Competing phases in a model of Pr-based cobaltites

NASA Astrophysics Data System (ADS)

Sotnikov, A.; Kuneš, J.

2017-12-01

Motivated by the physics of Pr-based cobaltites, we study the effect of the external magnetic field in the hole-doped two-band Hubbard model close to instabilities toward the excitonic condensation and ferromagnetic ordering. Using the dynamical mean-field theory we observe a field-driven suppression of the excitonic condensate. The onset of a magnetically ordered phase at the fixed chemical potential is accompanied by a sizable change of the electron density. This leads us to predict that Pr3 + abundance increases on the high-field side of the transition.

Phase diagram of a symmetric electron–hole bilayer system: a variational Monte Carlo study

NASA Astrophysics Data System (ADS)

Sharma, Rajesh O.; Saini, L. K.; Prasad Bahuguna, Bhagwati

2018-05-01

We study the phase diagram of a symmetric electron–hole bilayer system at absolute zero temperature and in zero magnetic field within the quantum Monte Carlo approach. In particular, we conduct variational Monte Carlo simulations for various phases, i.e. the paramagnetic fluid phase, the ferromagnetic fluid phase, the anti-ferromagnetic Wigner crystal phase, the ferromagnetic Wigner crystal phase and the excitonic phase, to estimate the ground-state energy at different values of in-layer density and inter-layer spacing. Slater–Jastrow style trial wave functions, with single-particle orbitals appropriate for different phases, are used to construct the phase diagram in the (r s , d) plane by finding the relative stability of trial wave functions. At very small layer separations, we find that the fluid phases are stable, with the paramagnetic fluid phase being particularly stable at and the ferromagnetic fluid phase being particularly stable at . As the layer spacing increases, we first find that there is a phase transition from the ferromagnetic fluid phase to the ferromagnetic Wigner crystal phase when d reaches 0.4 a.u. at r s   =  20, and before there is a return to the ferromagnetic fluid phase when d approaches 1 a.u. However, for r s   <  20 and a.u., the excitonic phase is found to be stable. We do not find that the anti-ferromagnetic Wigner crystal is stable over the considered range of r s and d. We also find that as r s increases, the critical layer separations for Wigner crystallization increase.

Polar-antipolar transition and weak ferromagnetism in Mn-doped Bi0.86La0.14FeO3

NASA Astrophysics Data System (ADS)

Khomchenko, V. A.; Karpinsky, D. V.; Troyanchuk, I. O.; Sikolenko, V. V.; Többens, D. M.; Ivanov, M. S.; Silibin, M. V.; Rai, R.; Paixão, J. A.

2018-04-01

Having been considered as a prime example of a room-temperature magnetoelectric multiferroic, BiFeO3 continues to attract much interest. Since functional properties of this material can be effectively influenced by chemical, electrical, magnetic, mechanical and thermal stimuli, it can serve as a model for the investigation of cross-coupling phenomena in solids. Special attention is currently paid to the study of chemical pressure-driven magneto-structural transformations. In this paper, we report on the effect of the Mn doping on the crystal structure and magnetic behavior of the Bi1‑x La x FeO3 multiferroics near their polar-antipolar (antiferromagnetic-weak ferromagnetic) phase boundary. Synchrotron x-ray and neutron powder diffraction measurements of the Bi0.86La0.14Fe1‑x Mn x O3 (x  =  0.05, 0.1, 0.15) compounds have been performed. The diffraction data suggest that the Mn substitution results in the suppression of the ferroelectric polarization and gives rise to the appearance of the antiferroelectric (generally, PbZrO3-related) phase characteristic of the phase diagrams of the Bi1‑x RE x FeO3 (RE  =  rare-earth) systems. Depending on the Mn concentration (determining phase composition of the Bi0.86La0.14Fe1‑x Mn x O3 samples at room temperature), either complete or partial revival of the polar phase can be observed with increasing temperature. Magnetic measurements of the samples indicate that the Mn doping affects the stability of the cycloidal antiferromagnetic order specific to the polar phase, thus resulting in the formation of a ferroelectric and weak ferromagnetic state.

Dimensionality Driven Enhancement of Ferromagnetic Superconductivity in URhGe.

PubMed

Braithwaite, Daniel; Aoki, Dai; Brison, Jean-Pascal; Flouquet, Jacques; Knebel, Georg; Nakamura, Ai; Pourret, Alexandre

2018-01-19

In most unconventional superconductors, like the high-T_{c} cuprates, iron pnictides, or heavy-fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the orthorhombic uranium ferromagnetic superconductors have a unique position, notably because magnetic fields couple directly to ferromagnetic order, leading to the fascinating discovery of the reemergence of superconductivity in URhGe at a high field. Here we show that uniaxial stress is a remarkable tool allowing the fine-tuning of the pairing strength. With a relatively small stress, the superconducting phase diagram is spectacularly modified, with a merging of the low- and high-field superconducting states and a significant enhancement of the superconductivity. The superconducting critical temperature increases both at zero field and under a field, reaching 1 K, more than twice higher than at ambient pressure. This enhancement of superconductivity is shown to be directly related to a change of the magnetic dimensionality detected from an increase of the transverse magnetic susceptibility: In addition to the Ising-type longitudinal ferromagnetic fluctuations, transverse magnetic fluctuations also play an important role in the superconducting pairing.

Phase diagram of a symmetric electron-hole bilayer system: a variational Monte Carlo study.

PubMed

Sharma, Rajesh O; Saini, L K; Bahuguna, Bhagwati Prasad

2018-05-10

We study the phase diagram of a symmetric electron-hole bilayer system at absolute zero temperature and in zero magnetic field within the quantum Monte Carlo approach. In particular, we conduct variational Monte Carlo simulations for various phases, i.e. the paramagnetic fluid phase, the ferromagnetic fluid phase, the anti-ferromagnetic Wigner crystal phase, the ferromagnetic Wigner crystal phase and the excitonic phase, to estimate the ground-state energy at different values of in-layer density and inter-layer spacing. Slater-Jastrow style trial wave functions, with single-particle orbitals appropriate for different phases, are used to construct the phase diagram in the (r s , d) plane by finding the relative stability of trial wave functions. At very small layer separations, we find that the fluid phases are stable, with the paramagnetic fluid phase being particularly stable at [Formula: see text] and the ferromagnetic fluid phase being particularly stable at [Formula: see text]. As the layer spacing increases, we first find that there is a phase transition from the ferromagnetic fluid phase to the ferromagnetic Wigner crystal phase when d reaches 0.4 a.u. at r s   =  20, and before there is a return to the ferromagnetic fluid phase when d approaches 1 a.u. However, for r s   <  20 and [Formula: see text] a.u., the excitonic phase is found to be stable. We do not find that the anti-ferromagnetic Wigner crystal is stable over the considered range of r s and d. We also find that as r s increases, the critical layer separations for Wigner crystallization increase.

Direct observation of isolated Damon-Eshbach and backward volume spin-wave packets in ferromagnetic microstripes

PubMed Central

Wessels, Philipp; Vogel, Andreas; Tödt, Jan-Niklas; Wieland, Marek; Meier, Guido; Drescher, Markus

2016-01-01

The analysis of isolated spin-wave packets is crucial for the understanding of magnetic transport phenomena and is particularly interesting for applications in spintronic and magnonic devices, where isolated spin-wave packets implement an information processing scheme with negligible residual heat loss. We have captured microscale magnetization dynamics of single spin-wave packets in metallic ferromagnets in space and time. Using an optically driven high-current picosecond pulse source in combination with time-resolved scanning Kerr microscopy probed by femtosecond laser pulses, we demonstrate phase-sensitive real-space observation of spin-wave packets in confined permalloy (Ni80Fe20) microstripes. Impulsive excitation permits extraction of the dynamical parameters, i.e. phase- and group velocities, frequencies and wave vectors. In addition to well-established Damon-Eshbach modes our study reveals waves with counterpropagating group- and phase-velocities. Such unusual spin-wave motion is expected for backward volume modes where the phase fronts approach the excitation volume rather than emerging out of it due to the negative slope of the dispersion relation. These modes are difficult to excite and observe directly but feature analogies to negative refractive index materials, thus enabling model studies of wave propagation inside metamaterials. PMID:26906113

Propagation of electromagnetic soliton in a spin polarized current driven weak ferromagnetic nanowire

NASA Astrophysics Data System (ADS)

Senthil Kumar, V.; Kavitha, L.; Gopi, D.

2017-11-01

We investigate the nonlinear spin dynamics of a spin polarized current driven anisotropic ferromagnetic nanowire with Dzyaloshinskii-Moriya interaction (DMI) under the influence of electromagnetic wave (EMW) propagating along the axis of the nanowire. The magnetization dynamics and electromagnetic wave propagation in the ferromagnetic nanowire with weak anti-symmetric interaction is governed by a coupled vector Landau-Lifshitz-Gilbert and Maxwell's equations. These coupled nonlinear vector equations are recasted into the extended derivative nonlinear Schrödinger (EDNLS) equation in the framework of reductive perturbation method. As it is well known, the modulational instability is a precursor for the emergence of localized envelope structures of various kinds, we compute the instability criteria for the weak ferromagnetic nanowire through linear stability analysis. Further, we invoke the homogeneous balance method to construct kink and anti-solitonic like electromagnetic (EM) soliton profiles for the EDNLS equation. We also explore the appreciable effect of the anti-symmetric weak interaction on the magnetization components of the propagating EM soliton. We find that the combination of spin-polarized current and the anti-symmetric DMI have a profound effect on the propagating EMW in a weak ferromagnetic nanowire. Thus, the anti-symmetric DMI in a spin polarized current driven ferromagnetic nanowire supports the lossless propagation of EM solitons, which may have potential applications in magnetic data storage devices.

Temperature dependent infrared nano-imaging of La0.67Sr0.33MnO3 thin film

NASA Astrophysics Data System (ADS)

Xu, Peng; Huffman, T. J.; Hae Kwak, In; Biswas, Amlan; Qazilbash, M. M.

2018-01-01

We investigate the temperature dependence of infrared properties at nanometer length scales in La0.67Sr0.33MnO3 (LSMO) thin film with a thickness of 47 unit cells grown on SrTiO3 substrate. The infrared nano-imaging experiments were performed using a near-field optical microscope in conjunction with a variable temperature heating stage. The near-field infrared data is consistent with the bulk of the LSMO film undergoing the thermally-driven non-percolative second-order transition from a metallic, ferromagnetic phase to an insulating, paramagnetic phase. We find persistent infrared contrast on the nanoscale that is independent of temperature and which we attribute to two novel phases with different conductivities coexisting in the vicinity of the film-substrate interface. These two coexisting phases at the film-substrate interface do not undergo the metal-insulator transition (MIT) and hence are different from the metallic, ferromagnetic and insulating, paramagnetic phases in the bulk of the film. At temperatures approaching the nominal MIT temperature, repeated scans of the same microscopic area at constant temperature reveal bimodal fluctuation of the near-field infrared amplitude. We interpret this phenomenon as slow, critical fluctuations of the conductivity in the bulk of the LSMO film.

Charge ordered ferromagnetic phase in La_0.5Ca_0.5MnO_3

NASA Astrophysics Data System (ADS)

Mathur, Neil

2003-03-01

Charge order and ferromagnetism should be mutually exclusive in the manganites, because ferromagnetism in these materials is normally promoted by delocalised electrons. Surprisingly, a phase that is both strongly charge ordered and fully ferromagnetic is observed [1] at 90 K in La_0.5Ca_0.5MnO_3, using Fresnel imaging, dark-field TEM and electron holography. This new phase coexists with the two low temperature phases that were already known to coexist in La_0.5Ca_0.5MnO_3. (One of these expected phases is ferromagnetic but not charge-ordered, the other is charge-ordered but not ferromagnetic.) Strain fields could be responsible for the novel microscopic texture presented here - perhaps creating conditions in which nearest neighbour hopping is sufficient to promote ferromagnetism. Similarly, strain fields are believed to cause sub-micron phase separation in the manganites. It therefore seems that the manganites can adapt to their environments over a wide range of length scales [2]. [1] http://xxx.lanl.gov/abs/cond-mat/0209436 [2] Neil Mathur and Peter Littlewood, Physics Today, early 2003.

Dynamics of magnetization in ferromagnet with spin-transfer torque

NASA Astrophysics Data System (ADS)

Li, Zai-Dong; He, Peng-Bin; Liu, Wu-Ming

2014-11-01

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. The precession frequency can be expressed as a function of the current and external magnetic field.

Fluctuations in a model ferromagnetic film driven by a slowly oscillating field with a constant bias

NASA Astrophysics Data System (ADS)

Buendía, Gloria M.; Rikvold, Per Arne

2017-10-01

We present a numerical and theoretical study that supports and explains recent experimental results on anomalous magnetization fluctuations of a uniaxial ferromagnetic film in its low-temperature phase, which is forced by an oscillating field above the critical period of the associated dynamic phase transition (DPT) [P. Riego, P. Vavassori, and A. Berger, Phys. Rev. Lett. 118, 117202 (2017), 10.1103/PhysRevLett.118.117202]. For this purpose, we perform kinetic Monte Carlo simulations of a two-dimensional Ising model with nearest-neighbor ferromagnetic interactions in the presence of a sinusoidally oscillating field, to which is added a constant bias field. We study a large range of system sizes and supercritical periods and analyze the data using a droplet-theoretical description of magnetization switching. We find that the period-averaged magnetization, which plays the role of the order parameter for the DPT, presents large fluctuations that give rise to well-defined peaks in its scaled variance and its susceptibility with respect to the bias field. The peaks are symmetric with respect to zero bias and located at values of the bias field that increase toward the field amplitude as an inverse logarithm of the field oscillation period. Our results indicate that this effect is independent of the system size for large systems, ruling out critical behavior associated with a phase transition. Rather, it is a stochastic-resonance phenomenon that has no counterpart in the corresponding thermodynamic phase transition, providing a reminder that the equivalence of the DPT to an equilibrium phase transition is limited to the critical region near the critical period and zero bias.

On the multiferroic skyrmion-host GaV4S8

NASA Astrophysics Data System (ADS)

Widmann, S.; Ruff, E.; Günther, A.; Krug von Nidda, H.-A.; Lunkenheimer, P.; Tsurkan, V.; Bordács, S.; Kézsmárki, I.; Loidl, A.

2017-12-01

The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, Néel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarisation and that excess spin-driven polarisations appear in all magnetic phases, including the skyrmion-lattice phase. Hence, GaV4S8 shows simultaneous magnetic and polar order and belongs to the class of multiferroics materials that attracted enormous attention in recent years. Here, we summarise the existing experimental information on the magnetic, electronic and dielectric properties of GaV4S8. By performing detailed magnetic susceptibility, resistivity, specific heat and dielectric experiments, we complement the low-temperature phase diagram. Specifically, we show that the low-temperature and low-field ground state of GaV4S8 seems to have a more complex spin configuration than purely collinear ferromagnetic spin order. In addition, at the structural Jahn-Teller transition the magnetic exchange interaction changes from antiferromagnetic to ferromagnetic. We also provide experimental evidence that the vanadium V4 clusters in GaV4S8 can be regarded as molecular units with spin 1/2. However, at high temperatures deviations in the susceptibility show up, indicating that either the magnetic moments of the vanadium atoms fluctuate independently or excited states of the V4 molecule become relevant.

Hybridized Kibble-Zurek scaling in the driven critical dynamics across an overlapping critical region

NASA Astrophysics Data System (ADS)

Zhai, Liang-Jun; Wang, Huai-Yu; Yin, Shuai

2018-04-01

The conventional Kibble-Zurek scaling describes the scaling behavior in the driven dynamics across a single critical region. In this paper, we study the driven dynamics across an overlapping critical region, in which a critical region (Region A) is overlaid by another critical region (Region B). We develop a hybridized Kibble-Zurek scaling (HKZS) to characterize the scaling behavior in the driven process. According to the HKZS, the driven dynamics in the overlapping region can be described by the critical theories for both Region A and Region B simultaneously. This results in a constraint on the scaling function in the overlapping critical region. We take the quantum Ising chain in an imaginary longitudinal field as an example. In this model, the critical region of the Yang-Lee edge singularity and the critical region of the ferromagnetic-paramagnetic phase transition overlap with each other. We numerically confirm the HKZS by simulating the driven dynamics in this overlapping critical region. The HKZSs in other models are also discussed.

Electric control of magnetization reorientation in FeRh /BaTiO3 mediated by a magnetic phase transition

NASA Astrophysics Data System (ADS)

Odkhuu, Dorj

2017-10-01

Employing first-principles calculations we predict magnetization reorientation in FeRh films epitaxially grown on BaTiO3 by reversing the electric polarization or applying the strain effect, which is associated with the recently discovered voltage-induced interfacial magnetic-phase transition by R. O. Cherifi et al. [Nat. Mater. 13, 345 (2014), 10.1038/nmat3870]. We propose that this transition from antiferromagnetic to ferromagnetic phase is the results of the mutual mechanisms of the polarization-reversal-induced volume/strain expansion in the interfacial FeRh layers and the competition between direct and indirect exchange interactions. These mechanisms are mainly driven by the ferroelectrically driven hybridization between Fe and Ti 3 d orbital states at the interface. Such a strong hybridization can further involve Rh 4 d states with large spin-orbit coupling, which, rather than the Fe 3 d orbitals, is responsible for magnetization reorientation at the magnetic-phase transition. These findings point toward the feasibility of electric field control of magnetization switching associated with the magnetic-phase transition in an antiferromagnet structure.

Vacancy-driven magnetocaloric effect in Prussian blue analogues

NASA Astrophysics Data System (ADS)

Evangelisti, Marco; Manuel, Espérança; Affronte, Marco; Okubo, Masashi; Train, Cyrille; Verdaguer, Michel

2007-09-01

We experimentally show that the magnetocaloric properties of molecule-based Prussian blue analogues can be adjusted by controlling during the synthesis the amount of intrinsic vacancies. For CsxNi4II[CrIII(CN)6], we find indeed that the ferromagnetic phase transition induces significantly large magnetic entropy changes, whose maxima shift from ˜68 to ˜95 K by varying the number of [CrIII(CN)6] vacancies, offering a unique tunability of the magnetocaloric effect in this complex.

Interplay of dopant, defects and electronic structure in driving ferromagnetism in Co-doped oxides: TiO(2), CeO(2) and ZnO.

PubMed

Ali, Bakhtyar; Shah, Lubna R; Ni, C; Xiao, J Q; Shah, S Ismat

2009-11-11

A comprehensive study of the defects and impurity (Co)-driven ferromagnetism is undertaken in the oxide semiconductors: TiO(2), ZnO and CeO(2). The effect of magnetic (Co(2+)) and non-magnetic (Cu(2+)) impurities in conjunction with defects, such as oxygen vacancies (V(o)), have been thoroughly investigated. Analyses of the x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) data reveal the incorporation of cobalt in the lattice, with no signature of cobalt segregation. It is shown that oxygen vacancies are necessary for the ferromagnetic coupling in the Co-doped oxides mentioned above. The possible exchange mechanisms responsible for the ferromagnetism are discussed in light of the energy levels of dopants in the host oxides. In addition, Co and Cu co-doped TiO(2) samples are studied in order to understand the role of point defects in establishing room temperature ferromagnetism. The parameters calculated from the bound magnetic polaron (BMP) and Jorgensen's optical electronegativity models offer a satisfactory explanation of the defect-driven ferromagnetism in the doped/co-doped samples.

The ferromagnetic-spin glass transition in PdMn alloys: symmetry breaking of ferromagnetism and spin glass studied by a multicanonical method.

PubMed

Kato, Tomohiko; Saita, Takahiro

2011-03-16

The magnetism of Pd(1-x)Mn(x) is investigated theoretically. A localized spin model for Mn spins that interact with short-range antiferromagnetic interactions and long-range ferromagnetic interactions via itinerant d electrons is set up, with no adjustable parameters. A multicanonical Monte Carlo simulation, combined with a procedure of symmetry breaking, is employed to discriminate between the ferromagnetic and spin glass orders. The transition temperature and the low-temperature phase are determined from the temperature variation of the specific heat and the probability distributions of the ferromagnetic order parameter and the spin glass order parameter at different concentrations. The calculation results reveal that only the ferromagnetic phase exists at x < 0.02, that only the spin glass phase exists at x > 0.04, and that the two phases coexist at intermediate concentrations. This result agrees semi-quantitatively with experimental results.

Ferromagnetic quantum critical point avoided by the appearance of another magnetic phase in LaCrGe 3 under pressure

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

Taufour, Valentin; Kaluarachchi, Udhara S.; Khasanov, Rustem

2016-07-13

Here, the temperature-pressure phase diagram of the ferromagnet LaCrGe 3 is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFMQ. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGemore » 3 is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.« less

An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes

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

Brook, David J. R.; Fleming, Connor; Chung, Dorothy

A single electron reduction of an iron bis(verdazyl) complex results in a large change in spin multiplicity resulting from a combination of spin crossover and exceptionally strong ferromagnetic exchange.

An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes

DOE PAGES

Brook, David J. R.; Fleming, Connor; Chung, Dorothy; ...

2018-01-01

A single electron reduction of an iron bis(verdazyl) complex results in a large change in spin multiplicity resulting from a combination of spin crossover and exceptionally strong ferromagnetic exchange.

Monte Carlo simulation of dynamic phase transitions and frequency dispersions of hysteresis curves in core/shell ferrimagnetic cubic nanoparticle

NASA Astrophysics Data System (ADS)

Vatansever, Erol

2017-05-01

By means of Monte Carlo simulation method with Metropolis algorithm, we elucidate the thermal and magnetic phase transition behaviors of a ferrimagnetic core/shell nanocubic system driven by a time dependent magnetic field. The particle core is composed of ferromagnetic spins, and it is surrounded by an antiferromagnetic shell. At the interface of the core/shell particle, we use antiferromagnetic spin-spin coupling. We simulate the nanoparticle using classical Heisenberg spins. After a detailed analysis, our Monte Carlo simulation results suggest that present system exhibits unusual and interesting magnetic behaviors. For example, at the relatively lower temperature regions, an increment in the amplitude of the external field destroys the antiferromagnetism in the shell part of the nanoparticle, leading to a ground state with ferromagnetic character. Moreover, particular attention has been dedicated to the hysteresis behaviors of the system. For the first time, we show that frequency dispersions can be categorized into three groups for a fixed temperature for finite core/shell systems, as in the case of the conventional bulk systems under the influence of an oscillating magnetic field.

Strain controlled ferromagnetic-ferrimagnetic transition and vacancy formation energy of defective graphene.

PubMed

Zhang, Yajun; Sahoo, Mpk; Wang, Jie

2016-09-23

Single vacancy (SV)-induced magnetism in graphene has attracted much attention motivated by its potential in achieving new functionalities. However, a much higher vacancy formation energy limits its direct application in electronic devices and the dependency of spin interaction on the strain is unclear. Here, through first-principles density-functional theory calculations, we investigate the possibility of strain engineering towards lowering vacancy formation energy and inducing new magnetic states in defective graphene. It is found that the SV-graphene undergoes a phase transition from an initial ferromagnetic state to a ferrimagnetic state under a biaxial tensile strain. At the same time, the biaxial tensile strain significantly lowers the vacancy formation energy. The charge density, density of states and band theory successfully identify the origin and underlying physics of the transition. The predicted magnetic phase transition is attributed to the strain driven spin flipping at the C-atoms nearest to the SV-site. The magnetic semiconducting graphene induced by defect and strain engineering suggests an effective way to modulate both spin and electronic degrees of freedom in future spintronic devices.

Dynamics of antiferromagnetic skyrmion driven by the spin Hall effect

NASA Astrophysics Data System (ADS)

Jin, Chendong; Song, Chengkun; Wang, Jianbo; Liu, Qingfang

2016-10-01

Magnetic skyrmion moved by the spin-Hall effect is promising for the application of the generation racetrack memories. However, the Magnus force causes a deflected motion of skyrmion, which limits its application. Here, we create an antiferromagnetic skyrmion by injecting a spin-polarized pulse in the nanostripe and investigate the spin Hall effect-induced motion of antiferromagnetic skyrmion by micromagnetic simulations. In contrast to ferromagnetic skyrmion, we find that the antiferromagnetic skyrmion has three evident advantages: (i) the minimum driving current density of antiferromagnetic skyrmion is about two orders smaller than the ferromagnetic skyrmion; (ii) the velocity of the antiferromagnetic skyrmion is about 57 times larger than the ferromagnetic skyrmion driven by the same value of current density; (iii) antiferromagnetic skyrmion can be driven by the spin Hall effect without the influence of Magnus force. In addition, antiferromagnetic skyrmion can move around the pinning sites due to its property of topological protection. Our results present the understanding of antiferromagnetic skyrmion motion driven by the spin Hall effect and may also contribute to the development of antiferromagnetic skyrmion-based racetrack memories.

Successive Phase Transitions and Magnetic Fluctuation in a Double-Perovskite NdBaMn2O6 Single Crystal

NASA Astrophysics Data System (ADS)

Yamada, S.; Sagayama, H.; Sugimoto, K.; Arima, T.

2018-03-01

We have succeeded in growing large high-quality single crystals of double-perovskite NdBaMn2O6 with c-axis aligned. Curie-Weiss paramagnetism and metallic conduction are observed above 290 K (TMI ). The magnetic susceptibility suddenly drops at TMI accompanied by a metal-insulator transition. Pervious studies using polycrystalline samples proposed that this material undergoes a ferromagnetic phase transition near 300K, and that the magnetic anomaly at TMI should be ascribed to layered antiferromagnetic phase transition. However, single-crystalline samples do not show any anomaly that indicates the ferromagnetic phase transition above TMI . We assign the onset of magnetic anisotropy at 235 K as antiferromagnetic transition temperature TN . Though the magnetization just above TMI shows the ferromagnetic-like magnetic-field dependence, the magnetization does not saturate under 70kOe at 300K. The magnetization behavior implies ferromagnetic fluctuation in the paramagnetic phase. The ferromagnetic fluctuation are also observed just below TMI . Because a metamagnetic transition is observed at a higher magnetic field, the ferromagnetic fluctuation competes with antiferromagnetic fluctuation in this temperature range.

Perfect Spin Filter by Periodic Drive of a Ferromagnetic Quantum Barrier

NASA Astrophysics Data System (ADS)

Thuberg, Daniel; Muñoz, Enrique; Eggert, Sebastian; Reyes, Sebastián A.

2017-12-01

We consider the problem of particle tunneling through a periodically driven ferromagnetic quantum barrier connected to two leads. The barrier is modeled by an impurity site representing a ferromagnetic layer or a quantum dot in a tight-binding Hamiltonian with a local magnetic field and an ac-driven potential, which is solved using the Floquet formalism. The repulsive interactions in the quantum barrier are also taken into account. Our results show that the time-periodic potential causes sharp resonances of perfect transmission and reflection, which can be tuned by the frequency, the driving strength, and the magnetic field. We demonstrate that a device based on this configuration could act as a highly tunable spin valve for spintronic applications.

Magnetic quantum phase transition in Cr-doped Bi2(SexTe1-x)3 driven by the Stark effect

NASA Astrophysics Data System (ADS)

Zhang, Zuocheng; Feng, Xiao; Wang, Jing; Lian, Biao; Zhang, Jinsong; Chang, Cuizu; Guo, Minghua; Ou, Yunbo; Feng, Yang; Zhang, Shou-Cheng; He, Ke; Ma, Xucun; Xue, Qi-Kun; Wang, Yayu

2017-10-01

The recent experimental observation of the quantum anomalous Hall effect has cast significant attention on magnetic topological insulators. In these magnetic counterparts of conventional topological insulators such as Bi2Te3, a long-range ferromagnetic state can be established by chemical doping with transition-metal elements. However, a much richer electronic phase diagram can emerge and, in the specific case of Cr-doped Bi2(SexTe1-x)3, a magnetic quantum phase transition tuned by the actual chemical composition has been reported. From an application-oriented perspective, the relevance of these results hinges on the possibility to manipulate magnetism and electronic band topology by external perturbations such as an electric field generated by gate electrodes—similar to what has been achieved in conventional diluted magnetic semiconductors. Here, we investigate the magneto-transport properties of Cr-doped Bi2(SexTe1-x)3 with different compositions under the effect of a gate voltage. The electric field has a negligible effect on magnetic order for all investigated compositions, with the remarkable exception of the sample close to the topological quantum critical point, where the gate voltage reversibly drives a ferromagnetic-to-paramagnetic phase transition. Theoretical calculations show that a perpendicular electric field causes a shift in the electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and, in turn, a magnetic phase transition.

Coupled multiferroic domain switching in the canted conical spin spiral system Mn2GeO4

NASA Astrophysics Data System (ADS)

Honda, T.; White, J. S.; Harris, A. B.; Chapon, L. C.; Fennell, A.; Roessli, B.; Zaharko, O.; Murakami, Y.; Kenzelmann, M.; Kimura, T.

2017-06-01

Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn2GeO4 using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180o commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn2GeO4 as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.

Phase Sensitive Measurements of Ferromagnetic Josephson Junctions for Cryogenic Memory Applications

NASA Astrophysics Data System (ADS)

Niedzielski, Bethany Maria

A Josephson junction is made up of two superconducting layers separated by a barrier. The original Josephson junctions, studied in the early 1960's, contained an insulating barrier. Soon thereafter, junctions with normal-metal barriers were also studied. Ferromagnetic materials were not even theoretically considered as a barrier layer until around 1980, due to the competing order between ferromagnetic and superconducting systems. However, many exciting physical phenomena arise in hybrid superconductor/ferromagnetic devices, including devices where the ground state phase difference between the two superconductors is shifted by pi. Since their experimental debut in 2001, so-called pi junctions have been demonstrated by many groups, including my own, in systems with a single ferromagnetic layer. In this type of system, the phase of the junction can be set to either 0 or pi depending on the thickness of the ferromagnetic layer. Of interest, however, is the ability to control the phase of a single junction between the 0 and pi states. This was theoretically shown to be possible in a system containing two ferromagnetic layers (spin-valve junctions). If the materials and their thicknesses are properly chosen to manipulate the electron pair correlation function, then the phase state of a spin-valve Josephson junction should be capable of switching between the 0 and ? phase states when the magnetization directions of the two ferromagnetic layers are oriented in the antiparallel and parallel configurations, respectively. Such a phase-controllable junction would have immediate applications in cryogenic memory, which is a necessary component to an ultra-low power superconducting computer. A fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers. The goal of this work was to experimentally verify this prediction for a phase-controllable ferromagnetic Josephson junction. To address this complicated system, first, studies of junctions with only a single ferromagnetic junction were required to determine the 0-pi transition thickness of that material, the decay of the critical current through the junction with thickness, and the switching field of the material. The materials studied included NiFeMo, NiFe, Ni, and NiFeCo. Additionally, roughness studies of several different superconducting base electrodes and normal metal buffer and spacer layers were performed to determine the optimum junction layers. The ferromagnetic layers used were on the order of 1-2 nm thick, so a smooth growth template is imperative to maintain continuous films with in-plane magnetizations. Lastly, single junction spin-valve samples were studied. We are not equipped to measure the phase of a single junction, but series of samples where one ferromagnetic layer is systematically varied in thickness can inform the proper thicknesses needed for 0-pi switching based on relative critical current values between the parallel and antiparallel magnetic configurations. Utilizing this background information, two spin-valve samples were incorporated in a superconducting loop so that the relative phase of the two junctions could be investigated. Through this process, the first phase-controllable ferromagnetic Josephson junctions were experimentally demonstrated using phase-sensitive measurement techniques. This provided the proof of concept for the Josephson Magnetic Random Access Memory (JMRAM), a superconducting memory system in development at Northrop Grumman, with whom we collaborate on this work. Phase-controllable systems were successfully demonstrated using two different magnetic material stacks and verified with several analysis techniques.

