds "Investments for the future". The program focuses on the exploration for geothermal resources in Guadeloupe, Lesser Antilles, where a geothermal power plant is in production since 1986 (Bouillante, Basse Terre). In Les Saintes archipelago, in the south of Guadeloupe, the outcrop analysis of Terre-de-Haut Island allows to point out an exhumed geothermal paleo-system that is thought to be an analogue of the Bouillante active geothermal system. We show that a detailed marine magnetic survey with a quantitative interpretation can bring information about the offshore structures around Les Saintes archipelago in order to extend the geological limits and structural elements. A similar survey and workflow is also conducted offshore Basse-Terre where more geophysical data is already available. In order to correctly link the offshore and onshore structures, the magnetic survey must be close enough to the shoreline and sufficiently detailed to correctly outline the tectonic structures. An appropriate solution for such a survey is to use a three component magnetometer aboard a speedboat. Such a boat allows more navigation flexibility than a classic oceanic vessel towing a magnetometer; it can sail at higher speed on calm seas and closer to the shoreline. This kind of magnetic acquisition is only viable because the magnetic effect of the ship can be compensated using the same algorithms than those used for airborne magnetometry. The use of potential field transforms allows a large variety of structures to be highlighted, providing insights to build a general understanding of the nature and distribution of the magnetic sources. In particular, we use the tilt angle operator to better identify the magnetic lineaments offshore in order to compare them to the faults identified onshore during the outcrop analysis. All the major faults and fractures directions observed onshore are well represented through the magnetic lineaments except the main N90-110 system which is almost inexistent. We also invert the magnetic data to obtain a magnetization intensity map. This inversion assumes a constant depth magnetized layer and a constant magnetization's direction. The calculated variations on the map are consistent with on-field measurements showing that hydrothermalized rocks have a lower magnetic susceptibility (2 orders of magnitude) than fresh ones. Our interpretation and the onshore structural and petrographic analysis allow us to recognize the offshore extension of the hydrothermalized area, as well as different structural orientations.

Neutron scattering investigations of frustated magnets

NASA Astrophysics Data System (ADS)

Fennell, Tom

This thesis describes the experimental investigation of frustrated magnetic systems based on the pyrochlore lattice of corner-sharing tetrahedra. Ho2Ti207 and Dy2Ti207 are examples of spin ices, in which the manifold of disordered magnetic groundstates maps onto that of the proton positions in ice. Using single crystal neutron scattering to measure Bragg and diffuse scattering, the effect of applying magnetic fields along different directions in the crystal was investigated. Different schemes of degeneracy removal were observed for different directions. Long and short range order, and the coexistence of both could be observed by this technique.The field and temperature dependence of magnetic ordering was studied in Ho2Ti207 and Dy2Ti207. Ho2Ti2()7 has been more extensively investigated. The field was applied on [00l], [hh0], [hhh] and [hh2h]. Dy2Ti207 was studied with the field applied on [00l] and [hho] but more detailed information about the evolution of the scattering pattern across a large area of reciprocal space was obtained.With the field applied on [00l] both materials showed complete degeneracy removal. A long range ordered structure was formed. Any magnetic diffuse scattering vanished and was entirely replaced by strong magnetic Bragg scattering. At T =0.05 K both materials show unusual magnetization curves, with a prominent step and hysteresis. This was attributed to the extremely slow dynamics of spin ice materials at this temperature.Both materials were studied in greatest detail with the field applied on [hh0]. The coexistence of long and short range order was observed when the field was raised at T = 0.05 K. The application of a field in this direction separated the spin system into two populations. One could be ordered by the field, and one remained disordered. However, via spin-spin interactions, the field restricted the degeneracy of the disordered spin population. The neutron scattering pattern of Dy2Ti207 shows that the spin system was separated into two populations of spin chains, one set ordered and the other only partly so. Cycling the field induced dynamics in these chains, again via spin-spin interactions, as the field acted on the ordered si)in chains. These field regulated dynamics were particularly noted in Ho2Ti207 where a full field cycle was executed. Raising the temperature in an applied field also activated the dynamics of the partially ordered spin chains. The continued evolution of the spin system toward a more ordered state, when dynamics can be induced, suggested that a spin ice does indeed have an energetic groundstate.The remaining two directions probed in Ho2Ti20y both have two populations of spins with different Zeeman energies. The competition of the field and the spin- spin interactions was used to investigate the onset of the ice rules regime (field on [hh2h] and the breaking of the ice rules by a strong field (field on [hhh]). It was shown that the behavior of Ho2Ti207 with field on [hhh] was consistent with the "kagome ice" hypothesis.

Multiphase magnetic systems: Measurement and simulation

NASA Astrophysics Data System (ADS)

Cao, Yue; Ahmadzadeh, Mostafa; Xu, Ke; Dodrill, Brad; McCloy, John S.

2018-01-01

Multiphase magnetic systems are common in nature and are increasingly being recognized in technical applications. One characterization method which has shown great promise for determining separate and collective effects of multiphase magnetic systems is first order reversal curves (FORCs). Several examples are given of FORC patterns which provide distinguishing evidence of multiple phases. In parallel, a visualization method for understanding multiphase magnetic interaction is given, which allocates Preisach magnetic elements as an input "Preisach hysteron distribution pattern" to enable simulation of different "wasp-waisted" magnetic behaviors. These simulated systems allow reproduction of different major hysteresis loops and FORC patterns of real systems and parameterized theoretical systems. The experimental FORC measurements and FORC diagrams of four commercially obtained magnetic materials, particularly those sold as nanopowders, show that these materials are often not phase pure. They exhibit complex hysteresis behaviors that are not predictable based on relative phase fraction obtained by characterization methods such as diffraction. These multiphase materials, consisting of various fractions of BaFe12O19, ɛ-Fe2O3, and γ-Fe2O3, are discussed.

Exact analytical modeling of magnetic vector potential in surface inset permanent magnet DC machines considering magnet segmentation

NASA Astrophysics Data System (ADS)

Jabbari, Ali

2018-01-01

Surface inset permanent magnet DC machine can be used as an alternative in automation systems due to their high efficiency and robustness. Magnet segmentation is a common technique in order to mitigate pulsating torque components in permanent magnet machines. An accurate computation of air-gap magnetic field distribution is necessary in order to calculate machine performance. An exact analytical method for magnetic vector potential calculation in surface inset permanent magnet machines considering magnet segmentation has been proposed in this paper. The analytical method is based on the resolution of Laplace and Poisson equations as well as Maxwell equation in polar coordinate by using sub-domain method. One of the main contributions of the paper is to derive an expression for the magnetic vector potential in the segmented PM region by using hyperbolic functions. The developed method is applied on the performance computation of two prototype surface inset magnet segmented motors with open circuit and on load conditions. The results of these models are validated through FEM method.

Influence of uniaxial single-ion anisotropy on the magnetic and thermal properties of Heisenberg antiferromagnets within unified molecular field theory

DOE PAGES

Johnston, David C.

2017-03-17

Here, the influence of uniaxial single-ion anisotropy –DS 2 z on the magnetic and thermal properties of Heisenberg antiferromagnets (AFMs) is investigated. The uniaxial anisotropy is treated exactly and the Heisenberg interactions are treated within unified molecular field theory (MFT), where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the z axis (D>0) in applied field H z = 0 are calculated versus D and temperature T, including the ordered moment μ, the Néel temperature T N, the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities χmore » ∥ and χ ⊥ in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin μ z(H z,D,T) is found, and the critical field H c(D,T) is derived at which the second-order AFM to PM phase transition occurs. The magnetic properties of the spin-flop (SF) phase are calculated, including the zero-field properties T N(D) and μ(D,T). The high-field μ z(H z,D,T) is determined, together with the associated spin-flop field H SF(D,T) at which a second-order SF to PM phase transition occurs. The free energies of the AFM, SF, and PM phases are derived from which H z–T phase diagrams are constructed. For f J =–1 and –0.75, where f J = θ pJ/T NJ and θ pJ and T NJ are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the H z–T plane similar to previous results are obtained. However, for f J = 0 we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite H z and T. Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility χ ⊥(D,T), the associated effective torque at low fields arising from the –DS 2 z term in the Hamiltonian, the high-field perpendicular magnetization μ ⊥, and the perpendicular critical field H c⊥ at which the second-order AFM to PM phase transition occurs. In addition to the above results for D > 0, the T N(D) and ordered moment μ(T,D) for collinear AFM ordering along the x axis with D < 0 are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with –DS 2 z uniaxial anisotropy with either sign of D, Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.« less

Relaxation dynamics of magnetization transitions in synthetic antiferromagnet with perpendicular anisotropy

NASA Astrophysics Data System (ADS)

Talantsev, A.; Lu, Y.; Fache, T.; Lavanant, M.; Hamadeh, A.; Aristov, A.; Koplak, O.; Morgunov, R.; Mangin, S.

2018-04-01

Two synthetic antiferromagnet bilayer systems with strong perpendicular anisotropy CoFeB/Ta/CoFeB and Pt/Co/Ir/Co/Pt have been grown using sputtering techniques. For both systems two types of magnetization transitions have been studied. The first one concerns transitions from a state where magnetizations of the two magnetic layers are parallel (P state) to a state where magnetizations of the two layers are aligned antiparallel (AP state). The second one concerns transitions between the two possible antiparallel alignments (AP+  to AP-). For both systems and both transitions after-effect measurements can be understood in the frame of nucleation—propagation model. Time derivative analysis of magnetic relaxation curves and mapping of the first order reversal curves at different temperature allowed us to demonstrate the presence of different pinning centers, which number can be controlled by magnetic field and temperature.

Complex magnetic structure of clusters and chains of Ni and Fe on Pt(111)

PubMed Central

Bezerra-Neto, Manoel M.; Ribeiro, Marcelo S.; Sanyal, Biplab; Bergman, Anders; Muniz, Roberto B.; Eriksson, Olle; Klautau, Angela B.

2013-01-01

We present an approach to control the magnetic structure of adatoms adsorbed on a substrate having a high magnetic susceptibility. Using finite Ni-Pt and Fe-Pt nanowires and nanostructures on Pt(111) surfaces, our ab initio results show that it is possible to tune the exchange interaction and magnetic configuration of magnetic adatoms (Fe or Ni) by introducing different numbers of Pt atoms to link them, or by including edge effects. The exchange interaction between Ni (or Fe) adatoms on Pt(111) can be considerably increased by introducing Pt chains to link them. The magnetic ordering can be regulated allowing for ferromagnetic or antiferromagnetic configurations. Noncollinear magnetic alignments can also be stabilized by changing the number of Pt-mediated atoms. An Fe-Pt triangularly-shaped nanostructure adsorbed on Pt(111) shows the most complex magnetic structure of the systems considered here: a spin-spiral type of magnetic order that changes its propagation direction at the triangle vertices. PMID:24165828

Motion stability of the magnetic levitation and suspension with YBa2Cu3O7-x high-Tc superconducting bulks and NdFeB magnets

NASA Astrophysics Data System (ADS)

Li, Jipeng; Zheng, Jun; Huang, Huan; Li, Yanxing; Li, Haitao; Deng, Zigang

2017-10-01

The flux pinning effect of YBa2Cu3O7-x high temperature superconducting (HTS) bulk can achieve self-stable levitation over a permanent magnet or magnet array. Devices based on this phenomenon have been widely developed. However, the self-stable flux pinning effect is not unconditional, under disturbances, for example. To disclose the roots of this amazing self-stable levitation phenomenon in theory, mathematical and mechanical calculations using Lyapunov's stability theorem and the Hurwitz criterion were performed under the conditions of magnetic levitation and suspension of HTS bulk near permanent magnets in Halbach array. It is found that the whole dynamical system, in the case of levitation, has only one equilibrium solution, and the singular point is a stable focus. In the general case of suspension, the system has two singular points: one is a stable focus, and the other is an unstable saddle. With the variation of suspension force, the two first-order singular points mentioned earlier will get closer and closer, and finally degenerate to a high-order singular point, which means the stable region gets smaller and smaller, and finally vanishes. According to the center manifold theorem, the high-order singular point is unstable. With the interaction force varying, the HTS suspension dynamical system undergoes a saddle-node bifurcation. Moreover, a deficient damping can also decrease the stable region. These findings, together with existing experiments, could enlighten the improvement of HTS devices with strong anti-interference ability.

Study on frequency characteristics of wireless power transmission system based on magnetic coupling resonance

NASA Astrophysics Data System (ADS)

Liang, L. H.; Liu, Z. Z.; Hou, Y. J.; Zeng, H.; Yue, Z. K.; Cui, S.

2017-11-01

In order to study the frequency characteristics of the wireless energy transmission system based on the magnetic coupling resonance, a circuit model based on the magnetic coupling resonant wireless energy transmission system is established. The influence of the load on the frequency characteristics of the wireless power transmission system is analysed. The circuit coupling theory is used to derive the minimum load required to suppress frequency splitting. Simulation and experimental results verify that when the load size is lower than a certain value, the system will appear frequency splitting, increasing the load size can effectively suppress the frequency splitting phenomenon. The power regulation scheme of the wireless charging system based on magnetic coupling resonance is given. This study provides a theoretical basis for load selection and power regulation of wireless power transmission systems.

Magnetic moment arrangement in amorphous Fe 0.66Er 0.19B 0.15

NASA Astrophysics Data System (ADS)

Szymański, K.; Kalska, B.; Satuła, D.; Dobrzyński, L.; Broddefalk, A.; Wäppling, R.; Nordblad, P.

2002-11-01

Magnetization measurements and Mössbauer spectroscopy with and without a monochromatic circularly polarized Mössbauer source (MCPMS) have been performed in order to determine the magnetic properties of the amorphous alloy Fe 0.66Er 0.19B 0.15. The system is found to order ferrimagnetically at TC=330 K and to show a compensation temperature ( Tcomp) at 120 K. A reorientation of the magnetic moments of iron and erbium during sample cooling through the compensation point in magnetic field is clearly displayed in the MCPMS data. The orientation of the net magnetic moment is due to the orientation of Fe moments above Tcomp and to Er moments at low temperatures. The results are compatible with a model of predominantly antiferromagnetic Fe-Er coupling accompanied by random local anisotropy acting on the Er moments.

Giant Controllable Magnetization Changes Induced by Structural Phase Transitions in a Metamagnetic Artificial Multiferroic

PubMed Central

Bennett, S. P.; Wong, A. T.; Glavic, A.; Herklotz, A.; Urban, C.; Valmianski, I.; Biegalski, M. D.; Christen, H. M.; Ward, T. Z.; Lauter, V.

2016-01-01

The realization of a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. This study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure. PMID:26940159

Giant Controllable Magnetization Changes Induced by Structural Phase Transitions in a Metamagnetic Artificial Multiferroic

DOE PAGES

Bennett, S. P.; Wong, A. T.; Glavic, A.; ...

2016-03-04

We realize that a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized and by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate.more » These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. In our study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure.« less

Development of superconducting high gradient magnetic separation system for highly viscous fluid for practical use

NASA Astrophysics Data System (ADS)

Hayashi, S.; Mishima, F.; Akiyama, Y.; Nishijima, S.

2011-11-01

In the industrial plants processing highly viscous fluid such as foods or industrial products, it is necessary to remove the metallic wear debris originating from pipe in manufacturing line which triggers quality loss. In this study, we developed a high gradient magnetic separation (HGMS) system which consists of superconducting magnet to remove the metallic wear debris. The magnetic separation experiment and the particle trajectory simulation were conducted with polyvinyl alcohol (PVA) as a model material (viscosity coefficient was 10 Pa s, which is 10,000 times higher than that in water). In order to develop a magnetic separation system for practical use, the particle trajectory simulation by using solenoidal superconducting magnet was conducted, and the possibility of the magnetic separation for removing ferromagnetic stainless steel (SUS) particles in highly viscous fluid of 10 Pa s was indicated. Based on the results, the number of filters to obtain required separation efficiency was examined to design the practical separation system.

Structure and magnetism in Co/X, Fe/Si, and Fe/(FeSi) multilayers

NASA Astrophysics Data System (ADS)

Franklin, Michael Ray

Previous studies have shown that magnetic behavior in multilayers formed by repeating a bilayer unit comprised of a ferromagnetic layer and a non-magnetic spacer layer can be affected by small structural differences. For example, a macroscopic property such as giant magnetoresistance (GMR) is believed to depend significantly upon interfacial roughness. In this study, several complimentary structural probes were used to carefully characterize the structure of several sputtered multilayer systems-Co/Ag, Co/Cu, Co/Mo, Fe/Si, and Fe//[FeSi/]. X-ray diffraction (XRD) studies were used to examine the long-range structural order of the multilayers perpendicular to the plane of the layers. Transmission electron diffraction (TED) studies were used to probe the long-range order parallel to the layer plane. X-ray Absorption Fine Structure (XAFS) studies were used to determine the average local structural environment of the ferromagnetic atoms. For the Co/X systems, a simple correlation between crystal structure and saturation magnetization is discovered for the Co/Mo system. For the Fe/X systems, direct evidence of an Fe-silicide is found for the /[FeSi/] spacer layer but not for the Si spacer layer. Additionally, differences were observed in the magnetic behavior between the Fe in the nominally pure Fe layer and the Fe contained in the /[FeSi/] spacer layers.

L a A l O3 / S r T i O3 -A Tale of Two Magnetisms

NASA Astrophysics Data System (ADS)

Pai, Yun-Yi; Tylan-Tyler, Anthony; Irvin, Patrick; Levy, Jeremy

Ten years since the first report of magnetism by Brinkman et al., a unified picture of magnetism at the two-dimensional electron system (2DES) between LaAlO3 / SrTiO3 is still lacking. The understanding is further hindered by the complex interplay of magnetism and many other aspects of this system: multi-band superconductivity, quantum paraelectricity, multiferroicity, to name but a few. We argue that the reported magnetic signatures in this system can come from two principal origins: (1) a ferromagnetic long-range order resulting from local magnetic moments mediated by itinerant electrons, and (2) metamagnetic phenomena associated with electron pairing without superconductivity. Finally, we discuss possible experimental tests of this framework. We gratefully acknowledge financial support from NSF (DMR-1124131, DMR-1609519) and ONR N00014-15-1-2847.

A β-Ta system for current induced magnetic switching in the absence of external magnetic field

NASA Astrophysics Data System (ADS)

Chen, Wenzhe; Qian, Lijuan; Xiao, Gang

2018-05-01

Magnetic switching via Giant Spin Hall Effect (GSHE) has received great interest for its role in developing future spintronics logic or memory devices. In this work, a new material system (i.e. a transition metal sandwiched between two ferromagnetic layers) with interlayer exchange coupling is introduced to realize the deterministic field-free perpendicular magnetic switching. This system uses β-Ta, as the GSHE agent to generate a spin current and as the interlayer exchange coupling medium to generate an internal field. The critical switching current density at zero field is on the order of 106 A/cm2 due to the large spin Hall angle of β-Ta. The internal field, along with switching efficiency, depends strongly on the orthogonal magnetization states of two ferromagnetic coupling layers in this system.

Removal of Iron Oxide Scale from Feed-water in Thermal Power Plant by Using Magnetic Separation

NASA Astrophysics Data System (ADS)

Nakanishi, Motohiro; Shibatani, Saori; Mishima, Fumihito; Akiyama, Yoko; Nishijima, Shigehiro

2017-09-01

One of the factors of deterioration in thermal power generation efficiency is adhesion of the scale to inner wall in feed-water system. Though thermal power plants have employed All Volatile Treatment (AVT) or Oxygen Treatment (OT) to prevent scale formation, these treatments cannot prevent it completely. In order to remove iron oxide scale, we proposed magnetic separation system using solenoidal superconducting magnet. Magnetic separation efficiency is influenced by component and morphology of scale which changes their property depending on the type of water treatment and temperature. In this study, we estimated component and morphology of iron oxide scale at each equipment in the feed-water system by analyzing simulated scale generated in the pressure vessel at 320 K to 550 K. Based on the results, we considered installation sites of the magnetic separation system.

John H. Dillon Medal Lecture: Magnetic Field Directed Self-Assembly of Block Copolymers and Surfactant Mesophases

NASA Astrophysics Data System (ADS)

Osuji, Chinedum

2015-03-01

Molecular self-assembly of block copolymers and small molecule surfactants gives rise to a rich phase behavior as a function of temperature, composition, and other variables. We consider the directed self-assembly of such soft mesophases using magnetic fields, principally through the use of in situ x-ray scattering studies. Field alignment is predicated on a sufficiently large product of magnetic anisotropy and grain size to produce magnetostatic interactions which are substantive relative to thermal forces. We examine the role of field strength on the phase behavior and alignment dynamics of a series of soft mesophases, outlining the possibility to readily create highly ordered functional materials over macroscopic length scales. We show that magnetic fields as large as 10 T have little discernable impact on the stability of block copolymer systems considered, with shifts in order-disorder transition temperatures of roughly 5 mK or smaller. Consequently, directed self-assembly in these systems proceeds by nucleation of randomly aligned grains which thereafter rotate into registry with the field. We highlight the tradeoff between decreasing mobility and increasing anisotropic field interaction that dictates alignment kinetics while transiting from a high temperature disordered state to an ordered system at lower temperatures. NSF support through DMR-0847534 is gratefully acknowledged.

Lack of multiferroic behavior in BaCuSi 2O 6 is consistent with the frustrated magnetic scenario for this material

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zapf, Vivien; Jaime, Marcelo; Chikara, Shalinee

2017-03-01

BaCuSi 2O 6 is a well-known quantum magnet that exhibits a Bose-Einstein Condensation quantum phase transition in applied magnetic fields. It contains Cu dimers that form singlets in zero magnetic field, and in applied fields as the singlet-triplet gap is suppressed a quantum phase transition occurs to canted XY antiferromagnetism between critical fields H c1 = 23 T and H c2 = 59 T. In addition, as the temperature is lowered, a rare frustrationinduced dimensional reduction has been proposed from three to two dimensions. Recently, however, a controversy has arisen about the details of the magnetic ordering due to themore » discovery of a tetragonal to orthorhombic structural transition at 100 K with an incommensurate modulation along the b-axis. Multiple magnon modes were observed in neutron diffraction studies, while NMR found modulation of the spin structure along both the ab plane and the c-axis. In this scenario the material is still a Bose-Einstein condensate system but the frustration is not perfect, calling into question the dimension reduction scenario. A recent study of BaCuSi 2O 6 combining inelastic neutron diffraction and density functional theory suggest that the material isn’t even frustrated at all and that the spins are ordered ferromagnetically in the a-b plane and antiferromagnetically along the c-axis. After a detailed symmetry analysis we have concluded that the magnetic scenario postulated by this most recent unfrustrated theory6 will render BaCuSi 2O 6 a multiferroic between H c1 and H c2, with electric polarization in easy axis of the a-b plane for magnetic fields along the c-axis via an inverse Dzyaloshinskii-Moriya mechanism. Electric polarization is a sensitive symmetry probe of magnetic order, since magnetic systems that break spatial inversion symmetry can induce an overall ferroelectricity in the crystalline lattice. In pulsed magnetic fields we can detect electric polarizations with unique sensitivity to sub-pC/m 2, which is orders of magnitude more sensitive than what can be detected in DC magnetic field.« less

Electronic and magnetic ordering induced by Mo- and Ru doping of the Mn site in CaMnO3 perovskite: EMR probing

NASA Astrophysics Data System (ADS)

Shames, A. I.; Auslender, M.; Rozenberg, E.; Gorodetsky, G.; Martin, C.; Maignan, A.

2005-05-01

X-band electron magnetic-resonance (EMR) measurements of polycrystalline CaMn1-yMoyO3 (0⩽y ⩽0.14) samples were performed at 120K⩽T⩽540K. The data obtained are compared with those of another electron-doped manganite system, CaMn1-xRuxO3 (0⩽x ⩽0.40). The observed anomalies of the EMR parameters correlate pretty well with the temperatures of antiferro-, ferromagneticlike, and orbital/charge-ordering transitions in these systems. However, a strong difference is observed between the resonant properties of Mo- and Ru doped series at both paramagnetic (PM) and magnetically ordered states. To describe such a difference, the energy-band diagrams, which comprise the deep impurity t2g-like states +eg-like conductive band for CaMn1-xRuxO3 and shallow impurity states+conductive band, both having eg-like symmetry, for CaMn1-yMoyO3, are proposed. Specific electrons' contribution to the EMR linewidth at PM temperatures is introduced for the considered systems.

Influence of the geometry on magnetic interactions in a retina fixator based on a magnetoactive elastomer seal

NASA Astrophysics Data System (ADS)

Nadzharyan, T. A.; Makarova, L. A.; Kazimirova, E. G.; Perov, N. S.; Kramarenko, E. Yu

2018-03-01

We study the effects the geometric configuration has on magnetic interactions between a magnetoactive elastomer (MAE) sample and various systems of permanent magnets for problems with both flat and curved geometry. MAEs consist of a silicone polymer matrix and iron filler microparticles embedded in it. Permanent magnets are cylindrical neodymium magnets arranged in a line on a flat or curved solid surfaces. We use computer simulations, namely the finite element method, in order to study the interaction force and magnetic pressure in a system with an MAE sample and permanent magnets. The model is based on classical Maxwell magnetostatics and two factors taking into account field dependence of MAE’s magnetic properties and inhomogeneities caused by local demagnetization. We calculate magnetic pressure dependences on various geometric parameters of the system, namely, the diameter and the height of permanent magnets, the distance between the magnets and dimensions of MAE samples. This research aims to create a set of guidelines for choosing the geometric configuration of a retina fixator based on MAE seals to be used in eye surgery for retinal detachment treatment.

Polarized neutron scattering study of the multiple order parameter system NdB4

NASA Astrophysics Data System (ADS)

Metoki, N.; Yamauchi, H.; Matsuda, M.; Fernandez-Baca, J. A.; Watanuki, R.; Hagihala, M.

2018-05-01

Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter f -electron system NdB4. We confirmed the noncollinear "all-in all-out" structure (Γ4) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the c -axis mc showed diagonally antiferromagnetic structure (Γ10), inconsistent with previously reported "vortex" structure (Γ2). The microscopic mixture of these two structures with q⃗0=(0 ,0 ,0 ) appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. The unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter Γ4 coupled with higher-order secondary order parameter Γ10. The magnetic moments were estimated to be 1.8 ±0.2 and 0.2 ±0.05 μB at T =7.5 K for Γ4 and Γ10, respectively. We also found a long-period incommensurate modulation of the q⃗1=(0 ,0 ,1 /2 ) antiferromagnetic structure of mc with the propagation q⃗s 1=(0.14 ,0.14 ,0.1 ) and q⃗s 2=(0.2 ,0 ,0.1 ) in phase III and IV, respectively. The amplitude of sinusoidal modulation was about mc=1.0 ±0.2 μB at T =1.5 K. The local (0 ,0 ,1 /2 ) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of mc, opposite to the coexisting Γ10. The mc of Γ10 is significantly enhanced up to 0.6 μB at T =1.5 K, which is accompanied by the incommensurate modulations. The Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet f -electron state play important roles.

Dynamic pathway of the photoinduced phase transition of TbMnO3

NASA Astrophysics Data System (ADS)

Bothschafter, Elisabeth M.; Abreu, Elsa; Rettig, Laurenz; Kubacka, Teresa; Parchenko, Sergii; Porer, Michael; Dornes, Christian; Windsor, Yoav William; Ramakrishnan, Mahesh; Alberca, Aurora; Manz, Sebastian; Saari, Jonathan; Koohpayeh, Seyed M.; Fiebig, Manfred; Forrest, Thomas; Werner, Philipp; Dhesi, Sarnjeet S.; Johnson, Steven L.; Staub, Urs

2017-11-01

We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic TbMnO3 by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. The use of orthogonal linear x-ray polarizations provides information on the contribution from the different magnetic moment directions, which can be interpreted as signatures from multiferroic cycloidal spin order and sinusoidal spin order. Tracking these signatures in the time domain enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond time scale. The transient phase is shown to exhibit mostly spin density wave character, as in the adiabatic case, while nevertheless retaining the wave vector of the cycloidal long-range order. Two different pump photon energies, 1.55 and 3.1 eV, lead to population of the conduction band predominantly via intersite d -d or intrasite p -d transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a time scale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic order and orbital reconstruction. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such theoretical studies.

Quasi-Periodic Oscillations in AM Herculis Binaries -- Cycle 3 Medium

NASA Astrophysics Data System (ADS)

Chanmugam, G.

1992-06-01

AM Her variables are close-binary systems in which a white dwarf with a magnetic field of 20-70 MG accretes matter from a companion star. Theoretical studies of magnetically channeled accretion flows in such systems predict that the shock formed near the white dwarf should oscillate with periods of order 0.1-1 sec. Optical high-speed photometry has indeed shown the existence of such rapid, quasi-periodic oscillations in some AM Her binaries, but not in others. We will use HST to obtain UV and optical high-speed photometry of several AM Her systems, in order to explore further the nature of the oscillations, and to extend the search into the UV. This proposal is a followup to an accepted Cycle 2 program. We are proposing it for Cycle 3 in order to complete our survey of the most suitable AM Her systems during this last opportunity for HSP observations.

Investigation of a Structural Phase Transition and Magnetic Structure of Na 2BaFe(VO 4) 2: A Triangular Magnetic Lattice with a Ferromagnetic Ground State

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sanjeewa, Liurukara D.; Garlea, Vasile O.; McGuire, Michael A.

The structural and magnetic properties of a glaserite-type Na 2BaFe(VO 4) 2 compound, featuring a triangular magnetic lattice of Fe 2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal Pmore » $$\\bar{3}$$ m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (T s = 288 K). This structural transition involves a tilting of Fe–O–V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na 2BaFe(VO 4) 2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Altogether, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO 6 octahedron.« less

Investigation of a Structural Phase Transition and Magnetic Structure of Na 2BaFe(VO 4) 2: A Triangular Magnetic Lattice with a Ferromagnetic Ground State

DOE PAGES

Sanjeewa, Liurukara D.; Garlea, Vasile O.; McGuire, Michael A.; ...

2017-12-07

The structural and magnetic properties of a glaserite-type Na 2BaFe(VO 4) 2 compound, featuring a triangular magnetic lattice of Fe 2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal Pmore » $$\\bar{3}$$ m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (T s = 288 K). This structural transition involves a tilting of Fe–O–V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na 2BaFe(VO 4) 2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Altogether, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO 6 octahedron.« less

Design and development of a magnetometer calibration device

NASA Astrophysics Data System (ADS)

Angelopoulos, S.

2017-12-01

This paper describes the development of a new magnetometer calibration device, which is able to provide accuracy of 1fT/√Hz. The mentioned device is able to eliminate the ambient magnetic field, using an active shielding technique. This can be achieved by the use of a solenoid or a pair of Helmholtz coils. In order to measure the existing magnetic field, it is necessary to develop and use accurate magnetometers with amorphous ribbons as core magnetic materials. The whole system works as a closed-loop system, which is able to control and adjust the produced counter magnetic field.

Hysteresis prediction inside magnetic shields and application

DOE Office of Scientific and Technical Information (OSTI.GOV)

Morić, Igor; CNES, Edouard Belin 18, 31400 Toulouse; De Graeve, Charles-Marie

2014-07-15

We have developed a simple model that is able to describe and predict hysteresis behavior inside Mumetal magnetic shields, when the shields are submitted to ultra-low frequency (<0.01 Hz) magnetic perturbations with amplitudes lower than 60 μT. This predictive model has been implemented in a software to perform an active compensation system. With this compensation the attenuation of longitudinal magnetic fields is increased by two orders of magnitude. The system is now integrated in the cold atom space clock called PHARAO. The clock will fly onboard the International Space Station in the frame of the ACES space mission.

Chemically ignited thermonuclear reactions—A near-term means for a high specific impulse—High thrust propulsion system

NASA Astrophysics Data System (ADS)

Winterberg, F.

The combination of metallic shells imploded with chemical explosives and the recently proposed magnetic booster target inertial fusion concept, could make possible the fissionless ignition of small thermonuclear explosions. In the magnetic booster concept a very dense but magnetically confined thermonuclear plasma of low yield serves as the trigger for an inertially confined thermonuclear plasma of high yield. For the most easily ignitable fusion reaction, the DT reaction, this could lead to a fissionless bomb propulsion system, with the advantage to have a much smaller yield of the pure fusion bombs as compared to either fission- or fission-induced fusion bombs, previously proposed for propulsion. Typically, the proposed propulsion concept should give a specific impulse of ˜ 3000 secs, corresponding to an exhaust velocity of ˜ 30 km/sec. If the energy released in each pure fusion bomb is of the order of 10 18 erg or the order of 100 tons of TNT, and if one fusion explosion per second takes place, the average thrust is of the order 10 3 tons. The propulsion system appears ideally suited for the fast economical transport of large spacecraft within the solar system.

Mossbauer spectroscopic studies in ferroboron

NASA Astrophysics Data System (ADS)

Yadav, Ravi Kumar; Govindaraj, R.; Amarendra, G.

2017-05-01

Mossbauer spectroscopic studies have been carried out in a detailed manner on ferroboron in order to understand the local structure and magnetic properties of the system. Evolution of the local structure and magnetic properties of the amorphous and crystalline phases and their thermal stability have been addressed in a detailed manner in this study. Role of bonding between Fe 4s and/or 4p electrons with valence electrons of boron (2s,2p) in influencing the stability and magnetic properties of Fe-B system is elucidated.

Magnetic Suspension Technology Workshop

NASA Technical Reports Server (NTRS)

Keckler, Claude R. (Editor); Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1993-01-01

In order to identify the state of magnetic suspension technology in such areas as rotating systems, pointing of experiments or subsystems, payload isolation, and superconducting materials, a workshop on Magnetic Suspension Technology was held at the Langley Research Center in Hampton, Virginia, on 2-4 Feb. 1988. The workshop included five technical sessions in which a total of 24 papers were presented. The technical sessions covered the areas of pointing, isolation, and measurement, rotating systems, modeling and control, and superconductors. A list of attendees is provided.

Probing equilibrium by nonequilibrium dynamics: Aging in Co/Cr superlattices

NASA Astrophysics Data System (ADS)

Binek, Christian

2013-03-01

Magnetic aging phenomena are investigated in a structurally ordered Co/Cr superlattice through measurements of magnetization relaxation, magnetic susceptibility, and hysteresis at various temperatures above and below the onset of collective magnetic order. We take advantage of the fact that controlled growth of magnetic multilayer thin films via molecular beam epitaxy allows tailoring the intra and inter-layer exchange interaction and thus enables tuning of magnetic properties including the spin-fluctuation spectra. Tailored nanoscale periodicity in Co/Cr multilayers creates mesoscopic spatial magnetic correlations with slow relaxation dynamics when quenching the system into a nonequilibrium state. Magnetization relaxation in weakly correlated spin systems depends on the microscopic spin-flip time of about 10 ns and is therefore a fast process. The spin correlations in our Co/Cr superlattice bring the magnetization dynamics to experimentally better accessible time scales of seconds or hours. In contrast to spin-glasses, where slow dynamics due to disorder and frustration is a well-known phenomenon, we tune and increase relaxation times in ordered structures. This is achieved by increasing spin-spin correlation between mesoscopically correlated regions rather than individual atomic spins, a concept with some similarity to block spin renormalization. Magnetization transients are measured after exposing the Co/Cr heterostructure to a magnetic set field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in nonequilibrium properties. Our findings suggest that scaling analysis of nonequilibrium data can serve as a probe for weak equilibrium phase transitions. Financial support by NRI, and NSF through EPSCoR, and MRSEC 0820521 is greatly acknowledged.

Resonant x-ray scattering reveals possible disappearance of magnetic order under hydrostatic pressure in the Kitaev candidate γ - Li 2 IrO 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Breznay, Nicholas P.; Ruiz, Alejandro; Frano, Alex

2017-07-05

Honeycomb iridates such as γ-Li 2IrO 3 are argued to realize Kitaev spin-anisotropic magnetic exchange, along with Heisenberg and possibly other couplings. While systems with pure Kitaev interactions are candidates to realize a quantum spin-liquid ground state, in γ-Li 2IrO 3 it has been shown that the presence of competing magnetic interactions leads to an incommensurate spiral spin order at ambient pressure below 38 K. In this paper, we study the pressure sensitivity of this magnetically ordered state in single crystals of γ-Li 2IrO 3 using resonant x-ray scattering (RXS) under applied hydrostatic pressures of up to 3 GPa. Finally,more » RXS is a direct probe of electronic order, and we observe the abrupt disappearance of the q sp = (0.57, 0, 0) spiral order at a critical pressure P c = 1.4 GPa with no accompanying change in the symmetry of the lattice.« less

Characterization of magnetic flux density in passive sources used in magnetic stimulation

NASA Astrophysics Data System (ADS)

Torres, J.; Hincapie, E.; Gilart, F.

2018-03-01

The spatial distribution of the magnetic flux density (B) was determined for the passive sources of magnetic field most used in magnetic stimulation of biological systems, toroidal dipole magnets and cylindrical dipole magnets, in order to find the spatial characteristics of the magnetic field within the volumes of interest for the treatment of biological systems. The perpendicular and parallel components of B regarding the polar surface of the magnets were measured, for which a FW Bell 5180 digital teslameter was used with longitudinal and transverse probes and a two-dimensional positioning system with millimeter scale. It was found that the magnets of this type, which are the most used, present a strong variation of the magnitude and direction of the magnetic flux density for spaces specified in millimeters, reason why the homogeneity of the magnetic field in the regions of interest was found to be relatively low, which makes them elements with a strong applicability for the stimulation of biological systems in which magnetic field gradients up to mT/mm are required in the case of cylindrical magnets, and up to tens of mT/mm in the case of toroidal magnets. Finally, it is concluded that a high percentage of experiments reported in the literature on magnetic treatment of biological systems may be presenting values of B in their doses with deviations of more than 100% of the real value, which raises an incongruence in the cause-effect proposed relation.

Observation of a second metastable spin-ordered state in ferrimagnet Cu2OSeO3

NASA Astrophysics Data System (ADS)

Huang, C. L.; Tseng, K. F.; Chou, C. C.; Mukherjee, S.; Her, J. L.; Matsuda, Y. H.; Kindo, K.; Berger, H.; Yang, H. D.

2011-02-01

dc and ac magnetization measurements were performed on single-crystal Cu2OSeO3 under magnetic field (H) and hydrostatic pressure (P) conditions. Increasing H shifts the ferrimagnetic transition observed at TC~60 K to a higher-temperature region. Moreover, the TC increases linearly and magnetization is enhanced with P. Features of the ladder in the M-vs-H curve or the peak in the dM/dH-vs-H curve are observed at HSF~0.5 kOe, suggesting a competing ordered state under magnetic fields below TC. Remarkably, a second shoulder is observed at ˜1 kOe in the dM/dH-vs-H curve, revealing another metastable spin-ordered state in Cu2OSeO3. This state is retained and enhanced by applying pressure. As H rises to 55 T, no further slope changes in the M-H curve are observed. These magnetic properties indicate a complex spin orientation in the geometrically spin-frustrated system Cu2OSeO3.

A first principles approach to magnetic and optical properties in single-layer graphene sandwiched between boron nitride monolayers

NASA Astrophysics Data System (ADS)

Das, Ritwika; Chowdhury, Suman; Jana, Debnarayan

2015-07-01

The dependence of the stability of single-layer graphene (SLG) sandwiched between hexagonal boron nitride bilayers (h-BN) has been described and investigated for different types of stacking in order to provide the fingerprint of the stacking order which affects the optical properties of such trilayer systems. Considering the four stacking models AAA-, AAB-, ABA-, and ABC-type stacking, the static dielectric functions (in case of parallel polarizations) for AAB-type stacking possesses maximum values, and minimum values are noticed for AAA. However, AAA-type stacking structures contribute the maximum magnetic moment while vanishing magnetic moments are observed for ABA and ABC stacking. The observed optical anisotropy and magnetic properties of these trilayer heterostructures (h-BN/SLG/h-BN) can be understood from the crystallographic stacking order and inherent crystal lattice symmetry. These optical and magnetic results suggest that the h-BN/SLG/h-BN could provide a viable route to graphene-based opto-electronic and spintronic devices.

Understanding temperature and magnetic-field actuated magnetization polarity reversal in the Prussian blue analogue Cu 0.73 Mn 0.77 [Fe(CN) 6 ]. z H 2 O, using XMCD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lahiri, Debdutta; Choi, Yongseong; Yusuf, S. M.

2016-02-23

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)(6)]center dot zH(2)O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for themore » observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.« less

Spin diffusion in the Mn2+ ion system of II-VI diluted magnetic semiconductor heterostructures

NASA Astrophysics Data System (ADS)

Maksimov, A. A.; Yakovlev, D. R.; Debus, J.; Tartakovskii, I. I.; Waag, A.; Karczewski, G.; Wojtowicz, T.; Kossut, J.; Bayer, M.

2010-07-01

The magnetization dynamics in diluted magnetic semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te were studied optically and simulated numerically. In samples with inhomogeneous magnetic ion distribution, these dynamics are contributed by spin-lattice relaxation and spin diffusion in the Mn spin system. A spin-diffusion coefficient of 7×10-8cm2/s was evaluated for Zn0.99Mn0.01Se from comparison of experiment and theory. Calculations of the exciton giant Zeeman splitting and the magnetization dynamics in ordered alloys and digitally grown parabolic quantum wells show perfect agreement with the experimental data. In both structure types, spin diffusion contributes essentially to the magnetization dynamics.

MCG measurement in the environment of active magnetic shield.

PubMed

Yamazaki, K; Kato, K; Kobayashi, K; Igarashi, A; Sato, T; Haga, A; Kasai, N

2004-11-30

MCG (Magnetocardiography) measurement by a SQUID gradiometer was attempted with only active magnetic shielding (active shielding). A three-axis-canceling-coil active shielding system, where three 16-10-16 turns-coil sets were put in the orthogonal directions, produces a homogeneous magnetic field in a considerable volume surrounding the center. Fluxgate sensors were used as the reference sensors of the system. The system can reduce environmental magnetic noise at low frequencies of less than a few Hz, at 50 Hz and at 150 Hz. Reducing such disturbances stabilizes biomagnetic measurement conditions for SQUIDs in the absence of magnetically shielded rooms (MSR). After filtering and averaging the measured MCG data by a first-order SQUID gradiometer with only the active shielding during the daytime, the QRS complex and T wave was clearly presented.

Low Dimensionality Effects in Complex Magnetic Oxides

NASA Astrophysics Data System (ADS)

Kelley, Paula J. Lampen

Complex magnetic oxides represent a unique intersection of immense technological importance and fascinating physical phenomena originating from interwoven structural, electronic and magnetic degrees of freedom. The resulting energetically close competing orders can be controllably selected through external fields. Competing interactions and disorder represent an additional opportunity to systematically manipulate the properties of pure magnetic systems, leading to frustration, glassiness, and other novel phenomena while finite sample dimension plays a similar role in systems with long-range cooperative effects or large correlation lengths. A rigorous understanding of these effects in strongly correlated oxides is key to manipulating their functionality and device performance, but remains a challenging task. In this dissertation, we examine a number of problems related to intrinsic and extrinsic low dimensionality, disorder, and competing interactions in magnetic oxides by applying a unique combination of standard magnetometry techniques and unconventional magnetocaloric effect and transverse susceptibility measurements. The influence of dimensionality and disorder on the nature and critical properties of phase transitions in manganites is illustrated in La0.7 Ca0.3MnO3, in which both size reduction to the nanoscale and chemically-controlled quenched disorder are observed to induce a progressive weakening of the first-order nature of the transition, despite acting through the distinct mechanisms of surface effects and site dilution. In the second-order material La0.8Ca0.2MnO3, a strong magnetic field is found to drive the system toward its tricritical point as competition between exchange interactions in the inhomogeneous ground state is suppressed. In the presence of large phase separation stabilized by chemical disorder and long-range strain, dimensionality has a profound effect. With the systematic reduction of particle size in microscale-phase-separated (La, Pr, Ca)MnO3 we observe a disruption of the long-range glassy strains associated with the charge-ordered phase in the bulk, lowering the field and pressure threshold for charge-order melting and increasing the ferromagnetic volume fraction as particle size is decreased. The long-range charge-ordered phase becomes completely suppressed when the particle size falls below 100 nm. In contrast, low dimensionality in the geometrically frustrated pseudo-1D spin chain compound Ca3Co2O6 is intrinsic, arising from the crystal lattice. We establish a comprehensive phase diagram for this exotic system consistent with recent reports of an incommensurate ground state and identify new sub-features of the ferrimagnetic phase. When defects in the form of grain boundaries are incorporated into the system the low-temperature slow-dynamic state is weakened, and new crossover phenomena emerge in the spin relaxation behavior along with an increased distribution of relaxation times. The presence of both disorder and randomness leads to a spin-glass-like state, as observed in gammaFe2O3 hollow nanoparticles, where freezing of surface spins at low temperature generates an irreversible magnetization component and an associated exchange-biasing effect. Our results point to distinct dynamic behaviors on the inner and outer surfaces of the hollow structures. Overall, these studies yield new physical insights into the role of dimensionality and disorder in these complex oxide systems and highlight the sensitivity of their manifested magnetic ground states to extrinsic factors, leading in many cases to crossover behaviors where the balance between competing phases is altered, or to the emergence of entirely new magnetic phenomena.

Biological effects of exposure to magnetic resonance imaging: an overview

PubMed Central

Formica, Domenico; Silvestri, Sergio

2004-01-01

The literature on biological effects of magnetic and electromagnetic fields commonly utilized in magnetic resonance imaging systems is surveyed here. After an introduction on the basic principles of magnetic resonance imaging and the electric and magnetic properties of biological tissues, the basic phenomena to understand the bio-effects are described in classical terms. Values of field strengths and frequencies commonly utilized in these diagnostic systems are reported in order to allow the integration of the specific literature on the bio-effects produced by magnetic resonance systems with the vast literature concerning the bio-effects produced by electromagnetic fields. This work gives an overview of the findings about the safety concerns of exposure to static magnetic fields, radio-frequency fields, and time varying magnetic field gradients, focusing primarily on the physics of the interactions between these electromagnetic fields and biological matter. The scientific literature is summarized, integrated, and critically analyzed with the help of authoritative reviews by recognized experts, international safety guidelines are also cited. PMID:15104797

Role of degeneracy, hybridization, and nesting in the properties of multiorbital systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nicholson, Andrew D; Liu, Jia-Ming; Ge, Weihao

2011-01-01

To understand the role that degeneracy, hybridization, and nesting play in the magnetic and pairing properties of multiorbital Hubbard models we here study numerically two types of two-orbital models, both with holelike and electron-like Fermi surfaces (FS s) that are related by nesting vectors ( ,0) and (0, ). In one case the bands that determine the FS s arise from strongly hybridized degenerate dxz and dyz orbitals, while in the other the two bands are determined by nondegenerate and nonhybridized s-like orbitals. Using a variety of techniques, in the weak-coupling regime it is shown that only the model withmore » hybridized bands develops metallic magnetic order, while the other model exhibits an ordered excitonic orbital-transverse spin state that is insulating and does not have a local magnetization. However, both models display similar insulating magnetic stripe ordering in the strong-coupling limit. These results indicate that nesting is a necessary but not sufficient condition for the development of ordered states with finite local magnetization in multiorbital Hubbard systems; the additional ingredient appears to be that the nested portions of the bands need to have the same orbital flavor. This condition can be achieved via strong hybridization of the orbitals in weak coupling or via the FS reconstruction induced by the Coulomb interactions in the strong-coupling regime. This effect also affects the pairing symmetry as demonstrated by the study of the dominant pairing channels for the two models.« less

Interplay between superconductivity and magnetism in Fe1−xPdxTe

PubMed Central

Karki, Amar B.; Garlea, V. Ovidiu; Custelcean, Radu; Stadler, Shane; Plummer, E. W.; Jin, Rongying

2013-01-01

The attractive/repulsive relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions, and Fe pnictides, showed superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. However, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds [RRh4B4 (R = Nd, Sm, Tm, Er), R′Mo6X8 (R′ = Tb, Dy, Er, Ho, and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn5, EuFe2(As1−xPx)2, etc.], providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe1−xPdxTe. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature TN/S, and turns into short-range AFM correlation with a characteristic peak in magnetic susceptibility at T′N. Superconductivity sets in when T′N reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (short-range) cross-over regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic (FM and AFM) interactions. PMID:23690601

Interplay between Superconductivity and Magnetism in Fe1-xPdxTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karki, A B; Garlea, Vasile O; Custelcean, Radu

The love/hate relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions and Fe pnictides, show superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. On the other hand, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds (RRh4B4 (R = Nd, Sm, Tm, Er), R'Mo6X8 (R' = Tb, Dy, Er, Ho,more » and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn5, EuFe2(As1-xPx)2 etc.), providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe1-xPdxTe. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature TN/S, and turns into short-range (SR) AFM correlation with a characteristic peak in magnetic susceptibility at T'N. Superconductivity sets in when T'N reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (SR) crossover regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic interactions (FM and AFM).« less

Giant reversible magnetocaloric effect in the pyrochlore Er 2 Mn 2 O 7 due to a cooperative two-sublattice ferromagnetic order

DOE PAGES

Cai, Y. Q.; Jiao, Y. Y.; Cui, Qi; ...