Raman spectroscopy of KxCo2-ySe2 single crystals near the ferromagnet-paramagnet transition

DOE PAGES

Opacic, M.; Lazarevic, N.; Radonjic, M. M.; ...

2016-10-05

Polarized Raman scattering spectra of the K xCo 2-ySe 2 (x = :::; y = :::) single crystals reveal the presence of two phonon modes, assigned as of the A1g and B1g symmetry. Absence of additional modes excludes the possibility of vacancy ordering, unlike in K xCo 2-ySe 2 . The ferromagnetic (FM) phase transition at Tc 74 K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For T > Tc the temperature dependence looks conventional, driven by the thermal expansion and anharmonicity. The Raman modes are rather broad due to the electron-phononmore » coupling increased by the disorder and spin fluctuation e ects. In the FM phase the phonon frequency of both modes increases, while an opposite trend is seen in their linewidth: the A1g mode narrows in the FM phase, whereas the B 1g mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B 1g mode is due to the coupling of spin fluctuations and vibration. Our density functional theory (DFT) calculations for the phonon frequencies agree rather well with the Raman measurements, with some discrepancy being expected since the DFT calculations neglect the spin fluctuations.« less

Ab initio description of the diluted magnetic semiconductor Ga1-xMnxAs: Ferromagnetism, electronic structure, and optical response

NASA Astrophysics Data System (ADS)

Craco, L.; Laad, M. S.; Müller-Hartmann, E.

2003-12-01

Motivated by a study of various experiments describing the electronic and magnetic properties of the diluted magnetic semiconductor Ga1-xMnxAs, we investigate its physical response in detail using a combination of first-principles band structure with methods based on dynamical mean field theory to incorporate strong, dynamical correlations, and intrinsic as well as extrinsic disorder in one single theoretical picture. We show how ferromagnetism is driven by double exchange (DE), in agreement with very recent observations, along with a good quantitative description of the details of the electronic structure, as probed by scanning tunneling microscopy and optical conductivity. Our results show how ferromagnetism can be driven by DE even in diluted magnetic semiconductors with small carrier concentration.

Chemical disorder influence on magnetic state of optimally-doped La0.7Ca0.3MnO3

NASA Astrophysics Data System (ADS)

Rozenberg, E.; Auslender, M.; Shames, A. I.; Jung, G.; Felner, I.; Tsindlekht, M. I.; Mogilyansky, D.; Sominski, E.; Gedanken, A.; Mukovskii, Ya. M.; Gorodetsky, G.

2011-10-01

X-band electron magnetic resonance and dc/ac magnetic measurements have been employed to study the effects of chemical disorder on magnetic ordering in bulk and nanometer-sized single crystals and bulk ceramics of optimally-doped La0.7Ca0.3MnO3 manganite. The magnetic ground state of bulk samples appeared to be ferromagnetic with the lower Curie temperature and higher magnetic homogeneity in the vicinity of the ferromagnetic-paramagnetic phase transition in the crystal, as compared with those characteristics in the ceramics. The influence of technological driven "macroscopic" fluctuations of Ca-dopant level in crystal and "mesoscopic" disorder within grain boundary regions in ceramics was proposed to be responsible for these effects. Surface spin disorder together with pronounced inter-particle interactions within agglomerated nano-sample results in well defined core/shell spin configuration in La0.7Ca0.3MnO3 nano-crystals. The analysis of the electron paramagnetic resonance data enlightened the reasons for the observed difference in the magnetic order. Lattice effects dominate the first-order nature of magnetic phase transition in bulk samples. However, mesoscale chemical disorder seems to be responsible for the appearance of small ferromagnetic polarons in the paramagnetic state of bulk ceramics. The experimental results and their analysis indicate that a chemical/magnetic disorder has a strong impact on the magnetic state even in the case of mostly stable optimally hole-doped manganites.

Anomalous metastability in a temperature-driven transition

NASA Astrophysics Data System (ADS)

Ibáñez Berganza, M.; Coletti, P.; Petri, A.

2014-06-01

The Langer theory of metastability provides a description of the lifetime and properties of the metastable phase of the Ising model field-driven transition, describing the magnetic-field-driven transition in ferromagnets and the chemical-potential-driven transition of fluids. An immediate further step is to apply it to the study of a transition driven by the temperature, as the one exhibited by the two-dimensional Potts model. For this model, a study based on the analytical continuation of the free energy (Meunier J. L. and Morel A., Eur. Phys. J. B, 13 (2000) 341) predicts the anomalous vanishing of the metastable temperature range in the large-system-size limit, an issue that has been controversial since the eighties. By a GPU algorithm we compare the Monte Carlo dynamics with the theory. For temperatures close to the transition we obtain agreement and characterize the dependence on the system size, which is essentially different with respect to the Ising case. For smaller temperatures, we observe the onset of stationary states with non-Boltzmann statistics, not predicted by the theory.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition.

PubMed

Chen, X Z; Feng, J F; Wang, Z C; Zhang, J; Zhong, X Y; Song, C; Jin, L; Zhang, B; Li, F; Jiang, M; Tan, Y Z; Zhou, X J; Shi, G Y; Zhou, X F; Han, X D; Mao, S C; Chen, Y H; Han, X F; Pan, F

2017-09-06

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α'-FeRh-based junctions driven by the magnetic phase transition of α'-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α'-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α'-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α'-FeRh magnetic electrode.

Transverse susceptibility as a probe of the magnetocrystalline anisotropy-driven phase transition in Pr0.5Sr0.5CoO3

NASA Astrophysics Data System (ADS)

Frey Huls, N. A.; Bingham, N. S.; Phan, M. H.; Srikanth, H.; Stauffer, D. D.; Leighton, C.

2011-01-01

Half-doped Pr1-xSrxCoO3 (x=0.5) displays anomalous magnetism, most notably manifest in the field-cooled magnetization versus temperature curves under different applied cooling fields. Recently, an explanation was advanced that a magnetocrystalline anisotropy transition driven by a structural transition at 120 K is the origin of this behavior. In this paper, we further elucidate the nature of the magnetic anisotropy across the low-temperature phase transition in this material by means of transverse susceptibility (TS) measurements performed using a self-resonant tunnel diode oscillator. TS probes magnetic materials by means of a small radio frequency oriented transverse to a dc field that sweeps from positive to negative saturation. TS scans as a function of field clearly reveal peaks associated with the anisotropy (HK) and switching fields (HS). When peak position is examined as a function of temperature, ˜120 K the signature of a ferromagnetic-to-ferromagnetic phase transition is evident as a sharp feature in HK and a corresponding cusp in HS. A third TS peak (not previously observed in other classes of magnetic oxides such as manganites and spinel ferrites) is found to be correlated with the crossover field (Hcr) in the unconventional magnetization versus temperature [M(T)] behavior. We observe a strong temperature dependence of Hcr at ˜120 K using this technique, which suggests the magnetic-field-influenced magnetocrystalline anisotropy transition. We show the switching between the high-field magnetization state and the low-field magnetization state associated with the magnetocrystalline anisotropy transition is irreversible when the magnetic field is recycled. Finally, we demonstrate that the TS peak magnitude indicates easy axis switching associated with this phase transition, even in these polycrystalline samples. Our results further confirm that TS provides new insights into the magnetic behavior of complex oxides.

High-Performance THz Emitters Based on Ferromagnetic/Nonmagnetic Heterostructures.

PubMed

Wu, Yang; Elyasi, Mehrdad; Qiu, Xuepeng; Chen, Mengji; Liu, Yang; Ke, Lin; Yang, Hyunsoo

2017-01-01

A low-cost, intense, broadband, noise resistive, magnetic field controllable, flexible, and low power driven THz emitter based on thin nonmagnetic/ferromagnetic metallic heterostructures is demonstrated. The THz emission origins from the inverse spin Hall Effect. The proposed devices are not only promising for a wide range of THz equipment, but also offer an alternative approach to characterize the spin-orbit interaction in nonmagnetic/ferromagnetic bilayers. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Simulation study on exchange interaction and unique magnetization near ferromagnetic morphotropic phase boundary.

PubMed

Wei, Songrui; Liao, Xiaoqi; Gao, Yipeng; Yang, Sen; Wang, Dong; Song, Xiaoping

2017-11-08

Extensive efforts have been made in searching enhanced functionalities near the so-called morphotropic phase boundaries (MPBs) in both ferroelectric and ferromagnetic materials. Due to the exchange anti-symmetry of the wave function of fermions, it is widely recognized that the exchange interaction plays a critical role in ferromagnetism. As a quantum effect, the exchange interaction is magnitudes larger than electric interaction, leading to a fundamental difference between ferroelectricity and ferromagnetism. In this paper, we establish an energetic model capturing the interplay among the anisotropy energy, magnetostatic energy and the exchange energy to investigate systematically the effects of the exchange energy on the behavior of the ferromagnetic MPB. For the first time, it is found that the exchange energy can narrow the width of MPB region in the composition temperature phase diagram for ferromagnetic MPB systems. As temperature increases, MPB region becomes wider because of the weakening of the exchange interaction. Our simulation results suggest that the exchange energy play a critical role on the unique behavior of ferromagnetic MPB, which is in contrast different from that of ferroelectric MPB.

Spontaneous exchange bias formation driven by a structural phase transition in the antiferromagnetic material.

PubMed

Migliorini, A; Kuerbanjiang, B; Huminiuc, T; Kepaptsoglou, D; Muñoz, M; Cuñado, J L F; Camarero, J; Aroca, C; Vallejo-Fernández, G; Lazarov, V K; Prieto, J L

2018-01-01

Most of the magnetic devices in advanced electronics rely on the exchange bias effect, a magnetic interaction that couples a ferromagnetic and an antiferromagnetic material, resulting in a unidirectional displacement of the ferromagnetic hysteresis loop by an amount called the 'exchange bias field'. Setting and optimizing exchange bias involves cooling through the Néel temperature of the antiferromagnetic material in the presence of a magnetic field. Here we demonstrate an alternative process for the generation of exchange bias. In IrMn/FeCo bilayers, a structural phase transition in the IrMn layer develops at room temperature, exchange biasing the FeCo layer as it propagates. Once the process is completed, the IrMn layer contains very large single-crystal grains, with a large density of structural defects within each grain, which are promoted by the FeCo layer. The magnetic characterization indicates that these structural defects in the antiferromagnetic layer are behind the resulting large value of the exchange bias field and its good thermal stability. This mechanism for establishing the exchange bias in such a system can contribute towards the clarification of fundamental aspects of this exchange interaction.

Itinerant ferromagnetism in an interacting Fermi gas with mass imbalance

NASA Astrophysics Data System (ADS)

von Keyserlingk, C. W.; Conduit, G. J.

2011-05-01

We study the emergence of itinerant ferromagnetism in an ultracold atomic gas with a variable mass ratio between the up- and down-spin species. Mass imbalance breaks the SU(2) spin symmetry, leading to a modified Stoner criterion. We first elucidate the phase behavior in both the grand canonical and canonical ensembles. Second, we apply the formalism to a harmonic trap to demonstrate how a mass imbalance delivers unique experimental signatures of ferromagnetism. These could help future experiments to better identify the putative ferromagnetic state. Furthermore, we highlight how a mass imbalance suppresses the three-body loss processes that handicap the formation of a ferromagnetic state. Finally, we study the time-dependent formation of the ferromagnetic phase following a quench in the interaction strength.

Ferromagnetic superconductors: A vortex phase in ternary rare-earth compounds

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

Kuper, C.G.; Revzen, M.; Ron, A.

1980-06-09

It is shown that the generalized Ginzburg-Landau free-energy functional of Blount and Varma admits self-consistent solutions with quantized-flux vortices, magnetized in a region about the cores. There is a temperature range where the new phase has a lower free energy than either the pure superconducting or ferromagnetic phases; it represents true coexistence of ferromagnetism and superconductivity. The main features of the specific heat and magnetic properties of some rare-earth ternary compounds can be explained qualitatively.

Numerical tests of local scale invariance in ageing q-state Potts models

NASA Astrophysics Data System (ADS)

Lorenz, E.; Janke, W.

2007-01-01

Much effort has been spent over the last years to achieve a coherent theoretical description of ageing as a non-linear dynamics process. Long supposed to be a consequence of the slow dynamics of glassy systems only, ageing phenomena could also be identified in the phase-ordering kinetics of simple ferromagnets. As a phenomenological approach Henkel et al. developed a group of local scale transformations under which two-time autocorrelation and response functions should transform covariantly. This work is to extend previous numerical tests of the predicted scaling functions for the Ising model by Monte Carlo simulations of two-dimensional q-state Potts models with q=3 and 8, which, in equilibrium, undergo temperature-driven phase transitions of second and first order, respectively.

Orbital ordering-driven ferromagnetism in LaCoO3 nanowires

NASA Astrophysics Data System (ADS)

Wang, Yang; Fan, Hong Jin

2010-09-01

The structure and magnetic properties of LaCoO3 nanowires are investigated as a function of the diameter in the temperature range of 5-300 K. Ferromagnetism below 85 K is observed in these nanowires, in agreement with the recent observations in LaCoO3 epitaxial thin films and nanoparticles. With the diameter of nanowires decreasing, the unit-cell volume increases, while both the global and local structural distortions lessen, accompanied by the gradual enhancement of ferromagnetism. The structure analysis reveals that LaCoO3 nanowires exhibit a monoclinic distorted structure with I2/a space group in the entire investigated temperature range. Different from bulks, there is no clear spin-state transition occurring with temperature in LaCoO3 nanowires. There exists a noticeable Jahn-Teller (JT) distortion in the nanowires even at the lowest temperature, namely, orbital-ordered JT active Co3+ ions with intermediate-spin (IS) state persist at low temperatures, which is not observed in bulk LaCoO3. These results indicate that the ferromagnetism in the nanowires is driven by the orbital ordering of IS Co3+.

Itinerant ferromagnetism in an interacting Fermi gas with mass imbalance

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

Keyserlingk, C. W. von; Conduit, G. J.; Physics Department, Ben Gurion University, Beer Sheva 84105

2011-05-15

We study the emergence of itinerant ferromagnetism in an ultracold atomic gas with a variable mass ratio between the up- and down-spin species. Mass imbalance breaks the SU(2) spin symmetry, leading to a modified Stoner criterion. We first elucidate the phase behavior in both the grand canonical and canonical ensembles. Second, we apply the formalism to a harmonic trap to demonstrate how a mass imbalance delivers unique experimental signatures of ferromagnetism. These could help future experiments to better identify the putative ferromagnetic state. Furthermore, we highlight how a mass imbalance suppresses the three-body loss processes that handicap the formation ofmore » a ferromagnetic state. Finally, we study the time-dependent formation of the ferromagnetic phase following a quench in the interaction strength.« less

Temperature limited heaters using phase transformation of ferromagnetic material

DOEpatents

Vitek, John Michael [Oak Ridge, TN; Brady, Michael Patrick [Oak Ridge, TN

2009-10-06

Systems, methods, and heaters for treating a subsurface formation are described herein. Systems and methods for making heaters are described herein. At least one heater includes a ferromagnetic conductor and an electrical conductor. The electrical conductor is electrically coupled to the ferromagnetic conductor. The heater provides a first amount of heat at a lower temperature. The heater may provide a second reduced amount of heat when the heater reaches a selected temperature, or enters a selected temperature range, at which the ferromagnetic conductor undergoes a phase transformation.

Intermittency and dynamical Lee-Yang zeros of open quantum systems.

PubMed

Hickey, James M; Flindt, Christian; Garrahan, Juan P

2014-12-01

We use high-order cumulants to investigate the Lee-Yang zeros of generating functions of dynamical observables in open quantum systems. At long times the generating functions take on a large-deviation form with singularities of the associated cumulant generating functions-or dynamical free energies-signifying phase transitions in the ensemble of dynamical trajectories. We consider a driven three-level system as well as the dissipative Ising model. Both systems exhibit dynamical intermittency in the statistics of quantum jumps. From the short-time behavior of the dynamical Lee-Yang zeros, we identify critical values of the counting field which we attribute to the observed intermittency and dynamical phase coexistence. Furthermore, for the dissipative Ising model we construct a trajectory phase diagram and estimate the value of the transverse field where the stationary state changes from being ferromagnetic (inactive) to paramagnetic (active).

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

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

Kaluarachchi, Udhara S.; Taufour, Valentin; Bud'ko, Sergey L.

We report the temperature-pressure-magnetic eld phase diagram of the ferromagnetic Kondolattice CeTiGe 3 determined by means of electrical resistivity measurements. Measurements up to ~5.8GPa reveal a rich phase diagram with multiple phase transitions. At ambient pressure, CeTiGe 3 orders ferromagnetically at T C =14 K. Application of pressure suppresses T C, but a pressure induced ferromagnetic quantum criticality is avoided by the appearance of two new successive transitions for p>4.1GPa that are probably antiferromagnetic in nature. These two transitions are suppressed under pressure, with the lower temperature phase being fully suppressed above 5.3GPa. The critical pressures for the presumed quantummore » phase transitions are p1≅4.1GPa and p2≅5.3GPa. Above 4.1GPa, application of magnetic eld shows a tricritical point evolving into a wing structure phase with a quantum tricritical point at 2.8T at 5.4GPa, where the rst order antiferromagneticferromagnetic transition changes into the second order antiferromagnetic-ferromagnetic transition.« less

Optical manifestation of the Stoner ferromagnetic transition in two-dimensional electron systems

NASA Astrophysics Data System (ADS)

Van'kov, A. B.; Kaysin, B. D.; Kukushkin, I. V.

2017-12-01

We perform a magneto-optical study of a two-dimensional electron systems in the regime of the Stoner ferromagnetic instability for even quantum Hall filling factors on MgxZn1 -xO /ZnO heterostructures. Under conditions of Landau-level crossing, caused by enhanced spin susceptibility in combination with the tilting of the magnetic field, the transition between two rivaling phases, paramagnetic and ferromagnetic, is traced in terms of optical spectra reconstruction. Synchronous sharp transformations are observed both in the photoluminescence structure and parameters of collective excitations upon transition from paramagnetic to ferromagnetic ordering. Based on these measurements, a phase diagram is constructed in terms of the two-dimensional electron density and tilt angle of the magnetic field. Apart from stable paramagnetic and ferromagnetic phases, an instability region is found at intermediate parameters with the Stoner transition occurring at ν ≈2 . The spin configuration in all cases is unambiguously determined by means of inelastic light scattering by spin-sensitive collective excitations. One indicator of the spin ordering is the intra-Landau-level spin exciton, which acquires a large spectral weight in the ferromagnetic phases. The other is an abrupt energy shift of the intersubband charge density excitation due to reconstruction of the many-particle energy contribution. From our analysis of photoluminescence and light scattering data, we estimate the ratio of surface areas occupied by the domains of the two phases in the vicinity of a transition point. In addition, the thermal smearing of a phase transition is characterized.

Intrinsic synchronization of an array of spin-torque oscillators driven by the spin-Hall effect

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

Siracusano, G., E-mail: giuliosiracusano@gmail.com; Puliafito, V.; Giordano, A.

2015-05-07

This paper micromagnetically studies the magnetization dynamics driven by the spin-Hall effect in a Platinum/Permalloy bi-layer. For a certain field and current range, the excitation of a uniform mode, characterized by a power with a spatial distribution in the whole ferromagnetic cross section, is observed. We suggest to use the ferromagnet of the bi-layer as basis for the realization of an array of spin-torque oscillators (STOs): the Permalloy ferromagnet will act as shared free layer, whereas the spacers and the polarizers are built on top of it. Following this strategy, the frequency of the uniform mode will be the samemore » for the whole device, creating an intrinsic synchronization. The synchronization of an array of parallely connected STOs will allow to increase the output power, as necessary for technological applications.« less

Oxygen octahedra distortion induced structural and magnetic phase transitions in Bi{sub 1−x}Ca{sub x}Fe{sub 1−x}Mn{sub x}O{sub 3} ceramics

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

Kumar, Pawan; Kar, Manoranjan, E-mail: mano@iitp.ac.in; Shankhwar, Nisha

2015-05-21

The co-doping of Ca and Mn in respective Bi and Fe-sites of BiFeO{sub 3} lattice leads to structural transition from rhombohedral (R3c space group) to orthorhombic (Pbnm space group) crystal symmetry. The tilt angle for anti-phase rotation of the oxygen octahedra of BiFeO{sub 3} at room temperature is observed to be ∼13.8°. It decreases with the increase in the co-doping percentage which suggests the composition-driven structural phase transition. The remnant magnetization for sample with 15% of co-doping becomes about 16 times that of BiFeO{sub 3}. It may be attributed to the suppression of cycloid spin structure and uncompensated spins atmore » the surface of nanocrystallites. Further increase in co-doping percentage results in the sharp reduction of remnant magnetization due to the dominant contribution from the collinear antiferromagnetic ordering in the Pbnm space group. The Arrott plot analysis clearly indicates the composition-driven crossover from the antiferromagnetic to weak ferromagnetic ordering and vice versa. Electron spin resonance results provide the evidence for the composition-driven phase transitions from an incommensurate spin cycloidal modulated state to one with nearly homogeneous spin order. The band gap (2.17 eV) of BiFeO{sub 3} measured using UV-Vis spectra was supported by the resonance Raman spectra.« less

Highly efficient photocatalysis by BiFeO3/α(γ)-Fe2O3 ferromagnetic nano p/n junctions formed by dopant-induced phase separation

NASA Astrophysics Data System (ADS)

Ramadan, Wegdan; Shaikh, Parvez A.; Ebrahim, Sh.; Ramadan, Abdallah; Hannoyer, Beatrice; Jouen, Samuel; Sauvage, Xavier; Ogale, Satishchandra

2013-08-01

A series of Bi1- x Ca x FeO3 (BCFO) nanoparticles (with x = 0.0, 0.03, 0.07, 0.10, 0.15, and 0.20) have been synthesized by sol-gel reaction. X-ray diffraction patterns establish the formation of hexagonal bismuth ferrite as the prominent phase, with a small contribution of the Bi2Fe4O9 phase (as reported by others as well) which diminishes rapidly with the increase in Ca concentration. Interestingly, above a calcium dopant concentration of about 10 % peaks of Fe2O3 (both α and γ components) are observed with a concomitant enhancement of ferromagnetism. Small contribution of the Bi6Ca4O13 phase is also noted in these samples. This phase evolution is driven by dopant-induced strain energy and increasing oxygen vacancy concentration for local charge balance. Transmission electron microscopy (with elemental scanning) and Mössbauer spectroscopy techniques bring out the evolution of nanoparticle morphology (and elemental distribution) and phase configuration, respectively. Measurements of photocatalytic activity (and photo-Fenton activity with H2O2) reveal that Ca doping at the Bi site in BFO enhances the activity significantly in the concentration regime where BFO/α(γ)-Fe2O3 phases coexist in the form of a nanocomposite. The enhancement can thus be attributed to the carrier transfer between BFO and α(γ)-Fe2O3 across nano p/n junctions leading to enhanced carrier lifetime. Importantly, the magnetization of the nanocomposite (about 16 emu gm-1 at x = 0.20) provides a convenient way to collect the photocatalyst with the help of an external magnet for reuse.

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

Quantum tricritical point in the temperature-pressure-magnetic field phase diagram of CeTiGe 3

DOE PAGES

Kaluarachchi, Udhara S.; Taufour, Valentin; Bud'ko, Sergey L.; ...

2018-01-22

We report the temperature-pressure-magnetic eld phase diagram of the ferromagnetic Kondolattice CeTiGe 3 determined by means of electrical resistivity measurements. Measurements up to ~5.8GPa reveal a rich phase diagram with multiple phase transitions. At ambient pressure, CeTiGe 3 orders ferromagnetically at T C =14 K. Application of pressure suppresses T C, but a pressure induced ferromagnetic quantum criticality is avoided by the appearance of two new successive transitions for p>4.1GPa that are probably antiferromagnetic in nature. These two transitions are suppressed under pressure, with the lower temperature phase being fully suppressed above 5.3GPa. The critical pressures for the presumed quantummore » phase transitions are p1≅4.1GPa and p2≅5.3GPa. Above 4.1GPa, application of magnetic eld shows a tricritical point evolving into a wing structure phase with a quantum tricritical point at 2.8T at 5.4GPa, where the rst order antiferromagneticferromagnetic transition changes into the second order antiferromagnetic-ferromagnetic transition.« less

Ferromagnetic Josephson Junctions for Cryogenic Memory

NASA Astrophysics Data System (ADS)

Niedzielski, Bethany M.; Gingrich, Eric C.; Khasawneh, Mazin A.; Loloee, Reza; Pratt, William P., Jr.; Birge, Norman O.

2015-03-01

Josephson junctions containing ferromagnetic materials are of interest for both scientific and technological purposes. In principle, either the amplitude of the critical current or superconducting phase shift across the junction can be controlled by the relative magnetization directions of the ferromagnetic layers in the junction. Our approach concentrates on phase control utilizing two junctions in a SQUID geometry. We will report on efforts to control the phase of junctions carrying either spin-singlet or spin-triplet supercurrent for cryogenic memory applications. Supported by Northorp Grumman Corporation and by IARPA under SPAWAR Contract N66001-12-C-2017.

Ferromagnetic phase in partially oxidized FeMn films

NASA Astrophysics Data System (ADS)

Svalov, A. V.; Savin, P. A.; Lepalovskij, V. N.; Vas'kovskiy, V. O.; Larrañaga, A.; Kurlyandskaya, G. V.

2018-04-01

The structure, magnetic and magnetoresistive properties of ferromagnetic phase in partially oxidized FeMn films was studied. The oxidation was performed by annealing of the samples under atmospheric pressure in a gas mixture (nitrogen with 0.5% oxygen) at the temperature of 300 °C. The resulting ferromagnetic phase was isotropic in the film plane. The value of the anisotropic magnetoresistance was similar to the value of the anisotropic magnetoresistance usually observed in films of pure iron. The oxidation of antiferromagnetic FeMn films resulted in the appearance of an exchange bias.

High temperature magnetism and microstructure of ferromagnetic alloy Si1-x Mn x

NASA Astrophysics Data System (ADS)

Aronzon, B. A.; Davydov, A. B.; Vasiliev, A. L.; Perov, N. S.; Novodvorsky, O. A.; Parshina, L. S.; Presniakov, M. Yu; Lahderanta, E.

2017-02-01

The results of a detailed study of magnetic properties and of the microstructure of SiMn films with a small deviation from stoichiometry are presented. The aim was to reveal the origin of the high temperature ferromagnetic ordering in such compounds. Unlike SiMn single crystals with the Curie temperature ~30 K, considered Si1-x Mn x compounds with x  =  0.5  +Δx and Δx in the range of 0.01-0.02 demonstrate a ferromagnetic state above room temperature. Such a ferromagnetic state can be explained by the existence of highly defective B20 SiMn nanocrystallites. These defects are Si vacancies, which are suggested to possess magnetic moments. The nanocrystallites interact with each other through paramagnons (magnetic fluctuations) inside a weakly magnetic manganese silicide matrix giving rise to a long range ferromagnetic percolation cluster. The studied structures with a higher value of Δx  ≈  0.05 contained three different magnetic phases: (a)—the low temperature ferromagnetic phase related to SiMn; (b)—the above mentioned high temperature phase with Curie temperature in the range of 200-300 K depending on the growth history and (c)—superparamagnetic phase formed by separated noninteracting SiMn nanocrystallites.

Modeling and calculation of RKKY exchange coupling to explain Ti-vacancy-induced ferromagnetism in Ta-doped TiO2

NASA Astrophysics Data System (ADS)

Majidi, Muhammad Aziz; Bupu, Annamaria; Fauzi, Angga Dito

2017-12-01

We present a theoretical study on Ti-vacancy-induced ferromagnetism in anatase TiO2. A recent experimental study has revealed room temperature ferromagnetism in Ta-doped anatase TiO2thin films (Rusydi et al., 2012) [7]. Ta doping assists the formation of Ti vacancies which then induce the formation of localized magnetic moments around the Ti vacancies. As neighboring Ti vacancies are a few unit cells apart, the ferromagnetic order is suspected to be mediated by itinerant electrons. We propose that such an electron-mediated ferromagnetism is driven by Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction. To examine our hypothesis, we construct a tight-binding based model Hamiltonian for the anatase TiO2 system. We calculate the RKKY exchange coupling constant of TiO2 as a function of distance between local magnetic moments at various temperatures. We model the system by taking only the layer containing a unit of TiO2, at which the Ti vacancy is believed to form, as our effective two-dimensional unit cell. Our model incorporates the Hubbard repulsive interactions between electrons occupying Ti d orbitals treated within mean-field approximation. The density of states profile resulting from the model captures the relevant electronic properties of TiO2, such as the energy gap of 3.4 eV and the n-type character, which may be a measure of the adequacy of the model. The calculated RKKY coupling constant shows that the ferromagnetic coupling extends up to 3-4 unit cells and enhances slightly as temperature is increased from 0 to 400 K. These results support our hypothesis that the ferromagnetism of this system is driven by RKKY mechanism.

FAST TRACK COMMUNICATION: Interlayer exchange coupling across a ferroelectric barrier

NASA Astrophysics Data System (ADS)

Zhuravlev, M. Ye; Vedyayev, A. V.; Tsymbal, E. Y.