2017-11-29

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. Furthermore, a stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore Er 2Mn 2O 7 near its second-order FM transition at T c ≈ 34K; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change –ΔS M ismore » 5.27 and 16.1Jkg –1K –1, and the estimated magnetic refrigerant capacity reaches 68 and 522Jkg –1, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in Er 2Mn 2O 7 is mainly attributed to the large saturation moment of 18.9μ B per formula unit owing to a simultaneous FM ordering of the rear-earth Er 3+ and transition-metal Mn 4+ localized moments. Our results suggest that Er 2Mn 2O 7 pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.« less

Giant reversible magnetocaloric effect in the pyrochlore Er 2 Mn 2 O 7 due to a cooperative two-sublattice ferromagnetic order

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cai, Y. Q.; Jiao, Y. Y.; Cui, Qi

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. Furthermore, a stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore Er 2Mn 2O 7 near its second-order FM transition at T c ≈ 34K; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change –ΔS M ismore » 5.27 and 16.1Jkg –1K –1, and the estimated magnetic refrigerant capacity reaches 68 and 522Jkg –1, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in Er 2Mn 2O 7 is mainly attributed to the large saturation moment of 18.9μ B per formula unit owing to a simultaneous FM ordering of the rear-earth Er 3+ and transition-metal Mn 4+ localized moments. Our results suggest that Er 2Mn 2O 7 pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.« less

Theory of anisotropic hybridization-broadened magnetic response in cerium and actinide systems

NASA Astrophysics Data System (ADS)

Hu, Gong-Jia; Cooper, Bernard R.

1993-11-01

Inelastic-neutron-scattering measurements on cerium and plutonium monopnictides, thought to have moderately delocalized f electrons, yield magnetic-excitation spectra with anisotropic dispersion; while reasonably sharp excitations have been observed only for USb and UTe among presumably more-delocalized uranium monopnictides and monochalcogenides. For UTe the broadening as well as the dispersion is quite anisotropic. We have now extended our previous theory for the magnetic behavior of hybridizing partially delocalized f-electron systems to include hybridization-induced relaxation effects in the magnetic response, and this work and results are reported in the present paper. Each partially delocalized f-electron ion is coupled by hybridization to the band sea; and this both leads to a hybridization-mediated anisotropic two-ion interaction giving magnetic ordering and also gives a damping mechanism, via the coupling to the band sea, for the excitations of the magnetically ordered lattice. This coupling also provides a strong renormalization of the magnetic-excitation energies obtained for the ionic lattice coupled by the two-ion interaction. To treat these effects on the magnetic response we have developed a formalism for calculating the dynamic susceptibility based on the projection-operator method developed by Mori and others. We have applied our model and theory to the behavior of CeSb, CeBi, PuSb, UP, UAs, and UTe; and excellent overall agreement with the wide range of unusual experimentally observed anisotropic magnetic-excitation behavior is obtained.

Magnetocaloric effects and electrical resistivity of Ni2Mn0.55CoxCr0.45-xGa - A Heusler alloy system exhibiting a partially-decoupled first-order phase transition

NASA Astrophysics Data System (ADS)

Brock, Jeffrey; Khan, Mahmud

2018-05-01

The phase transitions and associated magnetocaloric properties of the Ni2Mn0.55CoxCr0.45-xGa (0 ≤ x ≤ 0.25) Heusler alloy system have been investigated. All samples exhibit a first-order martensitic phase transition, evidenced by a sharp drop in the resistivity versus temperature data and a thermomagnetic irreversibility in the dc magnetization data of the respective samples. Large magnetic entropy changes have also been observed near the phase transitions. The martensitic transformation temperature increases as Cr is partially replaced with Co. Additionally, this substitution leads to a partial decoupling of the magnetic and structural phase transitions, dramatically suppressing any magnetic hysteresis losses. Furthermore, the change in electrical resistivity during the phase transition remains relatively constant across the system, despite major changes in the degree of structural disorder and magnetostructural phase transition coupling. Detailed experimental results and conjectures as to the origin of these behaviors have been provided.

Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering

NASA Astrophysics Data System (ADS)

Breunig, Daniel; Burset, Pablo; Trauzettel, Björn

2018-01-01

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction—the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.

Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering.

PubMed

Breunig, Daniel; Burset, Pablo; Trauzettel, Björn

2018-01-19

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction-the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.

Magnetic field effect on the structural properties of a peptide model: Molecular dynamics simulation study

NASA Astrophysics Data System (ADS)

Housaindokht, Mohammad Reza; Moosavi, Fatemeh

2018-06-01

The effect of magnetization on the properties of a system containing a peptide model is studied by molecular dynamics simulation at a range of 298-318 K. Two mole fractions of 0.001 and 0.002 of peptide were simulated and the variation of hydrogen bond number, orientational ordering parameter, gyration radius, mean square displacement, as well as radial distribution function, were under consideration. The results show that applying magnetic field will increase the number of hydrogen bonds between water molecules by clustering them and decreases the interaction of water and peptide. This reduction may cause more available free space and enhance the movement of the peptide. As a result, the diffusion coefficient of the peptide becomes greater and its conformation changes. Orientational ordering parameter besides radius of gyration demonstrates that peptide is expanded by static magnetic field and its orientational ordering parameter is affected.

Magnetostriction-driven ground-state stabilization in 2H perovskites

DOE PAGES

Porter, D. G.; Senn, M. S.; Khalyavin, D. D.; ...

2016-10-04

In this paper, the magnetic ground state of Sr 3ARuO 6, with A =(Li,Na), is studied using neutron diffraction, resonant x-ray scattering, and laboratory characterization measurements of high-quality crystals. Combining these results allows us to observe the onset of long-range magnetic order and distinguish the symmetrically allowed magnetic models, identifying in-plane antiferromagnetic moments and a small ferromagnetic component along the c axis. While the existence of magnetic domains masks the particular in-plane direction of the moments, it has been possible to elucidate the ground state using symmetry considerations. We find that due to the lack of local anisotropy, antisymmetric exchangemore » interactions control the magnetic order, first through structural distortions that couple to in-plane antiferromagnetic moments and second through a high-order magnetoelastic coupling that lifts the degeneracy of the in-plane moments. Finally, the symmetry considerations used to rationalize the magnetic ground state are very general and will apply to many systems in this family, such as Ca 3ARuO 6, with A = (Li,Na), and Ca 3LiOsO 6 whose magnetic ground states are still not completely understood.« less

Application of a local linearization technique for the solution of a system of stiff differential equations associated with the simulation of a magnetic bearing assembly

NASA Technical Reports Server (NTRS)

Kibler, K. S.; Mcdaniel, G. A.

1981-01-01

A digital local linearization technique was used to solve a system of stiff differential equations which simulate a magnetic bearing assembly. The results prove the technique to be accurate, stable, and efficient when compared to a general purpose variable order Adams method with a stiff option.

Study of magnetization switching in coupled magnetic nanostructured systems

NASA Astrophysics Data System (ADS)

Radu, Cosmin

A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2, FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored. Keywords: magnetization dynamics, magnetic susceptibility, tunnel diode oscillator, critical curve, synthetic antiferromagnet, coupled magnetic structures, MRAM.

Evidence for coexisting magnetic order in frustrated three-dimensional honeycomb iridates Li2IrO3

NASA Astrophysics Data System (ADS)

Breznay, Nicholas; Ruiz, Alejandro; Frano, Alex; Analytis, James

The search for unconventional magnetism has found a fertile hunting ground in 5d iridium oxide (iridate) materials. The competition between coulomb, spin-orbit, and crystal field energy scales in honeycomb iridates leads to a quantum magnetic system with localized spin-1/2 moments communicating through spin-anisotropic Kitaev exchange interactions. Although early and ongoing work has focused on layered two-dimensional honeycomb compounds such as Na2IrO3 and a 4d analog, RuCl3, recently discovered polytypes of Li2IrO3 take on three-dimensional honeycomb structures. Bulk thermodynamic studies, as well as recent resonant x-ray diffraction and absorption spectroscopy experiments, have uncovered a rich phase diagram for these three-dimensional honeycomb iridates. Low temperature incommensurate and commensurate magnetic orders can be stabilized by tuning the applied magnetic field, displaying a delicate coexistence that signals highly frustrated magnetism.

Remanent Magnetization: Signature of Many-Body Localization in Quantum Antiferromagnets

NASA Astrophysics Data System (ADS)

Ros, V.; Müller, M.

2017-06-01

We study the remanent magnetization in antiferromagnetic, many-body localized quantum spin chains, initialized in a fully magnetized state. Its long time limit is an order parameter for the localization transition, which is readily accessible by standard experimental probes in magnets. We analytically calculate its value in the strong-disorder regime exploiting the explicit construction of quasilocal conserved quantities of the localized phase. We discuss analogies in cold atomic systems.

Effects of Eu doping and O vacancy on the magnetic and optical properties of ZnO

NASA Astrophysics Data System (ADS)

Ling-Feng, Qu; Qing-Yu, Hou; Xiao-Fang, Jia; Zhen-Chao, Xu; Chun-Wang, Zhao

2018-02-01

We calculated the electronic structure and optical properties of Eu mono-doped ZnO systems with or without O vacancy. We also determined the relative energy of ferromagnetic and antiferromagnetic orders of Eu-double-doped ZnO systems. The double-doped systems possess high Curie temperature and achieve room temperature ferromagnetism. The magnetism in the Eu mono-doped system without O vacancy is caused by the -Eu3+-O2--Eu3+- bound magnetopolaron (BMP) model. The magnetism of Eu mono-doped ZnO systems with O vacancy is more stable than that without O vacancy, and such magnetism is attributed to the -Eu3+-VO++-Eu3+- BMP model. The absorption spectrum for mono-doped systems is red shifted, and this finding confirms that Eu-mono-doped ZnO is a candidate photocatalyst for various applications. Therefore, Eu-double-doped ZnO can be practically used as an unambiguous diluted magnetic semiconductor.

Hyperfine field and magnetic structure in the B phase of CeCoIn5

DOE Office of Scientific and Technical Information (OSTI.GOV)

Graf, Matthias J; Curro, Nicholas J; Young, Ben - Li

2009-01-01

We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B-phase of CeCoIn{sub 5}. The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H {parallel} [100] are consistent with magnetic order with wavevector Q = {pi}(1+{delta}/a, 1/a, 1/c) and Ce moments ordered antiferromagnetically along themore » [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15{micro}{sub B} along [001] and Q{sub n} = {pi}(1+{delta}/a, 1+{delta}c, 1/c) with incommensuration {delta} = 0.12 for field H {parallel} [1{bar 1}0]. Using these parameters, we find that the hyperfine field is consistent with both experiments. We speculate that the B phase of CeCoIn{sub 5} represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.« less

Neutron diffraction study and theoretical analysis of the antiferromagnetic order and the diffuse scattering in the layered kagome system CaBaCo2Fe2O7

NASA Astrophysics Data System (ADS)

Reim, J. D.; Rosén, E.; Zaharko, O.; Mostovoy, M.; Robert, J.; Valldor, M.; Schweika, W.

2018-04-01

The hexagonal swedenborgite, CaBaCo2Fe2O7 , is a chiral frustrated antiferromagnet, in which magnetic ions form alternating kagome and triangular layers. We observe a long-range √{3 }×√{3 } antiferromagnetic order setting in below TN=160 K by neutron diffraction on single crystals of CaBaCo2Fe2O7 . Both magnetization and polarized neutron single crystal diffraction measurements show that close to TN spins lie predominantly in the a b plane, while upon cooling the spin structure becomes increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered structure can be described and refined within the magnetic space group P 31 m' . Diffuse scattering between the magnetic peaks reveals that the spin order is partial. Monte Carlo simulations based on a Heisenberg model with two nearest-neighbor exchange interactions show a similar diffuse scattering and coexistence of the √{3 }×√{3 } order with disorder. The coexistence can be explained by the freedom to vary spins without affecting the long-range order, which gives rise to ground-state degeneracy. Polarization analysis of the magnetic peaks indicates the presence of long-period cycloidal spin correlations resulting from the broken inversion symmetry of the lattice, in agreement with our symmetry analysis.

Spin and charge ordering in organic conductors investigated by electron spin resonance

NASA Astrophysics Data System (ADS)

Tokumoto, Takahisa D.

This dissertation presents systematic studies on ordered states of organic conductors investigated mainly by Electron Spin Resonance (ESR). First, we describe an introduction to organic conductors. Organic conductors are based on conducting layers of highly planar donor molecules, separated by insulating layers of acceptors. The donor arrangements in the conducting layers determine the three simple parameters, transfer integral t between the donor molecules, onsite Coulomb interaction U and next neighboring Coulomb interaction V. Depending on the values of the above three parameters, a variety of ground states is realized and hence the organic conductors has become a main stream of condensed matter physics. Among many ground states, the main focus is on magnetic orders in this dissertation. Therefore we have employed ESR to probe local magnetic structures. And we cover a basic theory of ESR in paramagnetic/antiferromagnetically ordered states and the experimental realizations. Next, after an introduction to a system with an exchange interaction between d magnetic moments embedded at acceptor sites and pi spins at donor molecules is given, we discuss the effectiveness of systematic studies on isostructural magnetic and non-magnetic acceptor based organic conductors. Then, we go over one of the "exchange coupled" materials, beta-(BDA-TTP)2MCl 4 (M=Fe3+,Ga3+). We examine the origins of the Metal-Insulator transition and the long range antiferromangetic order in the magnetic acceptor based material, where we found the critical importance of the quantum fluctuations of pi spins. Finally, we delineate the magnetic order of alternating easy axes of a class of an organic conductor, tau-(P-(S,S)-DMEDT)2(AuBr2) 1+y, at low temperature/field by ESR. We briefly discuss the origin of this unprecedented magnetic structure in terms of the unstoichiometric ratio of donors to acceptors and the tetragonal symmetry of the unit cell. Then, we report the results of the ultra high field ESR to probe the magnetic structure changes around a hysteretic field induced metal insulator transition.

Analysis of tristable energy harvesting system having fractional order viscoelastic material

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oumbé Tékam, G. T.; Woafo, P.; Kitio Kwuimy, C. A.

2015-01-15

A particular attention is devoted to analyze the dynamics of a strongly nonlinear energy harvester having fractional order viscoelastic flexible material. The strong nonlinearity is obtained from the magnetic interaction between the end free of the flexible material and three equally spaced magnets. Periodic responses are computed using the KrylovBogoliubov averaging method, and the effects of fractional order damping on the output electric energy are analyzed. It is obtained that the harvested energy is enhanced for small order of the fractional derivative. Considering the order and strength of the fractional viscoelastic property as control parameter, the complexity of the systemmore » response is investigated through the Melnikov criteria for horseshoes chaos, which allows us to derive the mathematical expression of the boundary between intra-well motion and bifurcations appearance domain. We observe that the order and strength of the fractional viscoelastic property can be effectively used to control chaos in the system. The results are confirmed by the smooth and fractal shape of the basin of attraction as the order of derivative decreases. The bifurcation diagrams and the corresponding Lyapunov exponents are plotted to get insight into the nonlinear response of the system.« less

Charge ordering, ferroelectric, and magnetic domains in LuFe{sub 2}O{sub 4} observed by scanning probe microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, I. K.; Jeong, Y. H., E-mail: yhj@postech.ac.kr; Kim, Jeehoon

2015-04-13

LuFe{sub 2}O{sub 4} is a multiferroic system which exhibits charge order, ferroelectricity, and ferrimagnetism simultaneously below ∼230 K. The ferroelectric/charge order domains of LuFe{sub 2}O{sub 4} are imaged with both piezoresponse force microscopy (PFM) and electrostatic force microscopy (EFM), while the magnetic domains are characterized by magnetic force microscopy (MFM). Comparison of PFM and EFM results suggests that the proposed ferroelectricity in LuFe{sub 2}O{sub 4} is not of usual displacive type but of electronic origin. Simultaneous characterization of ferroelectric/charge order and magnetic domains by EFM and MFM, respectively, on the same surface of LuFe{sub 2}O{sub 4} reveals that both domains havemore » irregular patterns of similar shape, but the length scales are quite different. The domain size is approximately 100 nm for the ferroelectric domains, while the magnetic domain size is much larger and gets as large as 1 μm. We also demonstrate that the origin of the formation of irregular domains in LuFe{sub 2}O{sub 4} is not extrinsic but intrinsic.« less

Emergent geometric frustration of artificial magnetic skyrmion crystals

DOE PAGES

Ma, Fusheng; Reichhardt, Charles; Gan, Weiliang; ...

2016-10-05

Magnetic skyrmions have been receiving growing attention as potential information storage and magnetic logic devices since an increasing number of materials have been identified that support skyrmion phases. Explorations of artificial frustrated systems have led to new insights into controlling and engineering new emergent frustration phenomena in frustrated and disordered systems. Here, we propose a skyrmion spin ice, giving a unifying framework for the study of geometric frustration of skyrmion crystals (SCs) in a nonfrustrated artificial geometrical lattice as a consequence of the structural confinement of skyrmions in magnetic potential wells. The emergent ice rules from the geometrically frustrated SCsmore » highlight a novel phenomenon in this skyrmion system: emergent geometrical frustration. We demonstrate how SC topology transitions between a nonfrustrated periodic configuration and a frustrated icelike ordering can also be realized reversibly. The proposed artificial frustrated skyrmion systems can be annealed into different ice phases with an applied current-induced spin-transfer torque, including a long-range ordered ice rule obeying ground state, as-relaxed random state, biased state, and monopole state. In conclusion, the spin-torque reconfigurability of the artificial skyrmion ice states, difficult to achieve in other artificial spin ice systems, is compatible with standard spintronic device fabrication technology, which makes the semiconductor industrial integration straightforward.« less

Emergent geometric frustration of artificial magnetic skyrmion crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ma, Fusheng; Reichhardt, Charles; Gan, Weiliang

Magnetic skyrmions have been receiving growing attention as potential information storage and magnetic logic devices since an increasing number of materials have been identified that support skyrmion phases. Explorations of artificial frustrated systems have led to new insights into controlling and engineering new emergent frustration phenomena in frustrated and disordered systems. Here, we propose a skyrmion spin ice, giving a unifying framework for the study of geometric frustration of skyrmion crystals (SCs) in a nonfrustrated artificial geometrical lattice as a consequence of the structural confinement of skyrmions in magnetic potential wells. The emergent ice rules from the geometrically frustrated SCsmore » highlight a novel phenomenon in this skyrmion system: emergent geometrical frustration. We demonstrate how SC topology transitions between a nonfrustrated periodic configuration and a frustrated icelike ordering can also be realized reversibly. The proposed artificial frustrated skyrmion systems can be annealed into different ice phases with an applied current-induced spin-transfer torque, including a long-range ordered ice rule obeying ground state, as-relaxed random state, biased state, and monopole state. In conclusion, the spin-torque reconfigurability of the artificial skyrmion ice states, difficult to achieve in other artificial spin ice systems, is compatible with standard spintronic device fabrication technology, which makes the semiconductor industrial integration straightforward.« less

Probing the impact of magnetic interactions on the lattice dynamics of two-dimensional Ti2X (X = C, N) MXenes.

PubMed

Sternik, Małgorzata; Wdowik, Urszula D

2018-03-14

Dynamical properties of the two-dimensional Ti 2 C and Ti 2 N MXenes were investigated using density functional theory and discussed in connection with their structures and electronic properties. To elucidate the influence of magnetic interactions on the fundamental properties of these systems, the nonmagnetic, ferromagnetic and three distinct antiferromagnetic spin arrangements on titanium sublattice were considered. Each magnetic configuration was also studied at two directions of the spin magnetic moment with respect to the MXene layer. The zero-point energy motion, following from the phonon calculations, was taken into account while analyzing the energetic stability of the magnetic phases against the nonmagnetic solution. This contribution was found not to change a sequence of the energetic stability of the considered magnetic structures of Ti 2 X (X = C, N) MXenes. Both Ti 2 X (X = C, N) systems are shown to prefer antiferromagnetic arrangement of spins between Ti layers and the ferromagnetic order within each layer. This energetically privileged phase is semiconducting for Ti 2 C and metallic for Ti 2 N. The type of magnetic order as well as the in-plane or out-of-plane spin polarizations have a relatively small impact on the structural parameters, Ti-X bonding length, force constants and phonon spectra of both Ti 2 X systems, leading to observable differences only between the nonmagnetic and any other magnetic configurations. Nonetheless, a noticeable effect of the spin orientation on degeneracy of the Ti-3d orbitals is encountered. The magnetic interactions affect to a great extent the positions and intensities of the Raman-active modes, and hence one could exploit this effect for experimental verification of the theoretically predicted magnetic state of Ti 2 X monolayers. Theoretical phonon spectra of Ti 2 X (X = C, N) MXenes exhibit a linear dependence on energy in the long-wavelength limit, which is typical for a 2D system.

Magnetization of La2-xSrxNiO4+δ (0⩽x⩽0.5) : Spin-glass and memory effects

NASA Astrophysics Data System (ADS)

Freeman, P. G.; Boothroyd, A. T.; Prabhakaran, D.; Lorenzana, J.

2006-01-01

We have studied the magnetization of a series of spin-charge-ordered La2-xSrxNiO4+δ single crystals with 0⩽x⩽0.5 . For fields applied parallel to the ab plane there is a large irreversibility below a temperature TF1˜50K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition.

Mixed-Spin Diamond Chain Cu2FePO4F4(H2O)4 with a Noncollinear Spin Order and Possible Successive Phase Transitions.

PubMed

Lu, Hongcheng; Hayashi, Naoaki; Matsumoto, Yuki; Takatsu, Hiroshi; Kageyama, Hiroshi

2017-08-07

A diamond spin chain system, one of the one-dimensional frustrated lattices, is known to exhibit novel properties, but experimental studies have been exclusively confined to materials with a single spin component. Here, we report on the synthesis, structure, and magnetic properties of a new diamond chain compound Cu 2 FePO 4 F 4 (H 2 O) 4 1 composed of mixed-spins of Cu 2+ (S = 1/2 × 2) and Fe 3+ (S = 5/2). Compound 1 crystallizes in the space group C2/c of the monoclinic crystal system with a = 7.7546(4) Å, b = 12.1290(6) Å, c = 9.9209(6) Å, β = 105.29(1)°, and Z = 4. DC magnetization, Mössbauer spectroscopy, and heat capacity measurements revealed an antiferromagnetic order at 11.3 K with a small ferromagnetic component. It is suggested that ferrimagnetic diamond chains are arranged in an antiferromagnetic fashion (i.e., [...Fe(↑)-2Cu(↓↓)-Fe(↑)...] and [...Fe(↓)-2Cu(↑↑)-Fe(↓)...]) within the ab plane to cancel net magnetization, and the spin orientation of the diamond chains changes alternately along the c axis due to the magnetic anisotropy, leading to a noncollinear spin order. Furthermore, another anomaly is observed in the heat capacity at around 3 K, suggesting a successive magnetic transition or crossover due to competing magnetic interactions.

Conceptual study of the cryostats for the cold powering system for the triplets of the High Luminosity LHC

NASA Astrophysics Data System (ADS)

Ballarino, A.; Giannelli, S.; Jacquemod, A.; Leclercq, Y.; Ortiz Ferrer, C.; Parma, V.

2017-12-01

The High Luminosity LHC (HL-LHC) is a project aiming to upgrade the Large Hadron Collider (LHC) after 2020-2025 in order to increase the integrated luminosity by about one order of magnitude and extend the operational capabilities until 2035. The upgrade of the focusing triplet insertions for the Atlas and CMS experiments foresees using superconducting magnets operating in a pressurised superfluid helium bath at 1.9 K. The increased radiation levels from the particle debris produced by particle collisions in the experiments require that the power converters are placed in radiation shielded zones located in a service gallery adjacent to the main tunnel. The powering of the magnets from the gallery is achieved by means of MgB2 superconducting cables in a 100-m long flexible cryostat transfer line, actively cooled by 4.5 K to 20 K gaseous helium generated close to the magnets. At the highest temperature end, the helium flow cools the High Temperature Superconducting (HTS) current leads before being recovered at room temperature. At the magnet connection side, a dedicated connection box allows connection to the magnets and a controlled boil-off production of helium for the cooling needs of the powering system. This paper presents the overall concept of the cryostat system from the magnet connection boxes, through the flexible cryostat transfer line, to the connection box of the current leads.

Design and analysis of permanent magnet moving coil type generator used in a micro-CHP generation system

NASA Astrophysics Data System (ADS)

Oros Pop, Susana Teodora; Berinde, Ioan; Vadan, Ioan

2015-12-01

This paper presents the design and analysis of a permanent magnet moving coil type generator driven by a free piston Stirling engine. This assemble free piston Stirling engine - permanent magnet moving coil type generator will be used in a combined heat and power (CHP) system for producing heat and power in residential area. The design procedure for moving coil type linear generator starts from the rated power imposed and finally uses the Faraday law of induction. The magneto-static magnetic field generated by permanent magnets is analyzed by means of Reluctance method and Finite Element Method in order to evaluate the magnetic flux density in the air gap, which is a design data imposed in the design stage, and the results are compared.

Monte Carlo study of the magnetic properties in a bilayer dendrimer structure with non-magnetic layers

NASA Astrophysics Data System (ADS)

Jabar, A.; Masrour, R.

2017-12-01

In this paper, we study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions and magnetic layer effects on the bilayer transitions of a spin-5/2 Blume-Capel model formed by two magnetic blocs separated by a non-magnetic spacer of finite thickness. The thermalization process of magnetization for systems sizes has been given. We have shown that the magnetic order in the two magnetic blocs depend on the thickness of the magnetic layer. In the total magnetization profiles, the susceptibility peaks correspond to the reduced critical temperature. This critical temperature is displaced towards higher temperatures when increasing the number of magnetic layers. In addition, we have discussed and interpreted the behaviors of the magnetic hysteresis loops.

Magnetic Suspension and Balance Systems: A Selected, Annotated Bibliography

NASA Technical Reports Server (NTRS)

Tuttle Marie H.; Kilgore, Robert A.; Boyden, Richmond P.

1983-01-01

This publication, containing 206 entries, supersedes an earlier bibliography, NASA TM-80225 (April 1980). Citations for 18 documents have been added in this updated version. Most of the additions report results of recent studies aimed at increasing the research capabilities of magnetic suspension and balance systems, e.g., increasing force and torque capability, increasing angle of attack capability, and increasing overall system reliability. Some of the additions address the problem of scaling from the relatively small size of existing systems to much larger sizes. The purpose of this bibliography is to provide an up-to-date list of publications that might be helpful to persons interested in magnetic suspension and balance systems for use in wind tunnels. The arrangement is generally chronological by date of publication. However, papers presented at conferences or meetings are placed under dates of presentation. The numbers assigned to many of the citations have been changed from those used in the previous bibliography. This has been done in order to allow outdated citations to be removed and some recently discovered older works to be included in their proper chronological order.

Consecutive magnetic phase diagram of UCoGe-URhGe-UIrGe system

NASA Astrophysics Data System (ADS)

Pospíšil, Jiří; Haga, Yoshinori; Miyake, Atsushi; Kambe, Shinsaku; Tateiwa, Naoyuki; Tokunaga, Yo; Honda, Fuminori; Nakamura, Ai; Homma, Yoshiya; Tokunaga, Masashi; Aoki, Dai; Yamamoto, Etsuji

2018-05-01

We prepared single crystals in UCo1-xRhxGe and UIr1-xRhxGe systems to establish a complex dU-U-T (dU-U is the shortest interatomic uranium distance and T is temperature) magnetic phase diagram. This recognized a characteristic maximum in magnetic susceptibility at temperature Tmax along the b axis as an important parameter. Three magnetically ordered regions can be distinguished within this scope; first a ferromagnetic region with Curie temperature

Coupling between crystal structure and magnetism in transition-metal oxides

NASA Astrophysics Data System (ADS)

Barton, Phillip Thomas

Transition-metal oxides exhibit a fascinating array of phenomena ranging from superconductivity to negative thermal expansion to catalysis. This dissertation focuses on magnetism, which is integral to engineering applications such as data storage, electric motors/generators, and transformers. The investigative approach follows structure-property relationships from materials science and draws on intuition from solid-state chemistry. The interplay between crystal structure and magnetic properties is studied experimentally in order to enhance the understanding of magnetostructural coupling mechanisms and provide insight into avenues for tuning behavior. A combination of diffraction and physical property measurements were used to study structural and magnetic phase transitions as a function of chemical composition, temperature, and magnetic field. The systems examined are of importance in Li-ion battery electrochemistry, condensed-matter physics, solid-state chemistry, and p-type transparent conducting oxides. The materials were prepared by solid-state reaction of powder reagents at high temperatures for periods lasting tens of hours. The first project discussed is of a solid solution between NiO, a correlated insulator, and LiNiO2, a layered battery cathode. Despite the deceptive structural and compositional simplicity of this system, a complete understanding of its complex magnetic properties has remained elusive. This study shows that nanoscale domains of chemical order form at intermediate compositions, creating interfaces between antiferromagnetism and ferrimagnetism that give rise to magnetic exchange bias. A simple model of the magnetism is presented along with a comprehensive phase diagram. The second set of investigations focus on the Ge-Co-O system where the spin-orbit coupling of Co(II) plays a significant role. GeCo2O 4 is reported to exhibit unusual magnetic behavior that arises from Ising spin in its spinel crystal structure. Studies by variable-temperature synchrotron X-ray diffraction reveal a magnetostructural transition and capacitance measurements show evidence for magnetodielectric behavior. The above work uncovered a Co10Ge3O16 phase that had a known structure but whose physical properties were largely uncharacterized. This project examined its metamagnetic properties using detailed magnetometry experiments. Upon the application of a magnetic field, this material goes through a first-order phase transition from a noncollinear antiferromagnet to an unknown ferrimagnetic state. Lastly, this thesis explored the chemical dilution of magnetism in some perovskite and delafossite solid solutions. In the perovskite structure, compositions intermediate to the endmembers SrRuO3, a ferromagnetic metal, and LaRhO3, a diamagnetic semiconductor, were investigated. While the magnetism of this system is poised between localized and itinerant behavior, a compositionally-driven metal to insulator transition, revealed by electrical resistivity measurements, did not strongly impact the magnetic properties. Instead, both octahedral tilting and magnetic dilution had strong effects, and comparison of this characterization to Sr1-- x CaxRuO3 reinforces the important role of structural distortions in determining magnetic ground state. The final materials studied were of composition CuAl1-- xCrxO2 (0 < x < 1) in the delafossite structure. The primary interest was the geometric frustration of antiferromagnetism in CuCrO 2 and significant short-range correlations were observed above TN. The analysis found that reducing the number of degenerate states through Al substitution did not enhance magnetic ordering because of the weakening of magnetic exchange.

Magnetic Excitations and Continuum of a Possibly Field-Induced Quantum Spin Liquid in α -RuCl3

NASA Astrophysics Data System (ADS)

Wang, Zhe; Reschke, S.; Hüvonen, D.; Do, S.-H.; Choi, K.-Y.; Gensch, M.; Nagel, U.; Rõõm, T.; Loidl, A.

2017-12-01

We report on terahertz spectroscopy of quantum spin dynamics in α -RuCl3 , a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at Ts 2=62 K . With the onset of a long-range magnetic order at TN=6.5 K , spectral weight is transferred to a well-defined magnetic excitation at ℏω1=2.48 meV , which is accompanied by a higher-energy band at ℏω2=6.48 meV . Both excitations soften in a magnetic field, signaling a quantum phase transition close to Bc=7 T , where a broad continuum dominates the dynamical response. Above Bc, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

High Magnetic Field Heat Capacity of URu_2Si_2

NASA Astrophysics Data System (ADS)

Jaime, Marcelo; Kim, Kee Hoon; McCall, Scott; Mydosh, John A.

2002-03-01

URu_2Si2 is a heavy-fermion in which superconductivity (T_C= 1.3 K) and antiferromagnetism (TN = 17.5 K) coexist. Transport, thermal and magnetic data suggest the opening of a gap in the magnetic excitation spectrum at TN possibly due to the formation of an itinerant spin density wave. Neutron-scattering experiment indicate an ordered moment (0.03 μ_B/U) that is too small to explain the entropy loss and the size of the gap which develops at T_N. The presence of a hidden primary order parameter has been suggested to explain these discrepancies. (1) Permanent magnetic fields up to 45 T have been used to completely suppress the magnetic transition, which was observed with measurements of the magneto-caloric effect during adiabatic magnetization of the sample, and specific heat vs. temperature at constant magnetic field. These experiments provide clues to understand the ground state of the system. (1) N. Sha et al., Phys. Rev. B61, 564 (2000).

Thermal protection performance of magnetohydrodynamic heat shield system based on multipolar magnetic field

NASA Astrophysics Data System (ADS)

Kai, Li; Jun, Liu; Weiqiang, Liu

2017-07-01

In order to cover the shortage of dipole magnetic field in the magnetohydrodynamic(MHD) heat shield system, physical model of a multipolar magnetic field with central and peripheral solenoids is constructed. By employing the governing equations of three dimensional thermochemical nonequilibrium flow with electromagnetic source terms based on the low magneto-Reynolds assumption, the flow control performance of the dipole and multipolar magnetic fields are numerically simulated. To make the results comparable, two groups of cases are designed by first assuming equal stagnation magnetic induction strength and secondly assuming equal ampere-turns. Results show that, the five-magnet system, whose central polar orientation is the same with the peripheral ones, have stronger work capability and better shock control and thermal protection performance. Moreover, the five-solenoid systems are the best when the ampere-turns of the central solenoid are twice and fourth of the peripheral ones under those two circumstances respectively. Compared with the dipole magnetic field, the stagnation non-catalytic heat fluxes are decreased by a factor of 47.5% and 34.0% respectively.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Calabrese, G.; Capineri, L., E-mail: lorenzo.capineri@unifi.it; Granato, M.

This paper describes the design of a system for the characterization of magnetic hysteresis behavior in soft ferrite magnetic cores. The proposed setup can test magnetic materials exciting them with controlled arbitrary magnetic field waveforms, including the capability of providing a DC bias, in a frequency bandwidth up to 500 kHz, with voltages up to 32 V peak-to-peak, and currents up to 10 A peak-to-peak. In order to have an accurate control of the magnetic field waveform, the system is based on a voltage controlled current source. The electronic design is described focusing on closed loop feedback stabilization and passivemore » components choice. The system has real-time hysteretic loop acquisition and visualization. The comparisons between measured hysteresis loops of sample magnetic materials and datasheet available ones are shown. Results showing frequency and thermal behavior of the hysteresis of a test sample prove the system capabilities. Moreover, the B-H loops obtained with a multiple waveforms excitation signal, including DC bias, are reported. The proposal is a low-cost and replicable solution for hysteresis characterization of magnetic materials used in power electronics.« less

Optical spectroscopic study of multiferroic BiFeO3 and LuFe2O4

NASA Astrophysics Data System (ADS)

Xu, Xiaoshan

2010-03-01

Iron-based multiferroics such as BiFeO3 and LuFe2O4 exhibit the highest magnetic and ferroelectric ordering temperatures among known multiferroics. LuFe2O4 is a frustrated system with several phase transitions that result in electronically driven multiferroicity. To understand how this peculiar multiferroic mechanism correlates with magnetism, we studied electronic excitations by optical spectroscopy and other complementary techniques. We show that the charge order, which determines the dielectric properties, is due to the ``order by fluctuation'' mechanism, evidenced by the onset of charge fluctuation well below the charge ordering transition. We also find a low temperature monoclinic distortion driven by both temperature and magnetic field, indicating strong coupling between structure, magnetism and charge order. BiFeO3 is the only known single phase multiferroics with room temperature magnetism and ferroelectricity. To investigate the spin-charge coupling, we measured the optical properties of BiFeO3. We find that the absorption onset occurs due to on-site Fe^3+ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. The sensitivity of the magnon sidebands allows us to map out the magnetic-field temperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near 10 T. Work done in collaboration with T.V. Brinzari, R.C. Rai, M. Angst, R.P. Hermann, A.D. Christianson, J.-W. Kim, Z. Islam, B.C. Sales, D. Mandrus, S. Lee, Y.H. Chu, L. W. Martin, A. Kumar, R. Ramesh, S.W. Cheong, S. McGill, and J.L. Musfeldt.

Small-angle neutron scattering study of magnetic ordering and inhomogeneity across the martensitic phase transformation in Ni 50–xCo xMn₄₀Sn₁₀ alloys

DOE PAGES

Bhatti, Kanwal Preet; El-Khatib, S.; Srivastava, Vijay; ...

2012-04-27

The Heusler-derived multiferroic alloy Ni 50–xCo xMn₄₀Sn₁₀ has recently been shown to exhibit, at just above room temperature, a highly reversible martensitic phase transformation with an unusually large magnetization change. In this work the nature of the magnetic ordering above and below this transformation has been studied in detail in the critical composition range x = 6–8 via temperature-dependent (5–600 K) magnetometry and small-angle neutron scattering (SANS). We observe fairly typical paramagnetic to long-range-ordered ferromagnetic phase transitions on cooling to 420–430 K, with the expected critical spin fluctuations, followed by first-order martensitic phase transformations to a nonferromagnetic state below 360–390more » K. The static magnetization reveals complex magnetism in this low-temperature nonferromagnetic phase, including a Langevin-like field dependence, distinct spin freezing near 60 K, and significant exchange bias effects, consistent with superparamagnetic blocking of ferromagnetic clusters of nanoscopic dimensions. We demonstrate that these spin clusters, whose existence has been hypothesized in a variety of martensitic alloys exhibiting competition between ferromagnetic and antiferromagnetic exchange interactions, can be directly observed by SANS. The scattering data are consistent with a liquidlike spatial distribution of interacting magnetic clusters with a mean center-to-center spacing of 12 nm. Considering the behavior of the superparmagnetism, cooling-field and temperature-dependent exchange bias, and magnetic SANS, we discuss in detail the physical form and origin of these spin clusters, their intercluster interactions, the nature of the ground-state magnetic ordering in the martensitic phase, and the implications for our understanding of such alloy systems.« less

Dirac and non-Dirac conditions in the two-potential theory of magnetic charge

NASA Astrophysics Data System (ADS)

Scott, John; Evans, Timothy J.; Singleton, Douglas; Dzhunushaliev, Vladimir; Folomeev, Vladimir

2018-05-01

We investigate the Cabbibo-Ferrari, two-potential approach to magnetic charge coupled to two different complex scalar fields, Φ _1 and Φ _2, each having different electric and magnetic charges. The scalar field, Φ _1, is assumed to have a spontaneous symmetry breaking self-interaction potential which gives a mass to the "magnetic" gauge potential and "magnetic" photon, while the other "electric" gauge potential and "electric" photon remain massless. The magnetic photon is hidden until one reaches energies of the order of the magnetic photon rest mass. The second scalar field, Φ _2, is required in order to make the theory non-trivial. With only one field one can always use a duality rotation to rotate away either the electric or magnetic charge, and thus decouple either the associated electric or magnetic photon. In analyzing this system of two scalar fields in the Cabbibo-Ferrari approach we perform several duality and gauge transformations, which require introducing non-Dirac conditions on the initial electric and magnetic charges. We also find that due to the symmetry breaking the usual Dirac condition is altered to include the mass of the magnetic photon. We discuss the implications of these various conditions on the charges.

Development of non-destructive evaluation system using an HTS-SQUID gradiometer for magnetized materials

NASA Astrophysics Data System (ADS)

Kawano, J.; Tsukamoto, A.; Adachi, S.; Oshikubo, Y.; Hato, T.; Tanabe, K.; Okamura, T.

We have developed a new eddy-current non-destructive evaluation (NDE) system using an HTS SQUID gradiometer with the aim of applying it to practical materials with magnetization. The new NDE system employs a LN2-cooled external Cu pickup coil and an HTS SQUID chip placed in a magnetic shield made of HTS material. The HTS SQUID chip consists of an HTS planar gradiometer manufactured by using a ramp-edge junction technology and a multi-turn HTS thin film input coil coupled with the flip-chip configuration. The first-order coaxial gradiometric Cu pickup coil with a diameter of 16 mm and the baseline of 5.6 mm was used in the present NDE experiments. By using this NDE system, we could observe defect-induced magnetic signals without an appreciable influence of magnetization up to 10 mT. We also examined the ability of detecting deep-lying defects and compared with the results obtained using our previous NDE system.

Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer

NASA Astrophysics Data System (ADS)

Gross, I.; Akhtar, W.; Garcia, V.; Martínez, L. J.; Chouaieb, S.; Garcia, K.; Carrétéro, C.; Barthélémy, A.; Appel, P.; Maletinsky, P.; Kim, J.-V.; Chauleau, J. Y.; Jaouen, N.; Viret, M.; Bibes, M.; Fusil, S.; Jacques, V.

2017-09-01

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin-orbit effects that enable efficient spin-charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen-vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Thermodynamic properties of antiferromagnetic ordered states of π -d interacting systems of κ -(BETS) 2Fe X4(X =Br ,Cl )

NASA Astrophysics Data System (ADS)

Fukuoka, Shuhei; Yamashita, Satoshi; Nakazawa, Yasuhiro; Yamamoto, Takashi; Fujiwara, Hideki; Shirahata, Takashi; Takahashi, Kazuko

2016-06-01

The results are presented for systematic heat capacity measurements of π-d interacting systems of κ -(BETS) 2Fe Br4 and κ -(BETS) 2FeC l4 [BETS = bis(ethylenedithio)tetraselenafulvalene] performed under in-plane magnetic fields. We observed sharp thermal anomalies at 2.47 K for κ -(BETS) 2FeB r4 and at 0.47 K for κ -(BETS) 2FeC l4 at 0 T that are associated with antiferromagnetic transitions of the 3 d electrons in the anion layers. From analyses of the magnetic heat capacity data, we indicate that the two compounds show unconventional thermodynamic behaviors inherent in the π-d interacting layered system. In the case of κ -(BETS) 2FeB r4 , a small hump structure was observed in the magnetic heat capacity below the transition temperature when a magnetic field was applied parallel to the a axis. In the case of κ -(BETS) 2FeC l4 , a similar hump structure was observed at 0 T that remained in the data with magnetic fields applied parallel to the a axis. We demonstrate that the temperature dependencies of the magnetic heat capacities scale well by normalizing the temperatures with dominant one-dimensional direct interactions (Jdd/kB) of each compound. The field dependencies of the transition temperatures and the hump structures are elucidated in one simple magnetic field vs temperature (H -T ) phase diagram. These results indicate that the thermodynamic features of both κ-type BETS salts are essentially equivalent, and the observed hump structures are derived from the one-dimensional Jdd interaction characters, which are still influential for magnetic features even in the long-range magnetic ordered states.

Collapse of ferromagnetism in itinerant-electron system: A magnetic, transport properties, and high pressure study of (Hf,Ta)Fe2 compounds

NASA Astrophysics Data System (ADS)

Diop, L. V. B.; Kastil, J.; Isnard, O.; Arnold, Z.; Kamarad, J.

2014-10-01

The magnetism and transport properties were studied for Laves (Hf,Ta)Fe2 itinerant-electron compounds, which exhibit a temperature-induced first-order transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) state upon heating. At finite temperatures, the field-induced metamagnetic phase transition between the AFM and FM has considerable effects on the transport properties of these model metamagnetic compounds. A large negative magnetoresistance of about 14% is observed in accordance with the metamagnetic transition. The magnetic phase diagram is determined for the Laves Hf1-xTaxFe2 series and its Ta concentration dependence discussed. An unusual behavior is revealed in the paramagnetic state of intermediate compositions, it gives rise to the rapid increase and saturation of the local spin fluctuations of the 3d electrons. This new result is analysed in the frame of the theory of Moriya. For a chosen composition Hf0.825Ta0.175Fe2, exhibiting such remarkable features, a detailed investigation is carried out under hydrostatic pressure up to 1 GPa in order to investigate the volume effect on the magnetic properties. With increasing pressure, the magnetic transition temperature TFM-AFM from ferromagnetic to antiferromagnetic order decreases strongly non-linearly and disappears at a critical pressure of 0.75 GPa. In the pressure-induced AFM state, the field-induced first-order AFM-FM transition appears and the complex temperature dependence of the AFM-FM transition field is explained by the contribution from both the magnetic and elastic energies caused by the significant temperature variation of the amplitude of the local Fe magnetic moment. The application of an external pressure leads also to the progressive decrease of the Néel temperature TN. In addition, a large pressure effect on the spontaneous magnetization MS for pressures below 0.45 GPa, dln(Ms)/dP = -6.3 × 10-2 GPa-1 was discovered. The presented results are consistent with Moriya's theoretical predictions and can significantly help to better understand the underlying physics of itinerant electron magnetic systems nowadays widely investigated for both fundamental and applications purposes.

Interactive 3-D registration of ultrasound and magnetic resonance images based on a magnetic position sensor.