2010-09-01

A new magnetoelectric effect is predicted originating from the interlayer exchange coupling between two ferromagnetic layers separated by an ultrathin ferroelectric barrier. It is demonstrated that ferroelectric polarization switching driven by an external electric field leads to a sizable change in the interlayer exchange coupling. The effect occurs in asymmetric ferromagnet/ferroelectric/ferromagnet junctions due to a change in the electrostatic potential profile across the junction affecting the interlayer coupling. The predicted phenomenon indicates the possibility of switching the magnetic configuration by reversing the polarization of the ferroelectric barrier layer.

A Direct Method for Viewing Ferromagnetic Phase Transition.

ERIC Educational Resources Information Center

Lue, Chin-Shan

1994-01-01

Provides a method, using the Rowland ring as a specimen, to observe the phase transition process directly on the oscilloscope and even extract the critical exponent of ferromagnetic transition. Includes theory, experimental setup, and results. (MVL)

Ferromagnetic quantum critical point in the heavy-fermion metal YbNi4(P(1-x)As(x))2.

PubMed

Steppke, Alexander; Küchler, Robert; Lausberg, Stefan; Lengyel, Edit; Steinke, Lucia; Borth, Robert; Lühmann, Thomas; Krellner, Cornelius; Nicklas, Michael; Geibel, Christoph; Steglich, Frank; Brando, Manuel

2013-02-22

Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi(4)(P(0.92)As(0.08))(2) diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.

Magnetic quantum phase transition in Cr-doped Bi 2(Se xTe 1-x) 3 driven by the Stark effect

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

Zhang, Zuocheng; Feng, Xiao; Wang, Jing

The interplay between magnetism and topology, as exemplified in the magnetic skyrmion systems, has emerged as a rich playground for finding novel quantum phenomena and applications in future information technology. Magnetic topological insulators (TI) have attracted much recent attention, especially after the experimental realization of quantum anomalous Hall effect. Future applications of magnetic TI hinge on the accurate manipulation of magnetism and topology by external perturbations, preferably with a gate electric field. In this work, we investigate the magneto transport properties of Cr doped Bi 2(Se xTe 1-x) 3 TI across the topological quantum critical point (QCP). We find thatmore » the external gate voltage has negligible effect on the magnetic order for samples far away from the topological QCP. However, for the sample near the QCP, we observe a ferromagnetic (FM) to paramagnetic (PM) phase transition driven by the gate electric field. Theoretical calculations show that a perpendicular electric field causes a shift of electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and consequently a magnetic phase transition. Finally, the in situ electrical control of the topological and magnetic properties of TI shed important new lights on future topological electronic or spintronic device applications.« less

Magnetic quantum phase transition in Cr-doped Bi 2(Se xTe 1-x) 3 driven by the Stark effect

DOE PAGES

Zhang, Zuocheng; Feng, Xiao; Wang, Jing; ...

2017-08-07

The interplay between magnetism and topology, as exemplified in the magnetic skyrmion systems, has emerged as a rich playground for finding novel quantum phenomena and applications in future information technology. Magnetic topological insulators (TI) have attracted much recent attention, especially after the experimental realization of quantum anomalous Hall effect. Future applications of magnetic TI hinge on the accurate manipulation of magnetism and topology by external perturbations, preferably with a gate electric field. In this work, we investigate the magneto transport properties of Cr doped Bi 2(Se xTe 1-x) 3 TI across the topological quantum critical point (QCP). We find thatmore » the external gate voltage has negligible effect on the magnetic order for samples far away from the topological QCP. However, for the sample near the QCP, we observe a ferromagnetic (FM) to paramagnetic (PM) phase transition driven by the gate electric field. Theoretical calculations show that a perpendicular electric field causes a shift of electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and consequently a magnetic phase transition. Finally, the in situ electrical control of the topological and magnetic properties of TI shed important new lights on future topological electronic or spintronic device applications.« less

Random-field-induced disordering mechanism in a disordered ferromagnet: Between the Imry-Ma and the standard disordering mechanism

NASA Astrophysics Data System (ADS)

Andresen, Juan Carlos; Katzgraber, Helmut G.; Schechter, Moshe

2017-12-01

Random fields disorder Ising ferromagnets by aligning single spins in the direction of the random field in three space dimensions, or by flipping large ferromagnetic domains at dimensions two and below. While the former requires random fields of typical magnitude similar to the interaction strength, the latter Imry-Ma mechanism only requires infinitesimal random fields. Recently, it has been shown that for dilute anisotropic dipolar systems a third mechanism exists, where the ferromagnetic phase is disordered by finite-size glassy domains at a random field of finite magnitude that is considerably smaller than the typical interaction strength. Using large-scale Monte Carlo simulations and zero-temperature numerical approaches, we show that this mechanism applies to disordered ferromagnets with competing short-range ferromagnetic and antiferromagnetic interactions, suggesting its generality in ferromagnetic systems with competing interactions and an underlying spin-glass phase. A finite-size-scaling analysis of the magnetization distribution suggests that the transition might be first order.

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.

Phase Coexistence and Kinetic Arrest in the Magnetostructural Transition of the Ordered Alloy FeRh

DOE PAGES

Keavney, David J.; Choi, Yongseong; Holt, Martin V.; ...

2018-01-29

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging andmore » comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. In conclusion, the domains show evidence of inhibited growth after nucleation, resulting in a quasi- 2nd order temperature behavior.« less

Signature of Griffith phase in (Tb1-xCex)MnO3

NASA Astrophysics Data System (ADS)

Kumar, Abhishek; Dwivedi, G. D.; Singh, A.; Singh, R.; Shukla, K. K.; Yang, H. D.; Ghosh, A. K.; Chatterjee, Sandip

2016-05-01

Griffith phase phenomena is attributed to existence of FM (ferromagnetic) cluster in AFM (antiferromagnetic) ordering which usually occurs in ferromagnetic and antiferromagnetic bilayers or multilayers. In (Tb1-xCex)MnO3 evolution of Griffith phase have been observed. The observed Griffith phase might be due to the exchange interaction between Mn3+/Mn2+ states.

Micromagnetic modal analysis of spin-transfer-driven ferromagnetic resonance of individual nanomagnets

NASA Astrophysics Data System (ADS)

Torres, L.; Finocchio, G.; Lopez-Diaz, L.; Martinez, E.; Carpentieri, M.; Consolo, G.; Azzerboni, B.

2007-05-01

In a recent investigation Sankey et al. [Phys. Rev. Lett. 96, 227601 (2006)] demonstrated a technique for measuring spin-transfer-driven ferromagnetic resonance in individual ellipsoidal PyCu nanomagnets as small as 30×90×5.5nm3. In the present work, these experiments are analyzed by means of full micromagnetic modeling finding quantitative agreement and enlightening the spatial distribution of the normal modes found in the experiment. The magnetic parameter set used in the computations is obtained by fitting static magnetoresistance measurements. The temperature effect is also included together with all the nonuniform contributions to the effective field as the magnetostatic coupling and the Ampere field. The polarization function of Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)] is used including its spatial and angular dependences. Experimental spin-transfer-driven ferromagnetic resonance spectra are reproduced using the same currents as in the experiment. The use of full micromagnetic modeling allows us to further investigate the spatial dependence of the modes. The dependence of the normal mode frequency on the dc and the external field together with a comparison to the normal modes induced by a microwave current is also addressed.

Critical behavior study around the ferromagnetic phase transition in Pr2Pt2In

NASA Astrophysics Data System (ADS)

Tchokonté, M. B. Tchoula; Mboukam, J. J.; Sondezi, B. M.; Bashir, A. K. H.; Britz, D.; Strydom, A. M.; Kaczorowski, D.

2018-05-01

The magnetic ordering in Pr2Pt2In was investigated by means of magnetization and magnetic susceptibility measurements. The compound was found to order ferromagnetically at TC = 8.8(2) K with a second-order phase transition. The derived critical exponents β = 0.325(2), γ = 1.058(2) and δ = 4.26(4) are close to those expected for a 3D Ising ferromagnet.

Spin-orbit torque-driven magnetization switching in 2D-topological insulator heterostructure

NASA Astrophysics Data System (ADS)

Soleimani, Maryam; Jalili, Seifollah; Mahfouzi, Farzad; Kioussis, Nicholas

2017-02-01

Charge pumping and spin-orbit torque (SOT) are two reciprocal phenomena widely studied in ferromagnet (FM)/topological insulator (TI) heterostructures. However, the SOT and its corresponding switching phase diagram for a FM island in proximity to a two-dimensional topological insulator (2DTI) has not been explored yet. We have addressed these features, using the recently developed adiabatic expansion of time-dependent nonequilibrium Green's function (NEGF) in the presence of both precessing magnetization and bias voltage. We have calculated the angular and spatial dependence of different components of the SOT on the FM island. We determined the switching phase diagram of the FM for different orientations of the easy axis. The results can be used as a guideline for the future experiments on such systems.

Anomalous magnetoelastic behaviour near morphotropic phase boundary in ferromagnetic Tb 1-xNd xCo 2 system

DOE PAGES

Murtaza, Adil; Yang, Sen; Zhou, Chao; ...

2016-08-04

In this study, we report a morphotropic phase boundary (MPB) involved ferromagnetic system Tb 1-xNd xCo 2 and reveal the corresponding structural and magnetoelastic properties of this system. With high resolution synchrotron X-ray diffractometry, the crystal structure of the TbCo 2-rich side is detected to be rhombohedral and that of NdCo 2-rich side is tetragonal below their respective Curie temperatures TC. The MPB composition Tb 0.35Nd 0.65Co 2 corresponds to the coexistence of the rhombohedral phase (R-phase) and tetragonal phase ( T-phase). Contrary to previously reported MPB involved ferromagnetic systems, the MPB composition of Tb 0.35Nd 0.65Co 2 shows minimummore » magnetization which can be understood as compensation of sublattice moments between the R-phase and the T-phase. Furthermore, magnetostriction of Tb 1-xNd xCo 2 decreases with increasing Nd concentration until x = 0.8 and then increases in the negative direction with further increasing Nd concentration; the optimum point for magnetoelastic properties lies towards the rhombohedral phase. Finally, our work not only shows an anomalous type of ferromagnetic MPB but also provides an effective way to design functional materials.« less

Macroscopic phase separation of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2-x Se x revealed by μSR.

PubMed

Nikitin, A M; Grinenko, V; Sarkar, R; Orain, J-C; Salis, M V; Henke, J; Huang, Y K; Klauss, H-H; Amato, A; Visser, A de

2017-12-12

The compound Sr 0.5 Ce 0.5 FBiS 2 belongs to the intensively studied family of layered BiS 2 superconductors. It attracts special attention because superconductivity at T sc  = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T C  = 7.5 K. Recently it was reported that upon replacing S by Se T C drops and ferromagnetism becomes of an itinerant nature. At the same time T sc increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr 0.5 Ce 0.5 FBiS 2-x Se x with x = 0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x = 0.5 and x = 1.0 we find a ferromagnetic volume fraction of ~8 % and ~30 % at T = 0.25 K, well below T C  = 3.4 K and T C  = 3.3 K, respectively. For x = 1.0 (T sc  = 2.9 K) the superconducting phase occupies most (~64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.

Tunable ferromagnetic resonance behavior in Co2FeSi film by post-annealing

NASA Astrophysics Data System (ADS)

Xu, Zhan; Zhang, Zhi; Hu, Fang; Li, Xia; Liu, Peng; Liu, Er; Xu, Feng

2018-05-01

Co2FeSi film is potential in the spintronics applications, due to its low damping factor, which is reflected in the ferromagnetic resonance behavior. In this work, we demonstrate that the ferromagnetic resonance behavior in Co2FeSi film can be well engineered by post-annealing. After 450 °C post-annealing for 1 hour, the Gilbert damping factor decreases drastically from 0.039 at as-deposited state to 0.006, and the inhomogeneity contribution of ferromagnetic resonance linewidth decreases to 60.5 Oe. These decreases are ascribed to the crystallization of film from amorphous state to an ordered B2 phase. Higher annealing temperature, however, leads to the formation of the A2 phase with higher atomic disorder, instead of B2 phase, and brings about the increase of Gilbert damping.

Ferromagnetic quantum criticality: New aspects from the phase diagram of LaCrGe3

NASA Astrophysics Data System (ADS)

Taufour, Valentin; Kaluarachchi, Udhara S.; Bud'ko, Sergey L.; Canfield, Paul C.

2018-05-01

Recent theoretical and experimental studies have shown that ferromagnetic quantum criticality is always avoided in clean systems. Two possibilities have been identified. In the first scenario, the ferromagnetic transition becomes of the first order at a tricritical point before being suppressed. A wing structure phase diagram is observed indicating the possibility of a new type of quantum critical point under magnetic field. In a second scenario, a transition to a modulated magnetic phase occurs. Our recent studies on the compound LaCrGe3 illustrate a third scenario where not only a new magnetic phase occurs, but also a change of order of the transition at a tricritical point leading to a wing-structure phase diagram. Careful experimental study of the phase diagram near the tricritical point also illustrates new rules near this type of point.

Composition-driven magnetic and structural phase transitions in Bi1-xPrxFe1-xMnxO3 multiferroics

NASA Astrophysics Data System (ADS)

Khomchenko, V. A.; Ivanov, M. S.; Karpinsky, D. V.; Paixão, J. A.

2017-09-01

Magnetic ferroelectrics continue to attract much attention as promising multifunctional materials. Among them, BiFeO3 is distinguished by exceptionally high transition temperatures and, thus, is considered as a prototype room-temperature multiferroic. Since its properties are known to be strongly affected by chemical substitution, recognition of the doping-related factors determining the multiferroic behavior of the material would pave the way towards designing the structures with enhanced magnetoelectric functionality. In this paper, we report on the crystal structure and magnetic and local ferroelectric properties of the Bi1-xPrxFe1-xMnxO3 (x ≤ 0.3) compounds prepared by a solid state reaction method. The polar R3c structure specific to the parent BiFeO3 has been found to be unstable with respect to doping for x ≳ 0.1. Depending on the Pr/Mn concentration, either the antipolar PbZrO3-like or nonpolar PrMnO3-type structure can be observed. It has been shown that the non-ferroelectric compounds are weak ferromagnetic with the remanent/spontaneous magnetization linearly decreasing with an increase in x. The samples containing the polar R3c phase exhibit a mixed antiferromagnetic/weak ferromagnetic behavior. The origin of the magnetic phase separation taking place in the ferroelectric phase is discussed as related to the local, doping-introduced structural heterogeneity contributing to the suppression of the cycloidal antiferromagnetic ordering characteristic of the pure BiFeO3.

Interface-driven spin-torque ferromagnetic resonance by Rashba coupling at the interface between nonmagnetic materials

DOE PAGES

Jungfleisch, M. B.; Zhang, W.; Sklenar, J.; ...

2016-06-20

The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin current into a charge current. Here, we demonstrate the reverse process of a charge-to spin-current conversion at a Bi/Ag Rashba interface. We show that this interface-driven spin current can drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the directmore » Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni 80Fe 20) layer. As a result, electrical detection of the magnetization dynamics is achieved by a rectificationmechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.« less

Critical scaling of the mutual information in two-dimensional disordered Ising models

NASA Astrophysics Data System (ADS)

Sriluckshmy, P. V.; Mandal, Ipsita

2018-04-01

Rényi mutual information, computed from second Rényi entropies, can identify classical phase transitions from their finite-size scaling at critical points. We apply this technique to examine the presence or absence of finite temperature phase transitions in various two-dimensional models on a square lattice, which are extensions of the conventional Ising model by adding a quenched disorder. When the quenched disorder causes the nearest neighbor bonds to be both ferromagnetic and antiferromagnetic, (a) a spin glass phase exists only at zero temperature, and (b) a ferromagnetic phase exists at a finite temperature when the antiferromagnetic bond distributions are sufficiently dilute. Furthermore, finite temperature paramagnetic-ferromagnetic transitions can also occur when the disordered bonds involve only ferromagnetic couplings of random strengths. In our numerical simulations, the ‘zero temperature only’ phase transitions are identified when there is no consistent finite-size scaling of the Rényi mutual information curves, while for finite temperature critical points, the curves can identify the critical temperature T c by their crossings at T c and 2 Tc .

Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film

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

Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo

Interfaces with subtle difference in atomic and electronic structures in perovskite ABO3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr0.67Sr0.33MnO3 (PSMO) film on SrTiO3 (STO) substrate. The variations in out-of-plane lattice constant and BO6 octahedral rotation across the PSMO/STO interface strongly depend on the thickness of PSMO films. In the 12-nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI’) phase is formed during the cubic-to-tetragonal phase transition of STO, apparently due to enhanced electron-phonon interaction and atomic disorder in the film.more » The transport properties of the FI’ phase in the 30-nm film are masked because of the reduced interfacial effect and smaller interface-to-volume ratio. This work demonstrates how thickness-dependent interfacial coupling leads to formation of the theoretically predicted novel ferromagnetic-polaronic insulator in systems, as illustrated in a new phase diagram, that are otherwise ferromagnetic metals (FM) in bulk form.« less

Quantum Hall bilayer as pseudospin magnet

NASA Astrophysics Data System (ADS)

Kyriienko, O.; Wierschem, K.; Sengupta, P.; Shelykh, I. A.

2015-03-01

We revisit the physics of electron gas bilayers in the quantum Hall regime (MacDonald A. and Eisenstein J., Nature, 432 (2004) 691; Eisenstein J., Science, 305 (2004) 950), where transport and tunneling measurements provided evidence of a superfluid phase being present in the system. Previously, this behavior was explained by the possible formation of a BEC of excitons in the half-filled electron bilayers, where empty states play the role of holes. We discuss the fundamental difficulties with this scenario, and propose an alternative approach based on a treatment of the system as a pseudospin magnet. We show that the experimentally observed tunneling peak can be linked to the XY ferromagnet (FM) to Ising antiferromagnet (AFM) phase transition of the S = 1/2 XXZ pseudospin model, driven by the change in total electron density. This transition is accompanied by a qualitative change in the nature of the low-energy spin wave dispersion from a gapless linear mode in the XY-FM phase to a gapped, quadratic mode in the Ising AFM phase.

Quantum Hall ferromagnets and transport properties of buckled Dirac materials

NASA Astrophysics Data System (ADS)

Luo, Wenchen; Chakraborty, Tapash

2015-10-01

We study the ground states and low-energy excitations of a generic Dirac material with spin-orbit coupling and a buckling structure in the presence of a magnetic field. The ground states can be classified into three types under different conditions: SU(2), easy-plane, and Ising quantum Hall ferromagnets. For the SU(2) and the easy-plane quantum Hall ferromagnets there are goldstone modes in the collective excitations, while all the modes are gapped in an Ising-type ground state. We compare the Ising quantum Hall ferromagnet with that of bilayer graphene and present the domain-wall solution at finite temperatures. We then specify the phase transitions and transport gaps in silicene in Landau levels 0 and 1. The phase diagram depends strongly on the magnetic field and the dielectric constant. We note that there exist triple points in the phase diagrams in Landau level N =1 that could be observed in experiments.

Ferromagnetic quantum criticality: New aspects from the phase diagram of LaCrGe 3

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

Taufour, Valentin; Kaluarachchi, Udhara S.; Bud'ko, Sergey L.

Some Recent theoretical and experimental studies have shown that ferromagnetic quantum criticality is always avoided in clean systems. Two possibilities have been identified. In the first scenario, the ferromagnetic transition becomes of the first order at a tricritical point before being suppressed. A wing structure phase diagram is observed indicating the possibility of a new type of quantum critical point under magnetic field. In a second scenario, a transition to a modulated magnetic phase occurs. Our earlier studies on the compound LaCrGe 3 illustrate a third scenario where not only a new magnetic phase occurs, but also a change ofmore » order of the transition at a tricritical point leading to a wing-structure phase diagram. Careful experimental study of the phase diagram near the tricritical point also illustrates new rules near this type of point.« less

Ferromagnetic quantum criticality: New aspects from the phase diagram of LaCrGe 3

DOE PAGES

Taufour, Valentin; Kaluarachchi, Udhara S.; Bud'ko, Sergey L.; ...

2017-08-25

Some Recent theoretical and experimental studies have shown that ferromagnetic quantum criticality is always avoided in clean systems. Two possibilities have been identified. In the first scenario, the ferromagnetic transition becomes of the first order at a tricritical point before being suppressed. A wing structure phase diagram is observed indicating the possibility of a new type of quantum critical point under magnetic field. In a second scenario, a transition to a modulated magnetic phase occurs. Our earlier studies on the compound LaCrGe 3 illustrate a third scenario where not only a new magnetic phase occurs, but also a change ofmore » order of the transition at a tricritical point leading to a wing-structure phase diagram. Careful experimental study of the phase diagram near the tricritical point also illustrates new rules near this type of point.« less

Anomalous magnetoelastic behaviour near morphotropic phase boundary in ferromagnetic Tb{sub 1-x}Nd{sub x}Co{sub 2} system

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

Murtaza, Adil; Yang, Sen, E-mail: yang.sen@mail.xjtu.edu.cn; Zhou, Chao

2016-08-01

In this work, we report a morphotropic phase boundary (MPB) involved ferromagnetic system Tb{sub 1-x}Nd{sub x}Co{sub 2} and reveal the corresponding structural and magnetoelastic properties of this system. With high resolution synchrotron X-ray diffractometry, the crystal structure of the TbCo{sub 2}-rich side is detected to be rhombohedral and that of NdCo{sub 2}-rich side is tetragonal below their respective Curie temperatures T{sub C}. The MPB composition Tb{sub 0.35}Nd{sub 0.65}Co{sub 2} corresponds to the coexistence of the rhombohedral phase (R-phase) and tetragonal phase (T-phase). Contrary to previously reported MPB involved ferromagnetic systems, the MPB composition of Tb{sub 0.35}Nd{sub 0.65}Co{sub 2} shows minimummore » magnetization which can be understood as compensation of sublattice moments between the R-phase and the T-phase. Furthermore, magnetostriction of Tb{sub 1-x}Nd{sub x}Co{sub 2} decreases with increasing Nd concentration until x = 0.8 and then increases in the negative direction with further increasing Nd concentration; the optimum point for magnetoelastic properties lies towards the rhombohedral phase. Our work not only shows an anomalous type of ferromagnetic MPB but also provides an effective way to design functional materials.« less

Intrinsic Defect Ferromagnetism: The case of Hafnium Oxide

NASA Astrophysics Data System (ADS)

Das Pemmaraju, Chaitanya

2005-03-01

In view of the recent experimental reports of intrinsic ferromagnetism in Hafnium Oxide (HfO2) thin film systems ootnotetextM. Venkatesan, C. B. Fitzgerald, J. M. D. Coey Nature 430, 630 (2004) Brief Communications, we carried out first principles investigations to look for magnetic structure in HfO2 possibly brought about by the presence of small concentrations of intrinsic point defects. Ab initio electronic structure calculations using Density Functional Theory (DFT) show that isolated cation vacancy sites in HfO2 lead to the formation of high spin defect states which couple ferromagnetically to each other. Interestingly, these high spin states are observed in the low symmetry monoclinic and tetragonal phases while the highly symmetric cubic flourite phase exhibits a non-magnetic ground state. Detailed studies of the electronic structure of cation vacancies in the three crystalline phases of Hafnia show that symmetry leading to orbitally degenerate defect levels is not a pre-requsite for ferromagnetism and that the interplay between Kinetic, Coulomb and Exchange energy together with favourable coupling to the Crystalline environment can lead to high spin ferromagnetic ground states even in extreme low symmetry systems like monoclinic HfO2. These findings open up a much wider class of systems to the possibility of intrinsic defect ferromagnetism.

Ferromagnetism in LaCo O3

NASA Astrophysics Data System (ADS)

Yan, J.-Q.; Zhou, J.-S.; Goodenough, J. B.

2004-07-01

A systematic investigation of the low-temperature magnetic properties of LaCoO3 has demonstrated a ferromagnetism with Tc≈85K from surface cobalt atoms. The experimental investigation involved comparison of the magnetic susceptibility of (1) a single crystal, (2) a powder ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 400°C followed by a later anneal at 1000°C . The low-temperature magnetic susceptibility was found to have three contributions: a Curie-Weiss paramagnetism, a thermally driven spin-state transition, and a surface-related ferromagnetism with Tc≈85K . The ferromagnetic component has a remanence and coercivity at 5K that increases dramatically with increasing surface/volume ratio of the different samples. The presence of the surface ferromagnetism explains the discrepancies of the low-temperature magnetic susceptibility reported by different groups. An anion coordination at surface Co(III) ions that differs from that of the bulk cobalt is shown to be capable of stabilizing higher spin states. A Tc≈85K is argued to be too low for ferromagnetic coupling by oxidized clusters, and possible mechanisms for a ferromagnetic coupling between higher-spin Co(III) ions are discussed.

Layer Anti-Ferromagnetism on Bilayer Honeycomb Lattice

PubMed Central

Tao, Hong-Shuai; Chen, Yao-Hua; Lin, Heng-Fu; Liu, Hai-Di; Liu, Wu-Ming

2014-01-01

Bilayer honeycomb lattice, with inter-layer tunneling energy, has a parabolic dispersion relation, and the inter-layer hopping can cause the charge imbalance between two sublattices. Here, we investigate the metal-insulator and magnetic phase transitions on the strongly correlated bilayer honeycomb lattice by cellular dynamical mean-field theory combined with continuous time quantum Monte Carlo method. The procedures of magnetic spontaneous symmetry breaking on dimer and non-dimer sites are different, causing a novel phase transition between normal anti-ferromagnet and layer anti-ferromagnet. The whole phase diagrams about the magnetism, temperature, interaction and inter-layer hopping are obtained. Finally, we propose an experimental protocol to observe these phenomena in future optical lattice experiments. PMID:24947369

X-ray Characterization of Oxide-based Magnetic Semiconductors

NASA Astrophysics Data System (ADS)

Idzerda, Yves

2008-05-01

Although the evidence for magnetic semiconductors (not simply semiconductors which are ferromagnetic) is compelling, there is much uncertainty in the mechanism for the polarization of the carriers, suggesting that it must be quite novel. Recent experimental evidence suggests that this mechanism is similar to the polaron percolation theory proposed by Kaminski and Das Sarma,ootnotetextKaminski and S. Das Sarma, Physical Review Letters 88, 247202 (2002). which was recently applied specifically to doped oxides by Coey et al.ootnotetextJ. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, Nature Materials 4, 173 (2005). where the ferromagnetism is driven by the percolation of polarons generated by defects or dopants. We have used X-ray absorption spectroscopy at the L-edges and K-edges for low concentrations transition metal (TM) doped magnetic oxides (including TiO2, La1-xSrxO3, HfO2, and In2O3). We have found that in most cases, the transition metal assumes a valence consistent with being at a substitutional, and not interstitial site. We have also measured the X-ray Magnetic Circular Dichroism spectra. Although these materials show strong bulk magnetization, we are unable to detect a robust dichroism feature associated with magnetic elements in the host semiconductor. In the cases where a dichroism signal was observed, it was very weak and could be ascribed to a distinct ferromagnetic phase (TM metal cluster, TM oxide particulate, etc.) separate from the host material. This fascinating absence of a dichroic signal and its significant substantiation of important features of the polaron percolation model may help to finally resolve the issue of ferromagnetism in magnetically doped oxides.

Magnetic phase change in Mn-doped ZnSnAs2 thin films depending on Mn concentration

NASA Astrophysics Data System (ADS)

Uchitomi, Naotaka; Hidaka, Shiro; Saito, Shin; Asubar, Joel T.; Toyota, Hideyuki

2018-04-01

The relationship between Mn concentration and Curie temperature (TC) is studied for Mn-doped ZnSnAs2 ferromagnetic semiconductors, epitaxially grown on InP substrates by molecular beam epitaxy. In the ferromagnetic phase, Mn distributions in a (Zn,Mn,Sn)As2 thin film with 7.2 cation percent (cat. %) Mn are investigated using three-dimensional atom probe tomography. The results indicate an inhomogeneous distribution which spreads to a relatively high Mn concentration of 9.0 at. % (at. %). In the paramagnetic phase, it is found that the paramagnetic to ferromagnetic transition takes place sharply with a TC of 334 K when the Mn doping concentration increases to about 4 cat. % Mn, which corresponds to a magnetic percolation threshold for ferromagnetism in (Zn,Mn,Sn)As2. An effective Curie temperature ⟨TC⟩ is considered to bridge the Curie temperatures obtained experimentally to those calculated theoretically in inhomogeneous magnetic semiconductors. The behavior of magnetism in Mn-doped ZnSnAs2 can be explained by three different phases within the present framework.

Comparison of the ferromagnetic Blume-Emery-Griffiths model and the AF spin-1 longitudinal Ising model at low temperature

NASA Astrophysics Data System (ADS)

Thomaz, M. T.; Corrêa Silva, E. V.

2016-03-01

We derive the exact Helmholtz free energy (HFE) of the standard and staggered one-dimensional Blume-Emery-Griffiths (BEG) model in the presence of an external longitudinal magnetic field. We discuss in detail the thermodynamic behavior of the ferromagnetic version of the model, which exhibits magnetic field-dependent plateaux in the z-component of its magnetization at low temperatures. We also study the behavior of its specific heat and entropy, both per site, at finite temperature. The degeneracy of the ground state, at T=0, along the lines that separate distinct phases in the phase diagram of the ferromagnetic BEG model is calculated, extending the study of the phase diagram of the spin-1 antiferromagnetic (AF) Ising model in S.M. de Souza and M.T. Thomaz, J. Magn. and Magn. Mater. 354 (2014) 205 [5]. We explore the implications of the equality of phase diagrams, at T=0, of the ferromagnetic BEG model with K/|J| = - 2 and of the spin-1 AF Ising model for D/|J| > 1/2.

Pressure response of three-dimensional cyanide-bridged bimetallic magnets.