PubMed

Pagoulatos, N; Edwards, W S; Haynor, D R; Kim, Y

1999-12-01

The use of stereotactic systems has been one of the main approaches for image-based guidance of the surgical tool within the brain. The main limitation of stereotactic systems is that they are based on preoperative images that might become outdated and invalid during the course of surgery. Ultrasound (US) is considered the most practical and cost-effective intraoperative imaging modality, but US images inherently have a low signal-to-noise ratio. Integrating intraoperative US with stereotactic systems has recently been attempted. In this paper, we present a new system for interactively registering two-dimensional US and three-dimensional magnetic resonance (MR) images. This registration is based on tracking the US probe with a dc magnetic position sensor. We have performed an extensive analysis of the errors of our system by using a custom-built phantom. The registration error between the MR and the position sensor space was found to have a mean value of 1.78 mm and a standard deviation of 0.18 mm. The registration error between US and MR space was dependent on the distance of the target point from the US probe face. For a 3.5-MHz phased one-dimensional array transducer and a depth of 6 cm, the mean value of the registration error was 2.00 mm and the standard deviation was 0.75 mm. The registered MR images were reconstructed using either zeroth-order or first-order interpolation. The ease of use and the interactive nature of our system (approximately 6.5 frames/s for 344 x 310 images and first-order interpolation on a Pentium II 450 MHz) demonstrates its potential to be used in the operating room.

Cooper pair induced frustration and nematicity of two-dimensional magnetic adatom lattices

NASA Astrophysics Data System (ADS)

Schecter, Michael; Syljuâsen, Olav F.; Paaske, Jens

2018-05-01

We propose utilizing the Cooper pair to induce magnetic frustration in systems of two-dimensional (2D) magnetic adatom lattices on s -wave superconducting surfaces. The competition between singlet electron correlations and the RKKY coupling is shown to lead to a variety of hidden-order states that break the point-group symmetry of the 2D adatom lattice at finite temperature. The phase diagram is constructed using a newly developed effective bond theory [M. Schecter et al., Phys. Rev. Lett. 119, 157202 (2017), 10.1103/PhysRevLett.119.157202], and exhibits broad regions of long-range vestigial nematic order.

Second International Symposium on Magnetic Suspension Technology, part 1

NASA Technical Reports Server (NTRS)

Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1994-01-01

In order to examine the state of technology of all areas of magnetic suspension and to review related recent developments in sensors and controls approaches, superconducting magnet technology, and design/implementation practices, the Second International Symposium on Magnetic Suspension Technology was held. The symposium included 18 technical sessions in which 44 papers were presented. The technical sessions covered the areas of bearings, bearing modeling, controls, vibration isolation, micromachines, superconductivity, wind tunnel magnetic suspension systems, magnetically levitated trains (MAGLEV), rotating machinery and energy storage, and applications. A list of attendees appears at the end of the document.

Optical field induced rotation of polarization in rubidium atoms with the additional magnetic field

NASA Astrophysics Data System (ADS)

Ummal Momeen, M.; Hu, Jianping

2017-11-01

We present the magnetic and optical field induced rotation of polarization in 87Rb and 85Rb atoms at geophysical magnetic fields. The line shape varies considerably in the presence of a magnetic field of the order of a few mG. Multiple Zeeman sublevel EIT systems involving rubidium atoms are investigated. Theoretical formalism of optical field induced polarization rotation in the presence of a magnetic field is discussed by considering all the Zeeman sublevels. It is noted that the ground state population distribution also plays a major role.

The formation of high-mass binary star systems

NASA Astrophysics Data System (ADS)

Lund, Kristin; Bonnell, Ian A.

2018-06-01

We develop a semi-analytic model to investigate how accretion onto wide low-mass binary stars can result in a close high-mass binary system. The key ingredient is to allow mass accretion while limiting the gain in angular momentum. We envision this process as being regulated by an external magnetic field during infall. Molecular clouds are made to collapse spherically with material either accreting onto the stars or settling in a disk. Our aim is to determine what initial conditions are needed for the resulting binary to be both massive and close. Whether material accretes, and what happens to the binary separation as a result, depends on the relative size of its specific angular momentum, compared to the specific angular momentum of the binary. When we add a magnetic field we are introducing a torque to the system which is capable of stripping the molecular cloud of some of its angular momentum, and consequently easing the formation of high-mass binaries. Our results suggest that clouds in excess of 1000 M⊙ and radii of 0.5 pc or larger, can easily form binary systems with masses in excess of 25 M⊙ and separations of order 10 R⊙ with magnetic fields of order 100 μG (mass-to-flux ratios of order 5).

Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe

DOE PAGES

Kothapalli, K.; Bohmer, A. E.; Jayasekara, W. T.; ...

2016-09-01

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mossbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscentmore » of what has been found for the evolution of these transitions in the prototypical system Ba(Fe 1–xCo x) 2As 2. Lastly, our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.« less

A second metastable spin-ordered state on ferrimagnetic single crystal Cu2 OSeO 3

NASA Astrophysics Data System (ADS)

Chou, Chih Chieh; Huang, C. L.; Tseng, K. F.; Mukherjee, S.; Her, J. L.; Matsuda, Y. H.; Kindo, K.; Berger, H.; Yang, H. D.

2011-03-01

DC and AC susceptibilities were executed on ferrimagnetic single crystal Cu 2 OSe O3 under magnetic field (H) and hydrostatic pressure (P) circumstance. With increasing H , the ferrimagnetic transition at TC ~ 60 K tends to a higher temperature. Furthermore, the TC rises with a linear slope and magnetization is enhanced with increasing P . Features of the ladder shown in the M vs. H curve or the peak observed in the d M / d H vs. H curve are noted at HSF ~ 0.5 kOe, exhibiting a competing ordered state in magnetic fields below TC . Remarkably, another shoulder is observed at ~ 1 kOe in the d M / d H vs. H curve, revealing a metastable spin ordered state in Cu 2 OSe O3 . In addition, the novel state is retained and enhanced by applied pressure. However, at H up to 55 T, there is no more observable slop change in magnetization. These magnetic properties suggest a complex spin orientation in the spin-frustrated system Cu 2 OSe O3 .

Electron energy spectrum and magnetic interactions in high-Tc superconductors

NASA Technical Reports Server (NTRS)

Turshevski, S. A.; Liechtenstein, A. I.; Antropov, V. P.; Gubanov, V. A.

1991-01-01

The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T(sub N1) and T(sub N2). The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T(sub N2) reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T(sub N1) and T(sub N2) which depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg Hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J(sub ij) parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors.

Electron energy spectrum and magnetic interactions in high-T(sub c) superconductors

NASA Technical Reports Server (NTRS)

Turshevski, S. A.; Liechtenstein, A. I.; Antropov, V. P.; Gubanov, V. A.

1990-01-01

The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T sub N1 and T sub N2. The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T sub N2 reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T sub N1 and T sub N2 depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J sub ij parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors.

Modeling the global positioning system signal propagation through the ionosphere

NASA Technical Reports Server (NTRS)

Bassiri, S.; Hajj, G. A.

1992-01-01

Based on realistic modeling of the electron density of the ionosphere and using a dipole moment approximation for the Earth's magnetic field, one is able to estimate the effect of the ionosphere on the Global Positioning System (GPS) signal for a ground user. The lowest order effect, which is on the order of 0.1-100 m of group delay, is subtracted out by forming a linear combination of the dual frequencies of the GPS signal. One is left with second- and third-order effects that are estimated typically to be approximately 0-2 cm and approximately 0-2 mm at zenith, respectively, depending on the geographical location, the time of day, the time of year, the solar cycle, and the relative geometry of the magnetic field and the line of sight. Given the total electron content along a line of sight, the authors derive an approximation to the second-order term which is accurate to approximately 90 percent within the magnetic dipole moment model; this approximation can be used to reduce the second-order term to the millimeter level, thus potentially improving precise positioning in space and on the ground. The induced group delay, or phase advance, due to second- and third-order effects is examined for two ground receivers located at equatorial and mid-latitude regions tracking several GPS satellites.

Quantum origins of moment fragmentation in Nd2Zr2O7

NASA Astrophysics Data System (ADS)

Benton, Owen

2016-09-01

Spin-liquid states are often described as the antithesis of magnetic order. Recently, however, it has been proposed that in certain frustrated magnets the magnetic degrees of freedom may "fragment" in such a way as to give rise to a coexistence of spin liquid and ordered phases. Recent neutron-scattering results [S. Petit, E. Lhotel, B. Canals, M. Ciomaga Hatnean, J. Ollivier, H. Muttka, E. Ressouche, A. R. Wildes, M. R. Lees, and G. Balakrishnan, Nat. Phys. 12, 746 (2016), 10.1038/nphys3710] suggest that this scenario may be realized in the pyrochlore magnet Nd2Zr2O7 . These observations show the characteristic pinch-point features of a Coulombic spin liquid occurring alongside the Bragg peaks of an "all-in-all-out" ordered state. Here we explain the quantum origins of this apparent magnetic moment fragmentation, within the framework of a quantum model of nearest-neighbor exchange, appropriate to Nd2Zr2O7 . This model is able to capture both the ground-state order and the pinch points observed at finite energy. The observed fragmentation arises due to the combination of the unusual symmetry properties of the Nd3 + ionic wave functions and the structure of equations of motion of the magnetic degrees of freedom. The results of our analysis suggest that Nd2Zr2O7 is proximate to a U (1 ) spin-liquid phase and is a promising candidate for the observation of a Higgs transition in a magnetic system.

Topological superconductivity in an ultrathin, magnetically-doped topological insulator proximity coupled to a conventional superconductor

NASA Astrophysics Data System (ADS)

Kim, Youngseok; Philip, Timothy M.; Park, Moon Jip; Gilbert, Matthew J.

2016-12-01

As a promising candidate system to realize topological superconductivity, the system of a 3D topological insulator (TI) grown on top of the s -wave superconductor has been extensively studied. To access the topological superconductivity experimentally, the 3D TI sample must be thin enough to allow for Cooper pair tunneling to the exposed surface of TI. The use of magnetically ordered dopants to break time-reversal symmetry may allow the surface of a TI to host Majorana fermion, which are believed to be a signature of topological superconductivity. In this work, we study a magnetically-doped thin film TI-superconductor hybrid system. Considering the proximity induced order parameter in thin film of TI, we analyze the gap closing points of the Hamiltonian and draw the phase diagram as a function of relevant parameters: the hybridization gap, Zeeman energy, and chemical potential of the TI system. Our findings provide a useful guide in choosing relevant parameters to facilitate the observation of topological superconductivity in thin film TI-superconductor hybrid systems. In addition, we further perform numerical analysis on a TI proximity coupled to an s -wave superconductor and find that, due to the spin-momentum locked nature of the surface states in TI, the induced s -wave order parameter of the surface states persists even at large magnitude of the Zeeman energy.

The structural and magnetic properties of Fe2-xNiGa1+x Heusler alloys

NASA Astrophysics Data System (ADS)

Zhang (张玉洁), Y. J.; Xi (郗学奎), X. K.; Meng (孟凡斌), F. B.; Wang (王文洪), W. H.; Liu (刘恩克), E. K.; Chen (陈京兰), J. L.; Wu (吴光恒), G. H.

2015-04-01

The structural and magnetic properties of Fe2-xNiGa1+x (x=0~1) Heusler alloys have been investigated by experimental observation and calculation. In this system, a structural transition is found as a function of composition. A higher Ga content leads to an atomic-order transformation from Hg2CuTi to B2. The magnetization decreases due to the dilution effect and the competition between the magnetic interactions and enhanced covalent bonding. The calculation of electronic structure indicates that adding Ga enhances the p-d orbital hybridization between the transition-metal and main-group-element atoms at nearest-neighbor distance. A magnetic and a structural phase diagram have been obtained in which the composition dependences of the lattice constant, the ordering temperature and the Curie temperature show cusps at a critical composition of x=0.32.

Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions.

PubMed

Masuda, Hidetoshi; Sakai, Hideaki; Tokunaga, Masashi; Yamasaki, Yuichi; Miyake, Atsushi; Shiogai, Junichi; Nakamura, Shintaro; Awaji, Satoshi; Tsukazaki, Atsushi; Nakao, Hironori; Murakami, Youichi; Arima, Taka-hisa; Tokura, Yoshinori; Ishiwata, Shintaro

2016-01-01

For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm(2)/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.

Magnetic Separation Using HTS Bulk Magnet for Cs-Bearing Fe precipitates

NASA Astrophysics Data System (ADS)

Oka, T.; Ichiju, K.; Sasaki, S.; Ogawa, J.; Fukui, S.; Sato, T.; Ooizumi, M.; Yokoyama, K.; Aoki, S.; Ohnishi, N.

2017-09-01

A peculiar magnetic separation technique has been examined in order to remove the Cs-bearing Fe precipitates formed of the waste ash from the withdrawn incinerator furnaces in Fukushima. The separation system was constructed in combination with high temperature superconducting bulk magnets which generates the intensive magnetic field over 2 T, which was activated by the pulsed field magnetization process. The separation experiment has been operated with use of the newly-built alternating channel type magnetic separating device, which followed the high-gradient magnetic separation technique. The magnetic stainless steel filters installed in the water channels are magnetized by the applied magnetic fields, and are capable of attracting the precipitates bearing the Fe compound and thin Cs contamination. The experimental results clearly exhibited the positive feasibility of HTS bulk magnets.

Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase.

PubMed

Novoselova, Iuliia P; Petruhins, Andrejs; Wiedwald, Ulf; Ingason, Árni Sigurdur; Hase, Thomas; Magnus, Fridrik; Kapaklis, Vassilios; Palisaitis, Justinas; Spasova, Marina; Farle, Michael; Rosen, Johanna; Salikhov, Ruslan

2018-02-08

In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn 2 GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn 2 GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is T N  ~ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At T t  = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above T t and tensile (positive) below the T t . The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties.

Temporal Order in Periodically Driven Spins in Star-Shaped Clusters

NASA Astrophysics Data System (ADS)

Pal, Soham; Nishad, Naveen; Mahesh, T. S.; Sreejith, G. J.

2018-05-01

We experimentally study the response of star-shaped clusters of initially unentangled N =4 , 10, and 37 nuclear spin-1 /2 moments to an inexact π -pulse sequence and show that an Ising coupling between the center and the satellite spins results in robust period-2 magnetization oscillations. The period is stable against bath effects, but the amplitude decays with a timescale that depends on the inexactness of the pulse. Simulations reveal a semiclassical picture in which the rigidity of the period is due to a randomizing effect of the Larmor precession under the magnetization of surrounding spins. The timescales with stable periodicity increase with net initial magnetization, even in the presence of perturbations, indicating a robust temporal ordered phase for large systems with finite magnetization per spin.

The study of the coupling mechanism between antiferromagnetic and ferroelectric ordering and thermodynamic properties in ferroelectromagnets

NASA Astrophysics Data System (ADS)

Zhong, Chong Gui; Jiang, Qing

2002-09-01

We study the coupling mechanism between antiferromagnetic and ferroelectric ordering that coexist spontaneously at low temperatures. According to the results of experiment and previous theoretical considerations, we propose a possible coupling form related to a combination of electric polar and spin correlation and use it to calculate the thermodynamic properties of a ferroelectromagnetic system, including its magnetization m, polarization p, magnetization susceptibility χm, magnetoelectric susceptibility χme and polarization susceptibility χp, in the case of magnetization m perpendicular to polarization p. It is found that the relationship between m, χm and χme is in agreement with that of phenomenological theory, and polarization induced by magnetic coupling leads to an anomaly of χp at low temperature, which is consistent qualitatively with experimental results.

Active magnetic radiation shielding system analysis and key technologies.

PubMed

Washburn, S A; Blattnig, S R; Singleterry, R C; Westover, S C

2015-01-01

Many active magnetic shielding designs have been proposed in order to reduce the radiation exposure received by astronauts on long duration, deep space missions. While these designs are promising, they pose significant engineering challenges. This work presents a survey of the major systems required for such unconfined magnetic field design, allowing the identification of key technologies for future development. Basic mass calculations are developed for each system and are used to determine the resulting galactic cosmic radiation exposure for a generic solenoid design, using a range of magnetic field strength and thickness values, allowing some of the basic characteristics of such a design to be observed. This study focuses on a solenoid shaped, active magnetic shield design; however, many of the principles discussed are applicable regardless of the exact design configuration, particularly the key technologies cited. Copyright © 2015 The Committee on Space Research (COSPAR). All rights reserved.

Realistic finite temperature simulations of magnetic systems using quantum statistics

NASA Astrophysics Data System (ADS)

Bergqvist, Lars; Bergman, Anders

2018-01-01

We have performed realistic atomistic simulations at finite temperatures using Monte Carlo and atomistic spin dynamics simulations incorporating quantum (Bose-Einstein) statistics. The description is much improved at low temperatures compared to classical (Boltzmann) statistics normally used in these kind of simulations, while at higher temperatures the classical statistics are recovered. This corrected low-temperature description is reflected in both magnetization and the magnetic specific heat, the latter allowing for improved modeling of the magnetic contribution to free energies. A central property in the method is the magnon density of states at finite temperatures, and we have compared several different implementations for obtaining it. The method has no restrictions regarding chemical and magnetic order of the considered materials. This is demonstrated by applying the method to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co random alloys and the ferrimagnetic system GdFe3.

Impacts of Interior Permanent Magnet Machine Technology for Electric Vehicles

DTIC Science & Technology

2012-01-01

corrosion constraints of magnets  Minimum gear and more direct drive  Regenerative braking and short charging cycle of batteries  Impulse...be found in limited applications such as, antilock braking system (ABS) of the vehicles. Considering the performance enhancement and reliability of... system forms the backbone of modern society. Electricity and its accessibility is one of the major engineering achievements. In order to maintain and

Transmitted light relaxation and microstructure evolution of ferrofluids under gradient magnetic fields

NASA Astrophysics Data System (ADS)

Huang, Yan; Li, Decai; Li, Feng; Zhu, Quanshui; Xie, Yu

2015-03-01

Using light transmission experiments and optical microscope observations with a longitudinal gradient magnetic field configuration, the relationship between the behavior of the transmitted light relaxation and the microstructure evolution of ionic ferrofluids in the central region of an axisymmetric field is investigated. Under a low-gradient magnetic field, there are two types of relaxation process. When a field is applied, the transmitted light intensity decreases to a minimum within a time on the order of 101-102 s. It is then gradually restored, approaching its initial value within a time on the order of 102 s. This is type I relaxation, which corresponds to the formation of magnetic columns. After the transmission reaches this value, it either increases or decreases slowly, stabilizing within a time on the order of 103 s, according to the direction of the field gradient. This is a type II relaxation, which results from the shadowing effect, corresponding to the motion of the magnetic columns under the application of a gradient force. Under a magnetic field with a centripetal high-gradient (magnetic materials subjected to a force pointing toward the center of the axisymmetric field), the transmitted light intensity decreases monotonously and more slowly than that under a low-gradient field. Magnetic transport and separation resulted from magnetophoresis under high-gradient fields, changing the formation dynamics of the local columns and influencing the final state of the column system.

Two-order parameters theory of the metal-insulator phase transition kinetics in the magnetic field

NASA Astrophysics Data System (ADS)

Dubovskii, L. B.

2018-05-01

The metal-insulator phase transition is considered within the framework of the Ginzburg-Landau approach for the phase transition described with two coupled order parameters. One of the order parameters is the mass density which variation is responsible for the origin of nonzero overlapping of the two different electron bands and the appearance of free electron carriers. This transition is assumed to be a first-order phase one. The free electron carriers are described with the vector-function representing the second-order parameter responsible for the continuous phase transition. This order parameter determines mostly the physical properties of the metal-insulator transition and leads to a singularity of the surface tension at the metal-insulator interface. The magnetic field is involved into the consideration of the system. The magnetic field leads to new singularities of the surface tension at the metal-insulator interface and results in a drastic variation of the phase transition kinetics. A strong singularity in the surface tension results from the Landau diamagnetism and determines anomalous features of the metal-insulator transition kinetics.

Ordered defects in Fe1-xS generate additional magnetic anisotropy symmetries

NASA Astrophysics Data System (ADS)

Koulialias, D.; Charilaou, M.; Schäublin, R.; Mensing, C.; Weidler, P. G.; Löffler, J. F.; Gehring, A. U.

2018-01-01

Non-stoichiometric monoclinic 4C pyrrhotite (Fe7S8), a ferrimagnetic monosulfide that has been intensively used as a remanence carrier to infer the magnetization of the Earth's crust and extraterrestrial materials, exhibits a characteristic low-temperature transition accompanied by complex modifications in anisotropy and magnetization. We demonstrate that the magnetic rotational symmetry of the 4C pyrrhotite is critically affected by the order of the defective Fe-sites, and this in turn is a key to decipher the physics behind the low-temperature transition. Our torque experiments and numerical simulations show an emergent four-fold rotational symmetry in the c-plane of the 4C pyrrhotite at T < 30 K. This symmetry breaking associated with the transition is caused by the competitive interaction of two inherently hexagonal systems generated by two groups of Fe-sites with different local anisotropy fields that stem from the vacancy arrangement in the 4C stacking sequence, and it is triggered by changes in the spin orbit coupling due to the overlap of Fe-Fe electron orbitals at low-temperature. This mechanism provides a new explanation for the magnetic transition in 4C pyrrhotite at low temperature and could also cast light on non-trivial magnetic phenomena in defective systems.

Magnetic behavior of Fe(Se,Te) systems: First-principles calculations

NASA Astrophysics Data System (ADS)

Shi, Hongliang; Huang, Zhong-Bing; Tse, John S.; Lin, Hai-Qing

2011-08-01

The magnetic behaviors in Fe(Se,Te) systems have been investigated systematically using density functional calculations. At the experimental lattice parameters, the ground state is found to be in the double stripe magnetic phase for FeTe but in the single stripe magnetic phase for FeSe and FeSe0.5Te0.5, and there is no preference in the different easy axes of magnetization. Substitution of Se by Te enlarges the size of the Fermi surface in FeSe0.5Te0.5, resulting in a stronger nesting effect and thus enhancing the superconductivity. It is found that the double stripe order in FeTe1-xSex changes to the single stripe order when x > 0.18. Spiral calculations on FeSe0.5Te0.5 show that the lowest energy is at the commensurate point Q→= (0.5,0.5), accompanied by additional local minima at two incommensurate points near Q→= (0.5,0.5). This observation is consistent with the experimentally observed positions of low energy magnetic excitations. Geometry optimization calculations show that the tetragonal cell relaxes to orthorhombic and monoclinic cells for FeSe and FeTe, respectively, but remains unchanged for FeSe0.5Te0.5.

Strong-coupling phases of the spin-orbit-coupled spin-1 Bose-Hubbard chain: Odd-integer Mott lobes and helical magnetic phases

NASA Astrophysics Data System (ADS)

Pixley, J. H.; Cole, William S.; Spielman, I. B.; Rizzi, Matteo; Das Sarma, S.

2017-10-01

We study the odd-integer filled Mott phases of a spin-1 Bose-Hubbard chain and determine their fate in the presence of a Raman induced spin-orbit coupling which has been achieved in ultracold atomic gases; this system is described by a quantum spin-1 chain with a spiral magnetic field. The spiral magnetic field initially induces helical order with either ferromagnetic or dimer order parameters, giving rise to a spiral paramagnet at large field. The spiral ferromagnet-to-paramagnet phase transition is in a universality class with critical exponents associated with the divergence of the correlation length ν ≈2 /3 and the order-parameter susceptibility γ ≈1 /2 . We solve the effective spin model exactly using the density-matrix renormalization group, and compare with both a large-S classical solution and a phenomenological Landau theory. We discuss how these exotic bosonic magnetic phases can be produced and probed in ultracold atomic experiments in optical lattices.

Colossal magnetocapacitive effects in geometrically frustrated chalcogenide spinels

NASA Astrophysics Data System (ADS)

Lunkenheimer, Peter

2006-03-01

It is well known that the spinel structure is susceptible to the occurrence of geometrical frustration, which in the past was invoked to explain a number of unusual observations concerning the magnetic and orbital degrees of freedom in these materials. We demonstrate that several chalcogenide spinels also exhibit very unusual dielectric behavior, especially an extremely strong coupling of magnetic and dielectric properties and the simultaneous occurrence of magnetic and polar order. Especially, in CdCr2S4 a colossal magnetocapacitive effect is observed, which shows up as a sharp upturn of the dielectric constant ɛ' when the sample becomes ferromagnetic and as a variation of ɛ' up to a factor of 30 when the sample is subjected to external magnetic fields. As revealed by linear and non-linear dielectric measurements, this material shows the typical signatures of relaxor ferroelectrics, i.e. a strong increase of the static dielectric constant with decreasing temperature and considerable frequency dispersion of the complex permittivity. While in most relaxor ferroelectrics the freezing of polar moments is driven by frustrated interactions related to substitutional disorder, in the present pure system geometrical frustration seems a plausible mechanism to explain the relaxor behavior. However, one may also speculate on completely different mechanisms of ferroelectric polarization, e.g., the ordering of electronic degrees of freedom. The concomitant occurrence of polar and magnetic order makes CdCr2S4 another example of the rare species of multiferroic materials. In contrast to other members of this group of materials, it has sizable ordering temperatures and moments. A detailed investigation of the relaxational dynamics in this material provides clear evidence that the observed magnetocapacitive effect stems from an enormous acceleration of the relaxation dynamics induced by the development of magnetic order. In addition, recent results reveal even larger magnetocapacitive effects in In-doped CdCr2S4. In addition to CdCr2S4, we found similar effects also in ferromagnetic CdCr2Se4 and, most astonishing, in HgCr2S4, which exhibits a complex type of antiferromagnetic magnetic order at low temperatures. In the latter system, the magnetocapacitive effect, exemplified by the relative increase of ɛ' in a field of 5 T, reaches values up to 8x10^5 %.

Modification of electric and magnetic dipole emission in anisotropic plasmonic systems.

PubMed

Noginova, N; Hussain, R; Noginov, M A; Vella, J; Urbas, A

2013-10-07

In order to investigate the effects of plasmonic environments on spontaneous emission of magnetic and electric dipoles, we have studied luminescence of Eu³⁺ ions in close vicinity to gold nanostrip arrays. Significant changes in the emission kinetics, emission polarization, and radiation patterns have been observed in the wavelength range corresponding to the plasmonic resonance. The effect of the plasmonic resonance on the magnetic dipole transition ⁵D₀-->⁷F₁ is found to be very different from its effect on the electric dipole transitions. This makes Eu³⁺₋ containing complexes promising for mapping local distributions of magnetic and electric fields in metamaterials and plasmonic systems.

Rapid cycling medical synchrotron and beam delivery system

DOEpatents

Peggs, Stephen G [Port Jefferson, NY; Brennan, J Michael [East Northport, NY; Tuozzolo, Joseph E [Sayville, NY; Zaltsman, Alexander [Commack, NY

2008-10-07

A medical synchrotron which cycles rapidly in order to accelerate particles for delivery in a beam therapy system. The synchrotron generally includes a radiofrequency (RF) cavity for accelerating the particles as a beam and a plurality of combined function magnets arranged in a ring. Each of the combined function magnets performs two functions. The first function of the combined function magnet is to bend the particle beam along an orbital path around the ring. The second function of the combined function magnet is to focus or defocus the particle beam as it travels around the path. The radiofrequency (RF) cavity is a ferrite loaded cavity adapted for high speed frequency swings for rapid cycling acceleration of the particles.

Correlation of simulation/finite element analysis to the separation of intrinsically magnetic spores and red blood cells using a microfluidic magnetic deposition system.

PubMed

Sun, Jianxin; Moore, Lee; Xue, Wei; Kim, James; Zborowski, Maciej; Chalmers, Jeffrey J

2018-05-01

Magnetic separation of cells has been, and continues to be, widely used in a variety of applications, ranging from healthcare diagnostics to detection of food contamination. Typically, these technologies require cells labeled with antibody magnetic particle conjugate and a high magnetic energy gradient created in the flow containing the labeled cells (i.e., a column packed with magnetically inducible material), or dense packing of magnetic particles next to the flow cell. Such designs, while creating high magnetic energy gradients, are not amenable to easy, highly detailed, mathematic characterization. Our laboratories have been characterizing and developing analysis and separation technology that can be used on intrinsically magnetic cells or spores which are typically orders of magnitude weaker than typically immunomagnetically labeled cells. One such separation system is magnetic deposition microscopy (MDM) which not only separates cells, but deposits them in specific locations on slides for further microscopic analysis. In this study, the MDM system has been further characterized, using finite element and computational fluid mechanics software, and separation performance predicted, using a model which combines: 1) the distribution of the intrinsic magnetophoretic mobility of the cells (spores); 2) the fluid flow within the separation device; and 3) accurate maps of the values of the magnetic field (max 2.27 T), and magnetic energy gradient (max of 4.41 T 2 /mm) within the system. Guided by this model, experimental studies indicated that greater than 95% of the intrinsically magnetic Bacillus spores can be separated with the MDM system. Further, this model allows analysis of cell trajectories which can assist in the design of higher throughput systems. © 2018 Wiley Periodicals, Inc.

Influence of Chromium Doping on Electrical and Magnetic Behavior of Nd0.5Sr0.5MnO3 System

NASA Astrophysics Data System (ADS)

Lalitha, G.; Pavan Kumar, N.; Venugopal Reddy, P.

2018-04-01

With a view to understand the influence of chromium doping at the Mn site on the electrical and magnetic behavior of the Nd0.5Sr0.5MnO3 manganite system, a series of samples were prepared by the citrate sol-gel route method. The samples were characterized structurally by XRD. A systematic investigation of electrical resistivity over a temperature range 5-300 K was carried out mainly to understand the magneto-transport behavior in these materials. Studies on the variation of magnetization with temperature over a temperature range 80-330 K were undertaken. Investigation of magnetization at different magnetic fields at two different temperatures, viz. 80 and 300 K, was also carried out. The results show that chromium doping gave typical electrical and magnetic properties. It has been concluded that the coexistence of charge ordered and ferromagnetic phases induced by chromium doping plays an important role in the low-temperature behavior of the system.

Jahn-Teller versus quantum effects in the spin-orbital material LuVO 3

DOE PAGES

Skoulatos, M.; Toth, S.; Roessli, B.; ...

2015-04-13

In this article, we report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO 3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed studymore » enabled us to draw definite conclusions about the classical versus quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO 3 and similar systems.« less

Magnetic Correlations and Pairing in the 1/5-Depleted Square Lattice Hubbard Model

DOE PAGES

Khatemi, Ehsan; Singh, Rajiv R. P.; Pickett, Warren E.; ...

2014-09-04

We study the single-orbital Hubbard model on the 1/5-depleted square-lattice geometry, which arises in such diverse systems as the spin-gap magnetic insulator CaV 4O 9 and ordered-vacancy iron selenides, presenting new issues regarding the origin of both magnetic ordering and superconductivity in these materials. We find a rich phase diagram that includes a plaquette singlet phase, a dimer singlet phase, a Néel and a block-spin antiferromagnetic phase, and stripe phases. Quantum Monte Carlo simulations show that the dominant pairing correlations at half filling change character from d wave in the plaquette phase to extended s wave upon transition to themore » Néel phase. These findings have intriguing connections to iron-based superconductors, and suggest that some physics of multiorbital systems can be captured by a single-orbital model at different dopings.« less

Flightweight Carbon Nanotube Magnet Technology

NASA Technical Reports Server (NTRS)

Chapman, J. N.; Schmidt, H. J.; Ruoff, R. S.; Chandrasekhar, V.; Dikin, D. A.; Litchford, R. J.

2003-01-01

Virtually all plasma-based systems for advanced airborne/spaceborne propulsion and power depend upon the future availability of flightweight magnet technology. Unfortunately, current technology for resistive and superconducting magnets yields system weights that tend to counteract the performance advantages normally associated with advanced plasma-based concepts. The ongoing nanotechnology revolution and the continuing development of carbon nanotubes (CNT), however, may ultimately relieve this limitation in the near future. Projections based on recent research indicate that CNTs may achieve current densities at least three orders of magnitude larger than known superconductors and mechanical strength two orders of magnitude larger than steel. In fact, some published work suggests that CNTs are superconductors. Such attributes imply a dramatic increase in magnet performance-to-weight ratio and offer real hope for the construction of true flightweight magnets. This Technical Publication reviews the technology status of CNTs with respect to potential magnet applications and discusses potential techniques for using CNT wires and ropes as a winding material and as an integral component of the containment structure. The technology shortfalls are identified and a research and technology strategy is described that addresses the following major issues: (1) Investigation and verification of mechanical and electrical properties, (2) development of tools for manipulation and fabrication on the nanoscale, (3) continuum/molecular dynamics analysis of nanotube behavior when exposed to practical bending and twisting loads, and (4) exploration of innovative magnet fabrication techniques that exploit the natural attributes of CNTs.

Quantitative study of FORC diagrams in thermally corrected Stoner- Wohlfarth nanoparticles systems

NASA Astrophysics Data System (ADS)

De Biasi, E.; Curiale, J.; Zysler, R. D.

2016-12-01

The use of FORC diagrams is becoming increasingly popular among researchers devoted to magnetism and magnetic materials. However, a thorough interpretation of this kind of diagrams, in order to achieve quantitative information, requires an appropriate model of the studied system. For that reason most of the FORC studies are used for a qualitative analysis. In magnetic systems thermal fluctuations "blur" the signatures of the anisotropy, volume and particle interactions distributions, therefore thermal effects in nanoparticles systems conspire against a proper interpretation and analysis of these diagrams. Motivated by this fact, we have quantitatively studied the degree of accuracy of the information extracted from FORC diagrams for the special case of single-domain thermal corrected Stoner- Wohlfarth (easy axes along the external field orientation) nanoparticles systems. In this work, the starting point is an analytical model that describes the behavior of a magnetic nanoparticles system as a function of field, anisotropy, temperature and measurement time. In order to study the quantitative degree of accuracy of our model, we built FORC diagrams for different archetypical cases of magnetic nanoparticles. Our results show that from the quantitative information obtained from the diagrams, under the hypotheses of the proposed model, is possible to recover the features of the original system with accuracy above 95%. This accuracy is improved at low temperatures and also it is possible to access to the anisotropy distribution directly from the FORC coercive field profile. Indeed, our simulations predict that the volume distribution plays a secondary role being the mean value and its deviation the only important parameters. Therefore it is possible to obtain an accurate result for the inversion and interaction fields despite the features of the volume distribution.

Conceptual design of the cryostat for the new high luminosity (HL-LHC) triplet magnets

NASA Astrophysics Data System (ADS)

Ramos, D.; Parma, V.; Moretti, M.; Eymin, C.; Todesco, E.; Van Weelderen, R.; Prin, H.; Berkowitz Zamora, D.

2017-12-01

The High Luminosity LHC (HL-LHC) is a project to upgrade the LHC collider after 2020-2025 to increase the integrated luminosity by about one order of magnitude and extend the physics production until 2035. An upgrade of the focusing triplets insertion system for the ATLAS and CMS experiments is foreseen using superconducting magnets operating in a pressurised superfluid helium bath at 1.9 K. This will require the design and construction of four continuous cryostats, each about sixty meters in length and one meter in diameter, for the final beam focusing quadrupoles, corrector magnets and beam separation dipoles. The design is constrained by the dimensions of the existing tunnel and accessibility restrictions imposing the integration of cryogenic piping inside the cryostat, thus resulting in a very compact integration. As the alignment and position stability of the magnets is crucial for the luminosity performance of the machine, the magnet support system must be carefully designed in order to cope with parasitic forces and thermo-mechanical load cycles. In this paper, we present the conceptual design of the cryostat and discuss the approach to address the stringent and often conflicting requirements of alignment, integration and thermal aspects.

Visualization of superparamagnetic nanoparticles in vascular tissue using XμCT and histology.

PubMed

Tietze, Rainer; Rahn, Helene; Lyer, Stefan; Schreiber, Eveline; Mann, Jenny; Odenbach, Stefan; Alexiou, Christoph

2011-02-01

In order to increase the dose of antineoplastic agents in the tumor area, the concept of magnetic drug targeting (MDT) has been developed. Magnetic nanoparticles consisting of iron oxide and a biocompatible cover layer suspended in an aqueous solution (ferrofluid) serve as carriers for chemotherapeutics being enriched by an external magnetic field after intra-arterial application in desired body compartments (i.e., tumor). We established an ex vivo model to simulate in vivo conditions in a circulating system consisting of magnetic iron oxide nanoparticles passing an intact bovine artery and being focused by an external magnetic field to study their distribution in the vessel. Micro-computed X-ray tomography (XμCT) and histology can elucidate the arrangement of these particles after application. XμCT-analysis has been performed on arterial sections after MDT in order to determine the distribution of the nanoparticles. These measurements have been carried out with a cone X-ray source and corresponding histological sections were stained with Prussian blue. It could be shown that combining XμCT and histology offers the opportunity for a better understanding of the mechanisms of nanoparticle deposition in the vascular system after MDT.

Unconventional magnetism in the layered oxide LaSrRhO4

NASA Astrophysics Data System (ADS)

Furuta, Noriyasu; Asai, Shinichiro; Igarashi, Taichi; Okazaki, Ryuji; Yasui, Yukio; Terasaki, Ichiro; Ikeda, Masami; Fujita, Takahito; Hagiwara, Masayuki; Kobayashi, Kensuke; Kumai, Reiji; Nakao, Hironori; Murakami, Youichi

2014-10-01

We have prepared polycrystalline samples of LaSrRh1-xGaxO4 and LaSr1-xCaxRhO4, and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization, and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the g factor to be g =2.3 from the electron spin resonance, and the spin number to be S =1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the S =1 spins is 2%-5%, which depends on the degree of disorder in the La/Sr/Ca site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh3+ ions is discussed.

Tunable Weyl and Dirac states in the nonsymmorphic compound CeSbTe.

PubMed

Schoop, Leslie M; Topp, Andreas; Lippmann, Judith; Orlandi, Fabio; Müchler, Lukas; Vergniory, Maia G; Sun, Yan; Rost, Andreas W; Duppel, Viola; Krivenkov, Maxim; Sheoran, Shweta; Manuel, Pascal; Varykhalov, Andrei; Yan, Binghai; Kremer, Reinhard K; Ast, Christian R; Lotsch, Bettina V

2018-02-01

Recent interest in topological semimetals has led to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry, and the introduction of magnetic order can influence these states by breaking time-reversal symmetry. We show that we can realize a rich variety of different topological semimetal states in a single material, CeSbTe. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. Therefore, it allows for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological phase with an eightfold band crossing at a high-symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology. We also show that we can generally expand the numbers of space groups that allow for high-order band degeneracies by introducing antiferromagnetic order.

Optimization of a superconducting linear levitation system using a soft ferromagnet

NASA Astrophysics Data System (ADS)

Agramunt-Puig, Sebastia; Del-Valle, Nuria; Navau, Carles; Sanchez, Alvaro

2013-04-01

The use of guideways that combine permanent magnets and soft ferromagnetic materials is a common practice in magnetic levitation transport systems (maglevs) with bulk high-temperature superconductors. Theoretical tools to simulate in a realistic way both the behavior of all elements (permanent magnets, soft ferromagnet and superconductor) and their mutual effects are helpful to optimize the designs of real systems. Here we present a systematic study of the levitation of a maglev with translational symmetry consisting of a superconducting bar and a guideway with two identic permanent magnets and a soft ferromagnetic material between them. The system is simulated with a numerical model based on the energy minimization method that allows to analyze the mutual interaction of the superconductor, assumed to be in the critical state, and a soft ferromagnet with infinite susceptibility. Results indicate that introducing a soft ferromagnet within the permanent magnets not only increases the levitation force but also improves the stability. Besides, an estimation of the relative sizes and shapes of the soft ferromagnet, permanent magnets and the superconductor in order to obtain large levitation force with full stability is provided.

Relativistic calculation of nuclear magnetic shielding using normalized elimination of the small component

NASA Astrophysics Data System (ADS)

Kudo, K.; Maeda, H.; Kawakubo, T.; Ootani, Y.; Funaki, M.; Fukui, H.

2006-06-01

The normalized elimination of the small component (NESC) theory, recently proposed by Filatov and Cremer [J. Chem. Phys. 122, 064104 (2005)], is extended to include magnetic interactions and applied to the calculation of the nuclear magnetic shielding in HX (X =F,Cl,Br,I) systems. The NESC calculations are performed at the levels of the zeroth-order regular approximation (ZORA) and the second-order regular approximation (SORA). The calculations show that the NESC-ZORA results are very close to the NESC-SORA results, except for the shielding of the I nucleus. Both the NESC-ZORA and NESC-SORA calculations yield very similar results to the previously reported values obtained using the relativistic infinite-order two-component coupled Hartree-Fock method. The difference between NESC-ZORA and NESC-SORA results is significant for the shieldings of iodine.

Transport Studies of Quantum Magnetism: Physics and Methods

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Minhyea

The main goal of this project was to understand novel ground states of spin systems probed by thermal and electrical transport measurements. They are well-suited to characterize the nature of low-energy excitations as unique property of the ground state. More specifically, it was aimed to study the transverse electrical conductivity in the presence of non-collinear and non-coplanar spin ordering and the effects of gauge field as well as novel spin excitations as a coherent heat transport channel in insulating quantum magnets. Most of works done during the grant period focused on these topics. As a natural extension of the project'smore » initial goals, the scope was broadened to include transport studies on the spin systems with strong spin-orbit coupling. One particular focus was an exploration of systems with strong magnetic anisotropy combined with non-trivial spin configuration. Magnetic anisotropy is directly related to implement the non-collinear spin ordering to the existing common geometry of planar devices and thus poses a significant potential. Work in this direction includes the comparison of the topological Hall signal under hydrostatic pressure and chemical doping, as well as the angular dependence dependence of the non-collinear spin ordered phase and their evolution up on temperature and field strength. Another focus was centered around the experimental identification of spin-originated heat carrying excitation in quasi two dimensional honeycomb lattice, where Kitaev type of quantum spin liquid phase is expected to emerge. In fact, when its long range magnetic order is destroyed by the applied field, we discovered anomalously large enhancement of thermal conductivity, for which proximate Kitaev excitations in field-induced spin liquid state are responsible for. This work, combined with further investigations in materials in the similar class may help establish the experimental characterization of new quantum spin liquid and their unique low energy excitation, e.g. Majorana fermions.« less

Optimizing Power Density and Efficiency of a Double-Halbach Array Permanent-Magnet Ironless Axial-Flux Motor

NASA Technical Reports Server (NTRS)

Duffy, Kirsten P.

2016-01-01

NASA Glenn Research Center is investigating hybrid electric and turboelectric propulsion concepts for future aircraft to reduce fuel burn, emissions, and noise. Systems studies show that the weight and efficiency of the electric system components need to be improved for this concept to be feasible. This effort aims to identify design parameters that affect power density and efficiency for a double-Halbach array permanent-magnet ironless axial flux motor configuration. These parameters include both geometrical and higher-order parameters, including pole count, rotor speed, current density, and geometries of the magnets, windings, and air gap.

Computer aided design of digital controller for radial active magnetic bearings

NASA Technical Reports Server (NTRS)

Cai, Zhong; Shen, Zupei; Zhang, Zuming; Zhao, Hongbin

1992-01-01

A five degree of freedom Active Magnetic Bearing (AMB) system is developed which is controlled by digital controllers. The model of the radial AMB system is linearized and the state equation is derived. Based on the state variables feedback theory, digital controllers are designed. The performance of the controllers are evaluated according to experimental results. The Computer Aided Design (CAD) method is used to design controllers for magnetic bearings. The controllers are implemented with a digital signal processing (DSP) system. The control algorithms are realized with real-time programs. It is very easy to change the controller by changing or modifying the programs. In order to identify the dynamic parameters of the controlled magnetic system, a special experiment was carried out. Also, the online Recursive Least Squares (RLS) parameter identification method is studied. It can be realized with the digital controllers. Online parameter identification is essential for the realization of an adaptive controller.

Second International Symposium on Magnetic Suspension Technology, part 2

NASA Technical Reports Server (NTRS)

Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1994-01-01

In order to examine the state of technology of all areas of magnetic suspension and to review related recent developments in sensors and controls approaches, superconducting magnet technology, and design/implementation practices, the 2nd International Symposium on Magnetic Suspension Technology was held at the Westin Hotel in Seattle, WA, on 11-13 Aug. 1993. The symposium included 18 technical sessions in which 44 papers were presented. The technical sessions covered the areas of bearings, bearing modelling, controls, vibration isolation, micromachines, superconductivity, wind tunnel magnetic suspension systems, magnetically levitated trains (MAGLEV), rotating machinery and energy storage, and applications. A list of attendees appears at the end of the document.

Analysis and optimization of hybrid excitation permanent magnet synchronous generator for stand-alone power system

NASA Astrophysics Data System (ADS)

Wang, Huijun; Qu, Zheng; Tang, Shaofei; Pang, Mingqi; Zhang, Mingju

2017-08-01

In this paper, electromagnetic design and permanent magnet shape optimization for permanent magnet synchronous generator with hybrid excitation are investigated. Based on generator structure and principle, design outline is presented for obtaining high efficiency and low voltage fluctuation. In order to realize rapid design, equivalent magnetic circuits for permanent magnet and iron poles are developed. At the same time, finite element analysis is employed. Furthermore, by means of design of experiment (DOE) method, permanent magnet is optimized to reduce voltage waveform distortion. Finally, the validity of proposed design methods is validated by the analytical and experimental results.

A novel integrated structure with a radial displacement sensor and a permanent magnet biased radial magnetic bearing.

PubMed

Sun, Jinji; Zhang, Yin

2014-01-24

In this paper, a novel integrated structure is proposed in order to reduce the axial length of the high speed of a magnetically suspended motor (HSMSM) to ensure the maximum speed, which combines radial displacement sensor probes and the permanent magnet biased radial magnetic bearing in HSMSM. The sensor probes are integrated in the magnetic bearing, and the sensor preamplifiers are placed in the control system of the HSMSM, separate from the sensor probes. The proposed integrated structure can save space in HSMSMs, improve the working frequency, reduce the influence of temperature on the sensor circuit, and improve the stability of HSMSMs.

Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe

NASA Astrophysics Data System (ADS)

Kreyssig, Andreas

In iron-based superconductors, the lattice, magnetism and electronic system show a fascinating interplay. Nematic order breaks the tetragonal symmetry and yields an orthorhombic lattice distortion. The same symmetry is broken by the stripe-like antiferromagnetic order suggesting a symmetry-related coupling between both phenomena. The phase transitions in to both ordered states can be simultaneous and of first-order character like in CaFe2As2, or separated in temperature like in Co-doped BaFe2As2 with second or first-order character depending on the doping level. Stripe-type magnetic fluctuations are discussed as correlation-driven electronic mechanism of the nematicity and important for the superconducting electron pairing establishing a coupling mechanism. However, a universal picture has been confounded by measurements of FeSe where the nematic and magnetic transitions appear to be decoupled by the observation of the lattice distortion without antiferromagnetic order at ambient pressure. In this talk I will present our recent study on the relation between the nematic and magnetic order in FeSe single crystals investigated by synchrotron-based high-energy x-ray diffraction and time-domain Moessbauer spectroscopy as function of temperature and pressure. Distinct nematic and magnetic transitions are observed for low pressures and merge into a single first-order transition for higher pressures reminiscent of what has been found for the evolution of these transitions in Co-doped BaFe2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the interplay between the different ordering phenomena in the iron-based superconductors. This work was performed in collaboration with K. Kothapalli, A. E. Böhmer, W. T. Jayasekara, B. G. Ueland, P. Das, A. Sapkota, V. Taufour, Y. Xiao, E. Alp, S. L. Bud'ko, P.C. Canfield, and A.I. Goldman; and supported by the Department of Energy, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract No. DE-AC02-07CH11358.