PubMed

Ohba, Masaaki; Kaneko, Wakako; Kitagawa, Susumu; Maeda, Takuho; Mito, Masaki

2008-04-02

Effects of pressure on the structures and magnetic properties of three types of 3-D cyanide-bridged bimetallic coordination polymer magnets, MnIICrIII ferrimagnet [Mn(en)]3[Cr(CN)6]2.4H2O (1; en = ethylenediamine), NiIICrIII ferromagnet [Ni(dipn)]3[Cr(CN)6]2.3H2O (2; dipn = N,N-di(3-aminopropyl)amine), and NiIIFeIII ferromagnet [Ni(dipn)]2[Ni(dipn)(H2O)][Fe(CN)6]2.11H2O (3), were systematically examined under hydrostatic pressure up to 19.8 GPa using a piston-cylinder-type pressure cell and a diamond anvil cell. The ferrimagnet 1 showed the reversible crystalline-to-amorphous-like phase change, and the magnetic phase transition temperature (TC) was reversibly changed from 69 K at 0 GPa to 126 K at 4.7 GPa. At higher pressure, the net magnetization was suppressed with increasing pressure, and the magnetic state at 19.8 GPa was assumed to be paramagnetic. The initial ferrimagnetic phase of 1 was not recovered after releasing the pressure from 19.8 GPa. The magnetic phase of 2 was reversibly converted between ferromagnetic and paramagnetic-like phase in the range 0

Magnetisation studies of phase co-existence in Gd{sub 1-x}Ca{sub x}BaCo{sub 2}O{sub 5.5}

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

Thirumurugan, N.; Bharathi, A., E-mail: bharathi@igcar.gov.in; Arulraj, A.

2012-04-15

Highlights: Black-Right-Pointing-Pointer The series Gd{sub 1-x}Ca{sub x}BaCo{sub 2}O{sub 5.5} was synthesised by solid state reaction. Black-Right-Pointing-Pointer Magnetisation studies were carried out in the 4-300 K temperature range in magnetic fields upto 16 Tesla. Black-Right-Pointing-Pointer Results were used to formulate the T versus Ca fraction, phase diagram. Black-Right-Pointing-Pointer Evidence for Magnet-electronic phase separation is shown for the first time in the compound. -- Abstract: Magnetic properties of hole doped, oxygen deficient double perovskite compounds, Gd{sub 1-x}Ca{sub x}BaCo{sub 2}O{sub 5.5}, have been investigated. Ferromagnetic transition temperatures increase and the anti-ferromagnetic transition temperatures decrease with Ca substitution leading to stabilisation of ferromagnetisim formore » x {>=} 0.05. A detailed study of the ferromagnetic phase indicates the presence of double hysterisis loops for Ca fractions, 0.05 {<=} x {<=} 0.2 in the 50-200 K temperature range, suggestive of the co-existence of two ferromagnetic phases with different co-ercivities. Based on the magnetisation and transport measurements a phase diagram is proposed for Ca doped GdBaCo{sub 2}O{sub 5.5}.« less

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

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

Gingrich, E. C.; Niedzielski, Bethany M.; Glick, Joseph A.

Superconductivity and ferromagnetism are antagonistic forms of order, and rarely coexist. Many interesting new phenomena occur, however, in hybrid superconducting/ferromagnetic systems. For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of π in its ground state for certain thicknesses of the material. Such ‘π-junctions’ were first realized experimentally in 2001, and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing. Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and pi by changing the relativemore » orientation of the two magnetizations. These controllable 0–π junctions have immediate applications in cryogenic memory, where they serve as a necessary component to an ultralow power superconducting computer. Such a fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers. Here, phase-controllable junctions also open up new possibilities for superconducting circuit elements such as superconducting ‘programmable logic’, where they could function in superconducting analogues to field-programmable gate arrays.« less

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

DOE PAGES

Gingrich, E. C.; Niedzielski, Bethany M.; Glick, Joseph A.; ...

2016-03-14

Superconductivity and ferromagnetism are antagonistic forms of order, and rarely coexist. Many interesting new phenomena occur, however, in hybrid superconducting/ferromagnetic systems. For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of π in its ground state for certain thicknesses of the material. Such ‘π-junctions’ were first realized experimentally in 2001, and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing. Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and pi by changing the relativemore » orientation of the two magnetizations. These controllable 0–π junctions have immediate applications in cryogenic memory, where they serve as a necessary component to an ultralow power superconducting computer. Such a fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers. Here, phase-controllable junctions also open up new possibilities for superconducting circuit elements such as superconducting ‘programmable logic’, where they could function in superconducting analogues to field-programmable gate arrays.« less

Charge ordering in stoichiometric FeTe: Scanning tunneling microscopy and spectroscopy

DOE PAGES

Li, Wei; Yin, Wei -Guo; Wang, Lili; ...

2016-01-04

In this study, we use scanning tunneling microscopy and spectroscopy to reveal a unique stripy charge order in a parent phase of iron-based superconductors in stoichiometric FeTe epitaxy films. The charge order has unusually the same—usually half—period as the spin order. We also found highly anisotropic electron band dispersions being large and little along the ferromagnetic (crystallographic b) and antiferromagnetic (a) directions, respectively. Our data suggest that the microscopic mechanism is likely of the Stoner type driven by interatomic Coulomb repulsion V ij, and that V ij and charge fluctuations, so far much neglected, are important to the understanding ofmore » iron-based superconductors.« less

Room temperature ferromagnetism in liquid-phase pulsed laser ablation synthesized nanoparticles of nonmagnetic oxides

NASA Astrophysics Data System (ADS)

Singh, S. C.; Kotnala, R. K.; Gopal, R.

2015-08-01

Intrinsic Room Temperature Ferromagnetism (RTF) has been observed in undoped/uncapped zinc oxide and titanium dioxide spherical nanoparticles (NPs) obtained by a purely green approach of liquid phase pulsed laser ablation of corresponding metal targets in pure water. Saturation magnetization values observed for zinc oxide (average size, 9 ± 1.2 nm) and titanium dioxide (average size, 4.4 ± 0.3 nm) NPs are 62.37 and 42.17 memu/g, respectively, which are several orders of magnitude larger than those of previous reports. In contrast to the previous works, no postprocessing treatments or surface modification is required to induce ferromagnetism in the case of present communication. The most important result, related to the field of intrinsic ferromagnetism in nonmagnetic materials, is the observation of size dependent ferromagnetism. Degree of ferromagnetism in titanium dioxide increases with the increase in particle size, while it is reverse for zinc oxide. Surface and volume defects play significant roles for the origin of RTF in zinc oxide and titanium dioxide NPs, respectively. Single ionized oxygen and neutral zinc vacancies in zinc oxide and oxygen and neutral/ionized titanium vacancies in titanium dioxide are considered as predominant defect centres responsible for observed ferromagnetism. It is expected that origin of ferromagnetism is a consequence of exchange interactions between localized electron spin moments resulting from point defects.

Room temperature ferromagnetism in liquid-phase pulsed laser ablation synthesized nanoparticles of nonmagnetic oxides

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

Singh, S. C., E-mail: subhash.laserlab@gmail.com; Gopal, R.; Kotnala, R. K.

2015-08-14

Intrinsic Room Temperature Ferromagnetism (RTF) has been observed in undoped/uncapped zinc oxide and titanium dioxide spherical nanoparticles (NPs) obtained by a purely green approach of liquid phase pulsed laser ablation of corresponding metal targets in pure water. Saturation magnetization values observed for zinc oxide (average size, 9 ± 1.2 nm) and titanium dioxide (average size, 4.4 ± 0.3 nm) NPs are 62.37 and 42.17 memu/g, respectively, which are several orders of magnitude larger than those of previous reports. In contrast to the previous works, no postprocessing treatments or surface modification is required to induce ferromagnetism in the case of present communication. The most important result, relatedmore » to the field of intrinsic ferromagnetism in nonmagnetic materials, is the observation of size dependent ferromagnetism. Degree of ferromagnetism in titanium dioxide increases with the increase in particle size, while it is reverse for zinc oxide. Surface and volume defects play significant roles for the origin of RTF in zinc oxide and titanium dioxide NPs, respectively. Single ionized oxygen and neutral zinc vacancies in zinc oxide and oxygen and neutral/ionized titanium vacancies in titanium dioxide are considered as predominant defect centres responsible for observed ferromagnetism. It is expected that origin of ferromagnetism is a consequence of exchange interactions between localized electron spin moments resulting from point defects.« less

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates

PubMed Central

Shuai, M.; Klittnick, A.; Shen, Y.; Smith, G. P.; Tuchband, M. R.; Zhu, C.; Petschek, R. G.; Mertelj, A.; Lisjak, D.; Čopič, M.; Maclennan, J. E.; Glaser, M. A.; Clark, N. A.

2016-01-01

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth's magnetic field. PMID:26817823

Ferromagnetic resonance with long Josephson junction

NASA Astrophysics Data System (ADS)

Golovchanskiy, I. A.; Abramov, N. N.; Stolyarov, V. S.; Emelyanova, O. V.; Golubov, A. A.; Ustinov, A. V.; Ryazanov, V. V.

2017-05-01

In this work we propose a hybrid device based on a long Josephson junction (JJ) coupled inductively to an external ferromagnetic (FM) layer. The long JJ in a zero-field operation mode induces a localized AC magnetic field in the FM layer and enables a synchronized magnetostatic standing wave. The magnetostatic wave induces additional dissipation for soliton propagation in the junction and also enables a phase locking (resonant soliton synchronization) at a frequency of natural ferromagnetic resonance. The later manifests itself as an additional constant voltage step on the current-voltage characteristics at the corresponding voltage. The proposed device allows to study magnetization dynamics of individual micro-scaled FM samples using just DC technique, and also it provides additional phase locking frequency in the junction, determined exclusively by characteristics of the ferromagnet.

Itinerant ferromagnetism in fermionic systems with SP (2 N) symmetry

NASA Astrophysics Data System (ADS)

Yang, Wang; Wu, Congjun

The Ginzburg-Landau free energy of systems with SP (2 N) symmetry describes a second order phase transition on the mean field level, since the Casimir invariants of the SP (2 N) group can be only of even order combinations of the generators of the SP (2 N) group. This is in contrast with systems having the SU (N) symmetry, where the allowance of cubic term generally makes the phase transition into first order. In this work, we consider the Hertz-Millis type itinerant ferromagnetism in an interacting fermionic system with SP (2 N) symmetry, where the ferromagnetic orders are enriched by the multi-component nature of the system. The quantum criticality is discussed near the second order phase transition point.

Retrieval of phase information in neutron reflectometry

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

de Haan, V.; van Well, A.A.; Adenwalla, S.

Neutron reflectometry can determine unambiguously the chemical depth profile of a thin film if both phase and amplitude of the reflectance are known. The recovery of the phase information is achieved by adding to the unknown layered structure a known ferromagnetic layer. The ferromagnetic layer is magnetized by an external magnetic field in a direction lying in the plane of the layer and subsequently perpendicular to it. The neutrons are polarized either parallel or opposite to the magnetic field. In this way three measurements can be made, with different (and known) scattering-length densities of the ferromagnetic layer. The reflectivity obtainedmore » from each measurement can be represented by a circle in the (complex) reflectance plane. The intersections of these circles provide the reflectance.« less

The Jahn-Teller distortion influenced ferromagnetic order in Pr1-xLaxMnO3

NASA Astrophysics Data System (ADS)

He, Feifei; Mao, Zhongquan; Tang, Lingyun; Zhang, Jiang; Chen, Xi

2018-06-01

The structural and magnetic properties of Pr1-xLaxMnO3 (0 ≤ x ≤ 1) polycrystalline powders are investigated. A structural phase transition from a large Jahn-Teller (J-T) distorted orthorhombic structure to a small J-T distorted orthorhombic phase is found at x = 0.70, while the LaMnO3 is showed to have a rhombohedral structure. All the samples exhibit ferromagnetic ordering, and meanwhile, a reentrant spin glass behavior at low temperature. The relationship between J-T distortions and the ferromagnetic order is discussed.

Linear dynamics of classical spin as Mobius transformation

DOE PAGES

Galda, Alexey; Vinokur, Valerii М.

2017-04-26

Though the overwhelming majority of natural processes occur far from the equilibrium, general theoretical approaches to non-equilibrium phase transitions remain scarce. Recent breakthroughs introduced a description of open dissipative systems in terms of non-Hermitian quantum mechanics enabling the identification of a class of non-equilibrium phase transitions associated with the loss of combined parity (reflection) and time-reversal symmetries. Here we report that the time evolution of a single classical spin (e.g. monodomain ferromagnet) governed by the Landau-Lifshitz-Gilbert-Slonczewski equation in the absence of magnetic anisotropy terms is described by a Mobius transformation in complex stereographic coordinates. We identify the parity-time symmetry-breaking phasemore » transition occurring in spin-transfer torque-driven linear spin systems as a transition between hyperbolic and loxodromic classes of Mobius transformations, with the critical point of the transition corresponding to the parabolic transformation. However, this establishes the understanding of non-equilibrium phase transitions as topological transitions in configuration space.« less

Parity-time symmetry breaking in magnetic systems

DOE PAGES

Galda, Alexey; Vinokur, Valerii M.

2016-07-14

The understanding of out-of-equilibrium physics, especially dynamic instabilities and dynamic phase transitions, is one of the major challenges of contemporary science, spanning the broadest wealth of research areas that range from quantum optics to living organisms. By focusing on nonequilibrium dynamics of an open dissipative spin system, we introduce a non-Hermitian Hamiltonian approach, in which non-Hermiticity reflects dissipation and deviation from equilibrium. The imaginary part of the proposed spin Hamiltonian describes the effects of Gilbert damping and applied Slonczewski spin-transfer torque. In the classical limit, our approach reproduces Landau-Lifshitz-Gilbert-Slonczewski dynamics of a large macrospin. Here, we reveal the spin-transfer torque-drivenmore » parity-time symmetry-breaking phase transition corresponding to a transition from precessional to exponentially damped spin dynamics. Micromagnetic simulations for nanoscale ferromagnetic disks demonstrate the predicted effect. These findings can pave the way to a general quantitative description of out-of-equilibrium phase transitions driven by spontaneous parity-time symmetry breaking.« less

Model for multishot all-thermal all-optical switching in ferromagnets

NASA Astrophysics Data System (ADS)

Gorchon, J.; Yang, Y.; Bokor, J.

2016-07-01

All-optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promising technological applications. However, a fundamental understanding of the underlying mechanisms remains elusive. Here we present a model for multishot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of magnetic circular dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots heat the sample close to the Curie temperature. Finally, we qualitatively reproduce the all-optically switched domain patterns observed experimentally by numerically simulating the result of multiple laser shots on an FePtC granular ferromagnetic film.

Effects of the thermal and magnetic paths on first order martensite transition of disordered Ni45Mn44Sn9In2 Heusler alloy exhibiting a giant magnetocaloric effect and magnetoresistance near room temperature

NASA Astrophysics Data System (ADS)

Chabri, T.; Ghosh, A.; Nair, Sunil; Awasthi, A. M.; Venimadhav, A.; Nath, T. K.

2018-05-01

The existence of a first order martensite transition in off-stoichiometric Ni45Mn44Sn9In2 ferromagnetic shape memory Heusler alloy has been clearly observed by thermal, magnetic, and magneto-transport measurements. Field and thermal path dependence of the change in large magnetic entropy and negative magnetoresistance are observed, which originate due to the sharp change in magnetization driven by metamagnetic transition from the weakly magnetic martensite phase to the ferromagnetic austenite phase in the vicinity of the martensite transition. The noticeable shift in the martensite transition with the application of a magnetic field is the most significant feature of the present study. This shift is due to the interplay of the austenite and martensite phase fraction in the alloy. The different aspects of the first order martensite transition, e.g. broadening of the martensite transition and the field induced arrest of the austenite phase are mainly related to the dynamics of coexisting phases in the vicinity of the martensite transition. The alloy also shows a second order ferromagnetic  →  paramagnetic transition near the Curie temperature of the austenite phase. A noticeably large change in magnetic entropy (ΔS M   =  24 J kg‑1 K‑1 at 298 K) and magnetoresistance (=  ‑33% at 295 K) has been observed for the change in 5 and 8 T magnetic fields, respectively. The change in adiabatic temperature for the change in a magnetic field of 5 T is found to be  ‑3.8 K at 299 K. The low cost of the ingredients and the large change in magnetic entropy very near to the room temperature makes Ni45Mn44Sn9In2 alloy a promising magnetic refrigerant for real technological application.

Temperature dependence of spin-torque driven ferromagnetic resonance in MgO-based magnetic tunnel junction with a perpendicularly free layer

NASA Astrophysics Data System (ADS)

Wang, Xiao; Feng, Jiafeng; Guo, Peng; Wei, H. X.; Han, X. F.; Fang, B.; Zeng, Z. M.

2017-12-01

We report the temperature dependence of the spin-torque (ST) driven ferromagnetic resonance in MgO-based magnetic tunnel junction (MTJ) nanopillars with a perpendicularly free layer and an in-plane reference layer. From the evolution of the resonance frequency with magnetic field, we clearly identify the free-layer resonance mode and reference-layer mode. For the reference layer, we demonstrate a monotonic increase in resonance frequency and the effective damping with decreasing temperature, which suggests the saturated magnetization of the reference layer is dominant. However, for the free layer, the frequency and damping exhibit almost no change with temperature, indicating that the perpendicular magnetic anisotropy plays an important role in magnetization dynamics of the free layer.

Mechanical Signature of Heat Generated in a Current-Driven Ferromagnetic Resonance System

NASA Astrophysics Data System (ADS)

Cho, Sung Un; Jo, Myunglae; Park, Seondo; Lee, Jae-Hyun; Yang, Chanuk; Kang, Seokwon; Park, Yun Daniel

2017-07-01

In a current-driven ferromagnetic resonance (FMR) system, heat generated by time-dependent magnetoresistance effects, caused by magnetization precession, cannot be overlooked. Here, we describe the generated heat by magnetization motion under electric current in a freestanding nanoelectromechanical resonator fashioned from a permalloy (Py )/Pt bilayer. By piezoresistive transduction of Pt, the mechanical mode is electrically detected at room temperature and the internal heat in Py excluding thermoelectric effects is quantified as a shift of the mechanical resonance. We find that the measured spectral shifts correspond to the FMR, which is further verified from the spin-torque FMR measurement. Furthermore, the angular dependence of the mechanical reaction on an applied magnetic field reveals that the full accounting of FMR heat dissipation requires the time-dependent magnetoresistance effect.

Singular ferromagnetic susceptibility of the transverse-field Ising antiferromagnet on the triangular lattice

NASA Astrophysics Data System (ADS)

Biswas, Sounak; Damle, Kedar

2018-02-01

A transverse magnetic field Γ is known to induce antiferromagnetic three-sublattice order of the Ising spins σz in the triangular lattice Ising antiferromagnet at low enough temperature. This low-temperature order is known to melt on heating in a two-step manner, with a power-law ordered intermediate temperature phase characterized by power-law correlations at the three-sublattice wave vector Q : <σz(R ⃗) σz(0 ) > ˜cos(Q .R ⃗) /|R⃗| η (T ) with the temperature-dependent power-law exponent η (T )∈(1 /9 ,1 /4 ) . Here, we use a quantum cluster algorithm to study the ferromagnetic easy-axis susceptibility χu(L ) of an L ×L sample in this power-law ordered phase. Our numerical results are consistent with a recent prediction of a singular L dependence χu(L ) ˜L2 -9 η when η (T ) is in the range (1 /9 ,2 /9 ) . This finite-size result implies, via standard scaling arguments, that the ferromagnetic susceptibility χu(B ) to a uniform field B along the easy axis is singular at intermediate temperatures in the small B limit, χu(B ) ˜|B| -4/-18 η 4 -9 η for η (T )∈(1 /9 ,2 /9 ) , although there is no ferromagnetic long-range order in the low temperature state. Additionally we establish similar two-step melting behavior (via a study of the order parameter susceptibility χQ) in the case of the ferrimagnetic three-sublattice ordered phase which is stabilized by ferromagnetic next-neighbor couplings (J2) and confirm that the ferromagnetic susceptibility obeys the predicted singular form in the associated power-law ordered phase.

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

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

Phatak, C.; Petford-Long, A. K.; Zheng, H.

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here in this article, we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La 2-2xSr 1+2xMn 2O 7 with x = 0:38, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curiemore » temperature within the a - b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. Lastly, the ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.« less

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

DOE PAGES

Phatak, C.; Petford-Long, A. K.; Zheng, H.; ...

2015-12-14

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here in this article, we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La 2-2xSr 1+2xMn 2O 7 with x = 0:38, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curiemore » temperature within the a - b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. Lastly, the ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.« less

Ultrafast optically induced ferromagnetic/anti-ferromagnetic phase transition in GdTiO3 from first principles

NASA Astrophysics Data System (ADS)

Khalsa, Guru; Benedek, Nicole A.

2018-03-01

Epitaxial strain and chemical substitution have been the workhorses of functional materials design. These static techniques have shown immense success in controlling properties in complex oxides through the tuning of subtle structural distortions. Recently, an approach based on the excitation of an infrared active phonon with intense midinfrared light has created an opportunity for dynamical control of structure through special nonlinear coupling to Raman phonons. We use first-principles techniques to show that this approach can dynamically induce a magnetic phase transition from the ferromagnetic ground state to a hidden antiferromagnetic phase in the rare earth titanate GdTiO3 for realistic experimental parameters. We show that a combination of a Jahn-Teller distortion, Gd displacement, and infrared phonon motion dominate this phase transition with little effect from the octahedral rotations, contrary to conventional wisdom.

The phase diagram of a directed polymer in random media with p-spin ferromagnetic interactions

NASA Astrophysics Data System (ADS)

Wedagedera, J. R.

2011-01-01

We consider a directed polymer model with an additive p-spin (p>2) ferromagnetic term in the Hamiltonian. We give a rigorous proof for the specific free energy and derive the phase diagram. This model was proposed previously, and a detailed proof was given in the case p = 2, while the main result was only stated for p > 2. We give a detailed proof of the main result and show the behavior of the model as p → ∞ by constructing the phase diagram also in this case. These results are important in many applications, for instance, in telecommunication and immunology. Our major finding is that in the phase diagram for p > 2, a new transition curve (absent for p = 2) emerges between the paramagnetic region and the so-called mixed region and that the ferromagnetic region diminishes as p → ∞.

Observation of ferromagnetic ordering in a stable α -Co (OH) 2 phase grown on a Mo S2 surface

NASA Astrophysics Data System (ADS)

Debnath, Anup; Bhattacharya, Shatabda; Saha, Shyamal K.

2017-12-01

Because of the potential application of Co (OH) 2 in a magnetic cooling system as a result of its superior magnetocaloric effect many people have investigated magnetic properties of Co (OH) 2 . Unfortunately, most of the works have been carried out on the β -Co (OH) 2 phase due to the fact that the α -Co (OH) 2 phase is very unstable and continuously transformed into the stable β -Co (OH) 2 phase. However, in the present work, using a Mo S2 sheet as a two-dimensional template, we have been able to synthesize a stable α -Co (OH) 2 phase in addition to a β -Co (OH) 2 phase by varying the layer thickness. It is seen that for thinner samples the β phase, while for thicker samples α phase, is grown on the Mo S2 surface. Magnetic measurements are carried out for the samples over the temperature range from 2 to 300 K and it is seen that for the β phase, ferromagnetic ordering with fairly large coercivity (1271 Oe) at 2 K is obtained instead of the usual antiferromagnetism. The most interesting result is the observation of ferromagnetic ordering with a transition temperature (Curie temperature) more than 100 K in the α -Co (OH) 2 phase. Complete saturation in the hysteresis curve under application of very low field having coercivity of ˜162 Oe at 2 K and 60 Oe at 50 K is obtained. A thin stable α -Co (OH) 2 phase grown on Mo S2 surface with very soft ferromagnetic ordering will be very useful as the core material in electromagnets.

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates

DOE PAGES

Shuai, M.; Klittnick, A.; Shen, Y.; ...

2016-01-28

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. We find Its zero-field magnetization produces distinctive magnetic self-interaction effects, includingmore » liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth’s magnetic field.« less

Identification of novel compositions of ferromagnetic shape-memory alloys using composition spreads.

PubMed

Takeuchi, I; Famodu, O O; Read, J C; Aronova, M A; Chang, K-S; Craciunescu, C; Lofland, S E; Wuttig, M; Wellstood, F C; Knauss, L; Orozco, A

2003-03-01

Exploration of new ferroic (ferroelectric, ferromagnetic or ferroelastic) materials continues to be a central theme in condensed matter physics and to drive advances in key areas of technology. Here, using thin-film composition spreads, we have mapped the functional phase diagram of the Ni-Mn-Ga system whose Heusler composition Ni(2)MnGa is a well known ferromagnetic shape-memory alloy. A characterization technique that allows detection of martensitic transitions by visual inspection was combined with quantitative magnetization mapping using scanning SQUID (superconducting quantum interference device) microscopy. We find that a large, previously unexplored region outside the Heusler composition contains reversible martensites that are also ferromagnetic. A clear relationship between magnetization and the martensitic transition temperature is observed, revealing a strong thermodynamical coupling between magnetism and martensitic instability across a large fraction of the phase diagram.

Magnetic phase diagram and critical behavior of electron-doped LaxCa1-xMnO3(0⩽x⩽0.25) nanoparticles

NASA Astrophysics Data System (ADS)

Wang, Yang; Fan, Hong Jin

2011-06-01

A comparative study of electron-doped perovskite manganites LaxCa1-xMnO3 (0 ⩽ x ⩽ 0.25) in nanoparticle and bulk form is reported. The bulks and nanoparticles exhibit different magnetic evolutions. Overall with increasing x, the bulks have a phase-separated ground state with ferromagnetic (FM) clusters and antiferromagnetic (AFM) matrix coexisting. The FM clusters gradually grow, and the magnetization M peaks at x= 0.1. Subsequently, charge-ordering (CO) or local CO occurs, which suppresses the increase in FM clusters but favors the development of antiferromagnetism so M starts to decrease. Finally the system becomes a homogeneous AFM state at x > 0.18. For the nanoparticles in the range of 0 ⩽ x ⩽ 0.1, the ground state is similar to that of the bulks, but M is slightly increased because of a surface ferromagnetism. Nevertheless because of the structure distortion induced by surface pressure and the size effect, CO does not occur in the nanoparticles. Consequently, the ferromagnetism still gradually develops at x > 0.1 and thus M monotonously rises. M reaches a maximum at x= 0.18, after which the competition between ferromagnetism and antiferromagnetism induces a cluster-glass (CG) state. On the basis of these observations the phase diagrams for both bulks and nanoparticles are established. For the nanoparticles that display enhanced ferromagnetism the critical behavior analysis indicates that they fall into a three-dimensional (3D) Heisenberg ferromagnet class.

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

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

Glick, Joseph A.; Khasawneh, Mazin A.; Niedzielski, Bethany M.

We report that josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thicknesses. We present studies of Nb based micron-scale elliptically shaped Josephson junctions containing ferromagnetic barriers of Ni 81Fe 19 or Ni 65Fe 15Co 20. By applying an external magnetic field, the critical current of the junctions is found to follow characteristic Fraunhofer patterns and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extractmore » the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. Lastly, we compare the data to previous work and to models of the 0-π transitions based on existing theories.« less

Interaction energy and itinerant ferromagnetism in a strongly interacting Fermi gas in the absence of molecule formation

DOE PAGES

He, Lianyi

2014-11-26

In this study, we investigate the interaction energy and the possibility of itinerant ferromagnetism in a strongly interacting Fermi gas at zero temperature in the absence of molecule formation. The interaction energy is obtained by summing the perturbative contributions of Galitskii-Feynman type to all orders in the gas parameter. It can be expressed by a simple phase-space integral of an in-medium scattering phase shift. In both three and two dimensions (3D and 2D), the interaction energy shows a maximum before reaching the resonance from the Bose-Einstein condensate side, which provides a possible explanation of the experimental measurements of the interactionmore » energy. This phenomenon can be theoretically explained by the qualitative change of the nature of the binary interaction in the medium. The appearance of an energy maximum has significant effects on the itinerant ferromagnetism. In 3D, the ferromagnetic transition is reentrant and itinerant ferromagnetism exists in a narrow window around the energy maximum. In 2D, the present theoretical approach suggests that itinerant ferromagnetism does not exist, which reflects the fact that the energy maximum becomes much lower than the energy of the fully polarized state.« less

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

DOE PAGES

Glick, Joseph A.; Khasawneh, Mazin A.; Niedzielski, Bethany M.; ...

2017-10-06

We report that josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thicknesses. We present studies of Nb based micron-scale elliptically shaped Josephson junctions containing ferromagnetic barriers of Ni 81Fe 19 or Ni 65Fe 15Co 20. By applying an external magnetic field, the critical current of the junctions is found to follow characteristic Fraunhofer patterns and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extractmore » the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. Lastly, we compare the data to previous work and to models of the 0-π transitions based on existing theories.« less

Suppression of the ferromagnetic order in the Heusler alloy Ni{sub 50}Mn{sub 35}In{sub 15} by hydrostatic pressure

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

Salazar Mejía, C., E-mail: Catalina.Salazar@cpfs.mpg.de; Mydeen, K.; Naumov, P.

2016-06-27

We report on the effect of hydrostatic pressure on the magnetic and structural properties of the shape-memory Heusler alloy Ni{sub 50}Mn{sub 35}In{sub 15}. Magnetization and x-ray diffraction experiments were performed at hydrostatic pressures up to 5 GPa using diamond anvil cells. Pressure stabilizes the martensitic phase, shifting the martensitic transition to higher temperatures, and suppresses the ferromagnetic austenitic phase. Above 3 GPa, where the martensitic-transition temperature approaches the Curie temperature in the austenite, the magnetization shows no longer indications of ferromagnetic ordering. We further find an extended temperature region with a mixture of martensite and austenite phases, which directly relates to themore » magnetic properties.« less

A Monte Carlo study of the spin-1 Blume-Emery-Griffiths phase diagrams within biquadratic exchange anisotropy

NASA Astrophysics Data System (ADS)

Dani, Ibtissam; Tahiri, Najim; Ez-Zahraouy, Hamid; Benyoussef, Abdelilah

2014-08-01

The effect of the bi-quadratic exchange coupling anisotropy on the phase diagram of the spin-1 Blume-Emery-Griffiths model on simple-cubic lattice is investigated using mean field theory (MFT) and Monte Carlo simulation (MC). It is found that the anisotropy of the biquadratic coupling favors the stability of the ferromagnetic phase. By decreasing the parallel and/or perpendicular bi-quadratic coupling, the ferrimagnetic and the antiquadrupolar phases broaden in contrast, the ferromagnetic and the disordered phases become narrow. The behavior of magnetization and quadrupolar moment as a function of temperature is also computed, especially in the ferrimagnetic phase.

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

Colossal Magnetoelectric Effect with Competing Multiferroic and Weak-Ferromagnetic Phases

NASA Astrophysics Data System (ADS)

Choi, Young Jai; Zhang, Chenglin; Lee, Nara; Cheong, Sang-Wook

2011-03-01

From our investigation of magnetoelectric properties of Eu 0.75 Y0.25 Mn O3 , where a multiferroic phase competes with a weak ferromagnetic phase in magnetic fields, we found intriguing hysteretic behaviors of physical properties with variation of temperature and magnetic field. These hysteretic behaviors arise from the kinetic arrest/de-arrest processes of the first order magnetic transition, resulting in freezing or melting of a magnetoelectric glass state with the coexistence of two competing phases. We note that most of large magnetoelectric coupling effects in multiferroics are associated with the large change of polarization with magnetic fields, but the control of ferromagnetic-type magnetization by applying electric fields is most relevant to technological applications, which is scarcely observed. This important issue of mutual controllability is achieved in Eu 0.75 Y0.25 Mn O3 utilizing dynamical modulations of the coexistence of two contraindicative phases, highly susceptible to the external perturbations such as electric and magnetic fields.

Low temperature properties of spin filter NbN/GdN/NbN Josephson junctions

NASA Astrophysics Data System (ADS)

Massarotti, D.; Caruso, R.; Pal, A.; Rotoli, G.; Longobardi, L.; Pepe, G. P.; Blamire, M. G.; Tafuri, F.

2017-02-01

A ferromagnetic Josephson junction (JJ) represents a special class of hybrid system where different ordered phases meet and generate novel physics. In this work we report on the transport measurements of underdamped ferromagnetic NbN/GdN/NbN JJs at low temperatures. In these junctions the ferromagnetic insulator gadolinium nitride barrier generates spin-filtering properties and a dominant second harmonic component in the current-phase relation. These features make spin filter junctions quite interesting also in terms of fundamental studies on phase dynamics and dissipation. We discuss the fingerprints of spin filter JJs, through complementary transport measurements, and their implications on the phase dynamics, through standard measurements of switching current distributions. NbN/GdN/NbN JJs, where spin filter properties can be controllably tuned along with the critical current density (Jc), turn to be a very relevant term of reference to understand phase dynamics and dissipation in an enlarged class of JJs, not necessarily falling in the standard tunnel limit characterized by low Jc values.

Nonlinear heat transport in ferromagnetic-quantum dot-superconducting systems

NASA Astrophysics Data System (ADS)

Hwang, Sun-Yong; Sánchez, David

2018-03-01

We analyze the heat current traversing a quantum dot sandwiched between a ferromagnetic and a superconducting electrode. The heat flow generated in response to a voltage bias presents rectification as a function of the gate potential applied to the quantum dot. Remarkably, in the thermally driven case the heat shows a strong diode effect with large asymmetry ratios that can be externally tuned with magnetic fields or spin-polarized tunneling. Our results thus demonstrate the importance of hybrid systems as promising candidates for thermal applications.

Universal depinning transition of domain walls in ultrathin ferromagnets

NASA Astrophysics Data System (ADS)

Diaz Pardo, R.; Savero Torres, W.; Kolton, A. B.; Bustingorry, S.; Jeudy, V.

2017-05-01

We present a quantitative and comparative study of magnetic-field-driven domain-wall depinning transition in different ferromagnetic ultrathin films over a wide range of temperature. We reveal a universal scaling function accounting for both drive and thermal effects on the depinning transition, including critical exponents. The consistent description we obtain for both the depinning and subthreshold thermally activated creep motion should shed light on the universal glassy dynamics of thermally fluctuating elastic objects pinned by disordered energy landscapes.

Domain-wall guided nucleation of superconductivity in hybrid ferromagnet-superconductor-ferromagnet layered structures.

PubMed

Gillijns, W; Aladyshkin, A Yu; Lange, M; Van Bael, M J; Moshchalkov, V V

2005-11-25

Domain-wall superconductivity is studied in a superconducting Nb film placed between two ferromagnetic Co/Pd multilayers with perpendicular magnetization. The parameters of top and bottom ferromagnetic films are chosen to provide different coercive fields, so that the magnetic domain structure of the ferromagnets can be selectively controlled. From the dependence of the critical temperature Tc on the applied magnetic field H, we have found evidence for domain-wall superconductivity in this three-layered F/S/F structure for different magnetic domain patterns. The phase boundary, calculated numerically for this structure from the linearized Ginzburg-Landau equation, is in good agreement with the experimental data.

A nanomagnetic study of phase transition in manganite thin films and ballistic magnetoresistance in magnetic nanocontacts

NASA Astrophysics Data System (ADS)

Chung, Seok-Hwan

This work focuses on two largely unexplored phenomena in micromagnetics: the temperature-driven paramagnetic insulator to ferromagnetic (FM) metallic phase transition in perovskite manganite and ballistic magnetoresistance in spin-polarized nanocontacts. To investigate the phase transition, an off-the-shelf commercial scanning force microscope was redesigned for operation at temperatures from 350 K to 100 K. This adaptation is elaborated in this thesis. Using this system, both ferromagnetic and charge-ordered domain structures of (La 1-xPrx)0.67Ca0.33MnO3 thin film were observed by magnetic force microscopy (MFM) and electric force microscopy (EFM) operated in the vicinity of the peak resistance temperature (Tp). Predominantly in-plane oriented FM domains of sub-micrometer size emerge below Tp and their local magnetic moment increased as the temperature is reduced. Charge-ordered insulating regions show a strong electrostatic interaction with an EFM tip at a few degrees above Tp and the interaction correlates well with the temperature dependence of resistivity of the film. Cross-correlation analysis between topography and magnetic structure on several substrates indicates FM domains form on the flat regions of the surface, while charge ordering occurs at surface protrusions. In the investigation of ballistic magnetoresistance, new results on half-metallic ferromagnets formed by atomic or nanometer contacts of CrO2-CrO 2 and CrO2-Ni are presented showing magnetoconductance as high as 400%. Analysis of the magnetoconductance versus conductance data for all materials known to exhibit so-called ballistic magnetoresistance strongly suggests that magnetoconductance of nanocontacts follows a universal mechanism. If the maximum magnetoconductance is normalized to unity and the conductance is scaled with the resistivity of the material, then all data points fall onto a universal curve independent of the contact material and the transport mechanism. The analysis has been applied to all available magnetoconductance data of magnetic nanocontacts in the literature. The results are in agreement with a theory that takes into account only the spin-scattering within a magnetic domain wall and are independent of whether the transport is ballistic or diffusive.

Quasi-one-dimensional Bose-Einstein condensation in the spin-1/2 ferromagnetic-leg ladder 3-I-V

NASA Astrophysics Data System (ADS)

Kono, Y.; Kittaka, S.; Yamaguchi, H.; Hosokoshi, Y.; Sakakibara, T.

2018-03-01

Quantum criticality of the spin-1/2 ferromagnetic-leg ladder 3-I-V [=3-(3-iodophenyl)-1,5-diphenylverdazyl] has been examined with respect to the antiferromagnetic to paramagnetic phase transition near the saturation field Hc. The phase boundary Tc(H ) follows the power-law Tc(H ) ∝Hc-H for a wide temperature range. This characteristic behavior is discussed as a quasi-one-dimensional (quasi-1D) Bose-Einstein condensation, which is predicted theoretically for weakly coupled quasi-1D ferromagnets. Thus, 3-I-V provides the first promising candidate for this attractive prediction.

Repulsive atomic gas in a harmonic trap on the border of itinerant ferromagnetism.

PubMed

Conduit, G J; Simons, B D

2009-11-13

Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the most well-characterized phases of correlated Fermi systems. A recent experiment has reported the first evidence for novel phase behavior on the repulsive side of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting recent theoretical studies to the atomic trap geometry, we show that an adiabatic ferromagnetic transition would take place at a weaker interaction strength than is observed in experiment. This discrepancy motivates a simple nonequilibrium theory that takes account of the dynamics of magnetic defects and three-body losses. The formalism developed displays good quantitative agreement with experiment.

Electrical resistivity across the tricriticality in itinerant ferromagnet

NASA Astrophysics Data System (ADS)

Opletal, P.; Prokleška, J.; Valenta, J.; Sechovský, V.

2018-05-01

We investigate the discontinuous ferromagnetic phase diagram near tricritical point in UCo1-xRuxAl compounds by electrical resistivity measurements. Separation of phases in UCo0.995Ru0.005Al at ambient pressure and in UCo0.990Ru0.010Al at pressure of 0.2 GPa and disappearance of ferromagnetism at 0.4 GPa is confirmed. The exponent of temperature dependence of electrical resistivity implies change from Fermi liquid-like behavior to non-Fermi liquid at 0.2 GPa and reaches minimum at 0.4 GPa. Our results are compared to results obtained on the pure UCoAl and explanation for different exponents is given.

Microstructural Investigation, Raman and Magnetic Studies on Chemically Synthesized Nanocrystalline Ni-Doped Gadolinium Oxide (Gd1.90Ni0.10O3- δ )

NASA Astrophysics Data System (ADS)

Sarkar, B. J.; Mandal, J.; Dalal, M.; Bandyopadhyay, A.; Satpati, B.; Chakrabarti, P. K.

2018-03-01

Nanocrystalline Ni-doped gadolinium oxide (Gd1.90Ni0.10O3- δ , GNO) is synthesized by co-precipitation method. The as-prepared sample is annealed in vacuum at 700°C for 6 h. Analyses of the x-ray diffractogram by Rietveld refinement method, transmission electron microscopy and Raman spectroscopy of GNO recorded at room temperature confirmed the pure crystallographic phase and complete substitution of Ni-ions in Gd2O3 lattice. Magnetization ( M) as a function of temperature ( T) and magnetic field ( H) is measured by a superconducting quantum interference device magnetometer, which suggests the presence of ferromagnetic/antiferromagnetic phases together with a paramagnetic phase. From the M-T curve it can be shown that the ferromagnetic phase dominates over para-/antiferromagnetic phases in the temperature range of 300-100 K, but from 100 K to 50 K, the antiferromagnetic phase dominates over ferro-/paramagnetic phases. Hysteresis loops recorded at different temperatures indicate the presence of weak ferro-/antiferromagnetism, which dominates in the low field region (˜ 4000 Oe), above which magnetization increases linearly. The sharp increase of magnetization in M-T curve observed in the temperature range of 50-5 K confirms the presence of dominating ferromagnetic plus paramagnetic phase over antiferromagnetic part. For the first time a combined formula generated from three-dimensional (3D) spin wave model and Johnston formula is proposed to analyze the coexistence of different magnetic phases in different temperature ranges. Interestingly, the combined formula successfully explains the co-existence of different magnetic phases along with their contribution at different temperatures. The onset of ferromagnetism in Gd1.90Ni0.10O3- δ is explained by oxygen vacancy mediated F-centre exchange (FCE) coupling mechanism.

Room temperature ferromagnetism in Fe-doped CuO nanoparticles.

PubMed

Layek, Samar; Verma, H C

2013-03-01

The pure and Fe-doped CuO nanoparticles of the series Cu(1-x)Fe(x)O (x = 0.00, 0.02, 0.04, 0.06 and 0.08) were successfully prepared by a simple low temperature sol-gel method using metal nitrates and citric acid. Rietveld refinement of the X-ray diffraction data showed that all the samples were single phase crystallized in monoclinic structure of space group C2/c with average crystallite size of about 25 nm and unit cell volume decreases with increasing iron doping concentration. TEM micrograph showed nearly spherical shaped agglomerated particles of 4% Fe-doped CuO with average diameter 26 nm. Pure CuO showed weak ferromagnetic behavior at room temperature with coercive field of 67 Oe. The ferromagnetic properties were greatly enhanced with Fe-doping in the CuO matrix. All the doped samples showed ferromagnetism at room temperature with a noticeable coercive field. Saturation magnetization increases with increasing Fe-doping, becomes highest for 4% doping then decreases for further doping which confirms that the ferromagnetism in these nanoparticles are intrinsic and are not resulting from any impurity phases. The ZFC and FC branches of the temperature dependent magnetization (measured in the range of 10-350 K by SQUID magnetometer) look like typical ferromagnetic nanoparticles and indicates that the ferromagnetic Curie temperature is above 350 K.

Probing critical behavior of 2D Ising ferromagnet with diluted bonds using Wang-Landau algorithm

NASA Astrophysics Data System (ADS)

Ridha, N. A.; Mustamin, M. F.; Surungan, T.

2018-03-01

Randomness is an important subject in the study of phase transition as defect and impurity may present in any real material. The pre-existing ordered phase of a pure system can be affected or even ruined by the presence of randomness. Here we study ferromagnetic Ising model on a square lattice with a presence of randomness in the form of bond dilution. The pure system of this model is known to experience second order phase transition, separating between the high temperature paramagnetic and low-temperature ferromagnetic phase. We used Wang-Landau algorithm of Monte Carlo method to obtain the density of states from which we extract the ensemble average of energy and the specific heat. We observed the signature of phase transition indicated by the diverging peak of the specific heat as system sizes increase. These peaks shift to the lower temperature side as the dilution increases. The lower temperature ordered phase preserves up to certain concentration of dilution and is totally ruined when the bonds no longer percolates.

Strain-Induced Ferromagnetism in Antiferromagnetic LuMnO3 Thin Films

NASA Astrophysics Data System (ADS)

White, J. S.; Bator, M.; Hu, Y.; Luetkens, H.; Stahn, J.; Capelli, S.; Das, S.; Döbeli, M.; Lippert, Th.; Malik, V. K.; Martynczuk, J.; Wokaun, A.; Kenzelmann, M.; Niedermayer, Ch.; Schneider, C. W.

2013-07-01

Single phase and strained LuMnO3 thin films are discovered to display coexisting ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism (≈1μB), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth manganite thin films as promising candidate systems for new multifunctional devices.

Magnetic enhancement of ferroelectric polarization in a self-grown ferroelectric-ferromagnetic composite

NASA Astrophysics Data System (ADS)

Kumar, Amit; Narayan, Bastola; Pachat, Rohit; Ranjan, Rajeev

2018-02-01

Ferroelectric-ferromagnetic multiferroic composites are of great interest both from the scientific and technological standpoints. The extent of coupling between polarization and magnetization in such two-phase systems depends on how efficiently the magnetostrictive and electrostrictive/piezoelectric strain gets transferred from one phase to the other. This challenge is most profound in the easy to make 0-3 ferroelectric-ferromagnetic particulate composites. Here we report a self-grown ferroelectric-ferromagnetic 0-3 particulate composite through controlled spontaneous precipitation of ferrimagnetic barium hexaferrite phase (BaF e12O19 ) amid ferroelectric grains in the multiferroic alloy system BiFe O3-BaTi O3 . We demonstrate that a composite specimen exhibiting merely ˜1% hexaferrite phase exhibits ˜34% increase in saturation polarization in a dc magnetic field of ˜10 kOe. Using modified Rayleigh analysis of the polarization field loop in the subcoercive field region we argue that the substantial enhancement in the ferroelectric switching is associated with the reduction in the barrier heights of the pinning centers of the ferroelectric-ferroelastic domain walls in the stress field generated by magnetostriction in the hexaferrite grains when the magnetic field is turned on. Our study proves that controlled precipitation of the magnetic phase is a good strategy for synthesis of 0-3 ferroelectric-ferromagnetic particulate multiferroic composite as it not only helps in ensuring a good electrical insulating character of the composite, enabling it to sustain high enough electric field for ferroelectric switching, but also the factors associated with the spontaneity of the precipitation process ensure efficient transfer of the magnetostrictive strain/stress to the surrounding ferroelectric matrix making domain wall motion easy.

Magneto-electronic phase separation in doped cobaltites

NASA Astrophysics Data System (ADS)

He, Chunyong

This thesis work mainly focuses on magneto-electronic phase separation (MEPS), an effect where chemically homogeneous materials display inhomogeneous magnetic and electronic properties. A model system La1-xSrxCoO3 (LSCO) is chosen for the study of MEPS. The doping evolution of MEPS in LSCO single crystals is extensively studied through complementary experimental techniques including heat capacity, small angle neutron scattering, magnetometry, and transport. It is found that there exists a finite doping range over which MEPS occurs. The doping range determined from different experimental techniques is found to be in good agreement. Also, this same doping range is reproduced by statistical simulations incorporating local compositional fluctuations. The excellent agreement between experimental data and statistical simulations leads to the conclusion that the MEPS in LSCO is driven solely by inevitable local compositional fluctuations at nanoscopic length scales. Such a conclusion indicates that nanoscopic MEPS is doping fluctuation-driven rather than electronically-driven in LSCO. The effect of microscopic magneto-electronic phase separation on electrical transport in LSCO is also examined. It is demonstrated (i) that the T = 0 metal-insulator transition can be understood within double exchange-modified percolation framework, and, (ii) that the onset of a phase-pure low T ferromagnetic state at high x has a profound effect on the high T transport. In addition, a new origin for finite spin Co ions in LaCoO3 is revealed via a Schottky Anomaly in the heat capacity, which was not previously known. Such a discovery casts a new understanding of the spin state at low temperature. Via small-angle neutron scattering and d.c. susceptibility, it is revealed that short-range ordered FM clusters exist below a well-defined temperature (T*) in highly doped LSCO. It is demonstrated that the characteristics of this clustered state appear quite unlike those of a Griffiths phase. Finally, through magenetometry and SANS, the magneto-crystalline anisotropy of highly doped LSCO is studied and the easy and hard magnetization axes are determined.

Weak ferromagnetism in a high-pressure phase of FeTiO3 with polar lattice distortion

NASA Astrophysics Data System (ADS)

Varga, Tamas; Mitchell, John; Fennie, Craig; Streiffer, Stephen; Hong, Seungbum; Park, Moonkyu; Gopalan, Venkatraman; Kumar, Amit; Vlahos, Eftihia; Sanehira, Takeshi; Wang, Yanbin

2009-03-01

Today's challenge in multiferroics is to identify materials in which polarization and magnetization -- normally considered contraindicated properties - are strongly coupled. Recent density functional theory calculations have predicted that the family of compounds MTiO3 (M = Mn, Fe, Ni) are promising candidates where a polar lattice distortion can induce weak ferromagnetism. The crucial insight is that while the equilibrium one-atmosphere structure of these is ilmenite, they must be transformed to a closely related LiNbO3-type structure. We have prepared the corresponding FeTiO3 phase at 18 GPa and 1200 ^oC. It shows a sharp antiferromagnetic (AF) transition at 111.5 K. FeTiO3 also displays ferroelectric domains, and weak ferromagnetism coincident with the AF transition. Possible coupling between its polarization and weak ferromagnetism is discussed based on results of piezoelectric force microscopy (PFM), second harmonic generation (SHG), dielectric, and polarization measurements.

Demonstration of Ru as the 4th ferromagnetic element at room temperature.

PubMed

Quarterman, P; Sun, Congli; Garcia-Barriocanal, Javier; Dc, Mahendra; Lv, Yang; Manipatruni, Sasikanth; Nikonov, Dmitri E; Young, Ian A; Voyles, Paul M; Wang, Jian-Ping

2018-05-25

Development of novel magnetic materials is of interest for fundamental studies and applications such as spintronics, permanent magnetics, and sensors. We report on the first experimental realization of single element ferromagnetism, since Fe, Co, and Ni, in metastable tetragonal Ru, which has been predicted. Body-centered tetragonal Ru phase is realized by use of strain via seed layer engineering. X-ray diffraction and electron microscopy confirm the epitaxial mechanism to obtain tetragonal phase Ru. We observed a saturation magnetization of 148 and 160 emu cm -3 at room temperature and 10 K, respectively. Control samples ensure the ferromagnetism we report on is from tetragonal Ru and not from magnetic contamination. The effect of thickness on the magnetic properties is also studied, and it is observed that increasing thickness results in strain relaxation, and thus diluting the magnetization. Anomalous Hall measurements are used to confirm its ferromagnetic behavior.

Mapping the phase inhomogeneity across first order spin flop transition

NASA Astrophysics Data System (ADS)

Tripathi, Malvika; Majumder, Supriyo; Choudhary, R. J.; Phase, D. M.

2018-04-01

As a consequence of spin reorientation phase transition (SRPT, TSRPT = 34K) in SmCrO3, the two phases, high temperature uncompensated anti-ferromagnetic Γ4 configuration and low temperature collinear anti-ferromagnetic phase Γ1 coexist in the vicinity of transition. The observed unexpectedly huge coercivity (Hc ˜2T) below SRPT at 25K questions on the behavior of two co-existing phases. In the present study, we have used the FORC diagrams to monitor the distribution of clusters related to different phases and to understand the nature of interaction among the clusters of distinct phases. We observed that the nature of interaction has indeed magnetic effect and the pining across phase boundaries may cause the enhancement of coercivity at 25K.

RbEu (Fe1-xNix) 4As4 : From a ferromagnetic superconductor to a superconducting ferromagnet

NASA Astrophysics Data System (ADS)

Liu, Yi; Liu, Ya-Bin; Yu, Ya-Long; Tao, Qian; Feng, Chun-Mu; Cao, Guang-Han

2017-12-01

The intrinsically hole-doped RbEuFe4As4 exhibits bulk superconductivity at Tsc=36.5 K and ferromagnetic ordering in the Eu sublattice at Tm=15 K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor RbEuFe4As4 with Tsc>Tm . With the Ni doping, Tsc decreases rapidly, and the Eu-spin ferromagnetism and its Tm remain unchanged. Consequently, the system RbEu (Fe1-xNix) 4As4 transforms into a superconducting ferromagnet with Tm>Tsc for 0.07 ≤x ≤0.08 . The occurrence of superconducting ferromagnets is attributed to the decoupling between Eu2 + spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.

Half-Metallic Ferromagnetism and Stability of Transition Metal Pnictides and Chalcogenides

NASA Astrophysics Data System (ADS)

Liu, Bang-Gui

It is highly desirable to explore robust half-metallic ferromagnetic materials compatible with important semiconductors for spintronic applications. A state-of-the-art full potential augmented plane wave method within the densityfunctional theory is reliable enough for this purpose. In this chapter we review theoretical research on half-metallic ferromagnetism and structural stability of transition metal pnictides and chalcogenides. We show that some zincblende transition metal pnictides are half-metallic and the half-metallic gap can be fairly wide, which is consistent with experiment. Systematic calculations reveal that zincblende phases of CrTe, CrSe, and VTe are excellent half-metallic ferromagnets. These three materials have wide half-metallic gaps, are low in total energy with respect to the corresponding ground-state phases, and, importantly, are structurally stable. Halfmetallic ferromagnetism is also found in wurtzite transition metal pnictides and chalcogenides and in transition-metal doped semiconductors as well as deformed structures. Some of these half-metallic materials could be grown epitaxially in the form of ultrathin .lms or layers suitable for real spintronic applications.

Room Temperature Ferromagnetic Mn:Ge(001).

PubMed

Lungu, George Adrian; Stoflea, Laura Elena; Tanase, Liviu Cristian; Bucur, Ioana Cristina; Răduţoiu, Nicoleta; Vasiliu, Florin; Mercioniu, Ionel; Kuncser, Victor; Teodorescu, Cristian-Mihail

2013-12-27

We report the synthesis of a room temperature ferromagnetic Mn-Ge system obtained by simple deposition of manganese on Ge(001), heated at relatively high temperature (starting with 250 °C). The samples were characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and magneto-optical Kerr effect (MOKE). Samples deposited at relatively elevated temperature (350 °C) exhibited the formation of ~5-8 nm diameter Mn₅Ge₃ and Mn 11 Ge₈ agglomerates by HRTEM, while XPS identified at least two Mn-containing phases: the agglomerates, together with a Ge-rich MnGe ~2.5 phase, or manganese diluted into the Ge(001) crystal. LEED revealed the persistence of long range order after a relatively high amount of Mn (100 nm) deposited on the single crystal substrate. STM probed the existence of dimer rows on the surface, slightly elongated as compared with Ge-Ge dimers on Ge(001). The films exhibited a clear ferromagnetism at room temperature, opening the possibility of forming a magnetic phase behind a nearly ideally terminated Ge surface, which could find applications in integration of magnetic functionalities on semiconductor bases. SQUID probed the co-existence of a superparamagnetic phase, with one phase which may be attributed to a diluted magnetic semiconductor. The hypothesis that the room temperature ferromagnetic phase might be the one with manganese diluted into the Ge crystal is formulated and discussed.

Room Temperature Ferromagnetic Mn:Ge(001)

PubMed Central

Lungu, George Adrian; Stoflea, Laura Elena; Tanase, Liviu Cristian; Bucur, Ioana Cristina; Răduţoiu, Nicoleta; Vasiliu, Florin; Mercioniu, Ionel; Kuncser, Victor; Teodorescu, Cristian-Mihail

2014-01-01

We report the synthesis of a room temperature ferromagnetic Mn-Ge system obtained by simple deposition of manganese on Ge(001), heated at relatively high temperature (starting with 250 °C). The samples were characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and magneto-optical Kerr effect (MOKE). Samples deposited at relatively elevated temperature (350 °C) exhibited the formation of ~5–8 nm diameter Mn5Ge3 and Mn11Ge8 agglomerates by HRTEM, while XPS identified at least two Mn-containing phases: the agglomerates, together with a Ge-rich MnGe~2.5 phase, or manganese diluted into the Ge(001) crystal. LEED revealed the persistence of long range order after a relatively high amount of Mn (100 nm) deposited on the single crystal substrate. STM probed the existence of dimer rows on the surface, slightly elongated as compared with Ge–Ge dimers on Ge(001). The films exhibited a clear ferromagnetism at room temperature, opening the possibility of forming a magnetic phase behind a nearly ideally terminated Ge surface, which could find applications in integration of magnetic functionalities on semiconductor bases. SQUID probed the co-existence of a superparamagnetic phase, with one phase which may be attributed to a diluted magnetic semiconductor. The hypothesis that the room temperature ferromagnetic phase might be the one with manganese diluted into the Ge crystal is formulated and discussed. PMID:28788444

Phase Diagram of Spin-1/2 Alternating Ferromagnetic Chain with XY-Like Anisotropy

NASA Astrophysics Data System (ADS)

Yoshida, Satoru; Okamoto, Kiyomi

1989-12-01

By the use of the numerical method we investigate the ground state phase diagram of spin-1/2 alternating ferromagnetic chain. We numerically diagonalized the Hamiltonian of finite systems (up to 20 spins) and analyzed the numerical data for various physical quantities using the finite size scaling and the extrapolation methods. The ground state is either the effective singlet (ES) state or the spin fluid (SF) state depending on the value of the alternation parameter δ and the anisotropy parameter \\varDelta{\\equiv}Jz/J\\bot(\\varDelta{=}{-}1 for the isotropic ferromagnetic case and \\varDelta{=}0 for the XY case). The phase diagram obtained in this work strongly stupports the theoretical studies of Kohmoto-den Nijs-Kadanoff and Okamoto-Sugiyama. We also discuss the critical properties near the ES-SF transition line.

Thermoelectric Properties of a Ferromagnetic Semiconductor Based on a Dirac Semimetal (Cd3As2) under High Pressure

NASA Astrophysics Data System (ADS)

Melnikova, N. V.; Tebenkov, A. V.; Sukhanova, G. V.; Babushkin, A. N.; Saipulaeva, L. A.; Zakhvalinskii, V. S.; Gabibov, S. F.; Alibekov, A. G.; Mollaev, A. Yu.

2018-03-01

The pressure dependences of thermal emf (a parameter that ranks among the most sensitive to phase transformations) are studied for the purpose of identifying baric phase transitions in the 10-50 GPa interval in the Cd3As2 + MnAs (44.7% MnAs) structure formed by ferromagnetic MnAs granules in a semiconductor Cd3As2 matrix.

Agent-based spin model for financial markets on complex networks: Emergence of two-phase phenomena

NASA Astrophysics Data System (ADS)

Kim, Yup; Kim, Hong-Joo; Yook, Soon-Hyung

2008-09-01

We study a microscopic model for financial markets on complex networks, motivated by the dynamics of agents and their structure of interaction. The model consists of interacting agents (spins) with local ferromagnetic coupling and global antiferromagnetic coupling. In order to incorporate more realistic situations, we also introduce an external field which changes in time. From numerical simulations, we find that the model shows two-phase phenomena. When the local ferromagnetic interaction is balanced with the global antiferromagnetic interaction, the resulting return distribution satisfies a power law having a single peak at zero values of return, which corresponds to the market equilibrium phase. On the other hand, if local ferromagnetic interaction is dominant, then the return distribution becomes double peaked at nonzero values of return, which characterizes the out-of-equilibrium phase. On random networks, the crossover between two phases comes from the competition between two different interactions. However, on scale-free networks, not only the competition between the different interactions but also the heterogeneity of underlying topology causes the two-phase phenomena. Possible relationships between the critical phenomena of spin system and the two-phase phenomena are discussed.

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

Ma, Q. L., E-mail: maqinli@gmail.com, E-mail: mizukami@wpi-aimr.tohoku.ac.jp; Miyazaki, T.; Mizukami, S., E-mail: maqinli@gmail.com, E-mail: mizukami@wpi-aimr.tohoku.ac.jp

The laser-induced spin dynamics of FeCo in perpendicularly magnetized L1{sub 0}-MnGa/FeCo bilayers with ferromagnetic and antiferromagnetic interfacial exchange coupling (IEC) are examined using the time-resolved magneto-optical Kerr effect. We found a precessional phase reversal of the FeCo layer as the IEC changes from ferromagnetic to antiferromagnetic. Moreover, a precession-suspension window was observed when the magnetic field was applied in a certain direction for the bilayer with ferromagnetic IEC. Our observations reveal that the spin dynamics modulation is strongly dependent on the IEC type within the Landau-Lifshitz-Gilbert depiction. The IEC dependence of the precessional phase and amplitude suggests the interesting methodmore » for magnetization dynamics modulation.« less

Stable room-temperature ferromagnetic phase at the FeRh(100) surface

DOE PAGES

Pressacco, Federico; Uhlir, Vojtech; Gatti, Matteo; ...

2016-03-03

Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at ~400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. Furthermore, we find that the symmetry breaking induced at themore » Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that support the experimental findings.« less

Global mean-field phase diagram of the spin-1 Ising ferromagnet in a random crystal field

NASA Astrophysics Data System (ADS)

Borelli, M. E. S.; Carneiro, C. E. I.

1996-02-01

We study the phase diagram of the mean-field spin-1 Ising ferromagnet in a uniform magnetic field H and a random crystal field Δi, with probability distribution P( Δi) = pδ( Δi - Δ) + (1 - p) δ( Δi). We analyse the effects of randomness on the first-order surfaces of the Δ- T- H phase diagram for different values of the concentration p and show how these surfaces are affected by the dilution of the crystal field.