Nonlinear gyrokinetics: a powerful tool for the description of microturbulence in magnetized plasmas

NASA Astrophysics Data System (ADS)

Krommes, John A.

2010-12-01

Gyrokinetics is the description of low-frequency dynamics in magnetized plasmas. In magnetic-confinement fusion, it provides the most fundamental basis for numerical simulations of microturbulence; there are astrophysical applications as well. In this tutorial, a sketch of the derivation of the novel dynamical system comprising the nonlinear gyrokinetic (GK) equation (GKE) and the coupled electrostatic GK Poisson equation will be given by using modern Lagrangian and Lie perturbation methods. No background in plasma physics is required in order to appreciate the logical development. The GKE describes the evolution of an ensemble of gyrocenters moving in a weakly inhomogeneous background magnetic field and in the presence of electromagnetic perturbations with wavelength of the order of the ion gyroradius. Gyrocenters move with effective drifts, which may be obtained by an averaging procedure that systematically, order by order, removes gyrophase dependence. To that end, the use of the Lagrangian differential one-form as well as the content and advantages of Lie perturbation theory will be explained. The electromagnetic fields follow via Maxwell's equations from the charge and current density of the particles. Particle and gyrocenter densities differ by an important polarization effect. That is calculated formally by a 'pull-back' (a concept from differential geometry) of the gyrocenter distribution to the laboratory coordinate system. A natural truncation then leads to the closed GK dynamical system. Important properties such as GK energy conservation and fluctuation noise will be mentioned briefly, as will the possibility (and difficulties) of deriving nonlinear gyrofluid equations suitable for rapid numerical solution—although it is probably best to directly simulate the GKE. By the end of the tutorial, students should appreciate the GKE as an extremely powerful tool and will be prepared for later lectures describing its applications to physical problems.

Fourth International Symposium on Magnetic Suspension Technology

NASA Technical Reports Server (NTRS)

Groom, Nelson J. (Editor); Britcher, Colin P. (Editor)

1998-01-01

In order to examine the state of technology of all areas of magnetic suspension and to review recent developments in sensors, controls, superconducting magnet technology, and design/implementation practices, the Fourth International Symposium on Magnetic Suspension Technology was held at The Nagaragawa Convention Center in Gifu, Japan, on October 30 - November 1, 1997. The symposium included 13 sessions in which a total of 35 papers were presented. The technical sessions covered the areas of maglev, controls, high critical temperature (T(sub c)) superconductivity, bearings, magnetic suspension and balance systems (MSBS), levitation, modeling, and applications. A list of attendees is included in the document.

Passive Magnetic Bearing With Ferrofluid Stabilization

NASA Technical Reports Server (NTRS)

Jansen, Ralph; DiRusso, Eliseo

1996-01-01

A new class of magnetic bearings is shown to exist analytically and is demonstrated experimentally. The class of magnetic bearings utilize a ferrofluid/solid magnet interaction to stabilize the axial degree of freedom of a permanent magnet radial bearing. Twenty six permanent magnet bearing designs and twenty two ferrofluid stabilizer designs are evaluated. Two types of radial bearing designs are tested to determine their force and stiffness utilizing two methods. The first method is based on the use of frequency measurements to determine stiffness by utilizing an analytical model. The second method consisted of loading the system and measuring displacement in order to measure stiffness. Two ferrofluid stabilizers are tested and force displacement curves are measured. Two experimental test fixtures are designed and constructed in order to conduct the stiffness testing. Polynomial models of the data are generated and used to design the bearing prototype. The prototype was constructed and tested and shown to be stable. Further testing shows the possibility of using this technology for vibration isolation. The project successfully demonstrated the viability of the passive magnetic bearing with ferrofluid stabilization both experimentally and analytically.

Stability of a two-volume MRxMHD model in slab geometry

NASA Astrophysics Data System (ADS)

Tuen, Li Huey

Ideal MHD models are known to be inadequate to describe various physical attributes of a toroidal field with non-continuous symmetry, such as magnetic islands and stochastic regions. Motivated by this omission, a new variational principle MRXMHD was developed; rather than include an infinity of magnetic flux surfaces, MRxMHD has a finite number of flux surfaces, and thus supports partial plasma relaxation. The model comprises of relaxed plasma regions which are separated by nested ideal MHD interfaces (flux surfaces), and can be encased in a perfectly conducting wall. In each region the pressure is constant, but can jump across interfaces. The field and field pitch, or rotational transform, can also jump across the interfaces. Unlike ideal MHD, MRxMHD plasmas can support toroidally non-axisymmetric confined magnetic fields, magnetic islands and stochastic regions. In toroidally non-axisymmetric plasma, the existence of interfaces in MRxMHD is contingent on the irrationality of the rotational transform of flux surfaces. That is, the KAM theorem shows that invariant tori (flux surfaces) continue to exist for sufficiently small perturbations to an integrable system (which describes flux surfaces), provided that the rotational transform is sufficiently irrational. Building upon the MRxMHD stability model, we study the effects of irrationality of the rotational transform at interfaces in MRxMHD on plasma stability. We present an MRxMHD equilibrium model to investigate the effects of magnetic field pitch within the plasma and across the aforementioned flux surfaces within a chosen geometry. In this model, it is found that the 2D system stability conditions are dependent on the interface and resonant surface magnetic field pitch at minimised energy states, and the stability of a system as a function of magnetic field pitch destabilises at particular values of magnetic field pitch. We benchmark the treatment of a two-volume system, along with the calculations for background and perturbed magnetic fields to existing cylindrical working. An expression is formulated for the stability eigenvalues by creating a model for the slab geometry system. The eigenvalues for system stability at a minimum energy state are found to depend upon the rationality of the magnetic field pitch at resonant surfaces. Various system parameter scans are conducted to determine their affect upon system stability and their implications. While tearing instabilities exist at low order rational resonances, investigating the instability of high-order rationals requires study of pressure-driven instabilities.

Theoretical study on perpendicular magnetoelectric coupling in ferroelectromagnet system

NASA Astrophysics Data System (ADS)

Zhong, Chonggui; Jiang, Qing

2002-06-01

We apply the Heisenberg model for antiferromagnetic interaction and Diffour model for ferroelectric interaction to analyze the magnetic, electric, magnetoelectric property in the system with the spontaneous coexistence of the ferroelectric and antiferromagnetic orders below a certain temperature. The soft mode theory is used to calculate the on-site polarization and mean field theory is applied to deal with the on-site magnetization. We also present the perpendicular magnetoelectric susceptibility χme⊥, polarization susceptibility χp as a function of temperature, and discuss the effect of the inherent magnetoelectric coupling on them. In addition, it is found that an anomaly appears in the curve of the polarization susceptibility due to the coupling between the ferroelectric and antiferromagnetic orders.

Orbital-ordering-driven multiferroicity and magnetoelectric coupling in GeV₄S₈.

PubMed

Singh, Kiran; Simon, Charles; Cannuccia, Elena; Lepetit, Marie-Bernadette; Corraze, Benoit; Janod, Etienne; Cario, Laurent

2014-09-26

We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system GeV₄S₈. Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon.

Selective absorption processes as the origin of puzzling spectral line polarization from the Sun.

PubMed

Trujillo Bueno, J; Landi Degl'Innocenti, E; Collados, M; Merenda, L; Manso Sainz, R

2002-01-24

Magnetic fields play a key role in most astrophysical systems, from the Sun to active galactic nuclei. They can be studied through their effects on atomic energy levels, which produce polarized spectral lines. In particular, anisotropic radiation 'pumping' processes (which send electrons to higher atomic levels) induce population imbalances that are modified by weak magnetic fields. Here we report peculiarly polarized light in the He I 10,830-A multiplet observed in a coronal filament located at the centre of the solar disk. We show that the polarized light arises from selective absorption from the ground level of the triplet system of helium, and that it implies the presence of magnetic fields of the order of a few gauss that are highly inclined with respect to the solar radius vector. This disproves the common belief that population imbalances in long-lived atomic levels are insignificant in the presence of inclined fields of the order of a few gauss, and opens up a new diagnostic window for the investigation of solar magnetic fields.

Orbit Correction for the Newly Developed Polarization-Switching Undulator

NASA Astrophysics Data System (ADS)

Obina, Takashi; Honda, Tohru; Shioya, Tatsuro; Kobayashi, Yukinori; Tsuchiya, Kimichika; Yamamoto, Shigeru

2007-01-01

A new scheme of undulator magnet arrangements has been proposed and developed as a polarization-switching radiation source, and its test-stand was installed in the 2.5-GeV Photon Factory storage ring (PF ring) in order to investigate the effects on the beam orbit. The closed orbit distortion (COD) over 200 μm was produced in a vertical direction when we switched the polarization of the radiation from the test-stand. In a horizontal direction, the COD was less than 50μm. The results agreed well with the predictions from the magnetic-field measurement on the bench. In order to suppress the CODs and realize a stable operation of the ring with the polarization-switching, we developed an orbit correction system which consists of an encoder to detect motion of magnets, a pair of beam position monitors (BPMs), signal processing parts, and a pair of steering magnets. We succeeded in suppressing the CODs to the level below 3μm using the system even when we switch the polarization at a maximum frequency of 0.8 Hz.

Electronic and magnetic properties of SnS2 monolayer doped with 4d transition metals

NASA Astrophysics Data System (ADS)

Xiao, Wen-Zhi; Xiao, Gang; Rong, Qing-Yan; Chen, Qiao; Wang, Ling-Ling

2017-09-01

We investigate the electronic structures and magnetic properties of SnS2 monolayers substitutionally doped with 4-d transition-metal through systematic first principles calculations. The doped complexes exhibit interesting electronic and magnetic behaviors, depending on the interplay between crystal field splitting, Hund's rule, and 4d levels. The system doped with Y is nonmagnetic metal. Both the Zr- and Pd-doped systems remain nonmagnetic semiconductors. Doping results in half-metallic states for Nb-, Ru-, Rh-, Ag, and Cd doped cases, and magnetic semiconductors for systems with Mo and Tc dopants. In particular, the Nb- and Mo-doped systems display long-ranged ferromagnetic ordering with Curie temperature above room temperature, which are primarily attributable to the double-exchange mechanism, and the p-d/p-p hybridizations, respectively. Moreover, The Mo-doped system has excellent energetic stability and flexible mechanical stability, and also possesses remarkable dynamic and thermal (500 K) stability. Our studies demonstrate that Nb- and Mo-doped SnS2 monolayers are promising candidates for preparing 2D diluted magnetic semiconductors, and hence will be a helpful clue for experimentalists.

Multipeak low-temperature behavior of specific heat capacity in frustrated magnetic systems: An exact theoretical analysis

NASA Astrophysics Data System (ADS)

Jurčišinová, E.; Jurčišin, M.

2018-05-01

We investigate in detail the process of formation of the multipeak low-temperature structure in the behavior of the specific heat capacity in frustrated magnetic systems in the framework of the exactly solvable antiferromagnetic spin-1 /2 Ising model with the multisite interaction in the presence of the external magnetic field on the kagome-like Husimi lattice. The behavior of the entropy of the model is studied and exact values of the residual entropies of all ground states are found. It is shown that the multipeak structure in the behavior of the specific heat capacity is related to the formation of the multilevel hierarchical ordering in the system of all ground states of the model. Direct relation between the maximal number of peaks in the specific heat capacity behavior and the number of independent interactions in studied frustrated magnetic system is identified. The mechanism of the formation of the multipeak structure in the specific heat capacity is described and studied in detail, and it is generalized to frustrated magnetic systems with arbitrary numbers of independent interactions.

Magnetic anisotropy and spin-flop transition of NiWO4 single crystals

NASA Astrophysics Data System (ADS)

Liu, C. B.; He, Z. Z.; Liu, Y. J.; Chen, R.; Shi, M. M.; Zhu, H. P.; Dong, C.; Wang, J. F.

2017-12-01

NiWO4 exhibits a spin chain structure built by magnetic Ni2+ ions, which may be considered as a one dimensional S = 1 system. In this work, large-sized single crystals of NiWO4 were successfully synthesized by a flux method and the crystal quality was confirmed by X-ray diffraction. Magnetic properties of obtained single crystals were studied by means of magnetic susceptibility and high field magnetization along crystallographic axes. The results demonstrate that NiWO4 is highly magnetic anisotropic and possesses a three-dimensional long range ordering below 60 K, where a spin flop transition can be observed at 17.5 T in applied magnetic fields along the magnetic easy axis (c-axis).

Temperature and magnetic-field driven dynamics in artificial magnetic square ice

DOE PAGES

Drouhin, Henri-Jean; Wegrowe, Jean-Eric; Razeghi, Manijeh; ...

2015-09-08

Artificial spin ices are often spoken of as being realisations of some of the celebrated vertex models of statistical mechanics, where the exact microstate of the system can be imaged using advanced magnetic microscopy methods. The fact that a stable image can be formed means that the system is in fact athermal and not undergoing the usual finite-temperature fluctuations of a statistical mechanical system. In this paper we report on the preparation of artificial spin ices with islands that are thermally fluctuating due to their very small size. The relaxation rate of these islands was determined using variable frequency focusedmore » magneto-optic Kerr measurements. We performed magnetic imaging of artificial spin ice under varied temperature and magnetic field using X-ray transmission microscopy which uses X-ray magnetic circular dichroism to generate magnetic contrast. Furthermore, we have developed an on-membrane heater in order to apply temperatures in excess of 700 K and have shown increased dynamics due to higher temperature. Due to the ‘photon-in, photon-out' method employed here, it is the first report where it is possible to image the microstates of an ASI system under the simultaneous application of temperature and magnetic field, enabling the determination of relaxation rates, coercivties, and the analysis of vertex population during reversal.« less

Temperature and magnetic-field driven dynamics in artificial magnetic square ice

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drouhin, Henri-Jean; Wegrowe, Jean-Eric; Razeghi, Manijeh

Artificial spin ices are often spoken of as being realisations of some of the celebrated vertex models of statistical mechanics, where the exact microstate of the system can be imaged using advanced magnetic microscopy methods. The fact that a stable image can be formed means that the system is in fact athermal and not undergoing the usual finite-temperature fluctuations of a statistical mechanical system. In this paper we report on the preparation of artificial spin ices with islands that are thermally fluctuating due to their very small size. The relaxation rate of these islands was determined using variable frequency focusedmore » magneto-optic Kerr measurements. We performed magnetic imaging of artificial spin ice under varied temperature and magnetic field using X-ray transmission microscopy which uses X-ray magnetic circular dichroism to generate magnetic contrast. Furthermore, we have developed an on-membrane heater in order to apply temperatures in excess of 700 K and have shown increased dynamics due to higher temperature. Due to the ‘photon-in, photon-out' method employed here, it is the first report where it is possible to image the microstates of an ASI system under the simultaneous application of temperature and magnetic field, enabling the determination of relaxation rates, coercivties, and the analysis of vertex population during reversal.« less

Ultrafast Magnetism of Multi-component Ferromagnets and Ferrimagnets on the Time Scale of the Exchange Interaction

NASA Astrophysics Data System (ADS)

Radu, Ilie

2012-02-01

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for the magnetic recording industry [1]. Exchange interaction is the strongest force in magnetism, being ultimately responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically excited on a timescale of or even faster than the exchange interaction? Here, we demonstrate that femtosecond (fs) measurements of ferrimagnetic and ferromagnetic alloys using X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism on timescales pertinent to the exchange interaction. In particular, we show that upon fs optical excitation the ultrafast spin reversal of GdFeCo - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state [2]. The latter emerges due to different dynamics of the Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporarily switch from an antiferromagnetic to a ferromagnetic type of ordering. In order to understand whether the observation of this temporarily decoupled and element-specific dynamics is a general phenomenon or just something strictly related to the case of ferrimagnetic GdFeCo, we have investigated the demagnetization of the archetypal ferromagnetic NiFe alloys. Essentially, we observe the same distinct magnetization dynamics of the constituent magnetic moments: Ni demagnetizes within ˜300 fs being much faster than the demagnetization of Fe of ˜800 fs. This distinct demagnetization behavior leads to an apparent decoupling of the Fe and Ni magnetic moments on a few hundreds of fs time scale, despite the strong exchange interaction of 260meV (˜16 fs) that couples them. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction [3]. [4pt] [1] A. Kirilyuk, A.V. Kimel and Th. Rasing, Rev. Mod. Phys. 82, 2731 (2010). [0pt] [2] I. Radu et al., Nature 472, 205 (2011). [0pt] [3] I. Radu et al., submitted (2011).

Nanoscale Electronic Transport Studies of Novel Strongly Correlated Materials

NASA Astrophysics Data System (ADS)

Hardy, Will J.

Strongly correlated materials are those in which the electron-electron and electron-lattice interactions play pivotal roles in determining many aspects of observable physical behavior, including the electronic and magnetic properties. In this thesis, I describe electronic transport studies of novel strongly correlated materials at the nanoscale. After introducing some basic concepts, briefly reviewing historical development of the field, and discussing the process of making measurements on small length scales, I detail experimental results from studies of four specific materials: two transition metal oxide systems, and two layered transition metal dichalcogenides with intercalated magnetic moments. The first system is a modified version of a classic strongly correlated material, vanadium dioxide (VO2), which here is doped with hydrogen to suppress its metal-insulator transition and stabilize a poorly metallic phase down to liquid helium temperatures. Doped VO2 nanowires, micron flakes, and thin films display magnetoresistance (MR) consistent with weak localization physics, along with mesoscopic resistance fluctuations over short distances, raising questions about how to model transport in bad-metal correlated systems. A second transition metal oxide system is considered next: Quantum wells in SrTiO3 sandwiched between layers of SmTiO3, in which anomalous voltage fluctuation behavior is observed in etched nanostructures at low temperatures. After well-understood alternative origins are ruled out, an explanation is proposed involving a time-varying thermopower due to two-level fluctuations of etching-induced defects. Next, I shift to the topic of layered itinerant magnetic materials with intercalated moments, starting with Fe0.28TaS 2, a hard ferromagnet (FM) with strong spin-orbit coupling. Here, a surprisingly large MR of nearly 70% is observed, an especially striking feature given that the closely related compounds at Fe intercalation fractions of 1/4 or 1/3 have MR nearly two orders of magnitude smaller. In the latter compounds, the Fe atoms are arranged in ordered superlattices, whereas for the 0.28 case, a portion of the Fe moments deviate from ordered arrangement and are relatively easily flipped by an external magnetic field to be anti-aligned with neighboring ordered Fe moments. This situation, combined with strong spin-orbit coupling, results in enhanced charge carrier scattering and greatly increased resistance. The thesis concludes with a study of a second layered magnetic material, V5S8 (structurally equivalent to V0.25VS2), which is found to have a magnetic field driven phase transition at low temperatures, believed to be from antiferromagnetism to paramagnetism. This transition is first order in thick crystals, but becomes second order as the crystal thickness decreases toward 10 nm. Together, the experiments described in this thesis highlight the complexity and diversity of strongly correlated materials, while showcasing the power of nanoscale electronic transport in delivering an improved understanding of these systems.

Magnetic and crystal structures of the honeycomb lattice Na2IrO3 and single layer Sr2IrO4

NASA Astrophysics Data System (ADS)

Ye, Feng

2013-03-01

5 d based iridates have recently attracted great attention due to the large spin-orbit coupling (SOC). It is now recognized that the SOC that competes with other relevant energies, particularly the on-site Coulomb interaction U, and have driven novel electronic and magnetic phases. Combining single crystal neutron and x-ray diffractions, we have investigated the magnetic and crystal structures of the honeycomb lattice Na2IrO3. The system orders magnetically below 18.1 K with Ir4+ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of 0.22 μB /Ir site. Such a configuration sharply contrasts the Neel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the IrO6 octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5 d-electron based honeycomb lattice. Neutron diffraction experiments are also performed to investigate the magnetic and crystal structure of the single layer iridate Sr2IrO4, where new structural information and spin order are obtained that is not available from previous neutron powder diffraction measurement. This work was sponsored in part by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.

Magnetic ordering of nickel hydroxide layers 30 Å apart obtained by intercalating dodecyl sulfate

NASA Astrophysics Data System (ADS)

Seehra, M. S.; Singh, V.

2013-09-01

The nature of magnetic ordering in quasi-2D layered hydroxide of Ni (LH-Ni-DS) with hexagonal structure and synthesized by intercalating dodecyl sulfate (DS) ligand, (C12H25OSO3)-, between the layers using a hydrothermal technique is investigated. The observation of (00l) peaks up to l = 8 in x-ray diffraction on the sample yields an interlayer spacing c ≃ 30.5 Å and a crystallite size ≃ 16 nm. Assignment of the lines observed in the FTIR spectra to the various groups of the DS ligand confirms the intercalation. From the analysis of detailed investigations of the temperature dependence of the magnetization M at different magnetic fields, ac susceptibilities at frequencies from 0.1 to 1 kHz, and electron magnetic resonance spectra at 9.28 GHz, it is concluded that LH-Ni-DS orders ferromagnetically at TC ≃ 23 K. This TC is about 45% higher than TC ≃ 16 K reported for LH-Ni-Ac with c ≃ 8.6 Å obtained by intercalating an acetate ligand between the layers. The roles of the interlayer dipolar interaction, magnetic anisotropy and exchange interactions in determining TC in LH-Ni-L systems for several ligands L yielding different c-axes are discussed.

Emergent order in the kagome Ising magnet Dy3Mg2Sb3O14

PubMed Central

Paddison, Joseph A. M.; Ong, Harapan S.; Hamp, James O.; Mukherjee, Paromita; Bai, Xiaojian; Tucker, Matthew G.; Butch, Nicholas P.; Castelnovo, Claudio; Mourigal, Martin; Dutton, S. E.

2016-01-01

The Ising model—in which degrees of freedom (spins) are binary valued (up/down)—is a cornerstone of statistical physics that shows rich behaviour when spins occupy a highly frustrated lattice such as kagome. Here we show that the layered Ising magnet Dy3Mg2Sb3O14 hosts an emergent order predicted theoretically for individual kagome layers of in-plane Ising spins. Neutron-scattering and bulk thermomagnetic measurements reveal a phase transition at ∼0.3 K from a disordered spin-ice-like regime to an emergent charge ordered state, in which emergent magnetic charge degrees of freedom exhibit three-dimensional order while spins remain partially disordered. Monte Carlo simulations show that an interplay of inter-layer interactions, spin canting and chemical disorder stabilizes this state. Our results establish Dy3Mg2Sb3O14 as a tuneable system to study interacting emergent charges arising from kagome Ising frustration. PMID:27996012

Fatty acyl chain order in lecithin model membranes determined from proton magnetic resonance.

PubMed

Bloom, M; Burnell, E E; MacKay, A L; Nichol, C P; Valic, M I; Weeks, G

1978-12-26

Proton magnetic resonance (1H NMR) has been used to compare the local orientational order of acyl chains in phospholipid bilayers of multilamellar and small sonicated vesicular membranes of dipalmitoyllecithin (DPL) at 50 degrees C and egg yolk lecithin (EYL) at 31 degrees C. The orientational order of the multilamellar systems was characterized using deuterium magnetic resonance order parameters and 1H NMR second moments. 1H NMR line shapes in the vesicle samples were calculated using vesicle size distributions, determined directly using electron microscopy, and a theory of motional narrowing, which takes into account the symmetry properties of the bilayer systems. The predicted non-Lorentzian line shapes and widths were found to be in good agreement with experimental results, indicating that the local orientational order (called "packing" by many workers) in the bilayers of small vesicles and in multilamellar membranes is substantially the same. This results was found to be true not only for the largest 1H NMR line associated with the nonterminal methylene protons but also for the resolved 1H NMR lines due to the alpha-CH2 and the terminal CH3 positions on the acyl chain. Analysis of the vesicle 1H NMR spectra of EYL taken with different medium viscosities yielded a value of approximately 4 X 10(-8) cm2 s-1 for the lateral diffusion constant of the phospholipid molecules at 31 degrees C.

Cooperative magnetic behaviour in the new valence fluctuating compound Ce2Rh3Ge

NASA Astrophysics Data System (ADS)

Falkowski, M.; Strydom, A. M.

2015-10-01

In this study we report the physical properties of the new ternary compound Ce2Rh3Ge that crystallizes in the rhombohedral, triple hexagonal MgCu2-type of structure. The electronic ground state properties of Ce2Rh3Ge were characterized by magnetic susceptibility, specific heat, electrical resistivity and thermal transport measurements. The results indicate the presence of short range magnetic interaction, probably of ferromagnetic origin below T C  =  4 K. The shape of χ -1(T) deviates from the Curie-Weiss behavior with a broad minimum at about T\\min{{χ-1}}   =  450 K reminiscent of valence fluctuating cerium systems. At T  =  10 K, the magnetic part of the resistivity ρ 4 f (T) exhibits a shallow minimum followed by increase of resistivity ρ(T) \\propto   -lnT, which hints at a substantial Kondo screening effect. Ce2Rh3Ge belongs to a small group of strongly correlated cerium compounds in which the two competing effects of Kondo and RKKY interactions produce long-range magnetic order from strongly hybridized and intermediate-valent 4 f spins. At sufficiently low temperatures Ce2Rh3Ge scales well with the Kadowaki-Woods ratio A/γ 2 and the value of the Wilson ratio χ(T  →  0)/γ found for this compound classifies it as a mixed-valence compound. The presence of valence fluctuation and magnetic order it is rare for these attributes to be found simultaneously in same compound, in same temperature range. In our opinion a novelty of presented results of Ce2Rh3Ge is that this compound adds a new member to a small but growing class of systems bearing a strongly mixed- or intermediate-valent 4 f magnetic moment, but in which the lattice of spins nevertheless end up finding it possible to order magnetically.

Microscopic Description of Electric and Magnetic Toroidal Multipoles in Hybrid Orbitals

NASA Astrophysics Data System (ADS)

Hayami, Satoru; Kusunose, Hiroaki

2018-03-01

We derive the quantum-mechanical operator expressions of multipoles under the space-time inversion group. We elucidate that electric and magnetic toroidal multipoles, in addition to ordinary non-toroidal ones, are fundamental pieces to express arbitrary electronic degrees of freedom. We show that electric (magnetic) toroidal multipoles higher than the dipole (monopole) can become active in a hybridized-orbital system. We also demonstrate emergent cross-correlated couplings between the electric, magnetic, and elastic degrees of freedom, such as magneto-electric and magneto(electro)-elastic coupling, under toroidal multipole orders.

Magnetic susceptibility characterisation of superparamagnetic microspheres

NASA Astrophysics Data System (ADS)

Grob, David Tim; Wise, Naomi; Oduwole, Olayinka; Sheard, Steve

2018-04-01

The separation of magnetic materials in microsystems using magnetophoresis has increased in popularity. The wide variety and availability of magnetic beads has fuelled this drive. It is important to know the magnetic characteristics of the microspheres in order to accurately use them in separation processes integrated on a lab-on-a-chip device. To investigate the magnetic susceptibility of magnetic microspheres, the magnetic responsiveness of three types of Dynabeads microspheres were tested using two different approaches. The magnetophoretic mobility of individual microspheres is studied using a particle tracking system and the magnetization of each type of Dynabeads microsphere is measured using SQUID relaxometry. The magnetic beads' susceptibility is obtained at four different applied magnetic fields in the range of 38-70 mT for both the mobility and SQUID measurements. The susceptibility values in both approaches show a consistent magnetic field dependence.

The forward modelling and analysis of magnetic field on the East Asia area using tesseroids

NASA Astrophysics Data System (ADS)

Chen, Z.; Meng, X.; Xu, G.

2017-12-01

As the progress of airborne and satellite magnetic survey, high-resolution magnetic data could be measured at different scale. In order to test and improve the accuracy of the existing crustal model, the forward modeling method is usually used to simulate the magnetic field of the lithosphere. Traditional models to forward modelling the magnetic field are based on the Cartesian coordinate system, and are always used to calculate the magnetic field of the local and small area. However, the Cartesian coordinate system is not an ideal choice for calculating the magnetic field of the global or continental area at the height of the satellite and Earth's curvature cannot be ignored in this situation. The spherical element (called tesseroids) can be used as a model element in the spherical coordinate system to solve this problem. On the basis of studying the principle of this forward method, we focus the selection of data source and the mechanism of adaptive integration. Then we calculate the magnetic anomaly data of East Asia area based on the model Crust1.0. The results presented the crustal susceptibility distribution, which was well consistent with the basic tectonic features in the study area.

Chemically engineered graphene-based 2D organic molecular magnet.

PubMed

Hong, Jeongmin; Bekyarova, Elena; de Heer, Walt A; Haddon, Robert C; Khizroev, Sakhrat

2013-11-26

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization.

Correlations Between Magnetic Flux and Levitation Force of HTS Bulk Above a Permanent Magnet Guideway

NASA Astrophysics Data System (ADS)

Huang, Huan; Zheng, Jun; Zheng, Botian; Qian, Nan; Li, Haitao; Li, Jipeng; Deng, Zigang

2017-10-01

In order to clarify the correlations between magnetic flux and levitation force of the high-temperature superconducting (HTS) bulk, we measured the magnetic flux density on bottom and top surfaces of a bulk superconductor while vertically moving above a permanent magnet guideway (PMG). The levitation force of the bulk superconductor was measured simultaneously. In this study, the HTS bulk was moved down and up for three times between field-cooling position and working position above the PMG, followed by a relaxation measurement of 300 s at the minimum height position. During the whole processes, the magnetic flux density and levitation force of the bulk superconductor were recorded and collected by a multipoint magnetic field measurement platform and a self-developed maglev measurement system, respectively. The magnetic flux density on the bottom surface reflected the induced field in the superconductor bulk, while on the top, it reveals the penetrated magnetic flux. The results show that the magnetic flux density and levitation force of the bulk superconductor are in direct correlation from the viewpoint of inner supercurrent. In general, this work is instructive for understanding the connection of the magnetic flux density, the inner current density and the levitation behavior of HTS bulk employed in a maglev system. Meanwhile, this magnetic flux density measurement method has enriched present experimental evaluation methods of maglev system.

Measurement of the adhesion force between particles for high gradient magnetic separation of pneumatic conveyed powder products

NASA Astrophysics Data System (ADS)

Senkawa, K.; Nakai, Y.; Mishima, F.; Akiyama, Y.; Nishijima, S.

2011-11-01

In the industrial plants such as foods, medicines or industrial materials, there are big amount of issues on contamination by metallic wear debris originated from pipes of manufacturing lines. In this study, we developed a high gradient magnetic separation system (HGMS) under the dry process by using superconducting magnet to remove the ferromagnetic particles. One of the major problems of dry HGMS systems is, however, the blockage of magnetic filter caused by particle coagulation or deposition. In order to actualize the magnetic separation without blockage, we introduced pneumatic conveyance system as a new method to feed the powder. It is important to increase the drag force acting on the sufficiently dispersed particles, which require strong magnetic fields. To generate the strong magnetic fields, HGMS technique was examined which consists of a magnetic filter and a superconducting solenoid magnet. As a result of the magnetic separation experiment, it was shown that the separation efficiency changes due to the difference of the cohesive property of the particles. On the basis of the result, the adhesion force which acts between the ferromagnetic particles and the medium particles used for the magnetic separation was measured by Atomic Force Microscope (AFM), and cohesion of particles was studied from the aspect of interparticle interaction. We assessed a suitable flow velocity for magnetic separation according to the cohesive property of each particle based on the result.

Magnetic correlations in the intermetallic antiferromagnet Nd3Co4Sn13

NASA Astrophysics Data System (ADS)

Wang, C. W.; Lin, J. W.; Lue, C. S.; Liu, H. F.; Kuo, C. N.; Mole, R. A.; Gardner, J. S.

2017-11-01

Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd3Co4Sn13. At room temperature Nd3Co4Sn13 has a cubic, Pm-3n structure similar to Yb3Rh4Sn13. Antiferromagnetic interactions between, Nd3+ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q  =  (0 0 0) with an ordered moment of 1.78(2) µ B at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ~2.4 µ B at 350 mK. The average magnetic Nd3+ configuration corresponds to the 3D irreducible representation Γ7. This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV.

Structural changes concurrent with ferromagnetic transition

NASA Astrophysics Data System (ADS)

Yang, Sen; Bao, Hui-Xin; Zhou, Chao; Wang, Yu; Ren, Xiao-Bing; Song, Xiao-Ping; Yoshitaka, Matsushita; Yoshio, Katsuya; Masahiko, Tanaka; Keisuke, Kobayashi

2013-04-01

Ferromagnetic transition has generally been considered to involve only an ordering of magnetic moment with no change in the host crystal structure or symmetry, as evidenced by a wealth of crystal structure data from conventional X-ray diffractometry (XRD). However, the existence of magnetostriction in all known ferromagnetic systems indicates that the magnetic moment is coupled to the crystal lattice; hence there is a possibility that magnetic ordering may cause a change in crystal structure. With the development of high-resolution synchrotron XRD, more and more magnetic transitions have been found to be accompanied by simultaneous structural changes. In this article, we review our recent progress in understanding the structural change at a ferromagnetic transition, including synchrotron XRD evidence of structural changes at the ferromagnetic transition, a phenomenological theory of crystal structure changes accompanying ferromagnetic transitions, new insight into magnetic morphotropic phase boundaries (MPB) and so on. Two intriguing implications of non-centric symmetry in the ferromagnetic phase and the first-order nature of ferromagnetic transition are also discussed here. In short, this review is intended to give a self-consistent and logical account of structural change occurring simultaneously with a ferromagnetic transition, which may provide new insight for developing highly magneto-responsive materials.

Role of 4 f electrons in crystallographic and magnetic complexity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pathak, Arjun K.; Paudyal, Durga; Mudryk, Yaroslav

2017-08-09

Here, the functionality of many magnetic materials critically depends on first manipulating and then taking advantage of highly nonlinear changes of properties that occur during phase transformations. Unique to lanthanides, property-defining 4f electrons are highly localized and, as commonly accepted, play little to no role in chemical bonding. Yet here we demonstrate that the competition between 4f-electron energy landscapes of Dy (4f 9) and Er (4f 11) is the key element of the puzzle required to explain complex interplay of magnetic and structural features observed in Er 1–xDy xCo 2, and likely many other mixed lanthanide systems. Unlike the parentmore » binaries—DyCo 2 and ErCo 2—Er 1–xDy xCo 2 exhibits two successive magnetostructural transitions: a first order at TC, followed by a second order in the ferrimagnetically ordered state. Supported by first-principles calculations, our results offer new opportunities for targeted design of magnetic materials with multiple functionalities, and also provide a critical insight into the role of 4f electrons in controlling the magnetism and structure of lanthanide intermetallics.« less

Pressure measurements with a precision of 0.001 ppm in magnetic fields at low temperatures

NASA Astrophysics Data System (ADS)

Miura, Y.; Matsushima, N.; Ando, T.; Kuno, S.; Inoue, S.; Ito, K.; Mamiya, T.

1993-11-01

Pressure measurements made by an ac bridge technique with a precision of 0.001 ppm in magnetic fields at low temperatures using a Straty-Adams type gauge are described. In order to improve the sensitivity and the long-term stability of the bridge system, coaxial cables without dielectric insulator were developed, with a small cable capacitance temperature coefficient of the impedance. This pressure measurement system has a sensitivity of dP/P˜5×10-10 and a long-term stability of dP/P˜2.4×10-9 over 18 h. This is especially useful for measurements such as electric and magnetic susceptibility measurements in magnetic fields at low temperatures requiring a high precision.

Electric field control of magnon-induced magnetization dynamics in multiferroics.

PubMed

Risinggård, Vetle; Kulagina, Iryna; Linder, Jacob

2016-08-24

We consider theoretically the effect of an inhomogeneous magnetoelectric coupling on the magnon-induced dynamics of a ferromagnet. The magnon-mediated magnetoelectric torque affects both the homogeneous magnetization and magnon-driven domain wall motion. In the domains, we predict a reorientation of the magnetization, controllable by the applied electric field, which is almost an order of magnitude larger than that observed in other physical systems via the same mechanism. The applied electric field can also be used to tune the domain wall speed and direction of motion in a linear fashion, producing domain wall velocities several times the zero field velocity. These results show that multiferroic systems offer a promising arena to achieve low-dissipation magnetization rotation and domain wall motion by exciting spin-waves.

Cavity magnon polaritons with lithium ferrite and three-dimensional microwave resonators at millikelvin temperatures

NASA Astrophysics Data System (ADS)

Goryachev, Maxim; Watt, Stuart; Bourhill, Jeremy; Kostylev, Mikhail; Tobar, Michael E.

2018-04-01

Single crystal lithium ferrite (LiFe) spheres of sub-mm dimension are examined at mK temperatures, microwave frequencies, and variable dc magnetic field, for use in hybrid quantum systems and condensed matter and fundamental physics experiments. Strong coupling regimes of the photon-magnon interaction (cavity magnon polariton quasiparticles) were observed with coupling strength of up to 250 MHz at 9.5 GHz (2.6%) with magnon linewidths of order 4 MHz (with potential improvement to sub-MHz values). We show that the photon-magnon coupling can be significantly improved and exceed that of the widely used yttrium iron garnet crystal, due to the small unit cell of LiFe, allowing twice the spins per unit volume. Magnon mode softening was observed at low dc fields and, combined with the normal Zeeman effect, creates magnon spin-wave modes that are insensitive to first-order magnetic-field fluctuations. This effect is observed in the Kittel mode at 5.5 GHz (and another higher order mode at 6.5 GHz) with a dc magnetic field close to 0.19 tesla. We show that if the cavity is tuned close to this frequency, the magnon polariton particles exhibit an enhanced range of strong coupling and insensitivity to magnetic field fluctuations with both first-order and second-order insensitivity to magnetic field as a function of frequency (double magic point clock transition), which could potentially be exploited in cavity QED experiments.

A high-order positivity-preserving single-stage single-step method for the ideal magnetohydrodynamic equations

NASA Astrophysics Data System (ADS)

Christlieb, Andrew J.; Feng, Xiao; Seal, David C.; Tang, Qi

2016-07-01

We propose a high-order finite difference weighted ENO (WENO) method for the ideal magnetohydrodynamics (MHD) equations. The proposed method is single-stage (i.e., it has no internal stages to store), single-step (i.e., it has no time history that needs to be stored), maintains a discrete divergence-free condition on the magnetic field, and has the capacity to preserve the positivity of the density and pressure. To accomplish this, we use a Taylor discretization of the Picard integral formulation (PIF) of the finite difference WENO method proposed in Christlieb et al. (2015) [23], where the focus is on a high-order discretization of the fluxes (as opposed to the conserved variables). We use the version where fluxes are expanded to third-order accuracy in time, and for the fluid variables space is discretized using the classical fifth-order finite difference WENO discretization. We use constrained transport in order to obtain divergence-free magnetic fields, which means that we simultaneously evolve the magnetohydrodynamic (that has an evolution equation for the magnetic field) and magnetic potential equations alongside each other, and set the magnetic field to be the (discrete) curl of the magnetic potential after each time step. In this work, we compute these derivatives to fourth-order accuracy. In order to retain a single-stage, single-step method, we develop a novel Lax-Wendroff discretization for the evolution of the magnetic potential, where we start with technology used for Hamilton-Jacobi equations in order to construct a non-oscillatory magnetic field. The end result is an algorithm that is similar to our previous work Christlieb et al. (2014) [8], but this time the time stepping is replaced through a Taylor method with the addition of a positivity-preserving limiter. Finally, positivity preservation is realized by introducing a parameterized flux limiter that considers a linear combination of high and low-order numerical fluxes. The choice of the free parameter is then given in such a way that the fluxes are limited towards the low-order solver until positivity is attained. Given the lack of additional degrees of freedom in the system, this positivity limiter lacks energy conservation where the limiter turns on. However, this ingredient can be dropped for problems where the pressure does not become negative. We present two and three dimensional numerical results for several standard test problems including a smooth Alfvén wave (to verify formal order of accuracy), shock tube problems (to test the shock-capturing ability of the scheme), Orszag-Tang, and cloud shock interactions. These results assert the robustness and verify the high-order of accuracy of the proposed scheme.

Magnetic Properties of Restacked 2D Spin 1/2 honeycomb RuCl3 Nanosheets.

PubMed

Weber, Daniel; Schoop, Leslie M; Duppel, Viola; Lippmann, Judith M; Nuss, Jürgen; Lotsch, Bettina V

2016-06-08

Spin 1/2 honeycomb materials have gained substantial interest due to their exotic magnetism and possible application in quantum computing. However, in all current materials out-of-plane interactions are interfering with the in-plane order, hence a true 2D magnetic honeycomb system is still in demand. Here, we report the exfoliation of the magnetic semiconductor α-RuCl3 into the first halide monolayers and the magnetic characterization of the spin 1/2 honeycomb arrangement of turbostratically stacked RuCl3 monolayers. The exfoliation is based on a reductive lithiation/hydration approach, which gives rise to a loss of cooperative magnetism due to the disruption of the spin 1/2 state by electron injection into the layers. The restacked, macroscopic pellets of RuCl3 layers lack symmetry along the stacking direction. After an oxidative treatment, cooperative magnetism similar to the bulk is restored. The oxidized pellets of restacked single layers feature a magnetic transition at TN = 7 K if the field is aligned parallel to the ab-plane, while the magnetic properties differ from bulk α-RuCl3 if the field is aligned perpendicular to the ab-plane. The deliberate introduction of turbostratic disorder to manipulate the magnetic properties of RuCl3 is of interest for research in frustrated magnetism and complex magnetic order as predicted by the Kitaev-Heisenberg model.

Observation of anomalous Hall effect in a non-magnetic two-dimensional electron system

PubMed Central

Maryenko, D.; Mishchenko, A. S.; Bahramy, M. S.; Ernst, A.; Falson, J.; Kozuka, Y.; Tsukazaki, A.; Nagaosa, N.; Kawasaki, M.

2017-01-01

Anomalous Hall effect, a manifestation of Hall effect occurring in systems without time-reversal symmetry, has been mostly observed in ferromagnetically ordered materials. However, its realization in high-mobility two-dimensional electron system remains elusive, as the incorporation of magnetic moments deteriorates the device performance compared to non-doped structure. Here we observe systematic emergence of anomalous Hall effect in various MgZnO/ZnO heterostructures that exhibit quantum Hall effect. At low temperatures, our nominally non-magnetic heterostructures display an anomalous Hall effect response similar to that of a clean ferromagnetic metal, while keeping a large anomalous Hall effect angle θAHE≈20°. Such a behaviour is consistent with Giovannini–Kondo model in which the anomalous Hall effect arises from the skew scattering of electrons by localized paramagnetic centres. Our study unveils a new aspect of many-body interactions in two-dimensional electron systems and shows how the anomalous Hall effect can emerge in a non-magnetic system. PMID:28300133

Binary neutron star mergers and short gamma-ray bursts: Effects of magnetic field orientation, equation of state, and mass ratio

NASA Astrophysics Data System (ADS)

Kawamura, Takumu; Giacomazzo, Bruno; Kastaun, Wolfgang; Ciolfi, Riccardo; Endrizzi, Andrea; Baiotti, Luca; Perna, Rosalba

2016-09-01

We present fully general-relativistic magnetohydrodynamic simulations of the merger of binary neutron star (BNS) systems. We consider BNSs producing a hypermassive neutron star (HMNS) that collapses to a spinning black hole (BH) surrounded by a magnetized accretion disk in a few tens of ms. We investigate whether such systems may launch relativistic jets and hence power short gamma-ray bursts. We study the effects of different equations of state (EOSs), different mass ratios, and different magnetic field orientations. For all cases, we present a detailed investigation of the matter dynamics and of the magnetic field evolution, with particular attention to its global structure and possible emission of relativistic jets. The main result of this work is that we observe the formation of an organized magnetic field structure. This happens independently of EOS, mass ratio, and initial magnetic field orientation. We also show that those models that produce a longer-lived HMNS lead to a stronger magnetic field before collapse to a BH. Such larger fields make it possible, for at least one of our models, to resolve the magnetorotational instability and hence further amplify the magnetic field in the disk. However, by the end of our simulations, we do not (yet) observe a magnetically dominated funnel nor a relativistic outflow. With respect to the recent simulations of Ruiz et al. [Astrophys. J. 824, L6 (2016)], we evolve models with lower and more plausible initial magnetic field strengths and (for computational reasons) we do not evolve the accretion disk for the long time scales that seem to be required in order to see a relativistic outflow. Since all our models produce a similar ordered magnetic field structure aligned with the BH spin axis, we expect that the results found by Ruiz et al. (who only considered an equal-mass system with an ideal fluid EOS) should be general and—at least from a qualitative point of view—independent of the mass ratio, magnetic field orientation, and EOS.

Zn-site Substitution Effect in YbCo2Zn20

NASA Astrophysics Data System (ADS)

Kobayashi, Riki; Takamura, Haruki; Higa, Yasuyuki; Ikeda, Yoichi; Matsubayashi, Kazuyuki; Uwatoko, Yoshiya; Yoshizawa, Hideki; Aso, Naofumi

2017-04-01

We have investigated the substitution effect of YbCo2(Zn1-xTx)20 (T = Cu, Ga, and Cd) systems by using the experiments of X-ray powder diffraction (XRPD), specific heat, magnetic susceptibility, magnetization, and electrical resistivity in order to find out a material that approaches a quantum critical point by chemical pressure. The XRPD and electrical resistivity measurements clarify that the Cu-substitution makes the lattice constants shrink and keeps the magnetic electrical resistivity high, while the Ga- and the Cd-substitution show opposite relation of the Cu-substitution. However, we could not detect clear substitution effect in the specific heat, magnetic susceptibility, and magnetization measurements of Cu-substitution system within our experiments. It is necessary that to study the Cu-substitution samples that have higher x value at lower temperature.

Lattice dynamics, elasticity and magnetic abnormality in ordered crystalline alloys Fe3Pt at high pressures

NASA Astrophysics Data System (ADS)

Cheng, Tai-min; Yu, Guo-Liang; Su, Yong; Ge, Chong-Yuan; Zhang, Xin-Xin; Zhu, Lin; Li, Lin

2018-05-01

The ordered crystalline Invar alloy Fe3Pt is in a special magnetic critical state, under which the lattice dynamic stability of the system is extremely sensitive to external pressures. We studied the pressure dependence of enthalpy and magnetism of Fe3Pt in different crystalline alloys by using the first-principles projector augmented-wave method based on the density functional theory. Results show that the P4/mbm structure is the ground state structure and is more stable relative to other structures at pressures below 18.54 GPa. The total magnetic moments of L12, I4/mmm and DO22 structures decrease rapidly with pressure and oscillate near the ferromagnetic collapse critical pressure. At the pressure of 43 GPa, the ferrimagnetic property in DO22 structure becomes apparently strengthened and its volume increases rapidly. The lattice dynamics calculation for L12 structures at high pressures shows that the spontaneous magnetization of the system in ferromagnetic states induces the softening of the transverse acoustic phonon TA1 (M), and there exists a strong spontaneous volume magnetostriction at pressures below 26.95 GPa. Especially, the lattice dynamics stability is sensitive to pressure, in the pressure range between the ferromagnetic collapse critical pressure (41.9 GPa) and the magnetism completely disappearing pressure (57.25 GPa), and near the pressure of phase transition from L12 to P4/mbm structure (27.27 GPa). Moreover, the instability of magnetic structure leads to a prominent elastic modulus oscillation, and the spin polarizability of electrons near the Fermi level is very sensitive to pressures in that the pressure range. The pressure induces the stability of the phonon spectra of the system at pressures above 57.25 GPa.