Voltage-induced ferromagnetic resonance in magnetic tunnel junctions.

PubMed

Zhu, Jian; Katine, J A; Rowlands, Graham E; Chen, Yu-Jin; Duan, Zheng; Alzate, Juan G; Upadhyaya, Pramey; Langer, Juergen; Amiri, Pedram Khalili; Wang, Kang L; Krivorotov, Ilya N

2012-05-11

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.

Surface-State-Dominated Spin-Charge Current Conversion in Topological-Insulator-Ferromagnetic-Insulator Heterostructures.

PubMed

Wang, Hailong; Kally, James; Lee, Joon Sue; Liu, Tao; Chang, Houchen; Hickey, Danielle Reifsnyder; Mkhoyan, K Andre; Wu, Mingzhong; Richardella, Anthony; Samarth, Nitin

2016-08-12

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films-Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3}-deposited by molecular beam epitaxy on Y_{3}Fe_{5}O_{12} thin films. By systematically varying the Bi_{2}Se_{3} film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (λ_{IREE}), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator-ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of Y_{3}Fe_{5}O_{12}/(Bi,Sb)_{2}Te_{3} heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and λ_{IREE}.

Mn-based ferromagnetic semiconductors

NASA Astrophysics Data System (ADS)

Dietl, Tomasz; Sawicki, Maciej

2003-07-01

The present status of research and prospects for device applications of ferromagnetic (diluted magnetic) semiconductors (DMS) is presented. We review the nature of the electronic states and the mechanisms of the carrier-mediated exchange interactions (mean-field Zener model) in p-type Mn-based III-V and II-VI compounds, highlighting a good correspondence of experimental findings and theoretical predictions. An account of the latest progress on the road of increasing the Currie point to above the room temperature is given for both families of compounds. We comment on a possibility of obtaining ferromagnetism in n-type materials, taking (Zn,Mn)O:Al as the example. Concerning technologically important issue of easy axis and domain engineering, we present theoretical predictions and experimental results on the temperature and carrier concentration driven change of magnetic anisotropy in (Ga,Mn)As.

500 keV Ar2+ ion irradiation induced anatase to brookite phase transformation and ferromagnetism at room temperature in TiO2 thin films

NASA Astrophysics Data System (ADS)

Bharati, B.; Mishra, N. C.; Kanjilal, D.; Rath, Chandana

2018-01-01

In our earlier report, where we have demonstrated ferromagnetic behavior at room temperature (RT) in TiO2 thin films deposited through electron beam evaporation technique followed by annealing either in Ar or O2 atmosphere [Mohanty et al., Journal of Magnetism and Magnetic Materials 355 (2014) 240-245], here we have studied the evolution of structure and magnetic properties after irradiating the TiO2 thin films with 500 keV Ar2+ ions. The pristine film while exhibits anatase phase, the films become amorphous after irradiating at fluence in the range 1 × 1014 to 1 × 1016 ions/cm2. Increasing the fluence up to 5 × 1016 ions/cm2, amorphous to crystalline phase transformation occurs and the structure becomes brookite. Although anatase to rutile phase transformation is usually reported in literatures, anatase to brookite phase transformation is an unusual feature which we have reported here for the first time. Such anatase to brookite phase transformation is accompanied with grain growth without showing any change in film thickness evidenced from Rutherford's Back Scattering (RBS) measurement. From scanning probe micrographs (SPM), roughness is found to be more in amorphous films than in the crystalline ones. Anatase to brookite phase transformation could be realized by considering the importance of intermediate amorphous phase. Because due to amorphous phase, heat deposited by energetic ions are localized as dissipation of heat is less and as a result, the localized region crystallizes in brookite phase followed by grain growth as observed in highest fluence. Further, we have demonstrated ferromagnetic behavior at RT in irradiated films similar to pristine one, irrespective of their phase and crystallinity. Origin for room temperature ferromagnetism (RTFM) is attributed to the presence of oxygen vacancies which is confirmed by carrying out XPS measurement.

Spin-flop quasi-first order phase transition and putative tricritical point in Gd3Co

NASA Astrophysics Data System (ADS)

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

2017-12-01

Magnetic nature of Gd3Co is investigated using detailed measurements of temperature and field dependent magnetization. The antiferromagnetic phase is field-instable due to prevailing ferromagnetic exchange correlations above Néel temperature TN ∼ 130K . Below TN , with gradually increasing magnetic fields, the compound undergoes a quasi-first order phase transition from AFM to spin-flop over region and eventually acquires ferromagnetic phase in higher fields. Further the point at which the quasi-first order transition ends and second order transition sets in is the tricritical point, TTCP ∼ 125.6K , HTCP ∼ 4.4kOe .

Emerging single-phase state in small manganite nanodisks

DOE PAGES

Shao, Jian; Liu, Hao; Zhang, Kai; ...

2016-08-01

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La 0.325Pr 0.3Ca 0.375MnO 3 (LPCMO) single crystalline disks to study the effect of spatial confinementmore » on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.« less

Rhombohedral R3c to orthorhombic Pnma phase transition induced by Y-doping in BiFeO₃.

PubMed

Graf, Monica Elisabet; Di Napoli, Solange; Barral, Maria Andrea Andrea; Saleh Medina, Leila; Negri, R Martín; Sepliarsky, Marcelo; Llois, Ana María

2018-05-23

In this work we study, by means of <i>ab initio</i> calculations, the structural, electronic and magnetic properties of Y-doped BiFeO₃ compounds. We determine that there is a morphotropic phase boundary at an yttrium concentration of (18 ± 2)%, where the structure changes from <i>R3c</i> to <i>Pnma</i>. This structural transition is driven by the chemical pressure induced by the dopant. By analyzing the evolution of the oxygen octahedral tilts we find an enhanced antiferrodistortive distortion when increasing the Y-doping, together with a reduction of the ferroelectric distorsion, that gives rise to a smaller value of the electric polarization. These cooperative effects should lead to a larger canting of the Fe magnetic moments and to a larger ferromagnetic response in the <i>R3c</i> phase, as it is observed in the experiments. . © 2018 IOP Publishing Ltd.

Quasi-Particle Relaxation and Quantum Femtosecond Magnetism in Non-Equilibrium Phases of Insulating Manganites

NASA Astrophysics Data System (ADS)

Perakis, Ilias; Kapetanakis, Myron; Lingos, Panagiotis; Barmparis, George; Patz, A.; Li, T.; Wang, Jigang

We study the role of spin quantum fluctuations driven by photoelectrons during 100fs photo-excitation of colossal magneto-resistive manganites in anti-ferromagnetic (AFM) charge-ordered insulating states with Jahn-Teller distortions. Our mean-field calculation of composite fermion excitations demonstrates that spin fluctuations reduce the energy gap by quasi-instantaneously deforming the AFM background, thus opening a conductive electronic pathway via FM correlation. We obtain two quasi-particle bands with distinct spin-charge dynamics and dependence on lattice distortions. To connect with fs-resolved spectroscopy experiments, we note the emergence of fs magnetization in the low-temperature magneto-optical signal, with threshold dependence on laser intensity characteristic of a photo-induced phase transition. Simultaneously, the differential reflectivity shows bi-exponential relaxation, with fs component, small at low intensity, exceeding ps component above threshold for fs AFM-to-FM switching. This suggests the emergence of a non-equilibrium metallic FM phase prior to establishment of a new lattice structure, linked with quantum magnetism via spin/charge/lattice couplings for weak magnetic fields.

Topological-charge-driven reversal of ferromagnetic rings via 360∘ domain-wall formation

NASA Astrophysics Data System (ADS)

Oyarce, A. L. Gonzalez; Trypiniotis, T.; Roy, P. E.; Barnes, C. H. W.

2013-05-01

We study the reversal mechanism between opposite closed flux states of ferromagnetic nanorings driven by an azimuthal magnetic field. The reversal proceeds via the formation of 360∘ domain walls, and we show that the role of interacting nucleation sites is essential for the process to take place. Such nucleation is seen to create domain walls with the right topological charge conditions for 360∘ domain-wall formation. Given the symmetry of the system, we utilize an energetic description as a function of the azimuthal field magnitude, which clearly reveals the different stages of this reversal process. The annihilation of the 360∘ domain walls that is necessary for the reversal process to complete is controlling the field value at the final stage of the process. Such a fundamental mechanism for ring reversal has several implications and will guide the design of the various data-storage-device proposals based on nanorings.

All-optical switching in granular ferromagnets caused by magnetic circular dichroism

NASA Astrophysics Data System (ADS)

Ellis, Matthew O. A.; Fullerton, Eric E.; Chantrell, Roy W.

2016-07-01

Magnetic recording using circularly polarised femto-second laser pulses is an emerging technology that would allow write speeds much faster than existing field driven methods. However, the mechanism that drives the magnetisation switching in ferromagnets is unclear. Recent theories suggest that the interaction of the light with the magnetised media induces an opto-magnetic field within the media, known as the inverse Faraday effect. Here we show that an alternative mechanism, driven by thermal excitation over the anisotropy energy barrier and a difference in the energy absorption depending on polarisation, can create a net magnetisation over a series of laser pulses in an ensemble of single domain grains. Only a small difference in the absorption is required to reach magnetisation levels observed experimentally and the model does not preclude the role of the inverse Faraday effect but removes the necessity that the opto-magnetic field is 10 s of Tesla in strength.

Spin-torque driven magnetization switching in ferromagnetic nanopillar with pinned layer biasing configuration

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

Bhoomeeswaran, H.; Sabareesan, P., E-mail: sendtosabari@gmail.com; Bharathi, B. Divya

2016-05-06

Magnetization switching driven by spin transfer torque in a ferromagnetic nanopillar by biasing the angular polarizer with different orientation has been studied. The free layer dynamics includes the spin torque from the oscillating free layer with magneto crystalline anisotropy and shape anisotropy, which is governed by the Landau-Lifshitsz-Gilbert-Slonczweski (LLGS) equation and solving it numerically by using embedded Runge Kutta fourth order method. Results of numerical simulation shows that there is a drastic reduction of switching time in the free layer by the orientation of angular polarizer of the nano pillar device. We fixed the angular polarizer as 0°, 30°, 60°,more » 90° and the corresponding switching time is 6.53 ns, 4.36 ns, 2.25 ns and 1.21 ns respectively for an applied current density of 5 × 10{sup 11} Am{sup −2}.« less

Quantum critical scaling in the disordered itinerant ferromagnet UCo 1-xFe xGe

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

Huang, Kevin; Eley, Serena Merteen; Civale, Leonardo

The Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals the BKV theory predicts that the secondorder nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Lastly, our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo 1$-$xFe xGe as the first clear example that exhibits the associatedmore » critical exponents predicted by the BKV theory.« less

Quantum critical scaling in the disordered itinerant ferromagnet UCo 1-xFe xGe

DOE PAGES

Huang, Kevin; Eley, Serena Merteen; Civale, Leonardo; ...

2016-11-30

The Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals the BKV theory predicts that the secondorder nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Lastly, our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo 1$-$xFe xGe as the first clear example that exhibits the associatedmore » critical exponents predicted by the BKV theory.« less

Interstitial effects of B and Li on the magnetic phase transition and magnetocaloric effects in Gd2In alloy

NASA Astrophysics Data System (ADS)

Yang, Yang; Xie, Yigao; Zhou, Xiaoqian; Zhong, Hui; Jiang, Qingzheng; Ma, Shengcan; Zhong, Zhenchen; Cui, Weibin; Wang, Qiang

2018-05-01

Interstitial effects of B and Li on the phase transition and magnetocaloric effect in Gd2In alloys had been studied. The antiferromagnetic (AFM) - ferromagnetic (FM) phase transition was found to be of first-order nature while ferromagnetic - paramagnetic (PM) phase transition was of second-order nature in B- or Li-doped Gd2In alloys. AFM-FM phase transition temperature was increased while FM-PM phase transition was decreased with more doping concentrations. During AFM-FM phase transition, the slope of temperature-dependent critical field (μ0Hcr) was increased by increased doping amounts. The magnetic entropy changes under small field change were enhanced by B and Li addition, which showed the beneficial effects of B and Li additions.

Microstructure Evolution and Related Magnetic Properties of Cu-Zr-Al-Gd Phase-Separating Metallic Glasses

NASA Astrophysics Data System (ADS)

Kim, Sang Jun; Kim, Jinwoo; Park, Eun Soo

2018-04-01

We carefully investigated the correlation between microstructures and magnetic properties of Cu-Zr-Al-Gd phase-separating metallic glasses (PSMGs). The saturation magnetizations of the PSMGs were determined by total Gd contents of the alloys, while their coercivity exhibits a large deviation by the occurrence of phase separation due to the boundary pinning effect of hierarchically separated amorphous phases. Especially, the PSMGs containing Gd-rich amorphous nanoparticles show the highest coercivity which can be attributed to the size effect of the ferromagnetic amorphous phase. Furthermore, the selective crystallization of ferromagnetic amorphous phases can affect the magnetization behavior of the PSMGs. Our results could provide a novel strategy for tailoring unique soft magnetic properties of metallic glasses by introducing hierarchically separated amorphous phases and controlling their crystallinity.

Microstructure Evolution and Related Magnetic Properties of Cu-Zr-Al-Gd Phase-Separating Metallic Glasses

NASA Astrophysics Data System (ADS)

Kim, Sang Jun; Kim, Jinwoo; Park, Eun Soo

2018-06-01

We carefully investigated the correlation between microstructures and magnetic properties of Cu-Zr-Al-Gd phase-separating metallic glasses (PSMGs). The saturation magnetizations of the PSMGs were determined by total Gd contents of the alloys, while their coercivity exhibits a large deviation by the occurrence of phase separation due to the boundary pinning effect of hierarchically separated amorphous phases. Especially, the PSMGs containing Gd-rich amorphous nanoparticles show the highest coercivity which can be attributed to the size effect of the ferromagnetic amorphous phase. Furthermore, the selective crystallization of ferromagnetic amorphous phases can affect the magnetization behavior of the PSMGs. Our results could provide a novel strategy for tailoring unique soft magnetic properties of metallic glasses by introducing hierarchically separated amorphous phases and controlling their crystallinity.

Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites

NASA Astrophysics Data System (ADS)

Pradhan, Kalpataru; Yunoki, Seiji

2017-12-01

Using a two-band double-exchange model with Jahn-Teller lattice distortions and superexchange interactions, supplemented by quenched disorder, at an electron density n =0.65 , we explicitly demonstrate the coexistence of the n =1 /2 -type (π ,π ) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature Tc in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters upon decreasing the temperature down to Tc. The ferromagnetic nanoclusters start to grow and merge, and the volume fraction of the charge-ordered nanoclusters decreases below Tc, leading to the sharp drop in the resistivity. By applying a small external magnetic field h , we show that the resistivity above Tc increases, as compared with the case when h =0 , a fact that further confirms the coexistence of the charge-ordered and the ferromagnetic nanoclusters. In addition, we show that the volume fraction of the charge-ordered nanoclusters decreases upon increasing the bandwidth, and consequently the resistivity hump diminishes for large bandwidth manganites, in good qualitative agreement with experiments. The obtained insights from our calculations provide a complete pathway to understand the phase competition in CMR manganites.

Demonstrating the Curie Temperature in the Classroom

ERIC Educational Resources Information Center

Williams, David; Banks, Octavia; Eichmeyer, Livia; Wu, Cherrin

2018-01-01

Recent GCSE and IGCSE specifications include reference to both permanent and induced magnetism, giving the opportunity for novel classroom demonstrations based on ferromagnetism and paramagnetism, and the transition between these phases. Ferromagnetic materials lose their magnetism if raised above their Curie Temperature, a specific temperature…

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe1 - x Co x Si Ferromagnets

NASA Astrophysics Data System (ADS)

Povzner, A. A.; Volkov, A. G.; Nogovitsyna, T. A.

2018-02-01

LSDA + U + SO calculations of the electronic structure of helicoidal Fe1 - x Co x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T C and chiral fluctuations of the local magnetization at T > T C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T S ).

Formation and investigation of ultrathin layers of Co2FeSi ferromagnetic alloy synthesized on silicon covered with a CaF2 barrier layer

NASA Astrophysics Data System (ADS)

Grebenyuk, G. S.; Gomoyunova, M. V.; Pronin, I. I.; Vyalikh, D. V.; Molodtsov, S. L.

2016-03-01

Ultrathin (∼2 nm) films of Co2FeSi ferromagnetic alloy were formed on silicon by solid-phase epitaxy and studied in situ. Experiments were carried out in an ultrahigh vacuum (UHV) using substrates of Si(1 1 1) single crystals covered with a 5 nm thick CaF2 barrier layer. The elemental and phase composition as well as the magnetic properties of the synthesized films were analyzed by photoelectron spectroscopy using synchrotron radiation and by magnetic linear dichroism in photoemission of Fe 3p and Co 3p electrons. The study shows that the synthesis of the Co2FeSi ferromagnetic alloy occurs in the temperature range of 200-400 °C. At higher temperatures, the films become island-like and lose their ferromagnetic properties, as the CaF2 barrier layer is unable to prevent a mass transfer between the film and the Si substrate, which violates the stoichiometry of the alloy.

Adjustable magnetoelectric effect of self-assembled vertical multiferroic nanocomposite films by the in-plane misfit strain and ferromagnetic volume fraction

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

Wu, Huaping, E-mail: wuhuaping@gmail.com; Department of Mechanical Engineering and Science, Kyoto University, Nishikyo-ku, Kyoto 615-8540; Chai, Guozhong

The strain-mediated magnetoelectric (ME) property of self-assembled vertical multiferroic nanocomposite films epitaxially grown on cubic substrates was calculated by a nonlinear thermodynamic theory combined with the elastic theory. The dependent relations of phase state of ferroelectric films with the in-plane misfit strain, out-of-plane misfit strain, temperature, and volume fraction of ferromagnetic phase were confirmed. The effects of in-plane misfit strain and ferromagnetic volume fraction on the polarization and dielectric constant of ferroelectric films at room temperature were elaborately analyzed for the vertical BaTiO{sub 3}-CoFe{sub 2}O{sub 4} and PbTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films. Our calculated results confirmed the relationship amongmore » ME effect and in-plane misfit strain and ferromagnetic volume fraction in the nanocomposite films. The ME voltage coefficients of vertical BaTiO{sub 3}-CoFe{sub 2}O{sub 4} and PbTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films displayed various maximums and abrupt points at special phases and phase transition boundaries. The ME voltage coefficients of lead-free BaTiO{sub 3}-CoFe{sub 2}O{sub 4} nanocomposite films epitaxially grown on different substrates could reach a comparative value of ∼2 V·cm{sup −1}·Oe{sup −1} under the controllable in-plane misfit strain induced by substrate clamping. Our results provided an available method for the optimal design of vertical multiferroic nanocomposites with adjustable ME effect by optimizing the ferromagnetic volume fraction and substrate type.« less

Multiferroics and magnetoelectrics: thin films and nanostructures

NASA Astrophysics Data System (ADS)

Martin, L. W.; Crane, S. P.; Chu, Y.-H.; Holcomb, M. B.; Gajek, M.; Huijben, M.; Yang, C.-H.; Balke, N.; Ramesh, R.

2008-10-01

Multiferroic materials, or materials that simultaneously possess two or more ferroic order parameters, have returned to the forefront of materials research. Driven by the desire to achieve new functionalities—such as electrical control of ferromagnetism at room temperature—researchers have undertaken a concerted effort to identify and understand the complexities of multiferroic materials. The ability to create high quality thin film multiferroics stands as one of the single most important landmarks in this flurry of research activity. In this review we discuss the basics of multiferroics including the important order parameters and magnetoelectric coupling in materials. We then discuss in detail the growth of single phase, horizontal multilayer, and vertical heterostructure multiferroics. The review ends with a look to the future and how multiferroics can be used to create new functionalities in materials.

Width-Tuned Magnetic Order Oscillation on Zigzag Edges of Honeycomb Nanoribbons.

PubMed

Chen, Wen-Chao; Zhou, Yuan; Yu, Shun-Li; Yin, Wei-Guo; Gong, Chang-De

2017-07-12

Quantum confinement and interference often generate exotic properties in nanostructures. One recent highlight is the experimental indication of a magnetic phase transition in zigzag-edged graphene nanoribbons at the critical ribbon width of about 7 nm [ Magda , G. Z. et al. Nature 2014 , 514 , 608 ]. Here we show theoretically that with further increase in the ribbon width, the magnetic correlation of the two edges can exhibit an intriguing oscillatory behavior between antiferromagnetic and ferromagnetic, driven by acquiring the positive coherence between the two edges to lower the free energy. The oscillation effect is readily tunable in applied magnetic fields. These novel properties suggest new experimental manifestation of the edge magnetic orders in graphene nanoribbons and enhance the hopes of graphene-like spintronic nanodevices functioning at room temperature.

Magnetic fluctuations driven insulator-to-metal transition in Ca(Ir1−xRux)O3

PubMed Central

Gunasekera, J.; Harriger, L.; Dahal, A.; Heitmann, T.; Vignale, G.; Singh, D. K.

2015-01-01

Magnetic fluctuations in transition metal oxides are a subject of intensive research because of the key role they are expected to play in the transition from the Mott insulator to the unconventional metallic phase of these materials, and also as drivers of superconductivity. Despite much effort, a clear link between magnetic fluctuations and the insulator-to-metal transition has not yet been established. Here we report the discovery of a compelling link between magnetic fluctuations and the insulator-to-metal transition in Ca(Ir1−xRux)O3 perovskites as a function of the substitution coefficient x. We show that when the material turns from insulator to metal, at a critical value of x ~ 0.3, magnetic fluctuations tend to change their character from antiferromagnetic, a Mott insulator phase, to ferromagnetic, an itinerant electron state with Hund’s orbital coupling. These results are expected to have wide-ranging implications for our understanding of the unconventional properties of strongly correlated electrons systems. PMID:26647965

Magnetic fluctuations driven insulator-to-metal transition in Ca(Ir(1-x)Rux)O3.

PubMed

Gunasekera, J; Harriger, L; Dahal, A; Heitmann, T; Vignale, G; Singh, D K

2015-12-09

Magnetic fluctuations in transition metal oxides are a subject of intensive research because of the key role they are expected to play in the transition from the Mott insulator to the unconventional metallic phase of these materials, and also as drivers of superconductivity. Despite much effort, a clear link between magnetic fluctuations and the insulator-to-metal transition has not yet been established. Here we report the discovery of a compelling link between magnetic fluctuations and the insulator-to-metal transition in Ca(Ir1-xRux)O3 perovskites as a function of the substitution coefficient x. We show that when the material turns from insulator to metal, at a critical value of x ~ 0.3, magnetic fluctuations tend to change their character from antiferromagnetic, a Mott insulator phase, to ferromagnetic, an itinerant electron state with Hund's orbital coupling. These results are expected to have wide-ranging implications for our understanding of the unconventional properties of strongly correlated electrons systems.

Intrinsic Tunneling in Phase Separated Manganites

NASA Astrophysics Data System (ADS)

Singh-Bhalla, G.; Selcuk, S.; Dhakal, T.; Biswas, A.; Hebard, A. F.

2009-02-01

We present evidence of direct electron tunneling across intrinsic insulating regions in submicrometer wide bridges of the phase-separated ferromagnet (La,Pr,Ca)MnO3. Upon cooling below the Curie temperature, a predominantly ferromagnetic supercooled state persists where tunneling across the intrinsic tunnel barriers (ITBs) results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures. Upon application of a magnetic field, our data reveal that the ITBs are extinguished resulting in sharp, colossal, low-field resistance drops. Our results compare well to theoretical predictions of magnetic domain walls coinciding with the intrinsic insulating phase.

Phase-field-crystal model for magnetocrystalline interactions in isotropic ferromagnetic solids

NASA Astrophysics Data System (ADS)

Faghihi, Niloufar; Provatas, Nikolas; Elder, K. R.; Grant, Martin; Karttunen, Mikko

2013-09-01

An isotropic magnetoelastic phase-field-crystal model to study the relation between morphological structure and magnetic properties of pure ferromagnetic solids is introduced. Analytic calculations in two dimensions were used to determine the phase diagram and obtain the relationship between elastic strains and magnetization. Time-dependent numerical simulations in two dimensions were used to demonstrate the effect of grain boundaries on the formation of magnetic domains. It was shown that the grain boundaries act as nucleating sites for domains of reverse magnetization. Finally, we derive a relation for coercivity versus grain misorientation in the isotropic limit.

Interplay between current driven ferromagnetism in charge ordered antiferromagnetic Pr0.5Ca0.5MnO3 and superconducting YBa2Cu3O7-δ thin film multilayer

NASA Astrophysics Data System (ADS)

Baisnab, Dipak Kumar; Sardar, Manas; Amaladass, E. P.; Vaidhyanathan, L. S.; Baskaran, R.

2018-07-01

Thin film multilayer heterostructure of alternate YBa2Cu3O7-δ (YBCO) and Pr0.5Ca0.5MnO3 (PCMO) with thickness of each layer ∼60 nm has been deposited on (100) oriented SrTiO3 substrate by Pulsed Laser Deposition technique. A half portion of the base YBCO layer was masked in situ using mechanical shadow mask and in the remaining half portion, five alternate layers of PCMO and YBCO thin films were deposited. Magnetoresistance measurements were carried out under externally applied magnetic field and injection current. A noticeable damped oscillation of the superconducting transition temperature (TC) of this multilayer with respect to magnetic field is seen. Curiously, the field at which the first minimum in TC occurs, decreases as an injection current is driven perpendicular/parallel to the multilayers. Both these phenomena indicate that ferromagnetic correlation can be induced in antiferromagnetic PCMO thin films by (1) external magnetic field, or (2) injection current. While (1) is well researched, our study indicates that ferromagnetism can be induced by small amount of current in PCMO thin films. This unusual behavior points towards the strongly correlated nature of electrons in PCMO.

Observation of ferromagnetism in Mn doped KNbO3

NASA Astrophysics Data System (ADS)

Manikandan, M.; Venkateswaran, C.

2015-06-01

Pure and Mn doped KNbO3 have been prepared by ball milling assisted ceramic method. Mn ion had been doped at Nb site to induce ferromagnetism at room temperature. X-ray diffraction (XRD) patterns reveal the formation of orthorhombic phase. High resolution scanning electron micrograph (HR-SEM) of both pure and Mn doped samples show a mixture of spherical and plate like particles. Room temperature magnetic behavior of both the samples were analyzed using vibrating sample magnetometer (VSM). 5% Mn doped KNbO3 exhibits ferromagnetic behavior. Observed ferromagnetic feature has been explained by interactions between bound magnetic polarons which are created by Mn4+ ions.

Dynamics of heavy carriers in the ferromagnetic superconductor UGe2

NASA Astrophysics Data System (ADS)

Storchak, V. G.; Brewer, J. H.; Eshchenko, D. G.; Mengyan, P. W.; Parfenov, O. E.; Tokmachev, A. M.

2018-04-01

Superconductivity and ferromagnetism in a number of uranium-based materials come from the same f-electrons with a relatively large effective mass, suggesting the presence of a band of heavy quasiparticles, whose nature is still a mystery. Here, UGe2 dynamics in both ferromagnetic and paramagnetic phases is studied employing high-field μ +SR spectroscopy. The spectra exhibit a doublet structure characteristic to formation of subnanometer-sized magnetic polarons. This model is thoroughly explored here and correlated with the unconventional physics of UGe2. The heavy-fermion behaviour is ascribed to magnetic polarons; when coherent they form a narrow band, thus reconciling heavy carriers with superconductivity and itinerant ferromagnetism.

Effect of oxygen vacancies on magnetic and transport properties of Sr2IrO4

NASA Astrophysics Data System (ADS)

Dwivedi, Vinod Kumar; Mukhopadhyay, Soumik

2018-05-01

Iridates have recently attracted growing interest because of their potential for realizing various interesting phases like interaction driven Mott-type insulator and magnetically driven Slater-type. In this paper, we present the magnetic and electrical transport properties of polycrystalline Sr2IrO4 synthesized by solid state reaction route. We find a ferromagnetic transition at 240 K. The Curie-Weiss law behavior hold good above the magnetic transition temperature TMag = 240 K with a small effective paramagnetic magnetic moment μeff = 0.25 µB/f.u. and a Curie-Weiss temperature, θCW = +100 K. Zero field cooled (ZFC) magnetization shows a gradual dcrease below 150 K, while same for field cooled (FC) below 50 K. Interestingly, below temperatures, ⁓ 10 K, a sharp increase in ZFC and FC magnetization can be seen. A temperature dependent resistivity reveals insulating behavior followed by power law mechanism. The sintering of sample in air leads to the very low value of resistivity is likely related to Sr or oxygen vacancies.

Stability of the antiferromagnetic state in the electron doped iridates

NASA Astrophysics Data System (ADS)

Bhowal, Sayantika; Moradi Kurdestany, Jamshid; Satpathy, Sashi

2018-06-01

Iridates such as Sr2IrO4 are of considerable interest owing to the formation of the Mott insulating state driven by a large spin–orbit coupling. However, in contrast to the expectation from the Nagaoka theorem that a single doped hole or electron destroys the anti-ferromagnetic (AFM) state of the half-filled Hubbard model in the large U limit, the anti-ferromagnetism persists in the doped Iridates for a large dopant concentration beyond half-filling. With a tight-binding description of the relevant states by the third-neighbor (t 1, t 2, t 3, U) Hubbard model on the square lattice, we examine the stability of the AFM state to the formation of a spin spiral state in the strong coupling limit. The third-neighbor interaction t 3 is important for the description of the Fermi surface of the electron doped system. A phase diagram in the parameter space is obtained for the regions of stability of the AFM state. Our results qualitatively explain the robustness of the AFM state in the electron doped iridate (such as Sr2‑x La x IrO4), observed in many experiments, where the AFM state continues to be stable until a critical dopant concentration.

Ferromagnetism in spin-coated cobalt-doped TiO2 thin films and the role of crystalline phases

NASA Astrophysics Data System (ADS)

Salazar Cuaila, J. L.; Alayo, W.; Avellaneda, César O.