Engineering studies of vectorcardiographs in blood pressure measuring systems, appendix 3

NASA Technical Reports Server (NTRS)

Mark, R. G.

1975-01-01

The following subjects were covered: (1) ASM80 manual, (2) signal preprocessing as an aid to on-line EKG analysis, and (3) high speed evaluation of magnetic tape recordings of electrocardiograms. A description of the ASM80 symbolic assembly program for the INTEL 8080 microprocessor and a user's manual were presented. The capability of three redundancy reduction algorithms to produce adequate representations of electrocardiographic data was examined. A hardware device was constructed which carried out zero order interpolation on a signal. Examination of the zero order interpolators reconstructed signal indicated that this representation was adequate for analysis of rhythm. A system to analyze magnetic tapes of electrocardiograms recorded over 24 hour intervals was designed. The recordings are sampled 200 times per second using a Nova computer and a special interface system. This system was tested on several recordings of clinical data, containing over 75 premature ventricular contractions, each one of which was flagged.

New Methods of Low-Field Magnetic Resonance Imaging for Application to Traumatic Brain Injury

DTIC Science & Technology

2016-04-01

the need for high power radio - frequency (RF) to saturate the electron spins. Addition- ally, as EPR frequencies are two orders of magnitude higher...Crozier S. Electromechanical design and construction of a rotating radio - frequency coil system for applications in magnetic resonance. IEEE Trans Biomed...1 Award Number: W81XWH- 11 -2-0076 TITLE: New Methods of Low-Field Magnetic Resonance Imaging for Application to Traumatic Brain Injury PRINCIPAL

Electromagnetic imaging with an arbitrarily oriented magnetic dipole

NASA Astrophysics Data System (ADS)

Guillemoteau, Julien; Sailhac, Pascal; Behaegel, Mickael

2013-04-01

We present the theoretical background for the geophysical EM analysis with arbitrarily oriented magnetic dipoles. The first application of such a development is that we would now be able to correct the data when they are not acquired in accordance to the actual interpretation methods. In order to illustrate this case, we study the case of airborne TEM measurements over an inclined ground. This context can be encountered if the measurements are made in mountain area. We show in particular that transient central loop helicopter borne magnetic data should be corrected by a factor proportional to the angle of the slope under the system. In addition, we studied the sensitivity function of a grounded multi-angle frequency domain system. Our development leads to a general Jacobian kernel that could be used for all the induction number and all the position/orientation of both transmitter and receiver in the air layer. Indeed, if one could design a system controlling the angles of Tx and Rx, the present development would allow to interpret such a data set and enhance the ground analysis, especially in order to constrain the 3D anisotropic inverse problem.

A Novel Integrated Structure with a Radial Displacement Sensor and a Permanent Magnet Biased Radial Magnetic Bearing

PubMed Central

Sun, Jinji; Zhang, Yin

2014-01-01

In this paper, a novel integrated structure is proposed in order to reduce the axial length of the high speed of a magnetically suspended motor (HSMSM) to ensure the maximum speed, which combines radial displacement sensor probes and the permanent magnet biased radial magnetic bearing in HSMSM. The sensor probes are integrated in the magnetic bearing, and the sensor preamplifiers are placed in the control system of the HSMSM, separate from the sensor probes. The proposed integrated structure can save space in HSMSMs, improve the working frequency, reduce the influence of temperature on the sensor circuit, and improve the stability of HSMSMs. PMID:24469351

Electronic structure and magnetism of titanium substituted Cd3P2: An ab-initio study

NASA Astrophysics Data System (ADS)

Jaiganesh, G.; Jaya, S. Mathi

2018-05-01

Using the ab-initio computations that are based on the density functional theory, we have investigated the magnetism and electronic properties of one and two Ti atom substituted Cd3P2 compound. The magnetic stability of the substituted compounds was obtained by analyzing the minimum total energies in nonmagnetic, ferromagnetic and antiferromagnetic phases. Our results indicated the formation of magnetic order in one and two Ti atom substituted Cd3P2 as well as metallic characteristics in these systems. A significant value of the magnetic moment of Ti atom is observed from our calculations. We further find that the neighboring Cd and P atoms too acquire a small magnetic moment.

Simulation of High-Resolution Magnetic Resonance Images on the IBM Blue Gene/L Supercomputer Using SIMRI

DOE PAGES

Baum, K. G.; Menezes, G.; Helguera, M.

2011-01-01

Medical imaging system simulators are tools that provide a means to evaluate system architecture and create artificial image sets that are appropriate for specific applications. We have modified SIMRI, a Bloch equation-based magnetic resonance image simulator, in order to successfully generate high-resolution 3D MR images of the Montreal brain phantom using Blue Gene/L systems. Results show that redistribution of the workload allows an anatomically accurate 256 3 voxel spin-echo simulation in less than 5 hours when executed on an 8192-node partition of a Blue Gene/L system.

Simulation of High-Resolution Magnetic Resonance Images on the IBM Blue Gene/L Supercomputer Using SIMRI.

PubMed

Baum, K G; Menezes, G; Helguera, M

2011-01-01

Medical imaging system simulators are tools that provide a means to evaluate system architecture and create artificial image sets that are appropriate for specific applications. We have modified SIMRI, a Bloch equation-based magnetic resonance image simulator, in order to successfully generate high-resolution 3D MR images of the Montreal brain phantom using Blue Gene/L systems. Results show that redistribution of the workload allows an anatomically accurate 256(3) voxel spin-echo simulation in less than 5 hours when executed on an 8192-node partition of a Blue Gene/L system.

Progress towards extreme attitude testing with Magnetic Suspension and Balance Systems

NASA Technical Reports Server (NTRS)

Britcher, Colin P.; Parker, David H.

1988-01-01

Progress is reported in a research effort aimed towards demonstration of the feasibility of suspension and aerodynamic testing of models at high angles of attack in wind tunnel Magnetic Suspension and Balance Systems. Extensive modifications, described in this paper, have been made to the Southampton University suspension system in order to facilitate this work. They include revision of electromagnet configuration, installation of all-new position sensors and expansion of control system programs. An angle of attack range of 0 to 90 deg is expected for axisymmetric models. To date, suspension up to 80 deg angle of attack has been achieved.

Photovoltaic-wind hybrid system for permanent magnet DC motor

NASA Astrophysics Data System (ADS)

Nasir, M. N. M.; Lada, M. Y.; Baharom, M. F.; Jaafar, H. I.; Ramani, A. N.; Sulaima, M. F.

2015-05-01

Hybrid system of Photovoltaic (PV) - Wind turbine (WT) generation has more advantages and reliable compared to PV or wind turbine system alone. The aim of this paper is to model and design hybrid system of PV-WT supplying 100W permanent-magnet dc motor. To achieve the objective, both of PV and WT are connected to converter in order to get the same source of DC supply. Then both sources were combined and straightly connected to 100W permanent magnet dc motor. All the works in this paper is only applied in circuit simulator by using Matlab Simulink. The output produced from each converter is expected to be suit to the motor specification. The output produced from each renewable energy system is as expected to be high as it can support the motor if one of them is breakdown

Monte Carlo simulations of kagome lattices with magnetic dipolar interactions

NASA Astrophysics Data System (ADS)

Plumer, Martin; Holden, Mark; Way, Andrew; Saika-Voivod, Ivan; Southern, Byron

Monte Carlo simulations of classical spins on the two-dimensional kagome lattice with only dipolar interactions are presented. In addition to revealing the sixfold-degenerate ground state, the nature of the finite-temperature phase transition to long-range magnetic order is discussed. Low-temperature states consisting of mixtures of degenerate ground-state configurations separated by domain walls can be explained as a result of competing exchange-like and shape-anisotropy-like terms in the dipolar coupling. Fluctuations between pairs of degenerate spin configurations are found to persist well into the ordered state as the temperature is lowered until locking in to a low-energy state. Results suggest that the system undergoes a continuous phase transition at T ~ 0 . 43 in agreement with previous MC simulations but the nature of the ordering process differs. Preliminary results which extend this analysis to the 3D fcc ABC-stacked kagome systems will be presented.

Studies of superconducting materials with muon spin rotation

NASA Technical Reports Server (NTRS)

Davis, Michael R.; Stronach, Carey E.; Kossler, W. J.; Schone, H. E.; Yu, X. H.; Uemura, Y. J.; Sternlieb, B. J.; Kempton, J. R.; Oostens, J.; Lankford, W. F.

1989-01-01

The muon spin rotation/relaxation technique was found to be an exceptionally effective means of measuring the magnetic properties of superconductors, including the new high temperature superconductor materials, at the microscopic level. The technique directly measures the magnetic penetration depth (type II superconductors (SC's)) and detects the presence of magnetic ordering (antiferromagnetism or spin-glass ordering were observed in some high temperature superconductor (HTSC's) and in many closely related compounds). Extensive studies of HTSC materials were conducted by the Virginia State University - College of William and Mary - Columbia University collaboration at Brookhaven National Laboratory and TRIUMF (Vancouver). A survey of LaSrCuO and YBaCaCuO systems shows an essentially linear relationship between the transition temperature T(sub c) and the relaxation rate. This appears to be a manifestation of the proportionality between T(sub c) and the Fermi energy, which suggests a high energy scale for the SC coupling, and which is not consistent with the weak coupling of phonon-mediated SC. Studies of LaCuO and YBaCuO parent compounds show clear evidence of antiferromagnetism. YBa2Cu(3-x)CO(x)O7 shows the simultaneous presence of spin-glass magnetic ordering and superconductivity. Three-dimensional SC, (Ba, K) BiO3, unlike the layered CuO-based compounds, shows no suggestion of magnetic ordering. Experimental techniques and theoretical implications are discussed.

Gastric activity studies using a magnetic tracer.

PubMed

Cordova-Fraga, T; Bernal-Alvarado, J J; Gutierrez-Juarez, G; Sosa, M; Vargas-Luna, M

2004-10-01

A magnetic pulse generator has been set up in order to study gastric activity. Two coils 1.05 m in diameter, arranged in a Helmholtz configuration, were used. The system generated magnetic field pulses higher than 15 mT, of duration 17.3+/-1.2 ms. Measurements were performed in 11 male volunteers, with average age 29.3+/-6.4 years and body mass index 26.0+/-4.8 kg m(-2). Magnetite (Fe3O4) particles with diameters from 75 to 125 microm were used as magnetic tracers, which were mixed in 250 ml of yogurt in concentrations from 2 to 5 g. Signals were registered by using a high speed 3 axis fluxgate digital magnetometer and processed to determine the relaxation of the magnetic tracers by fitting a first-order exponential function to the data, a mean relaxation constant K = 116+/-40 s(-1) was obtained. Also, an average gastric peristaltic frequency was measured; a value of 3.2+/-0.3 cpm was determined.

Ab-initio calculation and experimental observation of room temperature ferromagnetism in 50 keV nitrogen implanted rutile TiO2

NASA Astrophysics Data System (ADS)

Luitel, Homnath; Chakrabarti, Mahuya; Sarkar, A.; Dechoudhury, S.; Bhowmick, D.; Naik, V.; Sanyal, D.

2018-02-01

Room temperature magnetic properties of 50 keV N4+ ion beam implanted rutile TiO2 have been theoretically and experimentally studied. Ab-initio calculation under the frame work of density functional theory has been carried out to study the magnetic properties of the different possible nitrogen related defects in TiO2. Spin polarized density of states calculation suggests that both Ninst and NO can induce ferromagnetic ordering in rutile TiO2. In both cases the 2p orbital electrons of nitrogen atom give rise to the magnetic moment in TiO2. The possibility of the formation of N2 molecule in TiO2 system is also studied but in this case no significant magnetic moment has been observed. The magnetic measurements, using SQUID magnetometer, results a ferromagnetic ordering even at room temperature for the 50 keV N4+ ion beam implanted rutile TiO2.

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20 K.

PubMed

Friedländer, Stefan; Liu, Jinxuan; Addicoat, Matt; Petkov, Petko; Vankova, Nina; Rüger, Robert; Kuc, Agnieszka; Guo, Wei; Zhou, Wencai; Lukose, Binit; Wang, Zhengbang; Weidler, Peter G; Pöppl, Andreas; Ziese, Michael; Heine, Thomas; Wöll, Christof

2016-10-04

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu(2+) ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu(2+) ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1/2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Itinerant and localized magnetization dynamics in antiferromagnetic Ho

DOE PAGES

Rettig, L.; Dornes, C.; Thielemann-Kuhn, N.; ...

2016-06-21

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L 3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Here, tuning the x-ray energy to the electric dipole (E1, 2p → 5d) or quadrupole (E2, 2p → 4f) transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic (2 1 3–τ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similarmore » spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4f–5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.« less

Physical realization of a quantum spin liquid based on a complex frustration mechanism

NASA Astrophysics Data System (ADS)

Reuther, Johannes; Balz, Christian; Lake, Bella

Unlike conventional magnets where the spins undergo magnetic long-range order in the ground state, in a quantum spin liquid they remain disordered down to the lowest temperatures without breaking local symmetries. Here, we investigate the novel, unexplored bilayer-kagome magnet Ca10Cr7O28, which has a complex Hamiltonian consisting of isotropic antiferromagnetic and ferromagnetic interactions where the ferromagnetic couplings are the dominant ones. We show both experimentally and theoretically that this compound displays all the features expected of a quantum spin liquid. In particular, experiments rule out static magnetic order down to 19mK and reveal a diffuse spinon-like excitation spectrum. Numerically simulating this material using the pseudo fermion functional renormalization group (PFFRG) method, we theoretically confirm the non-magnetic ground state of the system and qualitatively reproduce the measured spin correlation profile. By tuning the model parameters away from those realized in Ca10Cr7O28 we further show that the spin-liquid phase is of remarkable stability.

Experimental evidence for simultaneous relaxation processes in super spin glass γ-Fe2O3 nanoparticle system

NASA Astrophysics Data System (ADS)

Nikolic, V.; Perovic, M.; Kusigerski, V.; Boskovic, M.; Mrakovic, A.; Blanusa, J.; Spasojevic, V.

2015-03-01

Spherical γ-Fe2O3 nanoparticles with the narrow size distribution of (5 ± 1) nm were synthesized by the method of thermal decomposition from iron acetyl acetonate precursor. The existence of super spin-glass state at low temperatures and in low applied magnetic fields was confirmed by DC magnetization measurements on a SQUID magnetometer. The comprehensive investigation of magnetic relaxation dynamics in low-temperature region was conducted through the measurements of single-stop and multiple stop ZFC memory effects, ZFC magnetization relaxation, and AC susceptibility measurements. The experimental findings revealed the peculiar change of magnetic relaxation dynamics at T ≈ 10 K, which arose as a consequence of simultaneous existence of different relaxation processes in Fe2O3 nanoparticle system. Complementarity of the applied measurements was utilized in order to single out distinct relaxation processes as well as to elucidate complex relaxation mechanisms in the investigated interacting nanoparticle system.

Influence of radius of cylinder HTS bulk on guidance force in a maglev vehicle system

NASA Astrophysics Data System (ADS)

Longcai, Zhang

2014-07-01

Bulk superconductors had great potential for various engineering applications, especially in a high-temperature superconducting (HTS) maglev vehicle system. In such a system, the HTS bulks were always exposed to AC external magnetic field, which was generated by the inhomogeneous surface magnetic field of the NdFeB guideway. In our previous work, it was observed that the guidance force of the YBCO bulk over the NdFeB guideway used in the HTS maglev vehicle system was decayed by the application of the AC external magnetic field. In this paper, we investigated the influence of the radius of the cylinder HTS bulk exposed to an AC magnetic field perturbation on the guidance force in the maglev vehicle system. From the results, it was found that the guidance force was stronger for the bulk with bigger radius and the guidance force decay rates of the bulks were approximately equal despite of the different radius in the maglev vehicle system. Therefore, in order to obtain higher guidance force in the maglev vehicle system, we could use the cylinder HTS bulks with the bigger radius.

Superconductivity drives magnetism in δ -doped La2CuO4

NASA Astrophysics Data System (ADS)

Suter, A.; Logvenov, G.; Boris, A. V.; Baiutti, F.; Wrobel, F.; Howald, L.; Stilp, E.; Salman, Z.; Prokscha, T.; Keimer, B.

2018-04-01

Understanding the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on δ -doped La2CuO4 (δ -LCON ) superlattices. By means of molecular beam epitaxy whole LaO2 layers were periodically replaced by SrO2 layers, providing a charge reservoir yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muon spin rotation. We find a quasi-two-dimensional superconducting state which couples to the antiferromagnetic order in a nontrivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions.

Effect of Al doping on the magnetic and electrical properties of Zn(Cu)O based diluted magnetic semiconductors

NASA Astrophysics Data System (ADS)

Chakraborti, D.; Trichy, G.; Narayan, J.; Prater, J. T.; Kumar, D.

2007-12-01

The effect of Al doping on the magnetic properties of Zn(Cu)O based dilute magnetic semiconducting thin films has been systematically investigated. Epitaxial thin films have been deposited onto sapphire c-plane single crystals using pulsed laser deposition technique. X-ray diffraction and high resolution transmission electron microscopy studies show that the Zn(Cu,Al)O films are epitaxially grown onto (0001) sapphire substrates with a 30°/90° rotation in the basal plane. The large lattice misfit of the order of 16% is accommodated by matching integral multiples of lattice and substrate planes. In these large mismatch systems, the resulting films are fully relaxed following deposition of the first complete monolayer of ZnO (consistent with a critical thickness that is less than one monolayer). Magnetic hysteresis measurements indicate that the pure Zn(Cu)O thin films are ferromagnetic at room temperature. Doping with up to 5% Al (n type) does not significantly affect the ferromagnetism even though it results in an increase in carrier densities of more than 3 orders of magnitude, rising from 1×1017 to 1.5×1020 cm-3. However, for Al additions above 5%, a drop in net magnetization is observed. Annealing the films in an oxygen atmosphere at 600 °C also resulted in a dramatic drop in magnetic moment of the samples. These results strongly suggest that carrier induced exchange is not directly responsible for the magnetic properties of these materials. Rather, a defect mediated exchange mechanism needs to be invoked for this system.

Correlation of geothermal springs with sub-surface fault terminations revealed by high-resolution, UAV-acquired magnetic data

USGS Publications Warehouse

Glen, Jonathan; A.E. Egger,; C. Ippolito,; N.Athens,

2013-01-01

There is widespread agreement that geothermal springs in extensional geothermal systems are concentrated at fault tips and in fault interaction zones where porosity and permeability are dynamically maintained (Curewitz and Karson, 1997; Faulds et al., 2010). Making these spatial correlations typically involves geological and geophysical studies in order to map structures and their relationship to springs at the surface. Geophysical studies include gravity and magnetic surveys, which are useful for identifying buried, intra-basin structures, especially in areas where highly magnetic, dense mafic volcanic rocks are interbedded with, and faulted against less magnetic, less dense sedimentary rock. High-resolution magnetic data can also be collected from the air in order to provide continuous coverage. Unmanned aerial systems (UAS) are well-suited for conducting these surveys as they can provide uniform, low-altitude, high-resolution coverage of an area without endangering crew. In addition, they are more easily adaptable to changes in flight plans as data are collected, and improve efficiency. We have developed and tested a new system to collect magnetic data using small-platform UAS. We deployed this new system in Surprise Valley, CA, in September, 2012, on NASA's SIERRA UAS to perform a reconnaissance survey of the entire valley as well as detailed surveys in key transition zones. This survey has enabled us to trace magnetic anomalies seen in ground-based profiles along their length. Most prominent of these is an intra-basin magnetic high that we interpret as a buried, faulted mafic dike that runs a significant length of the valley. Though this feature lacks surface expression, it appears to control the location of geothermal springs. All of the major hot springs on the east side of the valley lie along the edge of the high, and more specifically, at structural transitions where the high undergoes steps, bends, or breaks. The close relationship between the springs and structure terminations revealed by this study is unprecedented. Collecting magnetic data via UAS represents a new capability in geothermal exploration of remote and dangerous areas that significantly enhances our ability to map the subsurface.

Cylinders vs. Spheres: Biofluid Shear Thinning in Driven Nanoparticle Transport

PubMed Central

Cribb, Jeremy A.; Meehan, Timothy D.; Shah, Sheel M.; Skinner, Kwan; Superfine, Richard

2011-01-01

Increasingly, the research community applies magnetophoresis to micro and nanoscale particles for drug delivery applications and the nanoscale rheological characterization of complex biological materials. Of particular interest is the design and transport of these magnetic particles through entangled polymeric fluids commonly found in biological systems. We report the magnetophoretic transport of spherical and rod-shaped particles through viscoelastic, entangled solutions using lambda-phage DNA (λ-DNA) as a model system. In order to understand and predict the observed phenomena, we fully characterize three fundamental components: the magnetic field and field gradient, the shape and magnetic properties of the probe particles, and the macroscopic rheology of the solution. Particle velocities obtained in Newtonian solutions correspond to macroscale rheology, with forces calculated via Stokes Law. In λ-DNA solutions, nanorod velocities are 100 times larger than predicted by measured zero-shear viscosity. These results are consistent with particles experiencing transport through a shear thinning fluid, indicating magnetically driven transport in shear thinning may be especially effective and favor narrow diameter, high aspect ratio particles. A complete framework for designing single-particle magnetic-based delivery systems results when we combine a quantified magnetic system with qualified particles embedded in a characterized viscoelastic medium. PMID:20571853

Optically switched magnetism in photovoltaic perovskite CH3NH3(Mn:Pb)I3

PubMed Central

Náfrádi, B.; Szirmai, P.; Spina, M.; Lee, H.; Yazyev, O. V.; Arakcheeva, A.; Chernyshov, D.; Gibert, M.; Forró, L.; Horváth, E.

2016-01-01

The demand for ever-increasing density of information storage and speed of manipulation boosts an intense search for new magnetic materials and novel ways of controlling the magnetic bit. Here, we report the synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I3 material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman–Kittel–Kasuya–Yosida interactions without heating up the spin system. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low-power, light controlling magnetic devices. PMID:27882917

Gradient nonlinearity calibration and correction for a compact, asymmetric magnetic resonance imaging gradient system.

PubMed

Tao, S; Trzasko, J D; Gunter, J L; Weavers, P T; Shu, Y; Huston, J; Lee, S K; Tan, E T; Bernstein, M A

2017-01-21

Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimer's Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26 cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.

Ultrasensitive interplay between ferromagnetism and superconductivity in NbGd composite thin films

PubMed Central

Bawa, Ambika; Gupta, Anurag; Singh, Sandeep; Awana, V.P.S.; Sahoo, Sangeeta

2016-01-01

A model binary hybrid system composed of a randomly distributed rare-earth ferromagnetic (Gd) part embedded in an s-wave superconducting (Nb) matrix is being manufactured to study the interplay between competing superconducting and ferromagnetic order parameters. The normal metallic to superconducting phase transition appears to be very sensitive to the magnetic counterpart and the modulation of the superconducing properties follow closely to the Abrikosov-Gor’kov (AG) theory of magnetic impurity induced pair breaking mechanism. A critical concentration of Gd is obtained for the studied NbGd based composite films (CFs) above which superconductivity disappears. Besides, a magnetic ordering resembling the paramagnetic Meissner effect (PME) appears in DC magnetization measurements at temperatures close to the superconducting transition temperature. The positive magnetization related to the PME emerges upon doping Nb with Gd. The temperature dependent resistance measurements evolve in a similar fashion with the concentration of Gd as that with an external magnetic field and in both the cases, the transition curves accompany several intermediate features indicating the traces of magnetism originated either from Gd or from the external field. Finally, the signatures of magnetism appear evidently in the magnetization and transport measurements for the CFs with very low (<1 at.%) doping of Gd. PMID:26725684

Magnetic pressure effects in a plasma-liner interface

NASA Astrophysics Data System (ADS)

García-Rubio, F.; Sanz, J.

2018-04-01

A theoretical analysis of magnetic pressure effects in a magnetized liner inertial fusion-like plasma is presented. In previous publications [F. García-Rubio and J. Sanz, Phys. Plasmas 24, 072710 (2017)], the evolution of a hot magnetized plasma in contact with a cold unmagnetized plasma, aiming to represent the hot spot and liner, respectively, was investigated in planar geometry. The analysis was made in a double limit low Mach and high thermal to magnetic pressure ratio β. In this paper, the analysis is extended to an arbitrary pressure ratio. Nernst, Ettingshausen, and Joule effects come into play in the energy balance. The region close to the liner is governed by thermal conduction, while the Joule dissipation becomes predominant far from it when the pressure ratio is low. Mass ablation, thermal energy, and magnetic flux losses are reduced with plasma magnetization, characterized by the electron Hall parameter ω e τ e , until β values of order unity are reached. From this point forward, increasing the electron Hall parameter no longer improves the magnetic flux conservation, and mass ablation is enhanced due to the magnetic pressure gradients. A thoughtful simplification of the problem that allows to reduce the order of the system of governing equations while still retaining the finite β effects is presented and compared to the exact case.

Enhanced terahertz magnetic dipole response by subwavelength fiber

NASA Astrophysics Data System (ADS)

Atakaramians, Shaghik; Shadrivov, Ilya V.; Miroshnichenko, Andrey E.; Stefani, Alessio; Ebendorff-Heidepriem, Heike; Monro, Tanya M.; Afshar V., Shahraam

2018-05-01

Dielectric sub-wavelength particles have opened up a new platform for realization of magnetic light. Recently, we have demonstrated that a dipole emitter by a sub-wavelength fiber leads to an enhanced magnetic response. Here, we experimentally demonstrate an enhanced magnetic dipole source in the terahertz frequency range. By placing the fiber next to the hole in a metal screen, we find that the radiation power can be enhanced more than one order of magnitude. The enhancement is due to the excitation of the Mie-type resonances in the fiber. We demonstrate that such a system is equivalent to a double-fiber system excited by a magnetic source. This coupled magnetic dipole and optical fiber system can be considered a unit cell of metasurfaces for manipulation of terahertz radiation and is a proof-of-concept of a possibility to achieve enhanced radiation of a dipole source in proximity of a sub-wavelength fiber. It can also be scaled down to optical frequencies opening up promising avenues for developing integrated nanophotonic devices such as nanoantennas or lasers on fibers.

Phase diagram of the underdoped cuprates at high magnetic field

NASA Astrophysics Data System (ADS)

Chakraborty, Debmalya; Morice, Corentin; Pépin, Catherine

2018-06-01

The experimentally measured phase diagram of cuprate superconductors in the temperature-applied magnetic field plane illuminates key issues in understanding the physics of these materials. At low temperature, the superconducting state gives way to a long-range charge order with increasing magnetic field; both the orders coexist in a small intermediate region. The charge order transition is strikingly insensitive to temperature and quickly reaches a transition temperature close to the zero-field superconducting Tc. We argue that such a transition along with the presence of the coexisting phase is difficult to obtain in a weak coupling competing orders formalism. We demonstrate that for some range of parameters there is an enlarged symmetry of the strongly coupled charge and superconducting orders in the system depending on their relative masses and the coupling strength of the two orders. We establish that this sharp switch from the superconducting phase to the charge order phase can be understood in the framework of a composite SU(2) order parameter comprising the charge and superconducting orders. Finally, we illustrate that there is a possibility of the coexisting phase of the competing charge and superconducting orders only when the SU(2) symmetry between them is weakly broken due to biquadratic terms in the free energy. The relation of this sharp transition to the proximity to the pseudogap quantum critical doping is also discussed.

Applied adaptive disturbance rejection using output redefinition on magnetic bearings

NASA Astrophysics Data System (ADS)

Matras, Alex Logan

Recent work has shown Adaptive Disturbance Rejection to be an effective technique for rejecting forces due to imbalance, runout and base motion disturbances on flywheels supported by magnetic bearings over a large span of frequencies. Often the applicability of some of the adaptive methods is limited because they require certain properties (such as almost-strict positive realness) that magnetic bearings do not possess. In this thesis, one method for adaptive disturbance rejection, called Adaptive Feedforward Cancellation (AFC), is modified to allow for a much wider range of frequencies to be rejected. This is accomplished by redefining the output of the original system to be the output from a reduced order state estimator instead. This can give a new system with an infinite gain margin. Additionally, the adaptation laws for the two disturbance rejection gains are slightly modified so that each adapts to a different signal in order to provide the best performance. A detailed model of a magnetic bearing is developed and computer simulations based on that model are performed to give an initial test of the new control law. A state-of-the-art magnetic bearing setup is then developed and used to implement the new control laws and determine their effectiveness. The results are successful and validate the new ideas that are presented.

Ab initio construction of magnetic phase diagrams in alloys: The case of Fe 1-xMn xPt

DOE PAGES

Pujari, B. S.; Larson, P.; Antropov, V. P.; ...

2015-07-28

A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. The application to the Fe 1–xMn xPt “magnetic chameleon” system yields the sequence of magnetic phases at T = 0 and the c-T magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of themore » magnetic phase diagram is demonstrated.« less

Vortex lattice structures in YNi{sub 2}B{sub 2}C

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yethiraj, M.; Paul, D.M.; Tomy, C.V.

The authors observe a flux lattice with square symmetry in the superconductor YNi{sub 2}B{sub 2}C when the applied field is parallel to the c-axis of the crystal. A square lattice observed previously in the isostructural magnetic analog ErNi{sub 2}B{sub 2}C was attributed to the interaction between magnetic order in that system and the flux lattice. Since the Y-based compound does not order magnetically, it is clear that the structure of the flux lattice is unrelated to magnetic order. In fact, they show that the flux lines have a square cross-section when the applied field is parallel to the c-axis ofmore » the crystal, since the measured penetration depth along the 100 crystal direction is larger than the penetration depth along the 110 by approximately 60%. This is the likely reason for the square symmetry of the lattice. Although they find considerable disorder in the arrangement of the flux lines at 2.5T, no melting of the vortex lattice was observed.« less

Vortex lattice structures in YNi{sub 2}B{sub 2}C

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yethiraj, M.; Paul, D.M.; Tomy, C.V.

We observe a flux lattice with square symmetry in the superconductor YNi{sub 2}B{sub 2}C when the applied field is parallel to the c-axis of the crystal. A square lattice observed previously in the isostructural magnetic analog ErNi{sub 2}B{sub 2}C was attributed to the interaction between magnetic order in that system and the flux lattice. Since the Y-based compound does not order magnetically, it is clear that the structure of the flux lattice is unrelated to magnetic order. In fact, we show that the flux lines have a square cross-section when the applied field is parallel to the c-axis of themore » crystal, since the measured penetration depth along the 110 crystal direction is smaller than the penetration depth along the 100 by approximately 30%. This causes the square symmetry of the lattice. Although we find considerable disorder in the arrangement of the flux lines at 2.5T, no melting of the vortex lattice was observed.« less

Evidence for strong enhancement of the magnetic ordering temperature of trivalent Nd metal under extreme pressure [Anomalous enhancement of the magnetic ordering temperature of trivalent Nd metal under extreme pressure: Possible Kondo lattice behavior

DOE PAGES

Song, J.; Bi, W.; Haskel, D.; ...

2017-05-15

Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5–295 K under pressures to 210 GPa. The magnetic ordering temperature T o of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking ΔT c as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30–40 GPa, the exchange coupling J between Nd ions and conduction electrons becomes negative, thusmore » activating Kondo physics in this highly correlated electron system. Furthermore, the rise and fall of T o and ΔT c with pressure can be accounted for in terms of an increase in the Kondo temperature.« less

Anisotropic antiferromagnetic order in the spin-orbit coupled trigonal-lattice Ca2Sr2IrO6

NASA Astrophysics Data System (ADS)

Sheng, Jieming; Ye, Feng; Hoffmann, Christina; Cooper, Valentino R.; Okamoto, Satoshi; Terzic, Jasminka; Zheng, Hao; Zhao, Hengdi; Cao, G.

2018-06-01

We used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca2Sr2IrO6 . The crystal structure is determined to be R 3 ¯ with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below TN=13.1 K. The magnetic wave vector is identified as (0,0.5,1) with ferromagnetic coupling along the a axis and antiferromagnetic correlation along the b axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34∘ toward the c axis. The ordered moment is 0.66(3) μB/Ir, larger than other iridates where iridium ions form corner- or edge-sharing IrO6 octahedral networks. The tilting angle is reduced to ≈19∘ when a magnetic field of 4.9 T is applied along the c axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap ˜0.5 eV.

Evidence for strong enhancement of the magnetic ordering temperature of trivalent Nd metal under extreme pressure [Anomalous enhancement of the magnetic ordering temperature of trivalent Nd metal under extreme pressure: Possible Kondo lattice behavior

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, J.; Bi, W.; Haskel, D.

Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5–295 K under pressures to 210 GPa. The magnetic ordering temperature T o of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking ΔT c as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30–40 GPa, the exchange coupling J between Nd ions and conduction electrons becomes negative, thusmore » activating Kondo physics in this highly correlated electron system. Furthermore, the rise and fall of T o and ΔT c with pressure can be accounted for in terms of an increase in the Kondo temperature.« less

Electrodeless Plasma Source: Phase II Update

NASA Astrophysics Data System (ADS)

Prager, James; Ziemba, Timothy; Miller, Kenneth

2012-10-01

Eagle Harbor Technologies, in collaboration with the University of Washington, has developed a low-impurity, electrode-less plasma source (EPS) for start-up and source plasma injection for fusion science applications. In order to not interfere with the experiment, a pre-ionizer/plasma source must meet a few critical criteria including low impurity production, low electromagnetic interference (EMI), and minimal disruption to the magnetic geometry of the experiment. This system was designed to be UHV compatible and bakable. Here we present the results of the EPS Phase II upgrade. The output plasma density was increased by two orders of magnitude to >10^17 m-3 in hydrogen with no magnetic field injected. EPS system integration with the HIT-SI experiment has begun.

Synthetic magnetoelectric coupling in a nanocomposite multiferroic

DOE PAGES

Jain, P.; Wang, Q.; Roldan, M.; ...

2015-03-13

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO₃ (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La₀.₇Sr₀.₃MnO₃ (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, whichmore » we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric data storage platforms.« less

Chiral battery, scaling laws and magnetic fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anand, Sampurn; Bhatt, Jitesh R.; Pandey, Arun Kumar, E-mail: sampurn@prl.res.in, E-mail: jeet@prl.res.in, E-mail: arunp@prl.res.in

2017-07-01

We study the generation and evolution of magnetic field in the presence of chiral imbalance and gravitational anomaly which gives an additional contribution to the vortical current. The contribution due to gravitational anomaly is proportional to T {sup 2} which can generate seed magnetic field irrespective of plasma being chirally charged or neutral. We estimate the order of magnitude of the magnetic field to be 10{sup 30} G at T ∼ 10{sup 9} GeV, with a typical length scale of the order of 10{sup −18} cm, which is much smaller than the Hubble radius at that temperature (10{sup −8} cm).more » Moreover, such a system possess scaling symmetry. We show that the T {sup 2} term in the vorticity current along with scaling symmetry leads to more power transfer from lower to higher length scale as compared to only chiral anomaly without scaling symmetry.« less

Topological RPdBi half-Heusler semimetals: A new family of noncentrosymmetric magnetic superconductors.

PubMed

Nakajima, Yasuyuki; Hu, Rongwei; Kirshenbaum, Kevin; Hughes, Alex; Syers, Paul; Wang, Xiangfeng; Wang, Kefeng; Wang, Renxiong; Saha, Shanta R; Pratt, Daniel; Lynn, Jeffrey W; Paglione, Johnpierre

2015-06-01

We report superconductivity and magnetism in a new family of topological semimetals, the ternary half-Heusler compound RPdBi (R: rare earth). In this series, tuning of the rare earth f-electron component allows for simultaneous control of both lattice density via lanthanide contraction and the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of the normal-state band inversion strength, superconducting pairing, and magnetically ordered ground states. Antiferromagnetism with ordering vector (½,½,½) occurs below a Néel temperature that scales with de Gennes factor dG, whereas a superconducting transition is simultaneously supressed with increasing dG. With superconductivity appearing in a system with noncentrosymmetric crystallographic symmetry, the possibility of spin-triplet Cooper pairing with nontrivial topology analogous to that predicted for the normal-state electronic structure provides a unique and rich opportunity to realize both predicted and new exotic excitations in topological materials.

Magnetic drug targeting through a realistic model of human tracheobronchial airways using computational fluid and particle dynamics.

PubMed

Pourmehran, Oveis; Gorji, Tahereh B; Gorji-Bandpy, Mofid

2016-10-01

Magnetic drug targeting (MDT) is a local drug delivery system which aims to concentrate a pharmacological agent at its site of action in order to minimize undesired side effects due to systemic distribution in the organism. Using magnetic drug particles under the influence of an external magnetic field, the drug particles are navigated toward the target region. Herein, computational fluid dynamics was used to simulate the air flow and magnetic particle deposition in a realistic human airway geometry obtained by CT scan images. Using discrete phase modeling and one-way coupling of particle-fluid phases, a Lagrangian approach for particle tracking in the presence of an external non-uniform magnetic field was applied. Polystyrene (PMS40) particles were utilized as the magnetic drug carrier. A parametric study was conducted, and the influence of particle diameter, magnetic source position, magnetic field strength and inhalation condition on the particle transport pattern and deposition efficiency (DE) was reported. Overall, the results show considerable promise of MDT in deposition enhancement at the target region (i.e., left lung). However, the positive effect of increasing particle size on DE enhancement was evident at smaller magnetic field strengths (Mn [Formula: see text] 1.5 T), whereas, at higher applied magnetic field strengths, increasing particle size has a inverse effect on DE. This implies that for efficient MTD in the human respiratory system, an optimal combination of magnetic drug career characteristics and magnetic field strength has to be achieved.

Errors and optics study of a permanent magnet quadrupole system

NASA Astrophysics Data System (ADS)

Schillaci, F.; Maggiore, M.; Rifuggiato, D.; Cirrone, G. A. P.; Cuttone, G.; Giove, D.

2015-05-01

Laser-based accelerators are gaining interest in recent years as an alternative to conventional machines [1]. Nowadays, energy and angular spread of the laser-driven beams are the main issues in application and different solutions for dedicated beam-transport lines have been proposed [2,3]. In this context a system of permanent magnet quadrupoles (PMQs) is going to be realized by INFN [2] researchers, in collaboration with SIGMAPHI [3] company in France, to be used as a collection and pre-selection system for laser driven proton beams. The definition of well specified characteristics, both in terms of performances and field quality, of the magnetic lenses is crucial for the system realization, for an accurate study of the beam dynamics and the proper matching with a magnetic selection system already realized [6,7]. Hence, different series of simulations have been used for studying the PMQs harmonic contents and stating the mechanical and magnetic tolerances in order to have reasonable good beam quality downstream the system. In this paper is reported the method used for the analysis of the PMQs errors and its validation. Also a preliminary optics characterization is presented in which are compared the effects of an ideal PMQs system with a perturbed system on a monochromatic proton beams.

Vibration and Control of Flexible Rotor Supported by Magnetic Bearings

NASA Technical Reports Server (NTRS)

Nonami, Kenzou

1988-01-01

Active vibration control of flexible rotors supported by magnetic bearings is discussed. Using a finite-element method for a mathematical model of the flexible rotor, the eigenvalue problem is formulated taking into account the interaction between a mechanical system of the flexible rotor and an electrical system of the magnetic bearings and the controller. However, for the sake of simplicity, gyroscopic effects are disregarded. It is possible to adapt this formulation to a general flexible rotor-magnetic bearing system. Controllability with and without collocation sensors and actuators located at the same distance along the rotor axis is discussed for the higher order flexible modes of the test rig. In conclusion, it is proposed that it is necessary to add new active control loops for the higher flexible modes even in the case of collocation. Then it is possible to stabilize for the case of uncollocation by means of this method.

High-frequency fluctuation measurements by far-infrared laser Faraday-effect polarimetry-interferometry and forward scattering system on MST.

PubMed

Ding, W X; Lin, L; Duff, J R; Brower, D L

2014-11-01

Magnetic fluctuation-induced transport driven by global tearing modes has been measured by Faraday-effect polarimetry and interferometry (phase measurements) in the MST reversed field pinch. However, the role of small-scale broadband magnetic and density turbulence in transport remains unknown. In order to investigate broadband magnetic turbulence, we plan to upgrade the existing detector system by using planar-diode fundamental waveguide mixers optimized for high sensitivity. Initial tests indicate these mixers have ×10 sensitivity improvement compared to currently employed corner-cube Schottky-diode mixers and ×5 lower noise. Compact mixer design will allow us to resolve the wavenumbers up to k ∼ 1-2 cm(-1) for beam width w = 1.5 cm and 15 cm(-1) for beam width w = 2 mm. The system can also be used to measure the scattered signal (amplitude measurement) induced by both plasma density and magnetic fluctuations.

Electrical Transport on the Shastry-Sutherland Lattice in Ising-type Rare Earth Tetraborides

NASA Astrophysics Data System (ADS)

Ye, Linda; Suzuki, Takehito; Checkelsky, Joseph. G.

In the presence of a magnetic field, frustrated spin systems may exhibit plateaus at fractional values of their saturation magnetization. Study of the magnetic ordering and excitations at such plateaus are key to understanding the nature of the underlying ground states in these systems. Here we study the magnetization plateaus in metallic rare earth tetraborides RB4 with Ising-type anisotropy (R = Er, Tm) in which R resides on a Shastry-Sutherland lattice. We focus on electrical transport and find that the response reflects scattering of charge carriers with the static and dynamic plateau structure. Modeling of these results is consistent with the expected strong uniaxial anisotropy and provides a framework for the study of plateau states in metallic frustrated systems. We thank NSF Grant No. DMR-1231319, Tsinghua Education Foundation, Moore foundation Grant No. GBMF3848 for support.

Frustration and thermalization in an artificial magnetic quasicrystal

NASA Astrophysics Data System (ADS)

Shi, Dong; Budrikis, Zoe; Stein, Aaron; Morley, Sophie A.; Olmsted, Peter D.; Burnell, Gavin; Marrows, Christopher H.

2018-03-01

Artificial frustrated systems offer a playground to study the emergent properties of interacting systems. Most work to date has been on spatially periodic systems, known as artificial spin ices when the interacting elements are magnetic. Here we have studied artificial magnetic quasicrystals based on quasiperiodic Penrose tiling patterns of interacting nanomagnets. We construct a low-energy configuration from a step-by-step approach that we propose as a ground state. Topologically induced emergent frustration means that this configuration cannot be constructed from vertices in their ground states. It has two parts, a quasi-one-dimensional `skeleton' that spans the entire pattern and is capable of long-range order, surrounding `flippable' clusters of macrospins that lead to macroscopic degeneracy. Magnetic force microscopy imaging of Penrose tiling arrays revealed superdomains that are larger for more strongly coupled arrays, especially after annealing the array above its blocking temperature.

Frustration and thermalization in an artificial magnetic quasicrystal

DOE PAGES

Shi, Dong; Budrikis, Zoe; Stein, Aaron; ...

2017-12-11

Here, artificial frustrated systems offer a playground to study the emergent properties of interacting systems. Most work to date has been on spatially periodic systems, known as artificial spin ices when the interacting elements are magnetic. Here we have studied artificial magnetic quasicrystals based on quasiperiodic Penrose tiling patterns of interacting nanomagnets. We construct a low-energy configuration from a step-by-step approach that we propose as a ground state. Topologically induced emergent frustration means that this configuration cannot be constructed from vertices in their ground states. It has two parts, a quasi-one-dimensional ‘skeleton’ that spans the entire pattern and is capablemore » of long-range order, surrounding ‘flippable’ clusters of macrospins that lead to macroscopic degeneracy. Magnetic force microscopy imaging of Penrose tiling arrays revealed superdomains that are larger for more strongly coupled arrays, especially after annealing the array above its blocking temperature.« less

Frustration and thermalization in an artificial magnetic quasicrystal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Dong; Budrikis, Zoe; Stein, Aaron

Here, artificial frustrated systems offer a playground to study the emergent properties of interacting systems. Most work to date has been on spatially periodic systems, known as artificial spin ices when the interacting elements are magnetic. Here we have studied artificial magnetic quasicrystals based on quasiperiodic Penrose tiling patterns of interacting nanomagnets. We construct a low-energy configuration from a step-by-step approach that we propose as a ground state. Topologically induced emergent frustration means that this configuration cannot be constructed from vertices in their ground states. It has two parts, a quasi-one-dimensional ‘skeleton’ that spans the entire pattern and is capablemore » of long-range order, surrounding ‘flippable’ clusters of macrospins that lead to macroscopic degeneracy. Magnetic force microscopy imaging of Penrose tiling arrays revealed superdomains that are larger for more strongly coupled arrays, especially after annealing the array above its blocking temperature.« less

Tsunami: ocean dynamo generator.

PubMed

Sugioka, Hiroko; Hamano, Yozo; Baba, Kiyoshi; Kasaya, Takafumi; Tada, Noriko; Suetsugu, Daisuke

2014-01-08

Secondary magnetic fields are induced by the flow of electrically conducting seawater through the Earth's primary magnetic field ('ocean dynamo effect'), and hence it has long been speculated that tsunami flows should produce measurable magnetic field perturbations, although the signal-to-noise ratio would be small because of the influence of the solar magnetic fields. Here, we report on the detection of deep-seafloor electromagnetic perturbations of 10-micron-order induced by a tsunami, which propagated through a seafloor electromagnetometer array network. The observed data extracted tsunami characteristics, including the direction and velocity of propagation as well as sea-level change, first to verify the induction theory. Presently, offshore observation systems for the early forecasting of tsunami are based on the sea-level measurement by seafloor pressure gauges. In terms of tsunami forecasting accuracy, the integration of vectored electromagnetic measurements into existing scalar observation systems would represent a substantial improvement in the performance of tsunami early-warning systems.

Magnetic storm effects in electric power systems and prediction needs

NASA Technical Reports Server (NTRS)

Albertson, V. D.; Kappenman, J. G.

1979-01-01

Geomagnetic field fluctuations produce spurious currents in electric power systems. These currents enter and exit through points remote from each other. The fundamental period of these currents is on the order of several minutes which is quasi-dc compared to the normal 60 Hz or 50 Hz power system frequency. Nearly all of the power systems problems caused by the geomagnetically induced currents result from the half-cycle saturation of power transformers due to simultaneous ac and dc excitation. The effects produced in power systems are presented, current research activity is discussed, and magnetic storm prediction needs of the power industry are listed.