2017-11-01

Two sets of Cobalt-doped (1-10% at) TiO2 thin films, for different molar concentrations of the Ti precursor (0.3 and 0.5 mol/L), have been deposited onto Si substrates by combining the Sol Gel process and the Spin Coating technique. The structure of the samples was studied by X-ray reflectivity (XRR) and X-ray diffraction (XRD) and their magnetic properties were analyzed by magnetization measurements as a function of the applied magnetic field. The XRR results provided the thickness and interfacial roughness of the films, while XRD patterns revealed the crystalline phases and lattice parameters. Room temperature ferromagnetic behaviour was observed for some of the atomic Co concentrations by the magnetization measurements. This behaviour has been correlated to the crystalline phases, which were found to be modified by both the molar ratio of Ti precursor and the concentration of the Co dopant. A suppression of ferromagnetism is observed for some atomic Co fractions and it was attributed to the presence of secondary crystalline phases.

Evolution of ferromagnetism in charge ordered manganite: An effect of external pressure

NASA Astrophysics Data System (ADS)

Dash, S.; Pradhan, M. K.; Rao, T. Lakshmana

2018-05-01

Detailed magnetic measurements of the Pr0.75Na0.25MnO3 polycrystalline sample have been carried out under external hydrostatic pressure upto 10kbar. Pressure strongly suppresses the first order magnetic transition, while thermal hysteresis narrows down progressively and then disappears with increase in pressure. The significant enhancement of the field cooled magnetization value at different pressures is due to the antiferromagnetic to ferromagnetic transformation, while ruling out any contribution from the domain alignment within the ferromagnetic phase.

Spin wave scattering and interference in ferromagnetic cross

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

Nanayakkara, Kasuni; Kozhanov, Alexander; Center for Nano Optics, Georgia State University, Atlanta, Georgia 30303

2015-10-28

Magnetostatic spin wave scattering and interference across a CoTaZr ferromagnetic spin wave waveguide cross junction were investigated experimentally and by micromagnetic simulations. It is observed that the phase of the scattered waves is dependent on the wavelength, geometry of the junction, and scattering direction. It is found that destructive and constructive interference of the spin waves generates switching characteristics modulated by the input phase of the spin waves. Micromagnetic simulations are used to analyze experimental data and simulate the spin wave scattering and interference.

Spin flip statistics and spin wave interference patterns in Ising ferromagnetic films: A Monte Carlo study.

PubMed

Acharyya, Muktish

2017-07-01

The spin wave interference is studied in two dimensional Ising ferromagnet driven by two coherent spherical magnetic field waves by Monte Carlo simulation. The spin waves are found to propagate and interfere according to the classic rule of interference pattern generated by two point sources. The interference pattern of spin wave is observed in one boundary of the lattice. The interference pattern is detected and studied by spin flip statistics at high and low temperatures. The destructive interference is manifested as the large number of spin flips and vice versa.

Ultrafast spin dynamics and switching via spin transfer torque in antiferromagnets with weak ferromagnetism

PubMed Central

Kim, Tae Heon; Grünberg, Peter; Han, Song Hee; Cho, Beongki

2016-01-01

The spin-torque driven dynamics of antiferromagnets with Dzyaloshinskii-Moriya interaction (DMI) were investigated based on the Landau-Lifshitz-Gilbert-Slonczewski equation with antiferromagnetic and ferromagnetic order parameters (l and m, respectively). We demonstrate that antiferromagnets including DMI can be described by a 2-dimensional pendulum model of l. Because m is coupled with l, together with DMI and exchange energy, close examination of m provides fundamental understanding of its dynamics in linear and nonlinear regimes. Furthermore, we discuss magnetization reversal as a function of DMI and anisotropy energy induced by a spin current pulse. PMID:27713522

Ferromagnetism regulated by edged cutting and optical identification in monolayer PtSe2 nanoribbons

NASA Astrophysics Data System (ADS)

Meng, Ming; Zhang, QiZhen; Wang, Lifen; Shan, Yun; Du, Yuandong; Qin, Nan; Liu, Lizhe

2018-06-01

Regulation of ferromagnetism and electronic structure in PtSe2 nanostructures has attracted much attention because of its potential in spintronics. The magnetic and optical properties of PtSe2 nanoribbons with different edge reconstruction and external deformations are calculated by density function theory. In 1 T phase PtSe2 nanoribbons, the ferromagnetism induced by spin polarization of exposed Pt or Se atoms is decreased with the reducing nanoribbon width. For smaller nanoribbon, the magnetism can be regulated by external strain more easily. However, the magnetism cannot occur in 1 H phase PtSe2 nanoribbon. The absorption spectra are suggested to identify the nanoribbon structural changes in detail. Our results suggest the use of edge reconstruction and strain engineering in spintronics applications.

Mode selection of magneto-Rayleigh-Taylor instability from the point of view of Landau phase transition theory

NASA Astrophysics Data System (ADS)

Dan, Jia Kun; Huang, Xian Bin; Ren, Xiao Dong; Wei, Bing

2017-08-01

A theoretical model referring to mode selection of Z-pinch-driven magneto-Rayleigh-Taylor (MRT) instability, which explains the generation of fundamental instability mode and evolution of fundamental wavelength in experiments, is proposed on the basis of the Landau theory of phase transition. The basic idea of this phase transition model lies in that the appearance of MRT instability pattern can be considered as a consequence of the spontaneous generation of interfacial structure like the spontaneous magnetization in a ferromagnetic system. It is demonstrated that the amplitude of instability is responsible for the order parameter in the Landau theory of phase transition and the fundamental wavelength appears to play a role analogous to inverse temperature in thermodynamics. Further analysis indicates that the MRT instability is characterized by first order phase transition and the fundamental wavelength is proportional to the square root of energy entering into the system from the driving source. The theory predicts that the fundamental wavelength grows rapidly and saturates reaching a limiting wavelength of the order of the liner's final outer radius. The results given by this theory show qualitative agreement with the available experimental data of MRT instability of liner implosions conducted on the Sandia Z machine as well as Primary Test Stand facility at the Institute of Fluid Physics.

Remanence carrying minerals in meteorites: a journey through an exotic jungle

NASA Astrophysics Data System (ADS)

Rochette, P.; Gattacceca, J.; Uehara, M.

2011-12-01

Well-known remanence carrying minerals in meteorites are magnetite and pyrrhotite, familiar on Earth, and Fe-Ni metal alloys. In Fe-Ni metal the difficulty in interpreting paleomagnetic data is due to the presence of multiple metastable phases which follow complex transformation paths during thermal treatment. A minor phase, tetrataenite (ordered Fe0.5Ni0.5), usually carries most of the remanence [1]. It is intimately mixed with high susceptibility phases (kamacite and taenite), implying strong interaction effects. FeNi phosphide and carbide (schreibersite and cohenite), often associated with metal, are usually overlooked although they may be responsible for the remanence of enstatite chondrites and some lunar basalts, with Tc around 200°C. They are also likely responsible for the claim of "magnetic carbon" found in Canyon Diablo meteorite [2]. Sulfides, a wide variety of which occurs in meteorites, provide even more thrill. Concerning pyrrhotite, there is still imperfect understanding of the observation that not monoclinic but hexagonal pyrrhotite is the ferromagnetic phase present in some martian meteorites and Rumuruti chondrites. The most common sulfide in meteorites, troilite (FeS), is an antiferromagnet (TN= 320°C), showing a susceptibility anomaly at 140°C. Recently a transition toward weak ferromagnetism has been proposed below 60-70 K [3]. However it has been shown subsequently that this weak ferromagnetism is due to impurities of chromite [4] an ubiquitous phase in meteorites that becomes ferromagnetic below a Tc of 40 to 150 K (a wide range linked to the various possible substitutions). Other sulfides found in meteorites show low temperature transitions. Alabandite ( (Fe,Mn)S) and Daubreelite (FeCr2S4) have been reviewed in [3]. Chalcopyrite (FeCuS2), an antiferromagnet at room temperature, shows magnetic ordering of Cu+ ions at 50 K with appearance of weak ferromagnetism [5]. Magnetic properties of cubanite (Fe2CuS3), a RT ferrimagnet found in CI chondrites and Martian meteorites will also be presented [6]. Most cited minerals exhibit high pressure phase transitions in the 3-5 GPa range and thus are remagnetized by moderate impact.

Intrinsic and spatially nonuniform ferromagnetism in Co-doped ZnO films

NASA Astrophysics Data System (ADS)

Tseng, L. T.; Suter, A.; Wang, Y. R.; Xiang, F. X.; Bian, P.; Ding, X.; Tseng, A.; Hu, H. L.; Fan, H. M.; Zheng, R. K.; Wang, X. L.; Salman, Z.; Prokscha, T.; Suzuki, K.; Liu, R.; Li, S.; Morenzoni, E.; Yi, J. B.

2017-09-01

Co doped ZnO films have been deposited by a laser-molecular beam epitaxy system. X-ray diffraction and UV spectra analysis show that Co effectively substitutes the Zn site. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy analysis indicate that there are no clusters. Co dopants are uniformly distributed in ZnO film. Ferromagnetic ordering is observed in all samples deposited under an oxygen partial pressure, PO2=10-3 , 10-5, and 10-7 torr, respectively. However, the magnetization of PO2=10-3 and 10-5 is very small at room temperature. At low temperature, the ferromagnetic ordering is enhanced. Muon spin relaxation (μ SR ) measurements confirm the ferromagnetism in all samples, and the results are consistent with magnetization measurements. From μ SR and TEM analysis, the film deposited under PO2=10-7 torr shows intrinsic ferromagnetism. However, the volume fraction of the ferromagnetism phase is approximately 70%, suggesting that the ferromagnetism is not carrier mediated. Resistivity versus temperature measurements indicate Efros variable range hopping dominates the conductivity. From the above results, we can confirm that a bound magnetic polaron is the origin of the ferromagnetism.

Current phase relation from graphs and diagrams and application to thick ferromagnetic Josephson junctions

NASA Astrophysics Data System (ADS)

Margaris, I.; Paltoglou, V.; Flytzanis, N.

2018-05-01

In this work we present a method of representing terms in the current-phase-relation of a ballistic Josephson junction by combinations of diagrams, used in previous work to represent an equivalent of the matching condition determinant of the junction. This is accomplished by the expansion of the logarithm of this determinant in Taylor series and keeping track of surviving terms, i.e. terms that do not annihilate each other. The types of the surviving terms are represented by connected graphs, whose points represent diagrammatic terms of the determinant expansion. Then the theory is applied to obtain approximations of the current-phase relation of relatively thick ballistic ferromagnetic Josephson junctions with non-collinear magnetizations. This demonstrates the versatility of the method in developing approximations schemes and providing physical insight into the nature of contributions to the supercurrent from the available particle excitations in the junction. We also discuss the strong second harmonic contribution to the supercurrent in junctions with three mutually orthogonal magnetization vectors and a weak intermediate ferromagnet.

Phase investigation in Pt supported off-stoichiometric iron-platinum thin films

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

Gupta, Rekha; Medwal, Rohit; Annapoorni, S., E-mail: annapoornis@yahoo.co.in

2013-10-15

Graphical abstract: - Highlights: • Low temperature FePt L1{sub 0} phase transformation using Pt/Fe{sub 3}Pt/Pt structure. • Temperature dependent FCC to FCT phase investigation using Rietveld refinement. • Estimation of soft and hard ferromagnetic contribution from demagnetization curve. • Interlayer diffusion and stoichiometry conformation of L1{sub 0} phase using RBS. • Correlation of structural, magnetic and RBS studies were successfully understood. - Abstract: The structural and magnetic phase transformation of Pt/Fe{sub 3}Pt/Pt films on Si <1 0 0> substrates prepared by DC magnetron sputtering is investigated as a function of annealing temperature. Pt diffusion driven low temperature phase transformation frommore » A1 to L1{sub 0} phase is achieved at 300 °C, attaining a very high coercivity of 9 kOe. At 300 °C, 85% L1{sub 0} phase transformation is observed using the X-ray diffraction profile fitting. The estimated phase content is also further verified by fitting the demagnetization curve. The underlayer promotes the ordering at lower temperature while overlayer induces growth along (0 0 1) preferred orientation. Rutherford back scattering study reveals interlayer diffusion and confirms the desired stoichiometry for L1{sub 0} phase. The presence of Pt under-overlayer provides the Pt source and further facilitates the Pt diffusion, which makes it effective in promoting the phase ordering at a lower temperature.« less

Electric-field control of magnetic moment in Pd

PubMed Central

Obinata, Aya; Hibino, Yuki; Hayakawa, Daichi; Koyama, Tomohiro; Miwa, Kazumoto; Ono, Shimpei; Chiba, Daichi

2015-01-01

Several magnetic properties have recently become tunable with an applied electric field. Particularly, electrically controlled magnetic phase transitions and/or magnetic moments have attracted attention because they are the most fundamental parameters in ferromagnetic materials. In this study, we showed that an electric field can be used to control the magnetic moment in films made of Pd, usually a non-magnetic element. Pd ultra-thin films were deposited on ferromagnetic Pt/Co layers. In the Pd layer, a ferromagnetically ordered magnetic moment was induced by the ferromagnetic proximity effect. By applying an electric field to the ferromagnetic surface of this Pd layer, a clear change was observed in the magnetic moment, which was measured directly using a superconducting quantum interference device magnetometer. The results indicate that magnetic moments extrinsically induced in non-magnetic elements by the proximity effect, as well as an intrinsically induced magnetic moments in ferromagnetic elements, as reported previously, are electrically tunable. The results of this study suggest a new avenue for answering the fundamental question of “can an electric field make naturally non-magnetic materials ferromagnetic?” PMID:26391306

Current-driven second-harmonic domain wall resonance in ferromagnetic metal/nonmagnetic metal bilayers: A field-free method for spin Hall angle measurements

NASA Astrophysics Data System (ADS)

Hajiali, M. R.; Hamdi, M.; Roozmeh, S. E.; Mohseni, S. M.

2017-10-01

We study the ac current-driven domain wall motion in bilayer ferromagnetic metal (FM)/nonmagnetic metal (NM) nanowires. The solution of the modified Landau-Lifshitz-Gilbert equation including all the spin transfer torques is used to describe motion of the domain wall in the presence of the spin Hall effect. We show that the domain wall center has a second-harmonic frequency response in addition to the known first-harmonic excitation. In contrast to the experimentally observed second-harmonic response in harmonic Hall measurements of spin-orbit torque in magnetic thin films, this second-harmonic response directly originates from spin-orbit torque driven domain wall dynamics. Based on the spin current generated by domain wall dynamics, the longitudinal spin motive force generated voltage across the length of the nanowire is determined. The second-harmonic response introduces additionally a practical field-free and all-electrical method to probe the effective spin Hall angle for FM/NM bilayer structures that could be applied in experiments. Our results also demonstrate the capability of utilizing FM/NM bilayer structures in domain wall based spin-torque signal generators and resonators.

Experimental and theoretical investigations on magnetic behavior of (Al,Co) co-doped ZnO nanoparticles.

PubMed

Jayakumar, O D; Achary, S N; Sudakar, C; Naik, R; Salunke, H G; Rao, Rekha; Peng, X; Ahuja, R; Tyagi, A K

2010-08-01

We present the structural and magnetic properties of Zn(0.95-x)Co(0.05)Al(x)O (x = 0.0 to 0.1) nanoparticles, synthesized by a novel sol-gel route followed by pyrolysis. Powder X-ray diffraction data confirms the formation of a single phase wurtzite type ZnO structure for all the compositions. The Zn(0.95)Co(0.05)O nanoparticles show diamagnetic behavior at room temperature. However, when Al is co-doped with Co with x = 0.0 to 0.10 in Zn(0.95-x)Co(0.05)Al(x)O, a systematic increase in ferromagnetic moment is observed up to x = 0.07 at 300 K. Above x = 0.07 (e.g. for x = 0.10) a drastic decrease in ferromagnetic nature is observed which is concomitant with the segregation of poorly crystalline Al rich ZnO phase as evidenced from TEM studies. Theoretical studies using density functional calculations on Zn(0.95-x)Co(0.05)Al(x)O suggest that the partial occupancy of S2 states leads to an increased double exchange interaction favoring the ferromagnetic ground states. Such ferromagnetic interactions are favorable beyond a threshold limit. At a high level doping of Al, the exchange splitting is reduced, which suppresses the ferromagnetic ordering.

Strain-Induced Extrinsic High-Temperature Ferromagnetism in the Fe-Doped Hexagonal Barium Titanate

PubMed Central

Zorko, A.; Pregelj, M.; Gomilšek, M.; Jagličić, Z.; Pajić, D.; Telling, M.; Arčon, I.; Mikulska, I.; Valant, M.

2015-01-01

Diluted magnetic semiconductors possessing intrinsic static magnetism at high temperatures represent a promising class of multifunctional materials with high application potential in spintronics and magneto-optics. In the hexagonal Fe-doped diluted magnetic oxide, 6H-BaTiO3-δ, room-temperature ferromagnetism has been previously reported. Ferromagnetism is broadly accepted as an intrinsic property of this material, despite its unusual dependence on doping concentration and processing conditions. However, the here reported combination of bulk magnetization and complementary in-depth local-probe electron spin resonance and muon spin relaxation measurements, challenges this conjecture. While a ferromagnetic transition occurs around 700 K, it does so only in additionally annealed samples and is accompanied by an extremely small average value of the ordered magnetic moment. Furthermore, several additional magnetic instabilities are detected at lower temperatures. These coincide with electronic instabilities of the Fe-doped 3C-BaTiO3-δ pseudocubic polymorph. Moreover, the distribution of iron dopants with frozen magnetic moments is found to be non-uniform. Our results demonstrate that the intricate static magnetism of the hexagonal phase is not intrinsic, but rather stems from sparse strain-induced pseudocubic regions. We point out the vital role of internal strain in establishing defect ferromagnetism in systems with competing structural phases. PMID:25572803

Active magnetic refrigerants based on Gd-Si-Ge material and refrigeration apparatus and process

DOEpatents

Gschneidner, Jr., Karl A.; Pecharsky, Vitalij K.

1998-04-28

Active magnetic regenerator and method using Gd.sub.5 (Si.sub.x Ge.sub.1-x).sub.4, where x is equal to or less than 0.5, as a magnetic refrigerant that exhibits a reversible ferromagnetic/antiferromagnetic or ferromagnetic-II/ferromagnetic-I first order phase transition and extraordinary magneto-thermal properties, such as a giant magnetocaloric effect, that renders the refrigerant more efficient and useful than existing magnetic refrigerants for commercialization of magnetic regenerators. The reversible first order phase transition is tunable from approximately 30 K to approximately 290 K (near room temperature) and above by compositional adjustments. The active magnetic regenerator and method can function for refrigerating, air conditioning, and liquefying low temperature cryogens with significantly improved efficiency and operating temperature range from approximately 10 K to 300 K and above. Also an active magnetic regenerator and method using Gd.sub.5 (Si.sub.x Ge.sub.1-x).sub.4, where x is equal to or greater than 0.5, as a magnetic heater/refrigerant that exhibits a reversible ferromagnetic/paramagnetic second order phase transition with large magneto-thermal properties, such as a large magnetocaloric effect that permits the commercialization of a magnetic heat pump and/or refrigerant. This second order phase transition is tunable from approximately 280 K (near room temperature) to approximately 350 K by composition adjustments. The active magnetic regenerator and method can function for low level heating for climate control for buildings, homes and automobile, and chemical processing.

Active magnetic refrigerants based on Gd-Si-Ge material and refrigeration apparatus and process

DOEpatents

Gschneidner, K.A. Jr.; Pecharsky, V.K.

1998-04-28

Active magnetic regenerator and method using Gd{sub 5} (Si{sub x}Ge{sub 1{minus}x}){sub 4}, where x is equal to or less than 0.5, as a magnetic refrigerant that exhibits a reversible ferromagnetic/antiferromagnetic or ferromagnetic-II/ferromagnetic-I first order phase transition and extraordinary magneto-thermal properties, such as a giant magnetocaloric effect, that renders the refrigerant more efficient and useful than existing magnetic refrigerants for commercialization of magnetic regenerators. The reversible first order phase transition is tunable from approximately 30 K to approximately 290 K (near room temperature) and above by compositional adjustments. The active magnetic regenerator and method can function for refrigerating, air conditioning, and liquefying low temperature cryogens with significantly improved efficiency and operating temperature range from approximately 10 K to 300 K and above. Also an active magnetic regenerator and method using Gd{sub 5} (Si{sub x} Ge{sub 1{minus}x}){sub 4}, where x is equal to or greater than 0.5, as a magnetic heater/refrigerant that exhibits a reversible ferromagnetic/paramagnetic second order phase transition with large magneto-thermal properties, such as a large magnetocaloric effect that permits the commercialization of a magnetic heat pump and/or refrigerant. This second order phase transition is tunable from approximately 280 K (near room temperature) to approximately 350 K by composition adjustments. The active magnetic regenerator and method can function for low level heating for climate control for buildings, homes and automobile, and chemical processing. 27 figs.

Transition from orbital liquid to Jahn-Teller insulator in orthorhombic perovskites RTiO3.

PubMed

Cheng, J-G; Sui, Y; Zhou, J-S; Goodenough, J B; Su, W H

2008-08-22

Following the same strategy used for RVO3, thermal conductivity measurements have been made on a series of single-crystal perovskites RTiO3 (R=La,Nd,...,Yb). Results reveal explicitly a transition from an orbital liquid to an orbitally ordered phase at a magnetic transition temperature, which is common for both the antiferromagnetic and ferromagnetic phases in the phase diagram of RTiO3. This spin/orbital transition is consistent with the mode softening at T_{N} in antiferromagnetic LaTiO3 and is supported by an anomalous critical behavior at T_{c} in ferromagnetic YTiO3.

First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi(1-x)Ge(x).

PubMed

Yeo, S; Nakatsuji, S; Bianchi, A D; Schlottmann, P; Fisk, Z; Balicas, L; Stampe, P A; Kennedy, R J

2003-07-25

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.

Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

NASA Astrophysics Data System (ADS)

White, J. S.; Butykai, Á.; Cubitt, R.; Honecker, D.; Dewhurst, C. D.; Kiss, L. F.; Tsurkan, V.; Bordács, S.

2018-01-01

We report small-angle neutron scattering studies of the lacunar spinel GaV4S8 , which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a recently proposed Néel-type skyrmion phase, transform into a simple ferromagnetic state. These results indicate the modulated phases in GaV4S8 gain their stability from thermal fluctuations, while at lower temperatures the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. Crucially, our study provides microscopic evidence that the skyrmions in GaV4S8 indeed display a Néel-type helicity. More generally, our approach can be applied to evidence the helicity of any bulk skyrmion phase.

Eigenstate Phase Transitions

NASA Astrophysics Data System (ADS)

Zhao, Bo

Phase transitions are one of the most exciting physical phenomena ever discovered. The understanding of phase transitions has long been of interest. Recently eigenstate phase transitions have been discovered and studied; they are drastically different from traditional thermal phase transitions. In eigenstate phase transitions, a sharp change is exhibited in properties of the many-body eigenstates of the Hamiltonian of a quantum system, but not the thermal equilibrium properties of the same system. In this thesis, we study two different types of eigenstate phase transitions. The first is the eigenstate phase transition within the ferromagnetic phase of an infinite-range spin model. By studying the interplay of the eigenstate thermalization hypothesis and Ising symmetry breaking, we find two eigenstate phase transitions within the ferromagnetic phase: In the lowest-temperature phase the magnetization can macroscopically oscillate by quantum tunneling between up and down. The relaxation of the magnetization is always overdamped in the remainder of the ferromagnetic phase, which is further divided into phases where the system thermally activates itself over the barrier between the up and down states, and where it quantum tunnels. The second is the many-body localization phase transition. The eigenstates on one side of the transition obey the eigenstate thermalization hypothesis; the eigenstates on the other side are many-body localized, and thus thermal equilibrium need not be achieved for an initial state even after evolving for an arbitrary long time. We study this many-body localization phase transition in the strong disorder renormalization group framework. After setting up a set of coarse-graining rules for a general one dimensional chain, we get a simple "toy model'' and obtain an almost purely analytical solution to the infinite-randomness critical fixed point renormalization group equation. We also get an estimate of the correlation length critical exponent nu ≈ 2.5.

Superfluid-ferromagnet-superfluid junction and the {pi} phase in a superfluid Fermi gas

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

Kashimura, Takashi; Tsuchiya, Shunji; CREST

2010-09-15

We investigate the possibility of a superfluid-ferromagnet-superfluid (SFS) junction in a superfluid Fermi gas. To examine this possibility in a simple manner, we consider an attractive Hubbard model at T=0 within the mean-field theory. When a potential barrier is embedded in a superfluid Fermi gas with population imbalance (N{sub {up_arrow}}>N{sub {down_arrow}}, where N{sub {sigma}} is the number of atoms with pseudospin {sigma}= {up_arrow}, {down_arrow}), this barrier is shown to be magnetized in the sense that excess {up_arrow}-spin atoms are localized around it. The resulting superfluid Fermi gas is spatially divided into two by this ferromagnet, so that one obtains amore » junction similar to the superconductor-ferromagnet-superconductor junction discussed in superconductivity. Indeed, we show that the so-called {pi} phase, which is a typical phenomenon in the SFS junction, is realized, where the superfluid order parameter changes its sign across the junction. Our results would be useful for the study of magnetic effects on fermion superfluidity using an ultracold Fermi gas.« less

Strain-tuned enhancement of ferromagnetic TC to 176 K in Sm-doped BiMnO3 thin films and determination of magnetic phase diagram.

PubMed

Choi, Eun-Mi; Kleibeuker, Josée E; MacManus-Driscoll, Judith L

2017-03-03

BiMnO 3 is a promising multiferroic material but it's ferromagnetic T C is well below room temperature and the magnetic phase diagram is unknown. In this work, the relationship between magnetic transition temperature (T C ) and the substrate induced (pseudo-) tetragonal distortion (ratio of out-of-plane to in-plane lattice parameters, c/a) in BiMnO 3 thin films, lightly doped to optimize lattice dimensions, was determined. For c/a > 0.99, hidden antiferromagnetism was revealed and the magnetisation versus temperature curves showed a tail behaviour, whereas for c/a < 0.99 clear ferromagnetism was observed. A peak T C of up to 176 K, more than 70 K higher than for bulk BiMnO 3 , was achieved through precise strain tuning. The T C was maximised for strong tensile in-plane strain which produced weak octahedral rotations in the out-of-plane direction, an orthorhombic-like structure, and strong ferromagnetic coupling.

Persistent ferromagnetism and topological phase transition at the interface of a superconductor and a topological insulator.

PubMed

Qin, Wei; Zhang, Zhenyu

2014-12-31

At the interface of an s-wave superconductor and a three-dimensional topological insulator, Majorana zero modes and Majorana helical states have been proposed to exist respectively around magnetic vortices and geometrical edges. Here we first show that randomly distributed magnetic impurities at such an interface will induce bound states that broaden into impurity bands inside (but near the edges of) the superconducting gap, which remains open unless the impurity concentration is too high. Next we find that an increase in the superconducting gap suppresses both the oscillation magnitude and the period of the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities. Within a mean-field approximation, the ferromagnetic Curie temperature is found to be essentially independent of the superconducting gap, an intriguing phenomenon due to a compensation effect between the short-range ferromagnetic and long-range antiferromagnetic interactions. The existence of robust superconductivity and persistent ferromagnetism at the interface allows realization of a novel topological phase transition from a nonchiral to a chiral superconducting state at sufficiently low temperatures, providing a new platform for topological quantum computation.

Preamble

NASA Astrophysics Data System (ADS)

Schmid, Hans

2015-03-01

What does the word "multiferroic" mean? When coining the term in 1994, this was done without ulterior motives, simply by enjoying the beauty of symmetries, permitting the cohabitation of two or all three primary ferroic properties, ferroelectric, ferromagnetic, and ferroelastic in the same phase, all forming domains and hysteresis loops, heralding intricate coupling properties. In this definition, the very cradle of "multiferroics" lies in Kêitsiro Aizu's colossal work of 1970, in which he gave all the 773 high-temperature (prototypic) phase/low-temperature (ferroic) phase point group pairs ("species") occurring due to phase transitions, with indication of the number of domain states, comprising all possible single-phase multiferroic combinations, including implicitly also antiferromagnetic and co-elastic ferroic phases. The original meaning of the word "multiferroic" has undergone a tacit, more realistic mutation. In its current usage, it designates all kinds of magnetic ferroelectrics, including hetero-phase systems, but with a mainstream trend in electric-field control of ferromagnetism and in all sorts of magnetoelectric interactions.

Thermodynamics around the first-order ferromagnetic phase transition of Fe2P single crystals

NASA Astrophysics Data System (ADS)

Hudl, M.; Campanini, D.; Caron, L.; Höglin, V.; Sahlberg, M.; Nordblad, P.; Rydh, A.

2014-10-01

The specific heat and thermodynamics of Fe2P single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at TC≃217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H -T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

Temperature dependence of ferromagnet-antiferromagnet spin alignment and coercivity in epitaxial micromagnet bilayers

DOE PAGES

Lee, Michael S.; Wynn, Thomas A.; Folven, Erik; ...

2017-06-26

In this paper, soft x-ray photoemission electron microscopy with an in situ magnetic field has been used to study the relationship between ferromagnetic and antiferromagnetic spin alignment and the switching/reversal field of epitaxial micromagnetic structures. We investigated a model system consisting of a bilayer of ferromagnetic La 0.7Sr 0.3MnO 3 and antiferromagnetic LaFeO 3 where the spin axes in each layer can be driven from mutually perpendicular (spin-flop) to parallel alignment by varying the temperature between 30 and 300 K. Results show that not only does this spin alignment noticeably influence the bilayer micromagnet coercivity compared to La 0.7Sr 0.3MnOmore » 3 single-layer micromagnets, but the coercivity within this materials system can be tuned over a wide range by careful balance of material properties.« less

Compact pulse generators with soft ferromagnetic cores driven by gunpowder and explosive.

PubMed

Ben, Chi; He, Yong; Pan, Xuchao; Chen, Hong; He, Yuan

2015-12-01

Compact pulse generators which utilized soft ferromagnets as an initial energy carrier inside multi-turn coil and hard ferromagnets to provide the initial magnetic field outside the coil have been studied. Two methods of reducing the magnetic flux in the generators have been studied: (1) by igniting gunpowder to launch the core out of the generator, and (2) by detonating explosives that demagnetize the core. Several types of compact generators were explored to verify the feasibility. The generators with an 80-turn coil that utilize gunpowder were capable of producing pulses with amplitude 78.6 V and the full width at half maximum was 0.41 ms. The generators with a 37-turn coil that utilize explosive were capable of producing pulses with amplitude 1.41 kV and the full width at half maximum was 11.68 μs. These two methods were both successful, but produce voltage waveforms with significantly different characteristics.