Tuning the magnetism of the top-layer FeAs on BaFe2As2 (001): First-principles study

NASA Astrophysics Data System (ADS)

Zhang, Bing-Jing; Liu, Kai; Lu, Zhong-Yi

2018-04-01

Magnetism may play an important role in inducing the superconductivity in iron-based superconductors. As a prototypical system, the surface of BaFe2As2 provides a good platform for studying related magnetic properties. We have designed systematic first-principles calculations to clarify the surface magnetism of BaFe2As2 (001), which previously has received little attention in comparison with surface structures and electronic states. We find that the surface environment has an important influence on the magnetic properties of the top-layer FeAs. For As-terminated surfaces, the magnetic ground state of the top-layer FeAs is in the staggered dimer antiferromagnetic (AFM) order, distinct from that of the bulk, while for Ba-terminated surfaces the collinear (single-stripe) AFM order is the most stable, the same as that in the bulk. When a certain coverage of Ba or K atoms is deposited onto the As-terminated surface, the calculated energy differences among different AFM orders for the top-layer FeAs on BaFe2As2 (001) can be much reduced, indicating enhanced spin fluctuations. To compare our results with available scanning tunneling microscopy (STM) measurements, we have simulated the STM images of several structural/magnetic terminations. Astonishingly, when the top-layer FeAs is in the staggered dimer AFM order, a stripe pattern appears in the simulated STM image even when the surface Ba atoms adopt a √{2 }×√{2 } structure, while a √{2 }×√{2 } square pattern comes out for the 1 ×1 full As termination. Our results suggest: (i) the magnetic state at the BaFe2As2 (001) surface can be quite different from that in the bulk; (ii) the magnetic properties of the top-layer FeAs can be tuned effectively by surface doping, which may likely induce superconductivity at the surface layer; (iii) both the surface termination and the AFM order in the top-layer FeAs can affect the STM image of BaFe2As2 (001), which needs to be taken into account when identifying the surface termination.

Magnetism of CrO overlayers on Fe(001)bcc surface: first principles calculations

NASA Astrophysics Data System (ADS)

Félix-Medina, Raúl Enrique; Leyva-Lucero, Manuel Andrés; Meza-Aguilar, Salvador; Demangeat, Claude

2018-04-01

Riva et al. [Surf. Sci. 621, 55 (2014)] as well as Calloni et al. [J. Phys.: Condens. Matter 26, 445001 (2014)] have studied the oxydation of Cr films deposited on Fe(001)bcc through low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy. In the present work we perform a density functional approach within Quantum Expresso code in order to study structural and magnetic properties of CrO overlayers on Fe(001)bcc. The calculations are performed using DFT+U. The investigated systems include O/Cr/Fe(001)bcc, Cr/O/Fe(001)bcc, Cr0.25O0.75/Fe(001)bcc, as well as the O coverage Ox/Cr/Fe(001)bcc (x = 0.25; 0.50). We have found that the ordered CrO overlayer presents an antiferromagnetic coupling between Cr and Fe atoms. The O atoms are located closer to the Fe atoms of the surface than the Cr atoms. The ground state of the systems O/Cr/Fe(001)bcc and Cr/O/Fe(001)bcc corresponds to the O/Cr/Fe(001)bcc system with a magnetic coupling c(2 × 2). The effect of the O monolayer on Cr/Fe(001)bcc changes the ground state from p(1 × 1) ↓ to c(2 × 2) and produces an enhancement of the magnetic moments. The Ox overlayer on Cr/Fe(001)bcc produces an enhancement of the Cr magnetic moments.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Buehler, Marc; Tartaglia, Michael; Tompkins, John

The Mu2e experiment at Fermilab is designed to explore charged lepton flavor violation by searching for muon-to-electron conversion. The magnetic field generated by a system of solenoids is crucial for Mu2e and requires accurate characterization to detect any flaws and to produce a detailed field map. Stringent physics goals are driving magnetic field specifications for the Mu2e solenoids. A field mapper is being designed, which will produce detailed magnetic field maps. The uniform field region of the spectrometer volume requires the highest level of precision (1 Gauss per 1 Tesla). During commissioning, multiple magnetic field maps will be generated tomore » verify proper alignment of all magnet coils, and to create the final magnetic field map. In order to design and build a precise field mapping system consisting of Hall and NRM probes, tolerances and precision for such a system need to be evaluated. In this paper we present a design for the Mu2e field mapping hardware, and discuss results from OPERA-3D simulations to specify parameters for Hall and NMR probes. We also present a fitting procedure for the analytical treatment of our expected magnetic measurements.« less

Magnetic structures of R5Ni2In4 and R11Ni4In9 (R  =  Tb and Ho): strong hierarchy in the temperature dependence of the magnetic ordering in the multiple rare-earth sublattices.

PubMed

Ritter, C; Provino, A; Manfrinetti, P; Pecharsky, V K; Gschneidner, K A; Dhar, S K

2015-12-02

The magnetic properties and magnetic structures of the R 5Ni2In4 and the microfibrous R 11Ni4In9 compounds with R  =  Tb and Ho have been examined using magnetization, heat capacity, and neutron diffraction data. Rare earth atoms occupy three and five symmetrically inequivalent rare earth sites in R 5Ni2In4 and R 11Ni4In9 compounds, respectively. As a result of the intra- and inter-magnetic sublattice interactions, the magnetic exchange interactions are different for various rare earth sites; this leads to a cascade of magnetic transitions with a strong hierarchy in the temperature dependence of the magnetic orderings. A transition at T C  =  125 K in Tb5Ni2In4 [κ 1  =  (0, 0, 0)] leads to a ferro/ferrimagnetic order where the magnetic ordering in one of the three R-sublattices leads to the ordering of another one; the third sublattice stays non-magnetic. New magnetic Bragg peaks appearing below T N  =  20 K can be indexed with the incommensurate magnetic propagation vector κ 2  =  (0, 0.636, ½); at T N  =  20 K a cycloidal spin order, which acts mostly upon the third R-sublattice, occurs. Ho5Ni2In4 establishes first antiferromagnetism [κ  =  (0, 0, 0)] at T N  =  31 K on two R-sublattices; then the system becomes ferro/ferrimagnetic at T C  =  25 K with the third sublattice ordering as well. Tb11Ni4In9 has three magnetic transitions at T C  =  135 K, T N1  =  35 K and at T N2  =  20 K; they are respectively coupled to the appearance of different propagation vectors [κ 1  =  (0, 0, 0), κ 2  =  (0, 0, ½), κ 3  =  (0, 1, ½)], which themselves are operating differently on the five different R-sublattices. Two sublattices remain mostly ferromagnetic down to lowest temperature while the three others are predominantly coupled antiferromagnetically. In Ho11Ni4In9 a purely antiferromagnetic order, described by four different magnetic propagation vectors [κ 1  =  (0, 0.62, 0), κ 2  =  (0, 1, 0), κ 3  =  (0, 0, ½), κ 4  =  (0, 1, ½)], succeedingly includes all five different sublattices on cooling through transitions at T N1  =  22 K, T N2  =  12 K, T N3  =  8 K and T N4  =  7 K. The strength of the magnetic interactions of the different sublattices can be linked to structural details for both R 5Ni2In4 and R 11Ni4In9 compounds.

Magnetic dipole interactions in crystals

NASA Astrophysics Data System (ADS)

Johnston, David C.

2016-01-01

The influence of magnetic dipole interactions (MDIs) on the magnetic properties of local-moment Heisenberg spin systems is investigated. A general formulation is presented for calculating the eigenvalues λ and eigenvectors μ ̂ of the MDI tensor of the magnetic dipoles in a line (one dimension, 1D), within a circle (2D) or a sphere (3D) of radius r surrounding a given moment μ⃗i for given magnetic propagation vectors k for collinear and coplanar noncollinear magnetic structures on both Bravais and non-Bravais spin lattices. Results are calculated for collinear ordering on 1D chains, 2D square and simple-hexagonal (triangular) Bravais lattices, 2D honeycomb and kagomé non-Bravais lattices, and 3D cubic Bravais lattices. The λ and μ ̂ values are compared with previously reported results. Calculations for collinear ordering on 3D simple tetragonal, body-centered tetragonal, and stacked triangular and honeycomb lattices are presented for c /a ratios from 0.5 to 3 in both graphical and tabular form to facilitate comparison of experimentally determined easy axes of ordering on these Bravais lattices with the predictions for MDIs. Comparisons with the easy axes measured for several illustrative collinear antiferromagnets (AFMs) are given. The calculations are extended to the cycloidal noncollinear 120∘ AFM ordering on the triangular lattice where λ is found to be the same as for collinear AFM ordering with the same k. The angular orientation of the ordered moments in the noncollinear coplanar AFM structure of GdB4 with a distorted stacked 3D Shastry-Sutherland spin-lattice geometry is calculated and found to be in disagreement with experimental observations, indicating the presence of another source of anisotropy. Similar calculations for the undistorted 2D and stacked 3D Shastry-Sutherland lattices are reported. The thermodynamics of dipolar magnets are calculated using the Weiss molecular field theory for quantum spins, including the magnetic transition temperature Tm and the ordered moment, magnetic heat capacity, and anisotropic magnetic susceptibility χ versus temperature T . The anisotropic Weiss temperature θp in the Curie-Weiss law for T >Tm is calculated. A quantitative study of the competition between FM and AFM ordering on cubic Bravais lattices versus the demagnetization factor in the absence of FM domain effects is presented. The contributions of Heisenberg exchange interactions and of the MDIs to Tm and to θp are found to be additive, which simplifies analysis of experimental data. Some properties in the magnetically-ordered state versus T are presented, including the ordered moment and magnetic heat capacity and, for AFMs, the dipolar anisotropy of the free energy and the perpendicular critical field. The anisotropic χ for dipolar AFMs is calculated both above and below the Néel temperature TN and the results are illustrated for a simple tetragonal lattice with c /a >1 , c /a =1 (cubic), and c /a <1 , where a change in sign of the χ anisotropy is found at c /a =1 . Finally, following the early work of Keffer [Phys. Rev. 87, 608 (1952), 10.1103/PhysRev.87.608], the dipolar anisotropy of χ above TN=69 K of the prototype collinear Heisenberg-exchange-coupled tetragonal compound MnF2 is calculated and found to be in excellent agreement with experimental single-crystal literature data above 130 K, where the smoothly increasing deviation of the experimental data from the theory on cooling from 130 K to TN is deduced to arise from dynamic short-range collinear c -axis AFM ordering in this temperature range driven by the exchange interactions.

Mobile metallic domain walls in an all-in-all-out magnetic insulator

DOE PAGES

Ma, Eric Yue; Cui, Yong -Tao; Ueda, Kentaro; ...

2015-10-30

Magnetic domain walls are boundaries between regions with different configurations of the same magnetic order. In a magnetic insulator, where the magnetic order is tied to its bulk insulating property, it has been postulated that electrical properties are drastically different along the domain walls, where the order is inevitably disturbed. Here we report the discovery of highly conductive magnetic domain walls in a magnetic insulator, Nd 2Ir 2O 7, that has an unusual all-in-all-out magnetic order, via transport and spatially resolved microwave impedance microscopy. The domain walls have a virtually temperature-independent sheet resistance of ~1 kilohm per square, show smoothmore » morphology with no preferred orientation, are free from pinning by disorders, and have strong thermal and magnetic field responses that agree with expectations for all-in-all-out magnetic order.« less

Mobile metallic domain walls in an all-in-all-out magnetic insulator.

PubMed

Ma, Eric Yue; Cui, Yong-Tao; Ueda, Kentaro; Tang, Shujie; Chen, Kai; Tamura, Nobumichi; Wu, Phillip M; Fujioka, Jun; Tokura, Yoshinori; Shen, Zhi-Xun

2015-10-30

Magnetic domain walls are boundaries between regions with different configurations of the same magnetic order. In a magnetic insulator, where the magnetic order is tied to its bulk insulating property, it has been postulated that electrical properties are drastically different along the domain walls, where the order is inevitably disturbed. Here we report the discovery of highly conductive magnetic domain walls in a magnetic insulator, Nd2Ir2O7, that has an unusual all-in-all-out magnetic order, via transport and spatially resolved microwave impedance microscopy. The domain walls have a virtually temperature-independent sheet resistance of ~1 kilohm per square, show smooth morphology with no preferred orientation, are free from pinning by disorders, and have strong thermal and magnetic field responses that agree with expectations for all-in-all-out magnetic order. Copyright © 2015, American Association for the Advancement of Science.

Role of quantum fluctuations on spin liquids and ordered phases in the Heisenberg model on the honeycomb lattice

NASA Astrophysics Data System (ADS)

Merino, Jaime; Ralko, Arnaud

2018-05-01

Motivated by the rich physics of honeycomb magnetic materials, we obtain the phase diagram and analyze magnetic properties of the spin-1 /2 and spin-1 J1-J2-J3 Heisenberg model on the honeycomb lattice. Based on the SU(2) and SU(3) symmetry representations of the Schwinger boson approach, which treats disordered spin liquids and magnetically ordered phases on an equal footing, we obtain the complete phase diagrams in the (J2,J3) plane. This is achieved using a fully unrestricted approach which does not assume any pre-defined Ansätze. For S =1 /2 , we find a quantum spin liquid (QSL) stabilized between the Néel, spiral, and collinear antiferromagnetic phases in agreement with previous theoretical work. However, by increasing S from 1 /2 to 1, the QSL is quickly destroyed due to the weakening of quantum fluctuations indicating that the model already behaves as a quasiclassical system. The dynamical structure factors and temperature dependence of the magnetic susceptibility are obtained in order to characterize all phases in the phase diagrams. Moreover, motivated by the relevance of the single-ion anisotropy, D , to various S =1 honeycomb compounds, we have analyzed the destruction of magnetic order based on an SU(3) representation of the Schwinger bosons. Our analysis provides a unified understanding of the magnetic properties of honeycomb materials realizing the J1-J2-J3 Heisenberg model from the strong quantum spin regime at S =1 /2 to the S =1 case. Neutron scattering and magnetic susceptibility experiments can be used to test the destruction of the QSL phase when replacing S =1 /2 by S =1 localized moments in certain honeycomb compounds.

Frustration and quantum criticality

NASA Astrophysics Data System (ADS)

Vojta, Matthias

2018-06-01

This review article is devoted to the interplay between frustrated magnetism and quantum critical phenomena, covering both theoretical concepts and ideas as well as recent experimental developments in correlated-electron materials. The first part deals with local-moment magnetism in Mott insulators and the second part with frustration in metallic systems. In both cases, frustration can either induce exotic phases accompanied by exotic quantum critical points or lead to conventional ordering with unconventional crossover phenomena. In addition, the competition of multiple phases inherent to frustrated systems can lead to multi-criticality.

Relativistic jets without large-scale magnetic fields

NASA Astrophysics Data System (ADS)

Parfrey, K.; Giannios, D.; Beloborodov, A.

2014-07-01

The canonical model of relativistic jets from black holes requires a large-scale ordered magnetic field to provide a significant magnetic flux through the ergosphere--in the Blandford-Znajek process, the jet power scales with the square of the magnetic flux. In many jet systems the presence of the required flux in the environment of the central engine is questionable. I will describe an alternative scenario, in which jets are produced by the continuous sequential accretion of small magnetic loops. The magnetic energy stored in these coronal flux systems is amplified by the differential rotation of the accretion disc and by the rotating spacetime of the black hole, leading to runaway field line inflation, magnetic reconnection in thin current layers, and the ejection of discrete bubbles of Poynting-flux-dominated plasma. For illustration I will show the results of general-relativistic force-free electrodynamic simulations of rotating black hole coronae, performed using a new resistivity model. The dissipation of magnetic energy by coronal reconnection events, as demonstrated in these simulations, is a potential source of the observed high-energy emission from accreting compact objects.

Chirality in Magnetic Multilayers Probed by the Symmetry and the Amplitude of Dichroism in X-Ray Resonant Magnetic Scattering

NASA Astrophysics Data System (ADS)

Chauleau, Jean-Yves; Legrand, William; Reyren, Nicolas; Maccariello, Davide; Collin, Sophie; Popescu, Horia; Bouzehouane, Karim; Cros, Vincent; Jaouen, Nicolas; Fert, Albert

2018-01-01

Chirality in condensed matter has recently become a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physical mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order parameters, are therefore essential. Here we report x-ray resonant magnetic scattering as a straightforward tool to reveal directly the properties of chiral magnetic systems. We show that it can straightforwardly and unambiguously determine the main characteristics of chiral magnetic distributions: i.e., its chiral nature, the quantitative winding sense (clockwise or counterclockwise), and its type, i.e., Néel [cycloidal] or Bloch [helical]. This method is model independent, does not require a priori knowledge of the magnetic parameters, and can be applied to any system with magnetic domains ranging from a few nanometers (wavelength limited) to several microns. By using prototypical multilayers with tailored magnetic chiralities driven by spin-orbit-related effects at Co |Pt interfaces, we illustrate the strength of this method.

Magnetoresistive detection of strongly pinned uncompensated magnetization in antiferromagnetic FeMn

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lapa, Pavel N.; Roshchin, Igor V.; Ding, Junjia

2017-01-17

Here we observed and studied pinned uncompensated magnetization in an antiferromagnet using magnetoresistance measurements. For this, we developed antiferromagnet-ferromagnet spin valves (AFSVs) that consist of an antiferromagnetic layer and a ferromagnetic one, separated by a nonmagnetic conducting spacer. In an AFSV, the uncompensated magnetization in the antiferromagnet affects scattering of spin-polarized electrons giving rise to giant magnetoresitance (GMR). By measuring angular dependence of AFSVs' resistance, we detected pinned uncompensated magnetization responsible for the exchange bias effect in an antiferromagnet- only exchange bias system Cu/FeMn/Cu. The fact that GMR measured in this system persists up to 110 kOe indicates that themore » scattering occurs on strongly pinned uncompensated magnetic moments in FeMn. This strong pinning can be explained if this pinned uncompensated magnetization is a thermodynamically stable state and coupled to the antiferromagnetic order parameter. Finally, using the AFSV technique, we confirmed that the two interfaces between FeMn and Cu are magnetically different: The uncompensated magnetization is pinned only at the interface with the bottom Cu layer.« less

Effective equations for the precession dynamics of electron spins and electron-impurity correlations in diluted magnetic semiconductors

NASA Astrophysics Data System (ADS)

Cygorek, M.; Axt, V. M.

2015-08-01

Starting from a quantum kinetic theory for the spin dynamics in diluted magnetic semiconductors, we derive simplified equations that effectively describe the spin transfer between carriers and magnetic impurities for an arbitrary initial impurity magnetization. Taking the Markov limit of these effective equations, we obtain good quantitative agreement with the full quantum kinetic theory for the spin dynamics in bulk systems at high magnetic doping. In contrast, the standard rate description where the carrier-dopant interaction is treated according to Fermi’s golden rule, which involves the assumption of a short memory as well as a perturbative argument, has been shown previously to fail if the impurity magnetization is non-zero. The Markov limit of the effective equations is derived, assuming only a short memory, while higher order terms are still accounted for. These higher order terms represent the precession of the carrier-dopant correlations in the effective magnetic field due to the impurity spins. Numerical calculations show that the Markov limit of our effective equations reproduces the results of the full quantum kinetic theory very well. Furthermore, this limit allows for analytical solutions and for a physically transparent interpretation.

Investigating Non-Equilibrium Fluctuations of Nanocolloids in a Magnetic Field Using Direct Imaging Methods

NASA Astrophysics Data System (ADS)

Rice, Ashley; Oprisan, Ana; Oprisan, Sorinel; Rice-Oprisan College of Charleston Team

Nanoparticles of iron oxide have a high surface area and can be controlled by an external magnetic field. Since they have a fast response to the applied magnetic field, these systems have been used for numerous in vivo applications, such as MRI contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery, and cell separation. We performed three direct imaging experiments in order to investigate the concentration-driven fluctuations using magnetic nanoparticles in the absence and in the presence of magnetic field. Our direct imaging experimental setup involved a glass cell filled with magnetic nanocolloidal suspension and water with the concentration gradient oriented against the gravitational field and a superluminescent diode (SLD) as the light source. Nonequilibrium concentration-driven fluctuations were recorded using a direct imaging technique. We used a dynamic structure factor algorithm for image processing in order to compute the structure factor and to find the power law exponents. We saw evidence of large concentration fluctuations and permanent magnetism. Further research will use the correlation time to approximate the diffusion coefficient for the free diffusion experiment. Funded by College of Charleston Department of Undergraduate Research and Creative Activities SURF grant.

Long-range interactions in magnetic bilayer above the critical temperature

NASA Astrophysics Data System (ADS)

de Souza, R. M. V.; Pereira, T. A. S.; Godoy, M.; de Arruda, A. S.

2018-01-01

In this paper we have studied the stabilization of the long-range order in (z ; x) -plane of two isotropic Heisenberg ferromagnetic monolayers coupled by a short-range exchange interaction (J⊥), by a long range dipole-dipole interactions and a magnetic field. We have applied a magnetic field along of the z-direction to study the thermodynamic properties above the critical temperature. The dispersion relation ω and the magnetization are given as function of dipolar anisotropy parameter defined as Ed =(gμ) 2 S /a3J∥ and for other Hamiltonian parameters, and they are calculated by the double-time Zubarev-Tyablikov Green's functions in the random-phase approximation (RPA). The results show that the system is unstable for values of Ed ≥ 0.012 with external magnetic field ranging between H /J∥ = 0 and 10-3. The instability appears for Ed larger then Edc = 0.0158 with H /J∥ = 10-5, Edc = 0.02885 with H /J∥ = 10-4, and Edc = 0.115 with H /J∥ = 10-3, i.e., a small magnetic field is sufficient to maintain the magnetic order in a greater range of the dipolar interaction.

Metamaterial-enhanced coupling between magnetic dipoles for efficient wireless power transfer

NASA Astrophysics Data System (ADS)

Urzhumov, Yaroslav; Smith, David R.

2011-05-01

Nonradiative coupling between conductive coils is a candidate mechanism for wireless energy transfer applications. In this paper we propose a power relay system based on a near-field metamaterial superlens and present a thorough theoretical analysis of this system. We use time-harmonic circuit formalism to describe all interactions between two coils attached to external circuits and a slab of anisotropic medium with homogeneous permittivity and permeability. The fields of the coils are found in the point-dipole approximation using Sommerfeld integrals which are reduced to standard special functions in the long-wavelength limit. We show that, even with a realistic magnetic loss tangent of order 0.1, the power transfer efficiency with the slab can be an order of magnitude greater than free-space efficiency when the load resistance exceeds a certain threshold value. We also find that the volume occupied by the metamaterial between the coils can be greatly compressed by employing magnetic permeability with a large anisotropy ratio.

Visualizing decoupling in nanocrystalline alloys: A FORC-temperature analysis

NASA Astrophysics Data System (ADS)

Rivas, M.; Martínez-García, J. C.; Gorria, P.

2016-02-01

Devitrifying ferromagnetic amorphous precursors in the adequate conditions may give rise to disordered assemblies of densely packed nanocrystals with extraordinary magnetic softness well explained by the exchange coupling among multiple crystallites. Whether the magnetic exchange interaction is produced by direct contact or mediated by the intergranular amorphous matrix has a strong influence on the behaviour of the system above room temperature. Multi-phase amorphous-nanocrystalline systems dramatically harden when approaching the amorphous Curie temperature (TC) due to the hard grains decoupling. The study of the thermally induced decoupling of nanosized crystallites embedded in an amorphous matrix has been performed in this work by the first-order reversal curves (FORCs) analysis. We selected a Fe-rich amorphous alloy with TC = 330 K, in order to follow the evolution of the FORC diagrams obtained below and above such temperature in samples with different percentages of nanocrystalline phase. The existence of up to four regions exhibiting unlike magnetic behaviours is unambiguously determined from the temperature evolution of the FORC.

Unconventional slowing down of electronic recovery in photoexcited charge-ordered La 1/3Sr 2/3FeO 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Yi; Hoffman, Jason; Rowland, Clare E.

Ordered electronic phases are intimately related to emerging phenomena such as high Tc superconductivity and colossal magnetoresistance. The coupling of electronic charge with other degrees of freedom such as lattice and spin are of central interest in correlated systems. Their correlations have been intensively studied from femtosecond to picosecond time scales, while the dynamics of ordered electronic phases beyond nanoseconds are usually assumed to follow a trivia thermally driven recovery. Here, we report an unusual slowing down of electronic phases across a first-order phase transition, far beyond thermal relaxation time. Following optical excitation, the recovery time of both transient opticalmore » reflectivity and x-ray diffraction intensity from a charge-ordered superstructure in a La 1/3Sr 2/3FeO 3 thin film increases by orders of magnitude longer than the independently measured lattice cooling time when the sample temperature approaches the phase transition temperature. The combined experimental and theoretical investigations show that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition. As a result, this extraordinary long electronic recovery time exemplifies an interplay of ordered electronic phases with magnetism beyond thermal processes in correlated systems.« less

Unconventional slowing down of electronic recovery in photoexcited charge-ordered La 1/3Sr 2/3FeO 3

DOE PAGES

Zhu, Yi; Hoffman, Jason; Rowland, Clare E.; ...

2018-05-04

Ordered electronic phases are intimately related to emerging phenomena such as high Tc superconductivity and colossal magnetoresistance. The coupling of electronic charge with other degrees of freedom such as lattice and spin are of central interest in correlated systems. Their correlations have been intensively studied from femtosecond to picosecond time scales, while the dynamics of ordered electronic phases beyond nanoseconds are usually assumed to follow a trivia thermally driven recovery. Here, we report an unusual slowing down of electronic phases across a first-order phase transition, far beyond thermal relaxation time. Following optical excitation, the recovery time of both transient opticalmore » reflectivity and x-ray diffraction intensity from a charge-ordered superstructure in a La 1/3Sr 2/3FeO 3 thin film increases by orders of magnitude longer than the independently measured lattice cooling time when the sample temperature approaches the phase transition temperature. The combined experimental and theoretical investigations show that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition. As a result, this extraordinary long electronic recovery time exemplifies an interplay of ordered electronic phases with magnetism beyond thermal processes in correlated systems.« less

Microfluidic magnetic bead conveyor belt.

PubMed

van Pelt, Stijn; Frijns, Arjan; den Toonder, Jaap

2017-11-07

Magnetic beads play an important role in the miniaturization of clinical diagnostics systems. In lab-on-chip platforms, beads can be made to link to a target species and can then be used for the manipulation and detection of this species. Current bead actuation systems utilize complex on-chip coil systems that offer low field strengths and little versatility. We demonstrate a novel system based on an external rotating magnetic field and on-chip soft-magnetic structures to focus the field locally. These structures were designed and optimized using finite element simulations in order to create a number of local flux density maxima. These maxima, to which the magnetic beads are attracted, move over the chip surface in a continuous way together with the rotation of the external field, resulting in a mechanism similar to that of a conveyor belt. A prototype was fabricated using PDMS molding techniques mixed with iron powder for the magnetic structures. In the subsequent experiments, a quadrupole electromagnet was used to create the rotating external field. We observed that beads formed agglomerates that rolled over the chip surface, just above the magnetic structures. Field rotation frequencies between 0.1-50 Hz were tested resulting in magnetic bead speeds of over 1 mm s -1 for the highest frequency. With this, we have shown that our novel concept works, combining a simple design and simple operation with a powerful and versatile method for bead actuation. This makes it a promising method for further research and utilization in lab-on-chip systems.

Topological superconductivity in an ultrathin, magnetically-doped topological insulator proximity coupled to a conventional superconductor

NASA Astrophysics Data System (ADS)

Kim, Youngseok; Philip, Timothy M.; Park, Moon Jip; Gilbert, Matthew J.; University of Illinois at Urbana; Champaign Team

As a promising candidate system to realize topological superconductivity (SC), 3D time-reversal invariant topological insulators (TI) proximity-coupled to s-wave superconductors have been intensively studied. Recent experiments on proximity-coupled TI have shown that superconductivity may be induced in ultrathin TI. One proposal to observe the topological SC in proximity-coupled ultrathin TI system is to add magnetic dopants to the TI. However, detailed study on the impact of the experimental parameters on possible topological phase is sparse. In this work, we investigate ultrathin, magnetically-doped, proximity-coupled TI in order to determine the experimentally relevant parameters needed to observe topological SC. We find that, due to the spin-momentum locked nature of the surface states in TI, the induced s-wave order parameter within the surface states persists even at large magnitudes of the Zeeman energy, allowing us to explore the system in parameter space. We elucidate the phase diagram as a function of: the hybridization gap, Zeeman energy, and chemical potential of the TI system. Our findings provide a useful guide in choosing relevant parameters to facilitate the observation of topological SC in thin film TI-superconductor hybrid systems. National Science Foundation (NSF) under Grant CAREER ECCS-1351871.

Electric field control of magnon-induced magnetization dynamics in multiferroics

PubMed Central

Risinggård, Vetle; Kulagina, Iryna; Linder, Jacob

2016-01-01

We consider theoretically the effect of an inhomogeneous magnetoelectric coupling on the magnon-induced dynamics of a ferromagnet. The magnon-mediated magnetoelectric torque affects both the homogeneous magnetization and magnon-driven domain wall motion. In the domains, we predict a reorientation of the magnetization, controllable by the applied electric field, which is almost an order of magnitude larger than that observed in other physical systems via the same mechanism. The applied electric field can also be used to tune the domain wall speed and direction of motion in a linear fashion, producing domain wall velocities several times the zero field velocity. These results show that multiferroic systems offer a promising arena to achieve low-dissipation magnetization rotation and domain wall motion by exciting spin-waves. PMID:27554064

Yttrium enrichment and improved magnetic properties in partially melted Y-Ba-Cu-O materials

NASA Technical Reports Server (NTRS)

Alterescu, Sidney; Hojaji, Hamid; Barkatt, Aaron; Michael, Karen A.; Hu, Shouxiang

1990-01-01

The yttrium-rich compositions in the Y-Ba-Cu-O system were mapped out in a systematic manner to quantify their magnetic properties and to correlate them with the microstructure and phase composition as determined by scanning electron microscopy and X-ray diffraction analysis. It is found that the microstructure of Y-Ba-Cu-O compositions is a sensitive function of both their composition and processing conditions. Measurements of magnetic susceptibility and maximum (low-field) and remanent magnetization for the system Y:Ba:Cu = x:2:3 show highest values for x = 2. The corresponding structures involve numerous small crystals of Y2BaCuO5 (211) embedded in highly ordered assemblages of continous YBa2Cu3O(7-y) (123) layers.

Coherent transport structures in magnetized plasmas. I. Theory

NASA Astrophysics Data System (ADS)

Di Giannatale, G.; Falessi, M. V.; Grasso, D.; Pegoraro, F.; Schep, T. J.

2018-05-01

In a pair of linked articles (called Papers I and II, respectively), we apply the concept of Lagrangian Coherent Structures (LCSs) borrowed from the study of dynamical systems to magnetic field configurations in order to separate regions where field lines have a different kind of behaviour. In the present article, Paper I, after recalling the definition and the properties of the LCSs, we show how this conceptual framework can be applied to the study of particle transport in a magnetized plasma. Furthermore, we introduce a simplified model that allows us to consider explicitly the case where the magnetic configuration evolves in time on time scales comparable to the particle transit time through the configuration. In contrast with previous works on this topic, this analysis requires that a system that is aperiodic in time be investigated.

Analysis of possible designs of processing units with radial plasma flows

NASA Astrophysics Data System (ADS)

Kolesnik, V. V.; Zaitsev, S. V.; Vashilin, V. S.; Limarenko, M. V.; Prochorenkov, D. S.

2018-03-01

Analysis of plasma-ion methods of obtaining thin-film coatings shows that their development goes along the path of the increasing use of sputter deposition processes, which allow one to obtain multicomponent coatings with varying percentage of particular components. One of the methods that allow one to form multicomponent coatings with virtually any composition of elementary components is the method of coating deposition using quasi-magnetron sputtering systems [1]. This requires the creation of an axial magnetic field of a defined configuration with the flux density within the range of 0.01-0.1 T [2]. In order to compare and analyze various configurations of processing unit magnetic systems, it is necessary to obtain the following dependencies: the dependency of magnetic core section on the input power to inductors, the distribution of magnetic induction within the equatorial plane in the corresponding sections, the distribution of the magnetic induction value in the area of cathode target location.

Magnetic properties of checkerboard lattice: a Monte Carlo study

NASA Astrophysics Data System (ADS)

Jabar, A.; Masrour, R.; Hamedoun, M.; Benyoussef, A.

2017-12-01

The magnetic properties of ferrimagnetic mixed-spin Ising model in the checkerboard lattice are studied using Monte Carlo simulations. The variation of total magnetization and magnetic susceptibility with the crystal field has been established. We have obtained a transition from an order to a disordered phase in some critical value of the physical variables. The reduced transition temperature is obtained for different exchange interactions. The magnetic hysteresis cycles have been established. The multiples hysteresis cycle in checkerboard lattice are obtained. The multiples hysteresis cycle have been established. The ferrimagnetic mixed-spin Ising model in checkerboard lattice is very interesting from the experimental point of view. The mixed spins system have many technological applications such as in domain opto-electronics, memory, nanomedicine and nano-biological systems. The obtained results show that that crystal field induce long-range spin-spin correlations even bellow the reduced transition temperature.

Interpreting inverse magnetic fabric in dikes from Eastern Iceland

NASA Astrophysics Data System (ADS)

Trippanera, Daniele; Urbani, Stefano; Porreca, Massimiliano; Acocella, Valerio; Kissel, Catherine; Sagnotti, Leonardo; Winkler, Aldo

2017-04-01

Since the 70's magnetic fabric analysis has been used to infer magma emplacement in dikes. However, the interpretation of magmatic flow orientation in dikes is often complicated by the occurrence of anomalous (i.e. inverse) magnetic fabric. This latter may either reflect the presence of single-domain (SD) grains or result from peculiar orientation mechanisms of magnetic minerals in magmas of different viscosities. Tertiary dike swarms of extinct volcanic systems in Eastern Iceland represent the ideal case study to clarify the origin of anomalous magnetic fabric. Here we present the results of a multidisciplinary study on dikes belonging to the Alftafjordur volcanic system (Eastern Iceland), including a: (1) structural field study in order to identify kinematic and thermal indicators of dikes; (2) anisotropy of low-field magnetic susceptibility (AMS) analysis, to investigate the magnetic fabric and reconstruct the flow direction of 25 dikes; (3) first order reversal curve (FORC) diagrams and thermomagnetic properties of selected dikes to define the magnetic mineralogy; (4) petrofabric and image analyses at different microscopic scales to investigate the origin of the magnetic fabric and compare the AMS results with mineral texture. Our results show that half of the dikes show a well defined inverse magnetic fabrics (k max orthogonal to the dike margins) and anomalous high anisotropy degrees. Only 7 dikes have a normal magnetic fabric and other 6 dikes have an intermediate magnetic fabric. No clear prevalence of SD grains, which could explain the inverse magnetic fabric, was observed. On the contrary, petrofabric and thermomagnetic analysis reveal the presence of low Ti-content coarse magnetite and high Ti-content elongated magnetite grains as the main contributors to most of the observed magnetic fabrics. In particular, the orientation of the elongated high Ti-content magnetite grains, though usually scattered, is partly comparable with that of the maximum and minimum axes of the AMS ellipsoids, suggesting that the preferential orientation of these minerals represent the main source of inverse and intermediate magnetic fabrics. The results of this study demonstrate that the interpretation of the magnetic fabric is not always straightforward and the origin of anomalous fabrics may be related to a variety of physical and chemical processes during magma emplacement.

The frustrated fcc antiferromagnet Ba 2 YOsO 6: Structural characterization, magnetic properties and neutron scattering studies

DOE PAGES

Kermarrec, E.; Marjerrison, Casey A.; Thompson, C. M.; ...

2015-02-26

Here we report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite Ba 2 YOsO 6. The Fmmore » $$\\bar{3}$$m space group is found both at 290 K and 3.5 K with cell constants a 0=8.3541(4) Å and 8.3435(4) Å, respectively. Os 5+ (5d 3) ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature θ ~₋700 K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below T N~69 K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) μ B on Os 5+. The ordered moment is much reduced from either the expected spin-only value of ~3 μ B or the value appropriate to 4d 3 Ru 5+ in isostructural Ba 2 YRuO 6 of 2.2(1) μ B, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at T=67.45 K, and the existence of a second-ordered state. We find time-of-flight inelastic neutron results reveal a large spin gap Δ~17 meV, unexpected for an orbitally quenched, d 3 electronic configuration. In conclusion, we discuss this in the context of the ~5 meV spin gap observed in the related Ru 5+,4d 3 cubic double perovskite Ba 2YRuO 6, and attribute the ~3 times larger gap to stronger SOC present in this heavier, 5d, osmate system.« less

Studies of magnetism in rhenium and manganese based perovskite oxides

NASA Astrophysics Data System (ADS)

Wiebe, Christopher Ryan

The bulk of this thesis consists of studies of geometric frustration in S = ½ FCC perovskites based upon the chemical formula A2BReO 6. The magnetism of these materials is expected to exhibit geometric frustration, a situation in which the ideal spin arrangements cannot be achieved for antiferromagnetic interactions between adjacent spins. It is proposed that subtle quantum effects are driving these systems to unique ground states in the absence of chemical disorder. Both compounds Sr2CaReO 6 and Sr2MgReO6 exhibit spin glass behaviour at low temperatures (TG ˜ 14 K and TG ˜ 50 K respectively), in which the magnetic moments freeze out in random orientations instead of an ordered array. This work shows that these materials possess several unconventional properties, which suggest that interesting spin dynamics may be present. Other perovskite and perovskite-related materials studied in this thesis include the magnetoresistive CaMnO3-delta and the "pillared" material La5Re3MnO16. Neutron diffraction studies have shown that both CaMnO2.94 and CaMnO2.89 order at TN ˜ 125 K, but possess unique yet related magnetic structures. CaMnO2.94 orders into a simple G-type magnetic structure, as observed in the compound CaMnO3. The slightly more doped sample CaMnO2.89, on the other hand, orders into a magnetic structure related to the G-type, and involves a Mn3+/Mn 4+ charge ordering over every four lattice spacings. The new material La5Re3MnO16 consists of layers of corner shared ReO6 and MnO6 octahedra that are separated by layers of Re2O10 dimer units. Metal-metal bonding involving Re atoms have been postulated for these dimers which separate the Re/Mn layers by approximately 10 A. The magnetic behaviour exhibited by this new class of materials is rich and complex. Despite the large distances separating the perovskite layers, the Re and Mn magnetic moments order into a ferrimagnetic Q = (0, 0, ½) structure below a relatively high T N of 161 K. There may be an additional spin rearrangement at lower temperatures as evidenced by weak magnetic Bragg peaks below ˜50 K.

Kinetic Simulations of the Lowest-order Unstable Mode of Relativistic Magnetostatic Equilibria

NASA Astrophysics Data System (ADS)

Nalewajko, Krzysztof; Zrake, Jonathan; Yuan, Yajie; East, William E.; Blandford, Roger D.

2016-08-01

We present the results of particle-in-cell numerical pair plasma simulations of relativistic two-dimensional magnetostatic equilibria known as the “Arnold-Beltrami-Childress” fields. In particular, we focus on the lowest-order unstable configuration consisting of two minima and two maxima of the magnetic vector potential. Breaking of the initial symmetry leads to exponential growth of the electric energy and to the formation of two current layers, which is consistent with the picture of “X-point collapse” first described by Syrovatskii. Magnetic reconnection within the layers heats a fraction of particles to very high energies. After the saturation of the linear instability, the current layers are disrupted and the system evolves chaotically, diffusing the particle energies in a stochastic second-order Fermi process, leading to the formation of power-law energy distributions. The power-law slopes harden with the increasing mean magnetization, but they are significantly softer than those produced in simulations initiated from Harris-type layers. The maximum particle energy is proportional to the mean magnetization, which is attributed partly to the increase of the effective electric field and partly to the increase of the acceleration timescale. We describe in detail the evolving structure of the dynamical current layers and report on the conservation of magnetic helicity. These results can be applied to highly magnetized astrophysical environments, where ideal plasma instabilities trigger rapid magnetic dissipation with efficient particle acceleration and flares of high-energy radiation.

KINETIC SIMULATIONS OF THE LOWEST-ORDER UNSTABLE MODE OF RELATIVISTIC MAGNETOSTATIC EQUILIBRIA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nalewajko, Krzysztof; Zrake, Jonathan; Yuan, Yajie

2016-08-01

We present the results of particle-in-cell numerical pair plasma simulations of relativistic two-dimensional magnetostatic equilibria known as the “Arnold–Beltrami–Childress” fields. In particular, we focus on the lowest-order unstable configuration consisting of two minima and two maxima of the magnetic vector potential. Breaking of the initial symmetry leads to exponential growth of the electric energy and to the formation of two current layers, which is consistent with the picture of “X-point collapse” first described by Syrovatskii. Magnetic reconnection within the layers heats a fraction of particles to very high energies. After the saturation of the linear instability, the current layers aremore » disrupted and the system evolves chaotically, diffusing the particle energies in a stochastic second-order Fermi process, leading to the formation of power-law energy distributions. The power-law slopes harden with the increasing mean magnetization, but they are significantly softer than those produced in simulations initiated from Harris-type layers. The maximum particle energy is proportional to the mean magnetization, which is attributed partly to the increase of the effective electric field and partly to the increase of the acceleration timescale. We describe in detail the evolving structure of the dynamical current layers and report on the conservation of magnetic helicity. These results can be applied to highly magnetized astrophysical environments, where ideal plasma instabilities trigger rapid magnetic dissipation with efficient particle acceleration and flares of high-energy radiation.« less

Spontaneous magnetic order in complex materials: Role of longitudinal spin-orbit interactions

NASA Astrophysics Data System (ADS)

Chakraborty, Subrata; Vijay, Amrendra

2017-06-01

We show that the longitudinal spin-orbit interactions (SOI) critically determine the fate of spontaneous magnetic order (SMO) in complex materials. To study the magnetic response of interacting electrons constituting the material, we implement an extension of the Hubbard model that faithfully accounts for the SOI. Next, we use the double-time Green functions of quantum statistical mechanics to obtain the spontaneous magnetization, Msp , and thence ascertain the possibility of SMO. For materials with quenched SOI, in an arbitrary dimension, Msp vanishes at finite temperatures, implying the presence of the disordered (paramagnetic) phase. This is consistent with and goes beyond the Bogolyubov's inequality based analysis in one and two dimensions. In the presence of longitudinal SOI, Msp , for materials in an arbitrary dimension, remains non-zero at finite temperatures, which indicates the existence of the ordered (ferromagnetic) phase. As a plausible experimental evidence of the present SOI-based phenomenology, we discuss, inter alia, a recent experimental study on Y4Mn1-xGa12-yGey, an intermetallic compound, which exhibits a magnetic phase transition (paramagnetic to ferromagnetic) upon tuning the fraction of Ge atoms and thence the vacancies of the magnetic centers in this system. The availability of Ge atoms to form a direct chemical bond with octahedral Mn in this material appears to quench the SOI and, as a consequence, favours the formation of the disordered (paramagnetic) phase.

Antiferromagnetism in the van der Waals layered spin-lozenge semiconductor CrTe 3

DOE PAGES

McGuire, Michael A.; Garlea, V. Ovidiu; KC, Santosh; ...

2017-04-14

We have investigated the crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe 3 on single-crystal and polycrystalline materials. Furthermore, the crystal structure contains layers made up of lozenge-shaped Cr 4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3 eV. The magnetic susceptibility exhibits a broad maximum near 300 K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55 K, and single-crystal neutron diffraction reveals the onset of long-range antiferromagnetic order at thismore » temperature. Strongly dispersive spin waves are observed in the ordered state. Significant magnetoelastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is evident in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first-principles calculations, which predict a cleavage energy 0.5 J / m 2 , similar to graphite. Based on our results, CrTe 3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.« less

Antiferromagnetism in the van der Waals layered spin-lozenge semiconductor CrTe 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGuire, Michael A.; Garlea, V. Ovidiu; KC, Santosh

We have investigated the crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe 3 on single-crystal and polycrystalline materials. Furthermore, the crystal structure contains layers made up of lozenge-shaped Cr 4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3 eV. The magnetic susceptibility exhibits a broad maximum near 300 K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55 K, and single-crystal neutron diffraction reveals the onset of long-range antiferromagnetic order at thismore » temperature. Strongly dispersive spin waves are observed in the ordered state. Significant magnetoelastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is evident in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first-principles calculations, which predict a cleavage energy 0.5 J / m 2 , similar to graphite. Based on our results, CrTe 3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.« less

Lanthanide contraction effect on crystal structures, magnetic, and dielectric properties in ordered double perovskites LnPbCoSbO{sub 6} (Ln = La, Pr, Nd)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Han, L.; Yao, C. G.; Meng, J. L.

The crystal structures, magnetic, and dielectric properties for the ordered double perovskites LnPbCoSbO{sub 6} (Ln = La, Pr, Nd) have been investigated. The crystal structure has been solved by Rietveld refinements of X-ray diffraction data in the monoclinic space group P2{sub 1}/n (No. 14). The Co{sup 2+} and Sb{sup 5+} ions are almost fully ordered over the B-site, and the octahedral framework displays significant tilting distortion according to the Glazer's tilt system a{sup –}a{sup –}c{sup +}. As the result of lanthanide contraction from La{sup 3+} to Nd{sup 3+}, the B-site sublattice distortions become stronger accompanying with the reduction of themore » tolerance factor and coordination number. The magnetization measurements show an antiferromagnetic ordering with large effective magnetic moments (μ{sub eff}) suggesting that the orbital component is significant. The maximum values of isothermal magnetization increase with the decrease in radii of rare earth ions, which is attributed to the weakening of antiferromagnetic interaction via Co{sup 2+}–O–Sb{sup 5+}–O–Co{sup 2+} paths. The dielectric constants present frequency dependence and monotonically decrease with the ionic radii reduction from La{sup 3+} to Nd{sup 3+} due to the suppression of electron transfer. These results indicate that the magnetic and dielectric properties can be tuned by controlling the degree of lattice distortion, which is realized by introducing different Ln{sup 3+} ions at the A-site.« less

Modeling and characterization of an electromagnetic system for the estimation of Frequency Response Function of spindle

NASA Astrophysics Data System (ADS)

Tlalolini, David; Ritou, Mathieu; Rabréau, Clément; Le Loch, Sébastien; Furet, Benoit

2018-05-01

The paper presents an electromagnetic system that has been developed to measure the quasi-static and dynamic behavior of machine-tool spindle, at different spindle speeds. This system consists in four Pulse Width Modulation amplifiers and four electromagnets to produce magnetic forces of ± 190 N for the static mode and ± 80 N for the dynamic mode up to 5 kHz. In order to measure the Frequency Response Function (FRF) of spindle, the applied force is required, which is a key issue. A dynamic force model is proposed in order to obtain the load from the measured current in the amplifiers. The model depends on the exciting frequency and on the magnetic characteristics of the system. The predicted force at high speed is validated with a specific experiment and the performance limits of the experimental device are investigated. The FRF obtained with the electromagnetic system is compared to a classical tap test measurement.

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning.

NASA Astrophysics Data System (ADS)

Uemura, Yasutomo

RENiO3 (RE = rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or hydrostatic pressure (V2O3) , they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. We demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These two cases correspond to the band-width tuning of Mott transitions, and also associated with structural phase transitions, Volume evolutions of antiferromagnetic transition from μSR will be compared to those of structure by x-ray and metallicity by nano optics, in first-order thermal Mott transition in a V2O3 film at ambient pressure. These results will be compared to the process of destruction of magnetic order in another Mott transition system Ba(Co,Ni)S2 in ``filling control'' without structural transition, and in parent compounds of high-Tc cuprates and Fe-based superconductors. We will also discuss roles of first-order quantum transition in generating soft modes relevant to magnetic resonance mode in unconventional superconductors. Work performed in collaboration with the groups of: J.A. Alonso (Madrid), H. Kageyama (Kyoto). E. Morenzoni (PSI), G.M. Luke (McMaster), C.Q. Jin (IOP Beijing), F.L. Ning (Zhejian), S.J.L. Billinge (Columbia), S. Shamoto, W. Higemoto (JAEA), A. Fujimori (Tokyo), A. Gauzzi (Paris), R. de Renzi (Parma), G. Kotliar (Rutgers), M. Imada (Tokyo), D. Basov (UCSD), I, Schuller (UCSD). supported by NSF DMR-1610633 and DMR-1436095 (DMREF).

Study of the effect of short ranged ordering on the magnetism in FeCr alloys

NASA Astrophysics Data System (ADS)

Jena, Ambika Prasad; Sanyal, Biplab; Mookerjee, Abhijit

2014-01-01

For the study of magnetism in systems where the local environment plays an important role, we propose a marriage between the Monte Carlo simulation and Zunger's special quasi-random structures. We apply this technique on disordered FeCr alloys and show that our estimates of the transition temperature is in good agreement with earlier experiments.

Superconducting Quantum Interferometers for Nondestructive Evaluation

PubMed Central

Kostyurina, E. A.; Kalashnikov, K. V.; Maslennikov, Yu. V.; Koshelets, V. P.

2017-01-01

We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-Tc DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-Tc DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna. PMID:29210980

Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn5.

PubMed

Park, Tuson; Ronning, F; Yuan, H Q; Salamon, M B; Movshovich, R; Sarrao, J L; Thompson, J D

2006-03-02

With only a few exceptions that are well understood, conventional superconductivity does not coexist with long-range magnetic order (for example, ref. 1). Unconventional superconductivity, on the other hand, develops near a phase boundary separating magnetically ordered and magnetically disordered phases. A maximum in the superconducting transition temperature T(c) develops where this boundary extrapolates to zero Kelvin, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity. Invariably, though, unconventional superconductivity masks the magnetic phase boundary when T < T(c), preventing proof of a magnetic quantum-critical point. Here we report specific-heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum-phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase, and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The T --> 0 K magnetic field-pressure phase diagram of CeRhIn5 is well described with a theoretical model developed to explain field-induced magnetism in the high-T(c) copper oxides, but in which a clear delineation of quantum-phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in copper oxides and heavy-electron systems such as CeRhIn5.

Magnifying lens for 800 MeV proton radiography.

PubMed

Merrill, F E; Campos, E; Espinoza, C; Hogan, G; Hollander, B; Lopez, J; Mariam, F G; Morley, D; Morris, C L; Murray, M; Saunders, A; Schwartz, C; Thompson, T N

2011-10-01

This article describes the design and performance of a magnifying magnetic-lens system designed, built, and commissioned at the Los Alamos National Laboratory (LANL) for 800 MeV flash proton radiography. The technique of flash proton radiography has been developed at LANL to study material properties under dynamic loading conditions through the analysis of time sequences of proton radiographs. The requirements of this growing experimental program have resulted in the need for improvements in spatial radiographic resolution. To meet these needs, a new magnetic lens system, consisting of four permanent magnet quadrupoles, has been developed. This new lens system was designed to reduce the second order chromatic aberrations, the dominant source of image blur in 800 MeV proton radiography, as well as magnifying the image to reduce the blur contribution from the detector and camera systems. The recently commissioned lens system performed as designed, providing nearly a factor of three improvement in radiographic resolution.

Magnifying lens for 800 MeV proton radiography

NASA Astrophysics Data System (ADS)

Merrill, F. E.; Campos, E.; Espinoza, C.; Hogan, G.; Hollander, B.; Lopez, J.; Mariam, F. G.; Morley, D.; Morris, C. L.; Murray, M.; Saunders, A.; Schwartz, C.; Thompson, T. N.

2011-10-01

This article describes the design and performance of a magnifying magnetic-lens system designed, built, and commissioned at the Los Alamos National Laboratory (LANL) for 800 MeV flash proton radiography. The technique of flash proton radiography has been developed at LANL to study material properties under dynamic loading conditions through the analysis of time sequences of proton radiographs. The requirements of this growing experimental program have resulted in the need for improvements in spatial radiographic resolution. To meet these needs, a new magnetic lens system, consisting of four permanent magnet quadrupoles, has been developed. This new lens system was designed to reduce the second order chromatic aberrations, the dominant source of image blur in 800 MeV proton radiography, as well as magnifying the image to reduce the blur contribution from the detector and camera systems. The recently commissioned lens system performed as designed, providing nearly a factor of three improvement in radiographic resolution.

First row transition metal atoms embedded in multivacancies in a rippled graphene system

NASA Astrophysics Data System (ADS)

Mombrú, Dominique; Faccio, Ricardo; Mombrú, Alvaro W.

2018-03-01

Ab-initio calculations based on density functional theory (DFT) have been performed to study systems where a first row transition metal atom is embedded in a rippled graphene due to the existence of an 8-order multivacancy. In addition to these cases, also the inclusion of a zinc atom, with a 3d10 electron configuration, was also studied. Structural distortions and magnetic response for each system were studied. A correlation was found for the magnitude of the rippling and the distortion in the vacancy. Variation in the trends was found for Cu and Zn cases, which were explained on the basis of the filling of the 3dx2-y2 orbital. All the systems exhibit lower magnetic moment in comparison to the metal-less system. The quenching of the magnetic moment due to the carbon atoms in the vacancy is observed for Sc and Cu.

Ordering and Excitations in the Field-Induced Magnetic Phase of Cs3Cr2Br9

NASA Astrophysics Data System (ADS)

Grenier, Beatrice

2006-03-01

Cs3Cr2Br9 is an interesting example of interacting spin-dimer system. As in other isotropic antiferromagnets such as Haldane or alternating chains and ladders, the ground state in zero field is a total spin singlet separated from the excited triplet by an energy gap. In a magnetic field H, a phase transition occurs at a critical field Hc1, where the gap to the lowest component of the Zeeman-split triplet closes. Above Hc1, field-induced magnetic order (FIMO) for spin components perpendicular to H is induced by inter-dimer or inter-chain couplings. The FIMO transition may be considered as a Bose-Einstein Condensation. Cs3Cr2Br9 differs from other dimer systems currently studied (e.g. PHCC, TlCuCl3) in two main ways: each Cr^3+ ion of the dimer has spin 3/2 rather than 1/2 for Cu-based systems and the arrangement of the dimers is hexagonal. This gives rise to anisotropy and frustration in a 3D lattice, respectively. The possibility of studying the magnetic ordering and the spin dynamics in a FIMO with sufficient detail to bring out features of frustration and anisotropy motivated the present neutron scattering study in Cs3Cr2Br9*. Two field orientations have been exploited, perpendicular and parallel to the easy axis c (direction of the dimers). First, I present the diffraction study: the FIMO displays large hysteresis incommensurability, showing the importance of frustration. The impact of anisotropy is seen in the magnetic structure, whose nature strongly depends on the field direction. Second, I focus on spin dynamics: it quantifies the presence of anisotropy and shows its crucial role on the energy gap at Hc1, which is measurably open or not, depending on whether H is perpendicular or parallel to c. Third, an explanation is proposed for the large value of the gap at higher field: it involves the mixing of higher order states (extended-FIMO), reflected by the absence of magnetization plateaus. Comparison with the sister Cs3Cr2Cl9 compound provides a test of this hypothesis. *B. Grenier et al., Phys. Rev. Lett. 92, 177202 (2004)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Larson, Amber M.; Wilfong, Brandon; Moetakef, Pouya

A metal–insulator transition tuned by application of an external magnetic field occurs in the quasi-one dimensional system Bi1.7V8O16, which contains a mix of S = 1 and S = 1/2 vanadium cations. Unlike all other known vanadates, the magnetic susceptibility of Bi1.7V8O16 diverges in its insulating state, although no long-range magnetic ordering is observed from neutron diffraction measurements, possibly due to the frustrated geometry of the triangular ladders. Magnetotransport measurements reveal that the transition temperature is suppressed upon application of an external magnetic field, from 62.5 K at zero field to 40 K at 8 T. This behavior is bothmore » hysteretic and anisotropic, suggesting t2g orbital ordering of the V3+ and V4+ cations drives a first-order structural transition. Single crystal X-ray diffraction reveals a charge density wave of Bi3+ cations with a propagation vector of 0.846c*, which runs parallel to the triangular chain direction. Neutron powder diffraction measurements show a first-order structural transition, characterized by the coexistence of two tetragonal phases near the metal–insulator transition. Finally, we discuss the likelihood that ferromagnetic V–V dimers coexist with a majority spin-singlet state below the transition in Bi1.7V8O16.« less

The physical mechanism of "inhomogeneous" magnetization transfer MRI

NASA Astrophysics Data System (ADS)

Manning, Alan P.; Chang, Kimberley L.; MacKay, Alex L.; Michal, Carl A.

2017-01-01

Inhomogeneous MT (ihMT) is a new magnetic resonance imaging technique that shows promise for myelin selectivity. Materials with a high proportion of lipids, such as white matter tissue, show a reduced intensity in magnetic resonance images acquired with selective prepulses at positive and negative offsets simultaneously compared to images with a single positive or negative offset prepulse of the same power. This effect was initially explained on the basis of hole-burning in inhomogeneously broadened lines of the lipid proton spin system. Our results contradict this explanation. ihMT in lipids can be understood with a simple spin-1 model of a coupled methylene proton pair. More generally, Provotorov theory can be used to consider the evolution of dipolar order in the non-aqueous spins during the prepulses. We show that the flip-angle dependence of the proton spectrum of a model lipid system (Prolipid-161) following dipolar order generation is in quantitative agreement with the model. In addition, we directly observe dipolar order and ihMT signals in the non-aqueous components of Prolipid-161 and homogeneously-broadened systems (hair, wood, and tendon) following ihMT prepulses. The observation of ihMT signals in tendon suggests that the technique may not be as specific to myelin as previously thought. Our work shows that ihMT occurs because of dipolar couplings alone, not from a specific type of spectral line broadening as its name suggests.

An induction heater device for studies of magnetic hyperthermia and specific absorption ratio measurements

NASA Astrophysics Data System (ADS)

Cano, M. E.; Barrera, A.; Estrada, J. C.; Hernandez, A.; Cordova, T.

2011-11-01

The development of a device for generating ac magnetic fields based on a resonant inverter is presented, which has been specially designed to carry out experiments of magnetic hyperthermia. By determining the electric current in the LC resonant circuit, a maximum intensity of magnetic field around of 15 mT is calculated, with a frequency around of 206 kHz. This ac magnetic field is able to heat powdered magnetic materials embedded in biological systems to be used in biomedical applications. Indeed, in order to evaluate the sensitivity of the device we also present the measurements of the specific absorption rate in phantoms performed with commercially prepared Fe3O4 and distilled water at different concentrations.

SYNTHESIS of MOLECULE/POLYMER-BASED MAGNETIC MATERIALS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miller, Joel S.

2016-02-01

We have synthesized and characterized several families of organic-based magnets, a new area showing that organic species can exhibit the technologically important property of magnetic ordering. Thin film magnets with ordering temperatures exceeding room temperature have been exceeded. Hence, organic-based magnets represent a new class of materials that exhibit magnetic ordering and do not require energy-intensive metallurgical processing and are based upon Earth-abundant elements.

Parameter Design and Optimal Control of an Open Core Flywheel Energy Storage System

NASA Technical Reports Server (NTRS)

Pang, D.; Anand, D. K.; Kirk, J. A.

1996-01-01

In low earth orbit (LEO) satellite applications spacecraft power is provided by photovoltaic cells and batteries. To overcome battery shortcomings the University of Maryland, working in cooperation with NASA/GSFC and NASA/LeRC, has developed a magnetically suspended flywheel for energy storage applications. The system is referred to as an Open Core Composite Flywheel (OCCF) energy storage system. Successful application of flywheel energy storage requires integration of several technologies, viz. bearings, rotor design, motor/generator, power conditioning, and system control. In this paper we present a parameter design method which has been developed for analyzing the linear SISO model of the magnetic bearing controller for the OCCF. The objective of this continued research is to principally analyze the magnetic bearing system for nonlinear effects in order to increase the region of stability, as determined by high speed and large air gap control. This is achieved by four tasks: (1) physical modeling, design, prototyping, and testing of an improved magnetically suspended flywheel energy storage system, (2) identification of problems that limit performance and their corresponding solutions, (3) development of a design methodology for magnetic bearings, and (4) design of an optimal controller for future high speed applications. Both nonlinear SISO and MIMO models of the magnetic system were built to study limit cycle oscillations and power amplifier saturation phenomenon observed in experiments. The nonlinear models include the inductance of EM coils, the power amplifier saturation, and the physical limitation of the flywheel movement as discussed earlier. The control program EASY5 is used to study the nonlinear SISO and MIMO models. Our results have shown that the characteristics and frequency responses of the magnetic bearing system obtained from modeling are comparable to those obtained experimentally. Although magnetic saturation is shown in the bearings, there are good correlations between the theoretical model and experimental data. Both simulation and experiment confirm large variations of the magnetic bearing characteristics due to air gap growth. Therefore, the gap growth effect should be considered in the magnetic bearing system design. Additionally, the magnetic bearing control system will be compared to other design methods using not only parameter design but H-infinity optimal control and mu synthesis.

The origin of transverse anisotropy in axially symmetric single molecule magnets.

PubMed

Barra, Anne-Laure; Caneschi, Andrea; Cornia, Andrea; Gatteschi, Dante; Gorini, Lapo; Heiniger, Leo-Philipp; Sessoli, Roberta; Sorace, Lorenzo

2007-09-05

Single-crystal high-frequency electron paramagnetic resonance spectroscopy has been employed on a truly axial single molecule magnet of formula [Mn(12)O(12)(tBu-CH(2)CO(2))16(CH(3)OH)4].CH(3)OH to investigate the origin of the transverse magnetic anisotropy, a crucial parameter that rules the quantum tunneling of the magnetization. The crystal structure, including the absolute structure of the crystal used for EPR experiments, has been fully determined and found to belong to I4 tetragonal space group. The angular dependence of the resonance fields in the crystallographic ab plane shows the presence of high-order tetragonal anisotropy and strong dependence on the MS sublevels with the second-highest-field transition being angular independent. This was rationalized including competing fourth- and sixth-order transverse parameters in a giant spin Hamiltonian which describes the magnetic anisotropy in the ground S = 10 spin state of the cluster. To establish the origin of these anisotropy terms, the experimental results have been further analyzed using a simplified multispin Hamiltonian which takes into account the exchange interactions and the single ion magnetic anisotropy of the Mn(III) centers. It has been possible to establish magnetostructural correlations with spin Hamiltonian parameters up to the sixth order. Transverse anisotropy in axial single molecule magnets was found to originate from the multispin nature of the system and from the breakdown of the strong exchange approximation. The tilting of the single-ion easy axes of magnetization with respect to the 4-fold molecular axis of the cluster plays the major role in determining the transverse anisotropy. Counterintuitively, the projections of the single ion easy axes on the ab plane correspond to hard axes of magnetization.

Heat flux measurements of Tb3M series (M=Co, Rh and Ru): Specific heat and magnetocaloric properties

NASA Astrophysics Data System (ADS)

Monteiro, J. C. B.; Lombardi, G. A.; dos Reis, R. D.; Freitas, H. E.; Cardoso, L. P.; Mansanares, A. M.; Gandra, F. G.

2016-12-01

We report on the magnetic properties and magnetocaloric effect (MCE) for the Tb3M series, with M=Co, Rh and Ru, obtained using a heat flux technique. The specific heat of Tb3Co and Tb3Rh are very similar, with a first order type transition occurring around 6 K below the magnetic ordering temperature without any corresponding feature on the magnetization. The slightly enhanced electronic specific heat, the Debye temperature around 150 K and the presence of the magnetic specific heat well above the ordering temperature are also characteristic of many other compounds of the R3M family (R=Rare Earth). The specific heat for Tb3Ru, however, presents two peaks at 37 K and 74 K. The magnetization shows that below the first peak the system presents an antiferromagnetic behavior and is paramagnetic above 74 K. We obtained a magnetocaloric effect for M=Co and Rh, -∆S=12 J/kg K, but for Tb3Ru it is less than 3 J/kg K (μ0∆H=5 T). We believe that the experimental results show that the MCE is directly related with the process of hybridization of the (R)5d-(M)d electrons that occurs in the R3M materials.

REVIEWS OF TOPICAL PROBLEMS: Order-disorder transformations and phase equilibria in strongly nonstoichiometric compounds

NASA Astrophysics Data System (ADS)

Gusev, Aleksandr I.

2000-01-01

Data on order-disorder phase transformations in strongly nonstoichiometric carbides and nitrides MXy (X=C, N) of Group IV and V transition metals at temperatures below 1300-1400 K are reviewed. The order-parameter functional method as applied to atomic and vacancy ordering in strongly nonstoichiometric MXy compounds and to phase equilibrium calculations for M-X systems is discussed. Phase diagram calculations for the Ti-C, Zr-C, Hf-C, V-C, Nb-C, Ta-C, Ti-N, and Ti-B-C systems (with the inclusion of the ordering of nonstoichiometric carbides and nitrides) and those for pseudobinary carbide M(1)C-M(2)C systems are presented. Heat capacity, electrical resistivity and magnetic susceptibility changes at reversible order-disorder phase transformations in nonstoichiometric carbides are considered.

Nonuniform radiation damage in permanent magnet quadrupoles.

PubMed

Danly, C R; Merrill, F E; Barlow, D; Mariam, F G

2014-08-01

We present data that indicate nonuniform magnetization loss due to radiation damage in neodymium-iron-boron Halbach-style permanent magnet quadrupoles. The proton radiography (pRad) facility at Los Alamos uses permanent-magnet quadrupoles for magnifying lenses, and a system recently commissioned at GSI-Darmsdadt uses permanent magnets for its primary lenses. Large fluences of spallation neutrons can be produced in close proximity to these magnets when the proton beam is, intentionally or unintentionally, directed into the tungsten beam collimators; imaging experiments at LANL's pRad have shown image degradation with these magnetic lenses at proton beam doses lower than those expected to cause damage through radiation-induced reduction of the quadrupole strength alone. We have observed preferential degradation in portions of the permanent magnet quadrupole where the field intensity is highest, resulting in increased high-order multipole components.

Itinerant G-type antiferromagnetic order in SrCr 2 As 2

DOE PAGES

Das, Pinaki; Sangeetha, N. S.; Lindemann, George R.; ...

2017-07-07

Here, neutron-diffraction and magnetic susceptibility studies of polycrystalline SrCr 2As 2 reveal that this compound is an itinerant G-type antiferromagnet below the Néel temperature T N = 590(5) K with the Cr magnetic moments aligned along the tetragonal c axis. The system remains tetragonal to the lowest measured temperature (~12 K). The lattice parameter ratio c/a and the magnetic moment saturate at about the same temperature below ~200 K, indicating a possible magnetoelastic coupling. The ordered moment μ = 1.9(1)μ B/Cr, measured at T = 12 K, is significantly reduced compared to its localized value (4μ B/Cr) due to themore » itinerant character brought about by hybridization between the Cr 3d and As 4p orbitals.« less

Itinerant G-type antiferromagnetic order in SrCr2As2

NASA Astrophysics Data System (ADS)

Das, Pinaki; Sangeetha, N. S.; Lindemann, George R.; Heitmann, T. W.; Kreyssig, A.; Goldman, A. I.; McQueeney, R. J.; Johnston, D. C.; Vaknin, D.

2017-07-01

Neutron-diffraction and magnetic susceptibility studies of polycrystalline SrCr2As2 reveal that this compound is an itinerant G-type antiferromagnet below the Néel temperature TN = 590(5) K with the Cr magnetic moments aligned along the tetragonal c axis. The system remains tetragonal to the lowest measured temperature (˜12 K). The lattice parameter ratio c /a and the magnetic moment saturate at about the same temperature below ˜200 K, indicating a possible magnetoelastic coupling. The ordered moment μ =1.9 (1 ) μB /Cr , measured at T =12 K, is significantly reduced compared to its localized value (4 μB /Cr ) due to the itinerant character brought about by hybridization between the Cr 3 d and As 4 p orbitals.

Itinerant G-type antiferromagnetic order in SrCr 2 As 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Das, Pinaki; Sangeetha, N. S.; Lindemann, George R.

Here, neutron-diffraction and magnetic susceptibility studies of polycrystalline SrCr 2As 2 reveal that this compound is an itinerant G-type antiferromagnet below the Néel temperature T N = 590(5) K with the Cr magnetic moments aligned along the tetragonal c axis. The system remains tetragonal to the lowest measured temperature (~12 K). The lattice parameter ratio c/a and the magnetic moment saturate at about the same temperature below ~200 K, indicating a possible magnetoelastic coupling. The ordered moment μ = 1.9(1)μ B/Cr, measured at T = 12 K, is significantly reduced compared to its localized value (4μ B/Cr) due to themore » itinerant character brought about by hybridization between the Cr 3d and As 4p orbitals.« less

Electron spin polarization by isospin ordering in correlated two-layer quantum Hall systems.

PubMed

Tiemann, L; Wegscheider, W; Hauser, M

2015-05-01

Enhancement of the electron spin polarization in a correlated two-layer, two-dimensional electron system at a total Landau level filling factor of 1 is reported. Using resistively detected nuclear magnetic resonance, we demonstrate that the electron spin polarization of two closely spaced two-dimensional electron systems becomes maximized when interlayer Coulomb correlations establish spontaneous isospin ferromagnetic order. This correlation-driven polarization dominates over the spin polarizations of competing single-layer fractional quantum Hall states under electron density imbalances.

Ferromagnetic spin-correlations in strained LaCoO3 thin films

NASA Astrophysics Data System (ADS)

Freeland, J. W.; Ma, J. X.; Shi, J.

2008-11-01

We present an element-resolved study of the valence and magnetic properties of LaCoO3 thin films grown via pulsed laser deposition. The Co L edge x-ray absorption shows that ferromagnetic (FM) order arises from a slight hole doping of the system presumably due to nonstoichiometry, which in the bulk system disrupts the low-spin state. However, even though the films are hole doped, the magnetic moments under tensile strain are much larger than the bulk system indicating that the strain can greatly increase the FM fraction observed in the spin-glass regime at low doping.

Optical investigation of the strong spin-orbit-coupled magnetic semimetal YbMnBi2

NASA Astrophysics Data System (ADS)

Chaudhuri, Dipanjan; Cheng, Bing; Yaresko, Alexander; Gibson, Quinn D.; Cava, R. J.; Armitage, N. P.

2017-08-01

Strong spin-orbit coupling (SOC) can result in ground states with nontrivial topological properties. The situation is even richer in magnetic systems where the magnetic ordering can potentially have strong influence over the electronic band structure. The class of A MnBi2 (A = Sr, Ca) compounds are important in this context as they are known to host massive Dirac fermions with strongly anisotropic dispersion, which is believed to be due to the interplay between strong SOC and magnetic degrees of freedom. We report the optical conductivity of YbMnBi2, a newly discovered member of this family and a proposed Weyl semimetal (WSM) candidate with broken time reversal symmetry. Together with density functional theory (DFT) band-structure calculations, we show that the complex conductivity can be interpreted as the sum of an intraband Drude response and interband transitions. We argue that the canting of the magnetic moments that has been proposed to be essential for the realization of the WSM in an otherwise antiferromagnetically ordered system is not necessary to explain the optical conductivity. We believe our data is explained qualitatively by the uncanted magnetic structure with a small offset of the chemical potential from strict stochiometry. We find no definitive evidence of a bulk Weyl nodes. Instead, we see signatures of a gapped Dirac dispersion, common in other members of A MnBi2 family or compounds with similar 2D network of Bi atoms. We speculate that the evidence for a WSM seen in ARPES arises through a surface magnetic phase. Such an assumption reconciles all known experimental data.

Antiferromagnetic Ordering in Quasi-Triangular Localized Spin System, β'-Et2Me2P[Pd(dmit)2]2, Studied by 13C NMR

NASA Astrophysics Data System (ADS)

Otsuka, Kei; Iikubo, Hideaki; Kogure, Takayuki; Takano, Yoshiki; Hiraki, Ko-ichi; Takahashi, Toshihiro; Cui, Hengbo; Kato, Reizo

2014-05-01

We performed 13C NMR measurements of a selectively 13C isotope-labeled single-crystal sample of a frustrated spin system, β'-Et2Me2P[Pd(dmit)2]2. A long-range antiferromagnetic (AF) ordering below 17 K was confirmed by the observation of NMR spectrum broadening and well split resonance lines at lower temperatures. NMR spectra in the AF state can be well explained by a two sublattice model. From the analysis of the angular dependence of the NMR spectrum, we clarified the magnetic structure in the AF state, where the easy and hard axes are the crystallographic c*- and b-axes, respectively, and the effective localized moments are quite small, ˜0.28 μB/dimer. This suggests a strong quantum fluctuation effect due to magnetic frustrations in a quasi-triangular spin-1/2 system.

Searching the Nearest Stars for Exoplanetary Radio Emission: VLA and LOFAR Observations

NASA Astrophysics Data System (ADS)

Knapp, Mary; Winterhalter, Daniel; Lazio, Joseph

2016-10-01

Six of the eight solar system planets and one moon (Ganymede) exhibit present-day dynamo magnetic fields. To date, however, there are no conclusive detections of exoplanetary magnetic fields. Low frequency radio emission via the cyclotron maser instability (CMI) from interactions between a planet and the solar/stellar wind is the most direct means of detecting and characterizing planetary/exoplanetary magnetic fields. We have undertaken a survey of the very nearest stars in low frequency radio (30 MHz - 4 GHz) in order to search for yet-undiscovered planets. The closest stars are chosen in order to reduce the attenuation of the magnetospheric radio signal by distance dilution, thereby increasing the chances of making a detection if a planet with a strong magnetic field is present. The VLA telescope (P-band: 230-470 MHz, L-band: 1-2 GHz, S-band: 2-4 GHz) and LOFAR telescope (LBA: 30-75 MHz) have been used to conduct this survey.This work focuses on VLA and LOFAR observations of an M-dwarf binary system: GJ 725. We present upper limits on radio flux as a function of frequency. Since the peak emission frequency of CMI-type emission is the local plasma frequency in the emission region, the peak frequency of planetary radio emission is a direct proxy for the magnetic field strength of the planet. Our spectral irradiance upper limits therefore represent upper limits on the magnetic field strengths of any planets in the GJ 725 system.Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Strugarek, A., E-mail: antoine.strugarek@cea.fr, E-mail: strugarek@astro.umontreal.ca

Planets in close-in orbit interact with the magnetized wind of their hosting star. This magnetic interaction was proposed to be a source for enhanced emissions in the chromosphere of the star, and to participate in setting the migration timescale of the close-in planet. The efficiency of the magnetic interaction is known to depend on the magnetic properties of the host star and of the planet, and on the magnetic topology of the interaction. We use a global, three-dimensional numerical model of close-in star–planet systems, based on the magnetohydrodynamics approximation, to compute a grid of simulations for varying properties of the orbitingmore » planet. We propose a simple parametrization of the magnetic torque that applies to the planet, and of the energy flux generated by the interaction. The dependency upon the planet properties and the wind properties is clearly identified in the derived scaling laws, which can be used in secular evolution codes to take into account the effect of magnetic interactions in planet migration. They can also be used to estimate a potential magnetic source of enhanced emissions in observed close-in star–planet systems, in order to constrain observationally possible exoplanetary magnetic fields.« less

Design of digital load torque observer in hybrid electric vehicle

NASA Astrophysics Data System (ADS)

Sun, Yukun; Zhang, Haoming; Wang, Yinghai

2008-12-01

In hybrid electric vehicle, engine begain to work only when motor was in high speed in order to decrease tail gas emission. However, permanent magnet motor was sensitive to its load, adding engine to the system always made its speed drop sharply, which caused engine to work in low efficiency again and produced much more environment pollution. Dynamic load torque model of permanent magnet synchronous motor is established on the basic of motor mechanical equation and permanent magnet synchronous motor vector control theory, Full- digital load torque observer and compensation control system is made based on TMS320F2407A. Experiment results prove load torque observer and compensation control system can detect and compensate torque disturbing effectively, which can solve load torque disturbing and decrease gas pollution of hybrid electric vehicle.

Comparative magnetic measurements on social insects

NASA Astrophysics Data System (ADS)

Ferreira, Jandira; Cernicchiaro, Geraldo; Winklhofer, Michael; Dutra, Humberto; de Oliveira, Paulo S.; S. Esquivel, Darci M.; Wajnberg, Eliane

2005-03-01

Biogenic magnetite has been detected in several species of social insects and may well form the basis of a magnetic sensory system in these animals, although other physiological functions are possible, too. We report here on hysteresis measurements on honeybees ( Apis mellifera) and the termite Neocapritermes opacus. The ratio of saturation remanence to saturation magnetization, Jrs/ Js, was determined as 0.11 (0.15) in bees (termite), the coercive force Hc as 90 (50 Oe). The magnetic remanence is generally low (of the order of 10 -6 emu per individual). The values obtained are similar to the ones reported previously on a migratory ant species, which suggests that biomineralization of magnetic material in social insects may underlie a generic process.

Beam director design report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Younger, F.C.

1986-08-01

A design and fabrication effort for a beam director is documented. The conceptual design provides for the beam to pass first through a bending and focusing system (or ''achromat''), through a second achromat, through an air-to-vacuum interface (the ''beam window''), and finally through the vernier steering system. Following an initial concept study for a beam director, a prototype permanent magnet 30/sup 0/ beam-bending achromat and prototype vernier steering magnet were designed and built. In volume II, copies are included of the funding instruments, requests for quotations, purchase orders, a complete set of as-built drawings, magnetic measurement reports, the concept designmore » report, and the final report on the design and fabrication project. (LEW)« less

Engineered materials for all-optical helicity-dependent magnetic switching

NASA Astrophysics Data System (ADS)

Mangin, S.; Gottwald, M.; Lambert, C.-H.; Steil, D.; Uhlíř, V.; Pang, L.; Hehn, M.; Alebrand, S.; Cinchetti, M.; Malinowski, G.; Fainman, Y.; Aeschlimann, M.; Fullerton, E. E.

2014-03-01

The possibility of manipulating magnetic systems without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of the magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization in engineered magnetic materials. We demonstrate that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. These results challenge present theories of AO-HDS and provide a pathway to engineering materials for future applications based on all-optical control of magnetic order.

Mixed-order phase transition in a colloidal crystal.

PubMed

Alert, Ricard; Tierno, Pietro; Casademunt, Jaume

2017-12-05

Mixed-order phase transitions display a discontinuity in the order parameter like first-order transitions yet feature critical behavior like second-order transitions. Such transitions have been predicted for a broad range of equilibrium and nonequilibrium systems, but their experimental observation has remained elusive. Here, we analytically predict and experimentally realize a mixed-order equilibrium phase transition. Specifically, a discontinuous solid-solid transition in a 2D crystal of paramagnetic colloidal particles is induced by a magnetic field [Formula: see text] At the transition field [Formula: see text], the energy landscape of the system becomes completely flat, which causes diverging fluctuations and correlation length [Formula: see text] Mean-field critical exponents are predicted, since the upper critical dimension of the transition is [Formula: see text] Our colloidal system provides an experimental test bed to probe the unconventional properties of mixed-order phase transitions.

Mixed-order phase transition in a colloidal crystal

NASA Astrophysics Data System (ADS)

Alert, Ricard; Tierno, Pietro; Casademunt, Jaume

2017-12-01

Mixed-order phase transitions display a discontinuity in the order parameter like first-order transitions yet feature critical behavior like second-order transitions. Such transitions have been predicted for a broad range of equilibrium and nonequilibrium systems, but their experimental observation has remained elusive. Here, we analytically predict and experimentally realize a mixed-order equilibrium phase transition. Specifically, a discontinuous solid-solid transition in a 2D crystal of paramagnetic colloidal particles is induced by a magnetic field H. At the transition field Hs, the energy landscape of the system becomes completely flat, which causes diverging fluctuations and correlation length ξ∝|H2-Hs2|-1/2. Mean-field critical exponents are predicted, since the upper critical dimension of the transition is du=2. Our colloidal system provides an experimental test bed to probe the unconventional properties of mixed-order phase transitions.

TU-H-BRA-02: The Physics of Magnetic Field Isolation in a Novel Compact Linear Accelerator Based MRI-Guided Radiation Therapy System

DOE Office of Scientific and Technical Information (OSTI.GOV)

Low, D; Mutic, S; Shvartsman, S

Purpose: To develop a method for isolating the MRI magnetic field from field-sensitive linear accelerator components at distances close to isocenter. Methods: A MRI-guided radiation therapy system has been designed that integrates a linear accelerator with simultaneous MR imaging. In order to accomplish this, the magnetron, port circulator, radiofrequency waveguide, gun driver, and linear accelerator needed to be placed in locations with low magnetic fields. The system was also required to be compact, so moving these components far from the main magnetic field and isocenter was not an option. The magnetic field sensitive components (exclusive of the waveguide) were placedmore » in coaxial steel sleeves that were electrically and mechanically isolated and whose thickness and placement were optimized using E&M modeling software. Six sets of sleeves were placed 60° apart, 85 cm from isocenter. The Faraday effect occurs when the direction of propagation is parallel to the magnetic RF field component, rotating the RF polarization, subsequently diminishing RF power. The Faraday effect was avoided by orienting the waveguides such that the magnetic field RF component was parallel to the magnetic field. Results: The magnetic field within the shields was measured to be less than 40 Gauss, significantly below the amount needed for the magnetron and port circulator. Additional mu-metal was employed to reduce the magnetic field at the linear accelerator to less than 1 Gauss. The orientation of the RF waveguides allowed the RT transport with minimal loss and reflection. Conclusion: One of the major challenges in designing a compact linear accelerator based MRI-guided radiation therapy system, that of creating low magnetic field environments for the magnetic-field sensitive components, has been solved. The measured magnetic fields are sufficiently small to enable system integration. This work supported by ViewRay, Inc.« less

Magnetically-induced electric polarization in an organo-metallic magnet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zapf, W S; Fabris, F W; Balakirev, F F

2009-01-01

The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition metal oxides. The strongest coupling is obtained in so-called magnetically induced multiferroics where ferroelectricity arises directly from magnetic order that breaks inversion symmetry. However, it has been difficult to find non-oxide based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organometallic quantum magnet containing S =1/1 Cu spins, in which a switchable electric polarization arises from field-tuned magnetic order. Fast magnetic field pulses allow us to perform sensitive measurements of the electricmore » polarization and demonstrate that the electric state is present only if the magnetic order is non-collinear. Furthermore, we show that the electric polarization can be switched in a stunning hysteretic fashion. Because the magnetic order in CDC is mediated by large organic molecules, our study shows that magnetoelectric interactions can exist in this important class of materials, opening the road to designing magnetoelectrics and multiferroics using large molecules as building blocks. Further, we demonstrate that CDC undergoes a magnetoelectric quantum phase transition -the first of its kind, where both ferroelectric and magnetic order emerge simultaneously as a function of magnetic field at very low temperatures.« less

Two-year magnetic monitoring in conjunction with geochemical and electron microscopic data of roadside dust in Seoul, Korea

NASA Astrophysics Data System (ADS)

Kim, Wonnyon; Doh, Seong-Jae; Park, Yong-Hee; Yun, Seong-Taek

Mineral magnetic properties of roadside dusts in Seoul, Korea, were measured and compared with the results of geochemical analyses in order to investigate the spatio-temporal patterns of urban pollution. Scanning electron microscope (SEM) observations and energy dispersive X-ray spectroscopy (EDS) analyses were carried out to verify the magnetic materials and their potential sources. A total of 1956 dust samples were collected monthly at eight sites, from June 1998 to June 2000. Thermomagnetic data and SEM observations for magnetic extracts indicated that the major magnetic phase was magnetite-like material. In particular, the highest and the lowest magnetic concentrations were observed in industrial areas and a park area, respectively, whereas, heavy traffic areas showed low to intermediate concentration. A linear correlation between enrichment indexes of magnetic susceptibility and heavy metals suggests that magnetic susceptibility can be used as a proxy for heavy metal pollution. The magnetic concentrations and magnetic particle sizes showed systematic seasonal fluctuations (high and large in winter versus low and small in summer) due to the seasonal influx variations of anthropogenic magnetic materials. On the basis of the morphology and elemental composition, the magnetic materials were grouped into three types: magnetic spherules possibly emitted from factories and domestic heating systems, aggregates derived from vehicle emission or motor vehicle brake system, and angular magnetic particles of natural origin.

Effect of the magnetic dipole interaction on a spin-1 system

NASA Astrophysics Data System (ADS)

Hu, Fangqi; Jia, Wei; Zhao, Qing

2018-05-01

We consider a hybrid system composed of a spin-1 triplet coupled to a nuclear spin. We study the effect of the axisymmetric and the quadrupole term of the magnetic dipole interaction between the two electrons forming the triplet on the energy spectrum in a static magnetic field. The energy spectrum obtained by directly diagonalizing the Hamiltonian of the system shows that these two terms not only remove the special crossings that appear in the absence of the magnetic dipole interaction, but also produce new (avoided) crossings by lifting the relevant levels. Specially, the gaps between the avoided crossing levels increase with the strength of the quadrupole term. In order to accurately illustrate these effects, we present the results for the discriminant and von Neumann entropy of one electron interacting with the rest of the whole system. Finally, by numerically solving the time-dependent Schrödinger equations of the system, we discover that the polarization oscillation of electron and nuclear spin is in-phase and the total average longitudinal spin is not conserved at location of avoided crossing, but the two results are opposite beyond that.

Magnetostructural coupling behavior at the ferromagnetic transition in double-perovskite S r2FeMo O6

NASA Astrophysics Data System (ADS)

Yang, Dexin; Harrison, Richard J.; Schiemer, Jason A.; Lampronti, Giulio I.; Liu, Xueyin; Zhang, Fenghua; Ding, Hao; Liu, Yan'gai; Carpenter, Michael A.

2016-01-01

The ordered double-perovskite S r2FeMo O6 (SFMO) possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combined structural and magnetic instabilities that are responsible for a cubic-tetragonal phase transition near 420 K. A formal strain analysis combined with measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to Γ4+ and m Γ4+ of parent space group F m 3 ¯m . The observed softening of the shear modulus by ˜50% is due to the classical effects of strain/order parameter coupling at an improper ferroelastic (Γ4+) transition which is second order in character, while the ferromagnetic order parameter (m Γ4+ ) couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B sites, is to change the strength of coupling between the Γ4+ order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.

Critical behavior of a quantum chain with four-spin interactions in the presence of longitudinal and transverse magnetic fields.

PubMed

Boechat, B; Florencio, J; Saguia, A; de Alcantara Bonfim, O F

2014-03-01

We study the ground-state properties of a spin-1/2 model on a chain containing four-spin Ising-like interactions in the presence of both transverse and longitudinal magnetic fields. We use entanglement entropy and finite-size scaling methods to obtain the phase diagrams of the model. Our numerical calculations reveal a rich variety of phases and the existence of multicritical points in the system. We identify phases with both ferromagnetic and antiferromagnetic orderings. We also find periodically modulated orderings formed by a cluster of like spins followed by another cluster of opposite like spins. The quantum phases in the model are found to be separated by either first- or second-order transition lines.

Micromagnetics and second-order reversal-curves as a route to understanding FORC diagrams of nanoparticles

NASA Astrophysics Data System (ADS)

Winklhofer, M.

2007-05-01

First-order-reversal curve (FORC) diagrams have proven useful in characterizing fine magnetic particle systems in terms of microscopic switching field distributions, characteristic interaction strengths and mean-field effects. Despite the profusion of measured FORC data, we still lack a simple, generally valid recipe for the quantitative analysis of FORC diagrams, the reason being that most samples do not act like classical linear Preisach systems, giving rise to reversible magnetization changes that tend to blur contributions from irreversible switching events. A good example illustrating the confounding influence of reversible contributions are FORC diagrams for particle systems in which vortex configurations occur as remanent states. For non-interacting Fe nanodots with well-defined grain sizes around the zero-field SD/PSD transition and random easy-axis orientation, we will show how a combination of micromagnetic modelling and second-order- reversal-curves can be used to disentangle reversible and irreversible contributions to the FORC diagram. It will also be shown that remanence-based Preisach diagrams do not fully capture the irreversible parts.

Ordering tendencies and electronic properties in quaternary Heusler derivatives

NASA Astrophysics Data System (ADS)

Neibecker, Pascal; Gruner, Markus E.; Xu, Xiao; Kainuma, Ryosuke; Petry, Winfried; Pentcheva, Rossitza; Leitner, Michael

2017-10-01

The phase stabilities and ordering tendencies in the quaternary full-Heusler alloys NiCoMnAl and NiCoMnGa have been investigated by in situ neutron diffraction, calorimetry, and magnetization measurements. NiCoMnGa was found to adopt the L 21 structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the B 2 phase with disorder also between Mn and Ga was observed at 1160 K . In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient L 21 ordering, otherwise the system displays only B 2 order. Linked to L 21 ordering, a drastic increase in the magnetic transition temperature was observed in NiCoMnAl, while annealing affected the magnetic behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder. First principles calculations were employed to study the thermodynamics as well as order-dependent electronic properties of both compounds. It was found that a near half-metallic pseudogap emerges in the minority spin channel only for the completely ordered Y structure. However, this structure is energetically unstable compared to a tetragonal structure with alternating layers of Ni and Co, which is predicted to be the low-temperature ground state. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies.

Magnetic phase transitions and magnetization reversal in MnRuP

NASA Astrophysics Data System (ADS)

Lampen-Kelley, P.; Mandrus, D.

The ternary phosphide MnRuP is an incommensurate antiferromagnetic metal crystallizing in the non-centrosymmetric Fe2P-type crystal structure. Below the Neel transition at 250 K, MnRuP exhibits hysteretic anomalies in resistivity and magnetic susceptibility curves as the propagation vectors of the spiral spin structure change discontinuously across T1 = 180 K and T2 = 100 K. Temperature-dependent X-ray diffraction data indicate that the first-order spin reorientation occurs in the absence of a structural transition. A strong magnetization reversal (MR) effect is observed upon cooling the system through TN in moderate dc magnetic fields. Positive magnetization is recovered on further cooling through T1 and maintained in subsequent warming curves. The field dependence and training of the MR effect in MnRuP will be discussed in terms of the underlying magnetic structures and compared to anomalous MR observed in vanadate systems. This work is supported by the Gordon and Betty Moore Foundation GBMF4416 and U.S. DOE, Office of Science, BES, Materials Science and Engineering Division.

Magnetism in thin transition metal alloys

NASA Astrophysics Data System (ADS)

Janke-Gilman, Nathaniel; Reade

Magnetic linear dichroism measurements allowed us to measure atomic moments and spin order in alloy magnetic systems with chemical specificity and surface sensitivity. The width of the dichroism spectrum is a measure of the atomic moment via the local exchange, while the dichroism amplitude is a measure of the elemental contribution to magnetic order in the alloy via the dipole selection rules. A novel method has been introduced to systematically determine the dichroism width and amplitude. Changing magnetic moments have been tracked with changing alloy composition, along with changes in the magnetic easy axis and Curie temperature. Measurements have been made of the bandstructure and band topology near the Fermi energy. Well defined spin and k states are selected using high energy and k resolution. The 'Stoner gap' in d bands near the Fermi energy is equal to the minimum energy spin-flip excitation available to d electrons in particular symmetry states. The size and shape of the sp band Fermi surface in momentum space determines the periodicity of oscillatory magnetic coupling. The exchange splitting in the sp bands is one measure of changing magnetization in a magnetic alloy, while the spin dependent mean free path is the inverse of the band width dk. The strong variation of these effects from one magnetic impurity to another supports the concept of magnetic impurity doping in magnetoelectronic devices. When the thickness of a magnetic system is sufficiently reduced, the finite size effect leads to reduction in the critical temperature Tc with decreasing thickness n according to the power law 1 - Tc(n)/Tc(bulk) = b n^lambda.Deviations from this power law have been observed by many authors in the ultrathin film limit (2--3 monolayers or less). We have shown that these deviations from power law behavior arise when the film thickness becomes less than the mean range of spin-spin interactions in the magnetic film, at which point the reduced surface free energy term dominates. The quantity b provides a measure of this range of spin-spin interactions. The range of magnetic interactions scales with the mean free path of minority spins.

Dynamics of Permanent-Magnet Biased Active Magnetic Bearings

NASA Technical Reports Server (NTRS)

Fukata, Satoru; Yutani, Kazuyuki

1996-01-01

Active magnetic radial bearings are constructed with a combination of permanent magnets to provide bias forces and electromagnets to generate control forces for the reduction of cost and the operating energy consumption. Ring-shaped permanent magnets with axial magnetization are attached to a shaft and share their magnet stators with the electromagnets. The magnet cores are made of solid iron for simplicity. A simplified magnetic circuit of the combined magnet system is analyzed with linear circuit theory by approximating the characteristics of permanent magnets with a linear relation. A linearized dynamical model of the control force is presented with the first-order approximation of the effects of eddy currents. Frequency responses of the rotor motion to disturbance inputs and the motion for impulsive forces are tested in the non-rotating state. The frequency responses are compared with numerical results. The decay of rotor speed due to magnetic braking is examined. The experimental results and the presented linearized model are similar to those of the all-electromagnetic design.

Contactless and absolute linear displacement detection based upon 3D printed magnets combined with passive radio-frequency identification

NASA Astrophysics Data System (ADS)

Windl, Roman; Abert, Claas; Bruckner, Florian; Huber, Christian; Vogler, Christoph; Weitensfelder, Herbert; Suess, Dieter

2017-11-01

Within this work a passive and wireless magnetic sensor, to monitor linear displacements, is proposed. We exploit recent advances in 3D printing and fabricate a polymer bonded magnet with a spatially linear magnetic field component corresponding to the length of the magnet. Regulating the magnetic compound fraction during printing allows specific shaping of the magnetic field distribution. A giant magnetoresistance magnetic field sensor is combined with a radio-frequency identification tag in order to passively monitor the exerted magnetic field of the printed magnet. Due to the tailored magnetic field, a displacement of the magnet with respect to the sensor can be detected within the sub-mm regime. The sensor design provides good flexibility by controlling the 3D printing process according to application needs. Absolute displacement detection using low cost components and providing passive operation, long term stability, and longevity renders the proposed sensor system ideal for structural health monitoring applications.

Destabilization of Magnetic Order in a Dilute Kitaev Spin Liquid Candidate

DOE PAGES

Lampen-Kelley, Paige; Banerjee, Arnab; Aczel, Adam A.; ...

2017-12-06

The insulating honeycomb magnet α–RuCl 3 exhibits fractionalized excitations that signal its proximity to a Kitaev quantum spin liquid state; however, at T=0, fragile long-range magnetic order arises from non-Kitaev terms in the Hamiltonian. Spin vacancies in the form of Ir 3+ substituted for Ru are found to destabilize this long-range order. Neutron diffraction and bulk characterization of Ru 1–xIr xCl 3 show that the magnetic ordering temperature is suppressed with increasing x, and evidence of zizag magnetic order is absent for x > 0.3. Inelastic neutron scattering demonstrates that the signature of fractionalized excitations is maintained over the fullmore » range of x investigated. In conclusion, the depleted lattice without magnetic order thus hosts a spin-liquid-like ground state that may indicate the relevance of Kitaev physics in the magnetically dilute limit of RuCl 3.« less

Destabilization of Magnetic Order in a Dilute Kitaev Spin Liquid Candidate

NASA Astrophysics Data System (ADS)

Lampen-Kelley, P.; Banerjee, A.; Aczel, A. A.; Cao, H. B.; Stone, M. B.; Bridges, C. A.; Yan, J.-Q.; Nagler, S. E.; Mandrus, D.

2017-12-01

The insulating honeycomb magnet α -RuCl3 exhibits fractionalized excitations that signal its proximity to a Kitaev quantum spin liquid state; however, at T =0 , fragile long-range magnetic order arises from non-Kitaev terms in the Hamiltonian. Spin vacancies in the form of Ir3 + substituted for Ru are found to destabilize this long-range order. Neutron diffraction and bulk characterization of Ru1 -xIrxCl3 show that the magnetic ordering temperature is suppressed with increasing x , and evidence of zizag magnetic order is absent for x >0.3 . Inelastic neutron scattering demonstrates that the signature of fractionalized excitations is maintained over the full range of x investigated. The depleted lattice without magnetic order thus hosts a spin-liquid-like ground state that may indicate the relevance of Kitaev physics in the magnetically dilute limit of RuCl3 .

Electrical and Magnetic Properties of Binary Amorphous Transition Metal Alloys.

NASA Astrophysics Data System (ADS)

Liou, Sy-Hwang

The electrical, superconductive and magnetic properties of several binary transition metal amorphous and metastable crystalline alloys, Fe(,x)Ti(,100-x) (30 (LESSTHEQ) x (LESSTHEQ) 100), Fe(,x)Zr(,100-x) (20 (LESSTHEQ) x (LESSTHEQ) 93), Fe(,x)Hf(,100-x) (20 (LESSTHEQ) x (LESSTHEQ) 100), Fe(,x)Nb(,100 -x) (22 (LESSTHEQ) x (LESSTHEQ) 85), Ni(,x)Nb(,100-x) (20 (LESSTHEQ) x (LESSTHEQ) 80), Cu(,x)Nb(,100-x) (10 (LESSTHEQ) x (LESSTHEQ) 90) were studied over a wide composition range. Films were made using a magnetron sputtering system, and the structure of the films was investigated by energy dispersive x-ray diffraction. The composition region of each amorphous alloys system was determined and found in good agreement with a model proposed by Egami and Waseda. The magnetic properties and hyperfine interactions in the films were investigated using a conventional Mossbauer spectrometer and a ('57)Co in Rh matrix source. In all Fe-early transition metal binary alloys systems, Fe does not retain its moment in the low iron concentration region and the result is that the critical concentration for magnetic order (x(,c)) is much larger than anticipated from percolation considerations. A direct comparison between crystalline alloys and their amorphous counterparts of the same composition illustrate no clear correlation between crystalline and amorphous states. Pronounced discontinuities in the magnetic properties with variation in Fe content of all Fe-early transition metal alloys at phase boundaries separating amorphous and crystalline states have been observed. This is caused by the differences in the atomic arrangement and the electronic structure between crystalline and amorphous solids. The temperature dependence of resistivity, (rho)(T), of several binary amorphous alloys of Fe-TM (where TM = Ti, Zr, Hf, Nb etc.) has been studied from 2K to 300K. The Fe-poor (x < x(,c)) samples and the Fe-rich (x > x(,c)) samples have distinctive differences in (rho)(T) at low temperature (below 30K). All the magnetic samples show a logarithmic dependence at low temperature that can be described by Kondo scattering. In addition, there is a change in slope of (rho)(T) at a temperature close to the magnetic ordering temperature, indicating a contribution attributed to magnetic ordering. Several Nb-based amorphous alloys (Fe-Nb, Ni-Nb, Cu-Nb) have also been systematically studied. The effect of the magnetic species on superconductivity is investigated. The value of superconducting transition temperature (T(,s)) increases linearly with increasing Nb concentration. (Abstract shortened with permission of author.).

Doped YbRh2Si2: not only ferromagnetic correlations but ferromagnetic order.

PubMed

Lausberg, S; Hannaske, A; Steppke, A; Steinke, L; Gruner, T; Pedrero, L; Krellner, C; Klingner, C; Brando, M; Geibel, C; Steglich, F

2013-06-21

YbRh2Si2 is a prototypical system for studying unconventional antiferromagnetic quantum criticality. However, ferromagnetic correlations are present which can be enhanced via isoelectronic cobalt substitution for rhodium in Yb(Rh(1-x)Co(x))2Si2. So far, the magnetic order with increasing x was believed to remain antiferromagnetic. Here, we present the discovery of ferromagnetism for x = 0.27 below T(C) = 1.30  K in single crystalline samples. Unexpectedly, ordering occurs along the c axis, the hard crystalline electric field direction, where the g factor is an order of magnitude smaller than in the basal plane. Although the spontaneous magnetization is only 0.1 μB/Yb it corresponds to the full expected saturation moment along c taking into account partial Kondo screening.

Low Temperature Magnetic Ordering of the Magnetic Ionic Plastic Crystal, Choline[FeCl4

NASA Astrophysics Data System (ADS)

de Pedro, I.; García-Saiz, A.; Andreica, D.; Fernández Barquín, L.; Fernández-Díaz, M. T.; Blanco, J. A.; Amato, A.; Rodríguez Fernández, J.

2015-11-01

We report on the nature of the low temperature magnetic ordering of a magnetic ionic plastic crystal, Choline[FeCl4]. This investigation was carried out using heat capacity measurements, neutron diffraction experiments and muon spin relaxation (μSR) spectroscopy. The calorimetric measurements show the onset of an unusual magnetic ordering below 4 K with a possible second magnetic phase transition below 2 K. Low temperature neutron diffraction data reveal a three dimensional antiferromagnetic ordering at 2 K compatible with the previous magnetometry results. The analysis of μSR spectra indicates a magnetic phase transition below 2.2 K. At 1.6 K, the analysis of the shape of the μSR spectra suggests the existence of an additional magnetic phase with features of a possible incommensurate magnetic structure.

Short- and long-range magnetic order in LaMnAsO

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGuire, Michael A.; Garlea, Vasile Ovidiu

2016-02-02

The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the Néel temperature T N = 360(1) K. Below T N the critical exponent describing the magnetic order parameter is β=0.33–0.35 , consistent with a three-dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to T SRO = 650(10) K. Morevoer, the magnetic susceptibility shows a weak anomaly at T SRO and no anomaly at T N. Analysis of the diffuse scattering data using a reverse Montemore » Carlo algorithm indicates that above T N nearly two-dimensional, short-range magnetic order is present with a correlation length of 9.3(3) Å within the Mn layers at 400 K. The inelastic scattering data reveal a spin gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasielastic) magnetic excitations emerging in the short-range ordered state. When we compared it with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above T N is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.« less

Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions - IV: the fine structure of self-reversed thermoremanent magnetization

NASA Astrophysics Data System (ADS)

Robinson, Peter; McEnroe, S. A.; Fabian, K.; Harrison, R. J.; Thomas, C. I.; Mukai, H.

2014-03-01

Magnetic experiments, a Monte Carlo simulation and transmission electron microscopy observations combine to confirm variable chemical phase separation during quench and annealing of metastable ferri-ilmenite compositions, caused by inhomogeneous Fe-Ti ordering and anti-ordering. Separation begins near interfaces between growing ordered and anti-ordered domains, the latter becoming progressively enriched in ilmenite component, moving the Ti-impoverished hematite component into Fe-enriched diffusion waves near the interfaces. Even when disordered regions are eliminated, Fe-enriched waves persist and enlarge on anti-phase boundaries between growing and shrinking ordered and anti-ordered domains. Magnetic results and conceptual models show that magnetic ordering with falling T initiates in the Fe-enriched wave crests. Although representing only a tiny fraction of material, identified at highest Ts on a field-cooling curve, they control the `pre-destiny' of progressive magnetization at lower T. They can provide a positive magnetic moment in a minority of ordered ferrimagnetic material, which, by exchange coupling, then creates a self-reversed negative moment in the remaining majority. Four Ts or T ranges are recognized on typical field-cooling curves: TPD is the T range of `pre-destination'; TC is the predominant Curie T where major positive magnetization increases sharply; TMAX is where magnetization reaches a positive maximum, beyond which it is outweighed by self-reversed magnetization and TZM is the T where total magnetization passes zero. Disposition of these Ts on cooling curves indicate the fine structure of self-reversed thermoremanent magnetization. These results confirm much earlier suspicions that the `x-phase' responsible for self-reversed magnetization resides in Fe-enriched phase boundaries.

A Definition of the Magnetic Transition Temperature Using Valence Bond Theory.

PubMed

Jornet-Somoza, Joaquim; Deumal, Mercè; Borge, Juan; Robb, Michael A

2018-03-01

Macroscopic magnetic properties are analyzed using Valence Bond theory. Commonly the critical temperature T C for magnetic systems is associated with a maximum in the energy-based heat capacity C p (T). Here a more broadly applicable definition of the magnetic transition temperature T C is described using the spin moment expectation value (i.e., applying the spin exchange density operator) instead of energy. Namely, the magnetic capacity C s (T) reflects variation in the spin multiplicity as a function of temperature, which is shown to be related to ∂[χT(T)]/∂T. Magnetic capacity C s (T) depends on long-range spin interactions that are not relevant in the energy-based heat capacity C p (T). Differences between C s (T) and C p (T) are shown to be due to spin order/disorder within the crystal that can be monitored via a Valence Bond analysis of the corresponding magnetic wave function. Indeed the concept of the Boltzmann spin-alignment order is used to provide information about the spin correlation between magnetic units. As a final illustration, the critical temperature is derived from the magnetic capacity for several molecular magnets presenting different magnetic topologies that have been experimentally studied. A systematic shift between the transition temperatures associated with C s (T) and C p (T) is observed. It is demonstrated that this shift can be attributed to the loss of long-range spin correlation. This suggests that the magnetic capacity C s (T) can be used as a predictive tool for the magnetic topology and thus for the synthetic chemists.

Multi-cathode unbalanced magnetron sputtering systems

NASA Technical Reports Server (NTRS)

Sproul, William D.

1991-01-01

Ion bombardment of a growing film during deposition is necessary in many instances to ensure a fully dense coating, particularly for hard coatings. Until the recent advent of unbalanced magnetron (UBM) cathodes, reactive sputtering had not been able to achieve the same degree of ion bombardment as other physical vapor deposition processes. The amount of ion bombardment of the substrate depends on the plasma density at the substrate, and in a UBM system the amount of bombardment will depend on the degree of unbalance of the cathode. In multi-cathode systems, the magnetic fields between the cathodes must be linked to confine the fast electrons that collide with the gas atoms. Any break in this linkage results in electrons being lost and a low plasma density. Modeling of the magnetic fields in a UBM cathode using a finite element analysis program has provided great insight into the interaction between the magnetic fields in multi-cathode systems. Large multi-cathode systems will require very strong magnets or many cathodes in order to maintain the magnetic field strength needed to achieve a high plasma density. Electromagnets offer the possibility of independent control of the plasma density. Such a system would be a large-scale version of an ion beam enhanced deposition (IBED) system, but, for the UBM system where the plasma would completely surround the substrate, the acronym IBED might now stand for Ion Blanket Enhanced Deposition.

Magnetic Nanoparticle Drug Carriers and their Study by Quadrupole Magnetic Field-Flow Fractionation

PubMed Central

Williams, P. Stephen; Carpino, Francesca; Zborowski, Maciej

2009-01-01

Magnetic nanoparticle drug carriers continue to attract considerable interest for drug targeting in the treatment of cancers and other pathological conditions. The efficient delivery of therapeutic levels of drug to a target site while limiting nonspecific, systemic toxicity requires optimization of the drug delivery materials, the applied magnetic field, and the treatment protocol. The history and current state of magnetic drug targeting is reviewed. While initial studies involved micron-sized and larger carriers, and work with these microcarriers continues, it is the sub-micron carriers or nanocarriers that are of increasing interest. An aspect of magnetic drug targeting using nanoparticle carriers that has not been considered is then addressed. This aspect involves the variation in the magnetic properties of the nanocarriers. Quadrupole magnetic field-flow fractionation (QMgFFF) is a relatively new technique for characterizing magnetic nanoparticles. It is unique in its capability of determining the distribution in magnetic properties of a nanoparticle sample in suspension. The development and current state of this technique is also reviewed. Magnetic nanoparticle drug carriers have been found by QMgFFF analysis to be highly polydisperse in their magnetic properties, and the strength of response of the particles to magnetic field gradients is predicted to vary by orders of magnitude. It is expected that the least magnetic fraction of a formulation will contribute the most to systemic toxicity, and the depletion of this fraction will result in a more effective drug carrying material. A material that has a reduced systemic toxicity will allow higher doses of cytotoxic drugs to be delivered to the tumor with reduced side effects. Preliminary experiments involving a novel method of refining a magnetic nanoparticle drug carrier to achieve this result are described. QMgFFF is used to characterize the refined and unrefined material. PMID:19591456

Visualization of a stable intermediate phase in photoinduced metal-to-insulator transition in manganites

NASA Astrophysics Data System (ADS)

Lin, Hanxuan; Liu, Hao; Bai, Yu; Miao, Tian; Yu, Yang; Zhu, Yinyan; Chen, Hongyan; Kou, Yunfang; Niu, Jiebin; Wang, Wenbin; Yin, Lifeng; Shen, Jian

First order metal-insulator transition, accounting for various intriguing phenomena, is one of the most important phase transitions in condensed matter systems. Aside from the initial and final states, i.e. the metallic and insulating phases, no stable intermediate phase has been experimentally identified in such first order phase transition, though some transient phases do exist at the ultrafast time scale. Here, using our unique low-temperature, high-field magnetic force microscopy with photoexcitation, we directly observed a stable intermediate phase emerging and mediating the photoinduced first order metal-insulator transition in manganites. This phase is characteristic of low net magnetization and high resistivity. Our observations unveil the microscopic details of the photoinduced metal-insulator transition in manganites, which may be insightful to study first order metal-insulator transition in other condensed matter systems. This work was supported by National Key Research Program of China (2016YFA0300702), National Basic Research Program of China (973 Program) under the Grant No. 2013CB932901 and 2014CB921104; National Natural Science Foundation of China (11274071, 11504053).

Syntheses, structure and magnetic properties of two vanadate garnets Ca{sub 5}M{sub 4}V{sub 6}O{sub 24} (M=Co, Ni)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Nannan; College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108; He, Zhangzhen, E-mail: hcz1988@hotmail.com

2015-08-15

Two vanadate compounds Ca{sub 5}M{sub 4}V{sub 6}O{sub 24} (M=Co, Ni) have been synthesized by a high-temperature solid-state reaction. The compounds are found to crystallize in the cubic system with a space group Ia-3d, which exhibit a typical garnet structural framework. Magnetic measurements show that Ca{sub 5}M{sub 4}V{sub 6}O{sub 24} (M=Co, Ni) exhibit similar magnetic behaviors, in which Ca{sub 5}Co{sub 4}V{sub 6}O{sub 24} possesses an antiferromagnetic ordering at T{sub N}=~6 K while Ca{sub 5}Ni{sub 4}V{sub 6}O{sub 24} shows an antiferromagnetic ordering at T{sub N}=~7 K. - Graphical abstract: Garnet vanadate compounds Ca{sub 5}M{sub 4}V{sub 6}O{sub 24} (M=Co, Ni) have been synthesizedmore » by a high-temperature solid-state reaction. Structural features and magnetic behaviors are also investigated. - Highlights: • New type of garnet vanadates Ca{sub 5}M{sub 4}V{sub 6}O{sub 24} (M=Co, Ni) are synthesized by a high-temperature solid-state reaction. • Structural features are confirmed by single crystal samples. • Magnetic behaviors are firstly investigated in the systems.« less

RADIO POLARIZATION OBSERVATIONS OF THE SNAIL: A CRUSHED PULSAR WIND NEBULA IN G327.1–1.1 WITH A HIGHLY ORDERED MAGNETIC FIELD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ma, Y. K.; Ng, C.-Y.; Bucciantini, N.

2016-04-01

Pulsar wind nebulae (PWNe) are suggested to be acceleration sites of cosmic rays in the Galaxy. While the magnetic field plays an important role in the acceleration process, previous observations of magnetic field configurations of PWNe are rare, particularly for evolved systems. We present a radio polarization study of the “Snail” PWN inside the supernova remnant G327.1−1.1 using the Australia Telescope Compact Array. This PWN is believed to have been recently crushed by the supernova (SN) reverse shock. The radio morphology is composed of a main circular body with a finger-like protrusion. We detected a strong linear polarization signal frommore » the emission, which reflects a highly ordered magnetic field in the PWN and is in contrast to the turbulent environment with a tangled magnetic field generally expected from hydrodynamical simulations. This could suggest that the characteristic turbulence scale is larger than the radio beam size. We built a toy model to explore this possibility, and found that a simulated PWN with a turbulence scale of about one-eighth to one-sixth of the nebula radius and a pulsar wind filling factor of 50%–75% provides the best match to observations. This implies substantial mixing between the SN ejecta and pulsar wind material in this system.« less

Transmission and transport properties in Cantor graphene structures: The case of magnetoelectric modulation

NASA Astrophysics Data System (ADS)

Rodríguez-González, R.; Rodríguez-Vargas, I.

2017-04-01

We discuss theoretically the transmission and transport properties of Dirac electrons in a Cantor graphene system under magnetoelectric effects. The transfer matrix method and the Landauer-Büttiker formalism have been implemented to compute the transmittance and the linear-regime conductance, respectively. The fractal order of Cantor type together with the magnetic and electric field are used to distribute and generate the magnetoelectric barriers. This system give us the possibility of compare the mentioned physical properties for magnetic and magnetoelectric barriers. We found a bifurcation process in the transmission spectra which is observable when the generation increases. Also, an asymmetrical and symmetrical behavior is presented for magnetic and magnetoelectric barriers, respectively. In general, an oscillatory behavior is manifested in the conductance. Moreover, we can describe the peaks (form and location) that give rise to the oscillations through the contour plots of the transmittance in the (E ,ky) space. Likewise, by increasing the generation of the system the conductance is enhanced, the oscillations reduced and less pronounced. In short, the magnetoelectric modulation along with the fractal order can be used to control the transmission and transport properties in graphene-based structures.

Naturally tuned quantum critical point in the S =1 kagomé YCa3(VO) 3(BO3)4

NASA Astrophysics Data System (ADS)

Silverstein, Harlyn J.; Sinclair, Ryan; Sharma, Arzoo; Qiu, Yiming; Heinmaa, Ivo; Leitmäe, Alexander; Wiebe, Christopher R.; Stern, Raivo; Zhou, Haidong

2018-04-01

Although S =1 /2 kagomé systems have been intensely studied theoretically, and within the past decade been realized experimentally, much less is known about the S =1 analogs. While the theoretical ground state is still under debate, it has been found experimentally that S =1 kagomé systems either order at low temperatures or enter a spin glass state. In this work, YCa3(VO) 3(BO3)4 (YCVBO) is presented, with trivalent vanadium. Owing to its unusual crystal structure, the metal-metal bonding is highly connected along all three crystallographic directions, atypical of other kagomé materials. Using neutron scattering it is shown that YCVBO fails to order down to at least 50 mK and exhibits broad and dispersionless excitations. 11B NMR provides evidence of fluctuating spins at low temperatures while dc magnetization shows critical scaling that is also observed in systems near a quantum critical point such as Herbertsmithite, despite its insulating nature and S =1 magnetism. The evidence shown indicates that YCVBO is naturally tuned to be a quantum disordered magnet in the limit of T =0 K.

Applied metrology in the production of superconducting model magnets for particle accelerators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ferradas Troitino, Jose; Bestmann, Patrick; Bourcey, Nicolas

2017-12-22

The production of superconducting magnets for particle accelerators involves high precision assemblies and tight tolerances, in order to achieve the requirements for their appropriate performance. It is therefore essential to have a strict control and traceability over the geometry of each component of the system, and also to be able to compensate possible inherent deviations coming from the production process.

On parasupersymmetric oscillators and relativistic vector mesons in constant magnetic fields

NASA Technical Reports Server (NTRS)

Debergh, Nathalie; Beckers, Jules

1995-01-01

Johnson-Lippmann considerations on oscillators and their connection with the minimal coupling schemes are visited in order to introduce a new Sakata-Taketani equation describing vector mesons in interaction with a constant magnetic field. This new proposal, based on a specific parasupersymmetric oscillator-like system, is characterized by real energies as opposed to previously pointed out relativistic equations corresponding to this interacting context.

Ground-state properties of Na2IrO3 determined from an ab initio Hamiltonian and its extensions containing Kitaev and extended Heisenberg interactions

NASA Astrophysics Data System (ADS)

Okubo, Tsuyoshi; Shinjo, Kazuya; Yamaji, Youhei; Kawashima, Naoki; Sota, Shigetoshi; Tohyama, Takami; Imada, Masatoshi

2017-08-01

We investigate the ground state properties of Na2IrO3 based on numerical calculations of the recently proposed ab initio Hamiltonian represented by Kitaev and extended Heisenberg interactions. To overcome the limitation posed by small tractable system sizes in the exact diagonalization study employed in a previous study [Y. Yamaji et al., Phys. Rev. Lett. 113, 107201 (2014), 10.1103/PhysRevLett.113.107201], we apply a two-dimensional density matrix renormalization group and an infinite-size tensor-network method. By calculating at much larger system sizes, we critically test the validity of the exact diagonalization results. The results consistently indicate that the ground state of Na2IrO3 is a magnetically ordered state with zigzag configuration in agreement with experimental observations and the previous diagonalization study. Applications of the two independent methods in addition to the exact diagonalization study further uncover a consistent and rich phase diagram near the zigzag phase beyond the accessibility of the exact diagonalization. For example, in the parameter space away from the ab initio value of Na2IrO3 controlled by the trigonal distortion, we find three phases: (i) an ordered phase with the magnetic moment aligned mutually in 120 degrees orientation on every third hexagon, (ii) a magnetically ordered phase with a 16-site unit cell, and (iii) an ordered phase with presumably incommensurate periodicity of the moment. It suggests that potentially rich magnetic structures may appear in A2IrO3 compounds for A other than Na. The present results also serve to establish the accuracy of the first-principles approach in reproducing the available experimental results thereby further contributing to finding a route to realize the Kitaev spin liquid.

Design of a conduction-cooled 4 T superconducting racetrack for a multi-field coupling measurement system

NASA Astrophysics Data System (ADS)

Chen, Yu-Quan; Ma, Li-Zhen; Wu, Wei; Guan, Ming-Zhi; Wu, Bei-Min; Mei, En-Ming; Xin, Can-Jie

2015-12-01

A conduction-cooled superconducting magnet producing a transverse field of 4 T has been designed for a new generation multi-field coupling measurement system, which will be used to study the mechanical behavior of superconducting samples at cryogenic temperatures and intense magnetic fields. A compact cryostat with a two-stage GM cryocooler is designed and manufactured for the superconducting magnet. The magnet is composed of a pair of flat racetrack coils wound by NbTi/Cu superconducting composite wires, a copper and stainless steel combinational former and two Bi2Sr2CaCu2Oy superconducting current leads. The two coils are connected in series and can be powered with a single power supply. In order to support the high stress and attain uniform thermal distribution in the superconducting magnet, a detailed finite element (FE) analysis has been performed. The results indicate that in the operating status the designed magnet system can sufficiently bear the electromagnetic forces and has a uniform temperature distribution. Supported by National Natural Science Foundation of China (11327802, 11302225), China Postdoctoral Science Foundation (2014M560820) and National Scholarship Foundation of China (201404910172)

Magnetism of metallacrown single-molecule magnets: From a simplest model to realistic systems

NASA Astrophysics Data System (ADS)

Pavlyukh, Y.; Rentschler, E.; Elmers, H. J.; Hübner, W.; Lefkidis, G.

2018-06-01

Electronic and magnetic properties of molecular nanomagnets are determined by competing energy scales due to the crystal field splitting, the exchange interactions between transition metal atoms, and relativistic effects. We present a comprehensive theory embracing all these phenomena based on first-principles calculations. In order to achieve this goal, we start from the FeNi4 cluster as a paradigm. The system can be accurately described on the ab initio level yielding all expected electronic states in a range of multiplicities from 1 to 9, with a ferromagnetic ground state. By adding the spin-orbit coupling between them we obtain the zero-field splitting. This allows to introduce a spin Hamiltonian of a giant spin model, which operates on a smaller energy scale. We compare the computed parameters of this Hamiltonian with the experimental and theoretical magnetic anisotropy energies of the monolayer Ni/Cu(001). In line with them, we find that the anisotropy almost entirely originates from the second-order spin-orbit coupling, the spin-spin coupling constitutes only a small fraction. Finally, we include the ligand atoms in our consideration. This component has a decisive role for the stabilization of molecules in experimental synthesis and characterization, and also substantially complicates the theory by bringing the superexchange mechanisms into play. Since they are higher-order effects involving two hopping matrix elements, not every theory can describe them. Our generalization of the corresponding perturbation theory substantiates the use of complete active space methods for the description of superexchange. At the same time, our numerical results for the {CuFe4} system demonstrate that the Goodenough-Kanamori rules, which are often used to determine the sign of these exchange interactions, cannot deliver quantitative predictions due to the interplay of other mechanisms, e. g., involving multicenter Coulomb integrals. We conclude by comparing ab initio values of the exchange interaction constants for the {CuCu4} and {CuFe4} metallacrown magnetic molecules with experimental values determined by fitting of the magnetic susceptibility curves χMT (T ) , and attribute the remaining discrepancy between them to the role of virtual electron excitations into and out of the active space (dynamical correlations).

Cryogen-free superconducting magnet system for multifrequency electron paramagnetic resonance up to 12.1 T

NASA Astrophysics Data System (ADS)

Smirnov, Alex I.; Smirnova, Tatyana I.; MacArthur, Ryan L.; Good, Jeremy A.; Hall, Renny

2006-03-01

Multifrequency and high field/high frequency (HF) electron paramagnetic resonance (EPR) is a powerful spectroscopy for studying paramagnetic spin systems ranging from organic-free radicals to catalytic paramagnetic metal ion centers in metalloproteins. Typically, HF EPR experiments are carried out at resonant frequencies ν =95-300GHz and this requires magnetic fields of 3.4-10.7T for electronic spins with g ≈2.0. Such fields could be easily achieved with superconducting magnets, but, unlike NMR, these magnets cannot operate in a persistent mode in order to satisfy a wide range of resonant fields required by the experiment. Operating and maintaining conventional passively cooled superconducting magnets in EPR laboratories require frequent transfer of cryogens by trained personnel. Here we describe and characterize a versatile cryogen-free magnet system for HF EPR at magnetic fields up to 12.1T that is suitable for ramping the magnetic field over the entire range, precision scans around the target field, and/or holding the field at the target value. We also demonstrate that in a nonpersistent mode of operation the magnetic field can be stabilized to better than 0.3ppm/h over 15h period by employing a transducer-controlled power supply. Such stability is sufficient for many HF EPR experiments. An important feature of the system is that it is virtually maintenance-free because it is based on a cryogen-free technology and therefore does not require any liquid cryogens (liquid helium or nitrogen) for operation. We believe that actively cooled superconducting magnets are ideally suited for a wide range of HF EPR experiments including studies of spin-labeled nucleic acids and proteins, single-molecule magnets, and metalloproteins.

Magnetite and magnetite/silver core/shell nanoparticles with diluted magnet-like behavior

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garza-Navarro, Marco; Torres-Castro, Alejandro, E-mail: alejandro.torrescs@uanl.edu.m; Centro de Innovacion, Investigacion y Desarrollo en Ingenieria y Tecnologia, Universidad Autonoma de Nuevo Leon, Apodaca, Nuevo Leon 66600

2010-01-15

In the present work is reported the use of the biopolymer chitosan as template for the preparation of magnetite and magnetite/silver core/shell nanoparticles systems, following a two step procedure of magnetite nanoparticles in situ precipitation and subsequent silver ions reduction. The crystalline and morphological characteristics of both magnetite and magnetite/silver core/shell nanoparticles systems were analyzed by high resolution transmission electron microscopy (HRTEM) and nanobeam diffraction patterns (NBD). The results of these studies corroborate the core/shell morphology and the crystalline structure of the magnetite core and the silver shell. Moreover, magnetization temperature dependent, M(T), measurements show an unusual diluted magnetic behaviormore » attributed to the dilution of the magnetic ordering in the magnetite and magnetite/silver core/shell nanoparticles systems. - Graphical abstract: Biopolymer chitosan was used as stabilization media to synthesize both magnetite and magnetite/silver core/shell nanoparticles. Results of HRTEM and NBD patterns confirm core/shell morphology of the obtained nanoparticles. It was found that the composites show diluted magnet-like behavior.« less

Maneuvering a reentry body via magneto-gasdynamic forces

NASA Astrophysics Data System (ADS)

Ohare, Leo Patrick

1992-04-01

Some of the characteristics of the interaction of an electrically conducting fluid with a non-uniform applied magnetic field and a potential magnetogasdynamic control system which may be used on future aerospace vehicles are presented. The flow through a two dimensional channel is predicted by numerically solving the magnetogasdynamic equations using a time marching technique. The fluid was modeled as a compressible, inviscid, supersonic gas with finite electrical conductivity. Development of the algorithm provided a means to predict and analyze phenomena associated with magnetogasdynamic flows which had not been previously explored using numerical methods. One such phenomena was the prediction of oblique waves resulting from the interaction of an electrically conducting fluid with a non-uniform applied magnetic field. Development of this tool provided a means to explore an application which might have potential use for future aerospace vehicle missions. In order to appreciate the significance of this technology, predictions were made of the pitching moment about a slender blunted cone, generated by a system relying on the fluid-magnetic interaction. These moments were compared to predictions of a pitching moment generated by a deflecting control surface on the same vehicle. It was shown that the proposed magnetogasdynamic system could produce moments which were on the same order as the moments produced by the flap systems at low deflection angles.

One dimensional spatial resolution optimization on a hybrid low field MRI-gamma detector

DOE Office of Scientific and Technical Information (OSTI.GOV)

Agulles-Pedrós, L., E-mail: lagullesp@unal.edu.co; Abril, A., E-mail: ajabrilf@unal.edu.co

Hybrid systems like Positron Emission Tomography/Magnetic Resonance Imaging (PET/MRI) and MRI/gamma camera, offer advantages combining the resolution and contrast capability of MRI with the better contrast and functional information of nuclear medicine techniques. However, the radiation detectors are expensive and need an electronic set-up, which can interfere with the MRI acquisition process or viceversa. In order to improve these drawbacks, in this work it is presented the design of a low field NMR system made up of permanent magnets compatible with a gamma radiation detector based on gel dosimetry. The design is performed using the software FEMM for estimation ofmore » the magnetic field, and GEANT4 for the physical process involved in radiation detection and effect of magnetic field. The homogeneity in magnetic field is achieved with an array of NbFeB magnets in a linear configuration with a separation between the magnets, minimizing the effect of Compton back scattering compared with a no-spacing linear configuration. The final magnetic field in the homogeneous zone is ca. 100 mT. In this hybrid proposal, although the gel detector do not have spatial resolution per se, it is possible to obtain a dose profile (1D image) as a function of the position by using a collimator array. As a result, the gamma detector system described allows a complete integrated radiation detector within the low field NMR (lfNMR) system. Finally we present the better configuration for the hybrid system considering the collimator parameters such as height, thickness and distance.« less

Investigation of Room temperature Ferromagnetism in Mn doped Ge

NASA Astrophysics Data System (ADS)

Colakerol Arslan, Leyla; Toydemir, Burcu; Onel, Aykut Can; Ertas, Merve; Doganay, Hatice; Gebze Inst of Tech Collaboration; Research Center Julich Collaboration

2014-03-01

We present a systematic investigation of structural, magnetic and electronic properties of MnxGe1 -x single crystals. MnxGe1-x films were grown by sequential deposition of Ge and Mn by molecular-beam epitaxy at low substrate temperatures in order to avoid precipitation of ferromagnetic Ge-Mn intermetallic compounds. Reflected high energy electron diffraction and x-ray diffraction observations revealed that films are epitaxially grown on Si (001) substrates from the initial stage without any other phase formation. Magnetic measurements carried out using a physical property measurement system showed that all samples exhibited ferromagnetism at room temperature. Electron spin resonance indicates the presence of magnetically ordered localized spins of divalent Mn ions. X-ray absorption measurements at the Mn L-edge confirm significant substitutional doping of Mn into Ge-sites. The ferromagnetism was mainly induced by Mn substitution for Ge site, and indirect exchange interaction of these magnetic ions with the intrinsic charge carriers is the origin of ferromagnetism. The magnetic interactions were better understood by codoping with nonmagnetic impurities. This work was supported by Marie-Curie Reintegration Grant (PIRG08-GA-2010-276973).

Structure and magnetism of Fe-doped BaSnO 3 thin films

DOE PAGES

Alaan, Urusa S.; N’Diaye, Alpha T.; Shafer, Padraic; ...

2017-02-28

BaSnO 3 is an excellent candidate system for developing a new class of perovskite-based dilute magnetic semiconductors. Here in this study, we show that BaSn 0.95Fe 0.05O 3 can be grown from a background pressure of ~2×10-3 mTorr to oxygen pressures of 300 mTorr with high crystallinity and excellent structural quality. When grown in vacuum, the films may be weakly ferromagnetic with a nonzero x-ray magnetic circular dichroism signal on the Fe L 3 edge. Growth with oxygen flow appears to suppress magnetic ordering. Even for very thick films grown in 100 mTorr O 2, the films are paramagnetic. Finally,more » the existence of ferromagnetism in vacuum-grown BaSnO 3 may be attributed to the F-center exchange mechanism, which relies on the presence of oxygen vacancies to facilitate the ferromagnetism. However, other possible extrinsic contributions to the magnetic ordering, such as clusters of Fe 3O 4 and FeO or contamination can also explain the observed behavior.« less

Electronic disorder and magnetic-field-induced superconductivity enhancement in Fe1+y(Te1-xSex)

NASA Astrophysics Data System (ADS)

Hu, Jin; Liu, Tijiang; Qian, Bin; Mao, Zhiqiang

2012-02-01

The iron chalcogenide Fe1+y(Te1-xSex) superconductor system exhibits a unique electronic and magnetic phase diagram distinct from those seen in iron pnictides: bulk superconductivity does not appear immediately following the suppression of long-range (π,0) AFM order. Instead, an intermediate phase with weak charge carrier localization appears between AFM order and bulk superconductivity (Liu et al., Nat. Mater. 9, 719 (2010)). In this talk, we report our recent studies on the relationship between the normal state and superconducting properties in Fe1+y(Te1-xSex). We show that the superconducting volume fraction VSC and normal state metallicity significantly increase while the normal state Sommerfeld coefficient γ and Hall coefficient RH drop drastically with increasing Se content in the underdoped superconducting region. Additionally, VSC is surprisingly enhanced by magnetic field in heavily underdoped superconducting samples. The implications of these results will be discussed. Our analyses suggest that the suppression of superconductivity in the underdoped region is associated with electronic disorder caused by incoherent magnetic scattering arising from (π,0) magnetic fluctuations.

Stellar fibril magnetic systems. I - Reduced energy state

NASA Technical Reports Server (NTRS)

Parker, E. N.

1984-01-01

The remarkable fibril structure of the magnetic fields at the surface of the sun (with fibrils compressed to 1,000-2,000 gauss) lies outside existing statistical theories of magnetohydrodynamic turbulence. The total energy of the fibril field is enhanced by a factor of more than 100 above the energy for the mean field in a continuum state. The magnetic energy density within a fibril is of the order of 100 times the local kinetic energy density, so that no simple application of equipartition principles is possible. It is pointed out that the total energy of the atmosphere (thermal + gravitational + magnetic) is reduced by the fibril state of the field by avoiding the magnetic inhibition of the convective overturning, suggesting that the formation of the observed intense fibril state may be in response to the associated energy reduction. Calculation of the minimum total energy of a polytropic atmosphere permeated by magnetic fibrils yields theoretical fibril fields of the order of 1-5 kilogauss when characteristics appropriate to the solar convective zone are introduced, in rough agreement with the actual fields of 1-2 kilogauss. The polytrope model, although crude, establishes that a large reduction in total energy is made possible by the fibril state.

Comment on ‘Oxygen vacancy-induced magnetic moment in edge-sharing CuO2 chains of Li2CuO2’

NASA Astrophysics Data System (ADS)

Kuzian, R. O.; Klingeler, R.; Lorenz, W. E. A.; Wizent, N.; Nishimoto, S.; Nitzsche, U.; Rosner, H.; Milosavljevic, D.; Hozoi, L.; Yadav, R.; Richter, J.; Hauser, A.; Geck, J.; Hayn, R.; Yushankhai, V.; Siurakshina, L.; Monney, C.; Schmitt, T.; Thar, J.; Roth, G.; Ito, T.; Yamaguchi, H.; Matsuda, M.; Johnston, S.; Málek, J.; Drechsler, S.-L.

2018-05-01

In a recent work devoted to the magnetism of Li2CuO2, Shu et al (2017 New J. Phys. 19, 023026) have proposed a ‘simplified’ unfrustrated microscopic model that differs considerably from the models refined through decades of prior work. We show that the proposed model is at odds with known experimental data, including the reported magnetic susceptibility χ(T) data up to 550 K. Using an 8th order high-temperature expansion for χ(T), we show that the experimental data for Li2CuO2 are consistent with the prior model derived from inelastic neutron scattering studies. We also establish the T-range of validity for a Curie–Weiss law for the real frustrated magnetic system. We argue that the knowledge of the long-range ordered magnetic structure for T < T N and of χ(T) in a restricted T-range provides insufficient information to extract all of the relevant couplings in frustrated magnets; the saturation field and INS data must also be used to determine several exchange couplings, including the weak but decisive frustrating antiferromagnetic interchain couplings.

Frustrated spin- 1 2 molecular magnetism in the mixed-valence antiferromagnets Ba 3 M Ru 2 O 9 ( M = In , Y, Lu)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ziat, D.; Aczel, Adam A.; Sinclair, R.

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron-scattering measurements on three members of the family Ba 3MRu 2O 9, where M=In, Y, and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocksmore » form a frustrated, quasi-two-dimensional triangular lattice. Our zero- and longitudinal-field μSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero-field muon spin polarization indicative of long-range magnetic order in the Lu sample. In conclusion, the static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.« less

Frustrated spin- 1 2 molecular magnetism in the mixed-valence antiferromagnets Ba 3 M Ru 2 O 9 ( M = In , Y, Lu)

DOE PAGES

Ziat, D.; Aczel, Adam A.; Sinclair, R.; ...

2017-05-22

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron-scattering measurements on three members of the family Ba 3MRu 2O 9, where M=In, Y, and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocksmore » form a frustrated, quasi-two-dimensional triangular lattice. Our zero- and longitudinal-field μSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero-field muon spin polarization indicative of long-range magnetic order in the Lu sample. In conclusion, the static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.« less

Isolated Power Generation System Using Permanent Magnet Synchronous Generator with Improved Power Quality

NASA Astrophysics Data System (ADS)

Arya, Sabha Raj; Patel, Ashish; Giri, Ashutosh

2018-06-01

This paper deals wind energy based power generation system using Permanent Magnet Synchronous Generator (PMSG). It is controlled using advanced enhanced phase-lock loop for power quality features using distribution static compensator to eliminate the harmonics and to provide KVAR compensation as well as load balancing. It also manages rated potential at the point of common interface under linear and non-linear loads. In order to have better efficiency and reliable operation of PMSG driven by wind turbine, it is necessary to analyze the governing equation of wind based turbine and PMSG under fixed and variable wind speed. For handling power quality problems, power electronics based shunt connected custom power device is used in three wire system. The simulations in MATLAB/Simulink environment have been carried out in order to demonstrate this model and control approach used for the power quality enhancement. The performance results show the adequate performance of PMSG based power generation system and control algorithm.

Magnetic phase transition in layered inorganic-organic hybrid (C12H25NH3)2CuCl4

NASA Astrophysics Data System (ADS)

Bochalya, Madhu; Kumar, Sunil; Kanaujia, Pawan K.; Prakash, G. Vijaya

2018-05-01

Inorganic-organic (IO) hybrids are material systems which have become an interesting theme of research for physicist and chemists recently due to the possibility of engineering specific magnetic, thermal or optoelectronic properties by playing around with the transition metal, halides and the organic components. Our experiments on (C12H25NH3)2CuCl4 show that the system exhibits a long range ferromagnetic order below ˜11 K. In such an inorganic-organic hybrid system, Jahn-Teller distortion of the copper ions results into a weak ferromagnetic order as compared to the antiferromagnetic spin-spin exchange in the pure inorganic CuCl2 compound. Moreover, this particular hybrid system also exhibits photoluminescence when excited below absorption maximum related to charge transfer peak though the effect is much weaker as compared to that in extensively studied other MX4-based (M = Sn, Pb; X = Cl, Br, I) counterparts.

Isolated Power Generation System Using Permanent Magnet Synchronous Generator with Improved Power Quality

NASA Astrophysics Data System (ADS)

Arya, Sabha Raj; Patel, Ashish; Giri, Ashutosh

2018-03-01

This paper deals wind energy based power generation system using Permanent Magnet Synchronous Generator (PMSG). It is controlled using advanced enhanced phase-lock loop for power quality features using distribution static compensator to eliminate the harmonics and to provide KVAR compensation as well as load balancing. It also manages rated potential at the point of common interface under linear and non-linear loads. In order to have better efficiency and reliable operation of PMSG driven by wind turbine, it is necessary to analyze the governing equation of wind based turbine and PMSG under fixed and variable wind speed. For handling power quality problems, power electronics based shunt connected custom power device is used in three wire system. The simulations in MATLAB/Simulink environment have been carried out in order to demonstrate this model and control approach used for the power quality enhancement. The performance results show the adequate performance of PMSG based power generation system and control algorithm.

Magnetic Signals of High-Temperature Superconductor Bulk During the Levitation Force Measurement Process

NASA Astrophysics Data System (ADS)

Huang, Huan; Zheng, Jun; Qian, Nan; Che, Tong; Zheng, Botian; Jin, Liwei; Deng, Zigang

2017-05-01

In order to study the commonly neglected magnetic field information in the course of levitation force measurement process in a superconducting maglev system, a multipoint magnetic field measurement platform was employed to acquire magnetic signals of a bulk high-Tc superconductor on both the top and the bottom surface. Working conditions including field cooling (FC) and zero field cooling were investigated for these vertical down and up motions above a permanent magnet guideway performed on a HTS maglev measurement system. We have discussed the magnetic flux variation process based on the Bean model. A magnetic hysteresis effect similar to the levitation force hysteresis loop of the bulk superconductor was displayed and analyzed in this paper. What is more valuable, there exists some available magnetic flux on the top surface of the bulk superconductor, and the proportion is as high as 62.42% in the FC condition, which provides an experimental hint to design the superconductor bulk and the applied field for practical use in a more efficient way. In particular, this work reveals real-time magnetic flux variation of the bulk superconductor in the levitation application, which is the other important information in contrast to the macroscopic levitation and guidance force investigations in previous studies, and it enriches the existing research methods. The results are significant for understanding the magnetic characteristic of superconductors, and they can contribute to optimize the present HTS maglev system design.

Superconducting Magnet Shielding of Astronauts from Cosmic Rays

NASA Astrophysics Data System (ADS)

Fisher, Peter; Hoffman, Jeffrey; Zhou, Feng; Batishchev, Oleg

2004-11-01

Protecting astronauts traveling outside the Earth's protective magnetic field from cosmic and solar radiation [1] is one of the critical problems that must be solved in order to realize the nation's new human space exploration vision. Superconducting magnets, such as those under construction for the ATLAS experiment [2] at CERN, have achieved sufficient size to be able to surround a reasonable habitable volume, and their field strength is high enough to deflect a significant portion of the incoming radiation. We have undertaken a research effort aimed at developing an accurate numerical model of a crew compartment surrounded by a large magnetic field, with which we can calculate the effect on incoming charged particles. We will use this model to optimize the magnetic configuration to produce the maximum shielding effect while minimizing the mass of the superconducting magnet system. We are also investigating some of the practical problems that must be solved if large, superconducting magnet systems are to be incorporated into human space systems. We will present preliminary results of our modeling, showing the reduction of radiation exposure as a function of energy and atomic species. [1] Review of Particle Physics, Ed. Particle Data Group, Phys. Lett. B, 1-4 (592) 1-1109, 2004 [2] http://atlasexperiment.org/

In situ Investigation of Magnetism in Metastable Phases of Levitated Fe83 B17 During Solidification

NASA Astrophysics Data System (ADS)

Quirinale, D. G.; Messina, D.; Rustan, G. E.; Kreyssig, A.; Prozorov, R.; Goldman, A. I.

2017-11-01

In situ measurements of structure, density, and magnetization on samples of Fe83 B17 using an electrostatic levitation furnace allow us to identify and correlate the magnetic and structural transitions in this system during its complex solidification process. In particular, we identify magnetic ordering in the metastable Fe23 B6 /fcc Fe coherently grown structures and primitive tetragonal Fe3 B metastable phase in addition to characterizing the equilibrium Fe2 B phase. Our measurements demonstrate that the incorporation of a tunnel-diode oscillator circuit within an electrostatic levitation furnace enables investigations of the physical properties of high-temperature metastable structures.

Chiral Spin Order in Kondo-Heisenberg Systems

NASA Astrophysics Data System (ADS)

Tsvelik, A. M.; Yevtushenko, O. M.

2017-12-01

We demonstrate that low dimensional Kondo-Heisenberg systems, consisting of itinerant electrons and localized magnetic moments (Kondo impurities), can be used as a principally new platform to realize scalar chiral spin order. The underlying physics is governed by a competition of the Ruderman-Kittel-Kosuya-Yosida (RKKY) indirect exchange interaction between the local moments with the direct Heisenberg one. When the direct exchange is weak and RKKY dominates, the isotropic system is in the disordered phase. A moderately large direct exchange leads to an Ising-type phase transition to the phase with chiral spin order. Our finding paves the way towards pioneering experimental realizations of the chiral spin liquid in systems with spontaneously broken time-reversal symmetry.

Design Method of Digital Optimal Control Scheme and Multiple Paralleled Bridge Type Current Amplifier for Generating Gradient Magnetic Fields in MRI Systems

NASA Astrophysics Data System (ADS)

Watanabe, Shuji; Takano, Hiroshi; Fukuda, Hiroya; Hiraki, Eiji; Nakaoka, Mutsuo

This paper deals with a digital control scheme of multiple paralleled high frequency switching current amplifier with four-quadrant chopper for generating gradient magnetic fields in MRI (Magnetic Resonance Imaging) systems. In order to track high precise current pattern in Gradient Coils (GC), the proposal current amplifier cancels the switching current ripples in GC with each other and designed optimum switching gate pulse patterns without influences of the large filter current ripple amplitude. The optimal control implementation and the linear control theory in GC current amplifiers have affinity to each other with excellent characteristics. The digital control system can be realized easily through the digital control implementation, DSPs or microprocessors. Multiple-parallel operational microprocessors realize two or higher paralleled GC current pattern tracking amplifier with optimal control design and excellent results are given for improving the image quality of MRI systems.