Fast nanoscale addressability of nitrogen-vacancy spins via coupling to a dynamic ferromagnetic vortex

PubMed Central

Wolf, M. S.; Badea, R.; Berezovsky, J.

2016-01-01

The core of a ferromagnetic vortex domain creates a strong, localized magnetic field, which can be manipulated on nanosecond timescales, providing a platform for addressing and controlling individual nitrogen-vacancy centre spins in diamond at room temperature, with nanometre-scale resolution. Here, we show that the ferromagnetic vortex can be driven into proximity with a nitrogen-vacancy defect using small applied magnetic fields, inducing significant nitrogen-vacancy spin splitting. We also find that the magnetic field gradient produced by the vortex is sufficient to address spins separated by nanometre-length scales. By applying a microwave-frequency magnetic field, we drive both the vortex and the nitrogen-vacancy spins, resulting in enhanced coherent rotation of the spin state. Finally, we demonstrate that by driving the vortex on fast timescales, sequential addressing and coherent manipulation of spins is possible on ∼100 ns timescales. PMID:27296550

Temperature dependence of ferromagnet-antiferromagnet spin alignment and coercivity in epitaxial micromagnet bilayers

NASA Astrophysics Data System (ADS)

Lee, Michael S.; Wynn, Thomas A.; Folven, Erik; Chopdekar, Rajesh V.; Scholl, Andreas; Retterer, Scott T.; Grepstad, Jostein K.; Takamura, Yayoi

2017-06-01

Soft x-ray photoemission electron microscopy with an in situ magnetic field has been used to study the relationship between ferromagnetic and antiferromagnetic spin alignment and the switching/reversal field of epitaxial micromagnetic structures. We investigated a model system consisting of a bilayer of ferromagnetic L a0.7S r0.3Mn O3 and antiferromagnetic LaFe O3 where the spin axes in each layer can be driven from mutually perpendicular (spin-flop) to parallel alignment by varying the temperature between 30 and 300 K. Results show that not only does this spin alignment noticeably influence the bilayer micromagnet coercivity compared to L a0.7S r0.3Mn O3 single-layer micromagnets, but the coercivity within this materials system can be tuned over a wide range by careful balance of material properties.

Fast nanoscale addressability of nitrogen-vacancy spins via coupling to a dynamic ferromagnetic vortex

DOE PAGES

Wolf, M. S.; Badea, R.; Berezovsky, J.

2016-06-14

The core of a ferromagnetic vortex domain creates a strong, localized magnetic field, which can be manipulated on nanosecond timescales, providing a platform for addressing and controlling individual nitrogen-vacancy centre spins in diamond at room temperature, with nanometre-scale resolution. Here, we show that the ferromagnetic vortex can be driven into proximity with a nitrogen-vacancy defect using small applied magnetic fields, inducing significant nitrogen-vacancy spin splitting. We also find that the magnetic field gradient produced by the vortex is sufficient to address spins separated by nanometre-length scales. By applying a microwave-frequency magnetic field, we drive both the vortex and the nitrogen-vacancymore » spins, resulting in enhanced coherent rotation of the spin state. Lastly, we demonstrate that by driving the vortex on fast timescales, sequential addressing and coherent manipulation of spins is possible on ~ 100 ns timescales.« less

Design of membrane actuators based on ferromagnetic shape memory alloy composite for the synthetic jet actuator

NASA Astrophysics Data System (ADS)

Liang, Yuanchang; Taya, Minoru; Kuga, Yasuo

2004-07-01

A new membrane actuator based on our previous diaphragm actuator was designed and constructed to improve the dynamic performance. The finite element analysis was used to estimate the frequency response of the composite membrane which will be driven close to its resonance to obtain a large stroke. The membrane is made of ferromagnetic shape memory alloy (FSMA) composite including a ferromagnetic soft iron pad and a superelastic grade of NiTi shape memory alloy (SMA). The actuation mechanism for the FSMA composite membrane of the actuator is the hybrid mechanism that we proposed previously. This membrane actuator is designed for a new synthetic jet actuator package that will be used for active flow control technology on airplane wings. Based on the FEM results, the new membrane actuator system was assembled and its static and dynamic performance was experimentally evaluated including the dynamic magnetic response of the hybrid magnet.

Electronic and magnetic structure of KNiF3 perovskite

NASA Astrophysics Data System (ADS)

Ricart, J. M.; Dovesi, R.; Roetti, C.; Saunders, V. R.

1995-07-01

The ground-state electronic structure of the ferromagnetic and antiferromagnetic phases of KNiF3 has been investigated using the ab initio periodic Hartree-Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is correctly predicted to be more stable than the ferromagnetic phase (0.031 eV per Ni pair at the experimental geometry). The energy difference between these phases is shown to obey a d-12 (d is the shortest Ni-Ni distance) power law, as suggested in the literature. The superexchange interaction turns out to be additive with respect to the number of Ni-Ni neighbors, as assumed in model spin Hamiltonians. Elastic properties, charge, and spin-density maps, and density of states plots are reported.

Universal Pinning Energy Barrier for Driven Domain Walls in Thin Ferromagnetic Films

NASA Astrophysics Data System (ADS)

Jeudy, V.; Mougin, A.; Bustingorry, S.; Savero Torres, W.; Gorchon, J.; Kolton, A. B.; Lemaître, A.; Jamet, J.-P.

2016-07-01

We report a comparative study of magnetic field driven domain wall motion in thin films made of different magnetic materials for a wide range of field and temperature. The full thermally activated creep motion, observed below the depinning threshold, is shown to be described by a unique universal energy barrier function. Our findings should be relevant for other systems whose dynamics can be modeled by elastic interfaces moving on disordered energy landscapes.

Coexistence of Weak Ferromagnetism and Ferroelectricity in the High Pressure LiNbO3-Type Phase of FeTiO3

NASA Astrophysics Data System (ADS)

Varga, T.; Kumar, A.; Vlahos, E.; Denev, S.; Park, M.; Hong, S.; Sanehira, T.; Wang, Y.; Fennie, C. J.; Streiffer, S. K.; Ke, X.; Schiffer, P.; Gopalan, V.; Mitchell, J. F.

2009-07-01

We report the magnetic and electrical characteristics of polycrystalline FeTiO3 synthesized at high pressure that is isostructural with acentric LiNbO3 (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate ferroelectricity at and below room temperature and weak ferromagnetism below ˜120K. These results validate symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure.

The Pressure Coefficients of the Superconducting Order Parameters at the Ground State of Ferromagnetic Superconductors

NASA Astrophysics Data System (ADS)

Konno, R.; Hatayama, N.; Chaudhury, R.

2014-04-01

We investigated the pressure coefficients of the superconducting order parameters at the ground state of ferromagnetic superconductors based on the microscopic single band model by Linder et al. The superconducting gaps (i) similar to the ones seen in the thin film of A2 phase in liquid 3He and (ii) with the line node were used. This study shows that we would be able to estimate the pressure coefficients of the superconducting and magnetic order parameters at the ground state of ferromagnetic superconductors.

Coexistence of weak ferromagnetism and ferroelectricity in the high pressure LiNbO3-type phase of FeTiO3.

PubMed

Varga, T; Kumar, A; Vlahos, E; Denev, S; Park, M; Hong, S; Sanehira, T; Wang, Y; Fennie, C J; Streiffer, S K; Ke, X; Schiffer, P; Gopalan, V; Mitchell, J F

2009-07-24

We report the magnetic and electrical characteristics of polycrystalline FeTiO_{3} synthesized at high pressure that is isostructural with acentric LiNbO_{3} (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate ferroelectricity at and below room temperature and weak ferromagnetism below approximately 120 K. These results validate symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure.

Different magnetic origins of (Mn, Fe)-codoped ZnO powders and thin films

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

Fan, Jiuping; Jiang, Fengxian; Quan, Zhiyong

2012-11-15

Graphical abstract: The effects of the sample forms, fabricated methods, and process conditions on the structural and magnetic properties of (Mn, Fe)-codoped ZnO powders and films were systematically studied. The origins of ferromagnetism in the vacuum-annealed powder and PLD-deposited film are different. The former originates from the impurities of magnetic clusters, whereas the latter comes from the almost homogenous phase. Highlights: ► The magnetic natures of Zn{sub 0.98}Mn{sub 0.01}Fe{sub 0.01}O powders and thin films come from different origins. ► The ferromagnetism of the powder is mainly from the contribution of magnetic clusters. ► Whereas the ferromagnetic behavior of the filmmore » comes from the almost homogenous phase. -- Abstract: The structural and magnetic properties of (Mn, Fe)-codoped ZnO powders as well as thin films were investigated. The X-ray diffraction and magnetic measurements indicated that the higher sintering temperature facilitates more Mn and Fe incorporation into ZnO. Magnetic measurements indicated that the powder sintered in air at 800 °C showed paramagnetic, but it exhibited obvious room temperature ferromagnetism after vacuum annealing at 600 °C. The results revealed that magnetic clusters were the major contributors to the observed ferromagnetism in vacuum-annealed Zn{sub 0.98}Mn{sub 0.01}Fe{sub 0.01}O powder. Interestingly, the room temperature ferromagnetism was also observed in the Zn{sub 0.98}Mn{sub 0.01}Fe{sub 0.01}O film deposited via pulsed laser deposition from the air-sintered paramagnetic target, but the secondary phases in the film were not detected from X-ray diffraction, transmission electron microscopy, and zero-field cooling and field cooling. Apparently, the magnetic natures of powders and films come from different origins.« less

Topological magnons in a one-dimensional itinerant flatband ferromagnet

NASA Astrophysics Data System (ADS)

Su, Xiao-Fei; Gu, Zhao-Long; Dong, Zhao-Yang; Li, Jian-Xin

2018-06-01

Different from previous scenarios that topological magnons emerge in local spin models, we propose an alternative that itinerant electron magnets can host topological magnons. A one-dimensional Tasaki model with a flatband is considered as the prototype. This model can be viewed as a quarter-filled periodic Anderson model with impurities located in between and hybridizing with the nearest-neighbor conducting electrons, together with a Hubbard repulsion for these electrons. By increasing the Hubbard interaction, the gap between the acoustic and optical magnons closes and reopens while the Berry phase of the acoustic band changes from 0 to π , leading to the occurrence of a topological transition. After this transition, there always exist in-gap edge magnonic modes, which is consistent with the bulk-edge correspondence. The Hubbard interaction-driven transition reveals a new mechanism to realize nontrivial magnon bands.

Width-Tuned Magnetic Order Oscillation on Zigzag Edges of Honeycomb Nanoribbons

DOE PAGES

Chen, Wen-Chao; Zhou, Yuan; Yu, Shun-Li; ...

2017-06-24

Quantum confinement and interference often generate exotic properties in nanostructures. One recent highlight is the experimental indication of a magnetic phase transition in zigzag-edged graphene nanoribbons at the critical ribbon width of about 7 nm [Magda, G. Z. et al. Nature 2014, 514, 608]. Here in this work, we show theoretically that with further increase in the ribbon width, the magnetic correlation of the two edges can exhibit an intriguing oscillatory behavior between antiferromagnetic and ferromagnetic, driven by acquiring the positive coherence between the two edges to lower the free energy. The oscillation effect is readily tunable in applied magneticmore » fields. In conclusion, these novel properties suggest new experimental manifestation of the edge magnetic orders in graphene nanoribbons and enhance the hopes of graphene-like spintronic nanodevices functioning at room temperature.« less

A microwave field-driven transistor-like skyrmionic device with the microwave current-assisted skyrmion creation

NASA Astrophysics Data System (ADS)

Xia, Jing; Huang, Yangqi; Zhang, Xichao; Kang, Wang; Zheng, Chentian; Liu, Xiaoxi; Zhao, Weisheng; Zhou, Yan

2017-10-01

Magnetic skyrmion is a topologically protected domain-wall structure at nanoscale, which could serve as a basic building block for advanced spintronic devices. Here, we propose a microwave field-driven skyrmionic device with the transistor-like function, where the motion of a skyrmion in a voltage-gated ferromagnetic nanotrack is studied by micromagnetic simulations. It is demonstrated that the microwave field can drive the motion of a skyrmion by exciting the propagating spin waves, and the skyrmion motion can be governed by a gate voltage. We also investigate the microwave current-assisted creation of a skyrmion to facilitate the operation of the transistor-like skyrmionic device on the source terminal. It is found that the microwave current with an appropriate frequency can reduce the threshold current density required for the creation of a skyrmion from the ferromagnetic background. The proposed transistor-like skyrmionic device operated with the microwave field and current could be useful for building future skyrmion-based circuits.

Development of compact explosively driven ferromagnetic seed source for helical magnetic flux compression generator

NASA Astrophysics Data System (ADS)

Liu, Peng; Zhang, He; Ma, Shaojie; Shi, Yunlei

2018-05-01

A compact explosively driven ferromagnetic generator (FMG) is developed for seed power source of helical magnetic flux compression generator (HMFCG). The mechanism of FMG is studied by establishing a magnetoelectric conversion model. Analytical calculations and numerical simulations are conducted on the magnetostatic field of open-circuit magnet in FMG. The calculation method for the magnet's cross-sectional magnetic flux is obtained. The pulse sources made of different materials and equipped with different initiation modes are experimentally explored. Besides, the dynamic coupling experiments of FMG and HMFCG are carried out. The results show that, N35 single-ended and double-ended initiating FMGs have an energy conversion efficiency ηt not less than 14.6% and 24.4%, respectively; FMG has an output pulse current not less than 4kA and an energy of about 3J on 320nH inductive load; HMFCG experiences energy gains of about 2-3 times. FMG and HMFCG can be coupled to form a full-blast electrical driving pulse source.

Berry phase mechanism of the anomalous Hall effect in a disordered two-dimensional magnetic semiconductor structure.

DOE PAGES

Oveshnikov, L. N.; Kulbachinskii, V. A.; Davydov, A. B.; ...

2015-11-24

In this study, the anomalous Hall effect (AHE) arises from the interplay of spin-orbit interactions and ferromagnetic order and is a potentially useful probe of electron spin polarization, especially in nanoscale systems where direct measurement is not feasible. While AHE is rather well-understood in metallic ferromagnets, much less is known about the relevance of different physical mechanisms governing AHE in insulators. As ferromagnetic insulators, but not metals, lend themselves to gatecontrol of electron spin polarization, understanding AHE in the insulating state is valuable from the point of view of spintronic applications. Among the mechanisms proposed in the literature for AHEmore » in insulators, the one related to a geometric (Berry) phase effect has been elusive in past studies. The recent discovery of quantized AHE in magnetically doped topological insulators - essentially a Berry phase effect - provides strong additional motivation to undertake more careful search for geometric phase effects in AHE in the magnetic semiconductors. Here we report our experiments on the temperature and magnetic field dependences of AHE in insulating, strongly-disordered two-dimensional Mn delta-doped semiconductor heterostructures in the hopping regime. In particular, it is shown that at sufficiently low temperatures, the mechanism of AHE related to the Berry phase is favoured.« less

Evaluation of phase transformation in ferromagnetic shape memory Fe-Pd alloy by magnetic Barkhausen noise

NASA Astrophysics Data System (ADS)

Furuya, Yasubumi; Tamoto, Shizuka; Kubota, Takeshi; Okazaki, Teiko; Hagood, Nesbitt W.; Spearing, S. Mark

2002-07-01

The possibility to detect the phase transformation with martensites by heating or cooling as well as stress-loading in ferromagnetic shape memory Fe-30at percent Pd alloy thin foil by using magnetic Markhausen noise sensor was studied. MBHN is caused by the irregular interactions between magnetic domain and thermally activated martensite twins during magnetization. In general, the envelope of the MBHN voltage versus time signals in Fe-29at percent Pd ribbon showed two peaks during magnetization, where secondary peak at intermediate state of magnetization process decreased with increasing temperature, while the MBHN envelopes in pure iron did not change with increasing temperature. The variety of MBHN due to the phase transformation was apt to arise at higher frequency part of spectrum during intermediate state of magnetization process and it decreased with disappearance of martensite twins. Besides, MBHN increased monotonically with increasing loading stress and then, it decreased with unloading, however MBHN showed large hysteresis between loading and unloading passes. Based on the experimental results from MBHN measurements for both thermoelastic and stress-induced martensite phase transformations in Fe-30at percent Pd ribbon samples, MBHN method seems a useful technique to non-destructive evaluation of martensite phase transformation of ferromagnetic shape memory alloy.

Berry phase mechanism of the anomalous Hall effect in a disordered two-dimensional magnetic semiconductor structure

PubMed Central

Oveshnikov, L. N.; Kulbachinskii, V. A.; Davydov, A. B.; Aronzon, B. A.; Rozhansky, I. V.; Averkiev, N. S.; Kugel, K. I.; Tripathi, V.

2015-01-01

The anomalous Hall effect (AHE) arises from the interplay of spin-orbit interactions and ferromagnetic order and is a potentially useful probe of electron spin polarization, especially in nanoscale systems where direct measurement is not feasible. While AHE is rather well-understood in metallic ferromagnets, much less is known about the relevance of different physical mechanisms governing AHE in insulators. As ferromagnetic insulators, but not metals, lend themselves to gate-control of electron spin polarization, understanding AHE in the insulating state is valuable from the point of view of spintronic applications. Among the mechanisms proposed in the literature for AHE in insulators, the one related to a geometric (Berry) phase effect has been elusive in past studies. The recent discovery of quantized AHE in magnetically doped topological insulators - essentially a Berry phase effect - provides strong additional motivation to undertake more careful search for geometric phase effects in AHE in the magnetic semiconductors. Here we report our experiments on the temperature and magnetic field dependences of AHE in insulating, strongly-disordered two-dimensional Mn delta-doped semiconductor heterostructures in the hopping regime. In particular, it is shown that at sufficiently low temperatures, the mechanism of AHE related to the Berry phase is favoured. PMID:26596472

Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film

DOE PAGES

Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo; ...

2016-06-08

Interfaces with subtle differences in atomic and electronic structures in perovskite ABO 3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr 0.67Sr 0.33MnO 3 film on SrTiO 3 substrate. The variations in the out-of-plane lattice constant and BO 6 octahedral rotation across the Pr 0.67Sr 0.33MnO 3/SrTiO 3 interface strongly depend on the thickness of the Pr 0.67Sr 0.33MnO 3 film. In the 12-nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI') phase is formed during the cubic-to-tetragonal phase transition ofmore » SrTiO 3, apparently due to the enhanced electron–phonon interaction and atomic disorder in the film. The transport properties of the FI' phase in the 30-nm film are masked because of the reduced interfacial coupling and smaller interface-to-volume ratio. In conclusion, this work demonstrates how thickness-dependent interfacial coupling leads to the formation of a theoretically predicted ferromagnetic–polaronic insulator, as illustrated in a new phase diagram, that is otherwise ferromagnetic metal (FM) in bulk form.« less

Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film

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

Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo

Interfaces with subtle differences in atomic and electronic structures in perovskite ABO 3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr 0.67Sr 0.33MnO 3 film on SrTiO 3 substrate. The variations in the out-of-plane lattice constant and BO 6 octahedral rotation across the Pr 0.67Sr 0.33MnO 3/SrTiO 3 interface strongly depend on the thickness of the Pr 0.67Sr 0.33MnO 3 film. In the 12-nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI') phase is formed during the cubic-to-tetragonal phase transition ofmore » SrTiO 3, apparently due to the enhanced electron–phonon interaction and atomic disorder in the film. The transport properties of the FI' phase in the 30-nm film are masked because of the reduced interfacial coupling and smaller interface-to-volume ratio. In conclusion, this work demonstrates how thickness-dependent interfacial coupling leads to the formation of a theoretically predicted ferromagnetic–polaronic insulator, as illustrated in a new phase diagram, that is otherwise ferromagnetic metal (FM) in bulk form.« less

Physical properties and spin excitations in the lacunar spinels AV4S8(A =Ga, Ge)

NASA Astrophysics Data System (ADS)

Pokharel, Ganesh; Christianson, Andrew; Mandrus, David; Liusuo Wu Team; Mark Lumsden Collaboration; Rupam Mukherjee Collaboration; Matthew Stone Collaboration; Georg Ehlers Collaboration

In the lacunar spinels AV4S8 (A = Ga, Ge), the interplay of spin, charge, and orbital degrees of freedom results in a complex phase diagram which includes: ferroelectric, orbitally ordered, and Néel type skyrmion phases. Below 12.7 K GaV4S8 exhibits cycloidal and ferromagnetic order and the application of a magnetic field results in a Néel type skyrmion spin structure. On the other hand, GeV4S8 orders antiferromagentically below 18 K. To illuminate the underlying physics driving the formation of these novel phases, we have measured the magnetization, resistivity, thermal conductivity, and inelastic neutron scattering spectrum of these spinels. The inelastic neutron scattering data shows broadened spin excitations which extend to 6 meV within the magnetically order phases for both GaV4S8 and GeV4S8. The similarity of the spectra for ferromagnetic GaV4S8 and antiferromagnetic GeV4S8 reflects the close balance of ferromagnetic and antiferromagnetic interactions in these materials. This research is funded by the Gordon and Betty Moore Foundation's EPIQS Initiative through Grant GBMF4416.

Surface Spin Glass Ordering and Exchange Bias in Nanometric Sm0.09Ca0.91MnO3 Manganites

NASA Astrophysics Data System (ADS)

Giri, S. K.; Nath, T. K.

2011-07-01

We have thoroughly investigated the entire magnetic state of under doped ferromagnetic insulating manganite Sm0.09Ca0.91MnO3 through temperature dependent linear and non-linear ac magnetic susceptibility and magnetization measurements. This ferromagnetic insulating manganite is found to have frequency dependent ferromagnetic to paramagnetic transition temperature at around 108 K. Exchange- bias effect are observed in field -cooled magnetic hysteresis loops for this nanoparticle. We have attributed our observation to the formation of ferromagnetic cluster which are formed as a consequence of intrinsic phase separation below certain temperature in this under doped manganites. We have carried out electronic- and magneto-transport measurements to support these observed results.

Growth of room temperature ferromagnetic Ge1-xMnx quantum dots on hydrogen passivated Si (100) surfaces

NASA Astrophysics Data System (ADS)

Gastaldo, Daniele; Conta, Gianluca; Coïsson, Marco; Amato, Giampiero; Tiberto, Paola; Allia, Paolo

2018-05-01

A method for the synthesis of room-temperature ferromagnetic dilute semiconductor Ge1-xMnx (5 % < x < 8 %) quantum dots by molecular beam epitaxy by selective growth on hydrogen terminated silicon (100) surface is presented. The functionalized substrates, as well as the nanostructures, were characterized in situ by reflection high-energy electron diffraction. The quantum dots density and equivalent radius were extracted from field emission scanning electron microscope pictures, obtained ex-situ. Magnetic characterizations were performed by superconducting quantum interference device vibrating sample magnetometry revealing that ferromagnetic order is maintained up to room temperature: two different ferromagnetic phases were identified by the analysis of the field cooled - zero field cooled measurements.

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

Zhang, Zuocheng; Feng, Xiao; Wang, Jing

The interplay between magnetism and topology, as exemplified in the magnetic skyrmion systems, has emerged as a rich playground for finding novel quantum phenomena and applications in future information technology. Magnetic topological insulators (TI) have attracted much recent attention, especially after the experimental realization of quantum anomalous Hall effect. Future applications of magnetic TI hinge on the accurate manipulation of magnetism and topology by external perturbations, preferably with a gate electric field. In this work, we investigate the magneto transport properties of Cr doped Bi 2(Se xTe 1-x) 3 TI across the topological quantum critical point (QCP). We find thatmore » the external gate voltage has negligible effect on the magnetic order for samples far away from the topological QCP. However, for the sample near the QCP, we observe a ferromagnetic (FM) to paramagnetic (PM) phase transition driven by the gate electric field. Theoretical calculations show that a perpendicular electric field causes a shift of electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and consequently a magnetic phase transition. Finally, the in situ electrical control of the topological and magnetic properties of TI shed important new lights on future topological electronic or spintronic device applications.« less

Unconventional Magnetic Domain Structure in the Ferromagnetic Phase of MnP Single Crystals

NASA Astrophysics Data System (ADS)

Koyama, Tsukasa; Yano, Shin-ichiro; Togawa, Yoshihiko; Kousaka, Yusuke; Mori, Shigeo; Inoue, Katsuya; Kishine, Jun-ichiro; Akimitsu, Jun

2012-04-01

We have studied ferromagnetic (FM) structures in the FM phase of MnP single crystals by low-temperature Lorentz transmission electron microscopy and small-angle electron diffraction analysis. In Lorentz Fresnel micrographs, striped FM domain structures were observed at an external magnetic field less than 10 Oe in specimens with the ab-plane in their plane. From real- and reciprocal-space analyses, it was clearly identified that striped FM domains oriented to the c-axis appear with Bloch-type domain walls in the b-direction and order regularly along the a-axis with a constant separation less than 100 nm. Moreover, the magnetic chirality reverses in alternate FM domain walls. These specific spin configuration of striped FM domains will affect the magnetic phase transition from the FM phase to the proper screw spiral phase at low temperature or to the FAN phase in magnetic fields in MnP.

Tuning exchange bias by Co doping in Mn₅₀Ni{sub 41–x}Sn₉Co{sub x} melt-spun ribbons

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

Zhao, D. W.; Li, G. K.; Wang, S. Q.

2014-09-14

In Mn₅₀Ni{sub 41–x}Sn₉Co{sub x} ribbons, the exchange bias field is very sensitive to the Co content. Based on both theoretical and experimental studies, it has been found that with increasing Co content, the pinned phase (ferromagnetic phase) remains almost unchanged while the pinning phase is changed from a canonical spin glass to a cluster spin glass and finally to a ferromagnetic phase. Changing the Co content in Mn₅₀Ni{sub 41–x}Sn₉Co{sub x} alloys has been proven to be an effective way of tuning the magnetic anisotropy and the phase structure of the pinning phase. With different Co contents, a continuous tuning ofmore » the exchange bias field from 345 Oe to 3154 Oe is realized.« less

Magnetism and phase transitions in LaCoO3

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

Belanger, David P; Durand, Alice M; Booth, C

2013-01-01

Neutron scattering and magnetometry measurements have been used to study phase transitions in LaCoO3 (LCO). For H 100 Oe, evidence for a ferromagnetic (FM) transition is observed at Tc 87 K. For 1 kOe H 60 kOe, no transition is apparent. For all H, Curie Weiss analysis shows predominantly antiferromagnetic (AFM) interactions for T > Tc, but the lack of long-range AFM order indicates magnetic frustration. We argue that the weak ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and nanoparticles. The lattice strain is present at the bulk surfaces and atmore » the interfaces between the LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly affected by the Co O Co angle ( ), in agreement with recent band calculations which predict that ferromagnetic long-range order can only take place above a critical value, C. Consistent with recent thin film estimations, we find C D 162:8. For > C, we observe power-law behavior in the structural parameters. decreases with T until the critical temperature, To 37 K; below To the rate of change becomes very small. For T < To, FM order appears to be confined to regions close to the surfaces, likely due to the lattice strain keeping the local Co O Co angle above C.« less

FAST TRACK COMMUNICATION: Magnetic exchange hardening in polycrystalline GdN thin films

NASA Astrophysics Data System (ADS)

Senapati, K.; Fix, T.; Vickers, M. E.; Blamire, M. G.; Barber, Z. H.

2010-08-01

We report the observation of intrinsic exchange hardening in polycrystalline GdN thin films grown at room temperature by magnetron sputtering. We find, in addition to the ferromagnetic phase, that a fraction of GdN crystallizes in a structural polymorphic form which orders antiferromagnetically. The relative fraction of these two phases was controlled by varying the relative abundance of reactive species in the sputtering plasma by means of the sputtering power and N2 partial pressure. An exchange bias of ~ 30 Oe was observed at 10 K. The exchange coupling between the ferromagnetic and the antiferromagnetic phases resulted in an order of magnitude enhancement in the coercive field in these films.

Spin reorientation and magnetoelastic properties of ferromagnetic T b1 -xN dxC o2 systems with a morphotropic phase boundary

NASA Astrophysics Data System (ADS)

Murtaza, Adil; Yang, Sen; Chang, Tieyan; Ghani, Awais; Khan, Muhammad Tahir; Zhang, Rui; Zhou, Chao; Song, Xiaoping; Suchomel, Matthew; Ren, Yang

2018-03-01

The spin reorientation (SR) and magnetoelastic properties of pseudobinary ferromagnetic T b1 -xN dxC o2 (0 ≤x ≤1.0 ) systems involving a morphotropic phase boundary (MPB) were studied by high-resolution synchrotron x-ray diffraction (XRD), magnetization, and magnetostriction measurements. The easy magnetization direction of the Laves phase lies along the 〈111 〉 axis with x <0.65 , whereas it lies along the 〈100 〉 axis for x >0.65 below Curie temperature (TC). The temperature-dependent magnetization curves showed SR; this can be explained by a two-sublattice model. Based on the synchrotron (XRD) and magnetization measurements, the SR phase diagram for a MPB composition of T b0.35N d0.65C o2 was obtained. Contrary to previously reported ferromagnetic systems involving MPB, the MPB composition of T b0.35N d0.65C o2 exhibits a low saturation magnetization (MS), indicating a compensation of the Tb and Nd magnetic moments at MPB. The anisotropic magnetostriction (λS) first decreased until x =0.8 and then continuously increased in the negative direction with further increase of Nd concentration. The decrease in magnetostriction can be attributed to the decrease of spontaneous magnetostriction λ111 and increase of λ100 with opposite sign to λ111. This paper indicates an anomalous type of MPB in the ferromagnetic T b1 -xN dxC o2 system and provides an active way to design novel functional materials with exotic properties.

Lattice effects on ferromagnetism in perovskite ruthenates

PubMed Central

Cheng, J.-G.; Zhou, J.-S.; Goodenough, John B.

2013-01-01

Ferromagnetism and its evolution in the orthorhombic perovskite system Sr1–xCaxRuO3 have been widely believed to correlate with structural distortion. The recent development of high-pressure synthesis of the Ba-substituted Sr1–yBayRuO3 makes it possible to study ferromagnetism over a broader phase diagram, which includes the orthorhombic Imma and the cubic phases. However, the chemical substitutions introduce the A-site disorder effect on Tc, which complicates determination of the relationship between ferromagnetism and structural distortion. By clarifying the site disorder effect on Tc in several unique series of ruthenates in which the average bond length 〈A–O〉 remains the same but the bond-length variance varies, we are able to demonstrate a parabolic curve of Tc versus mean bond length 〈A–O〉. A much higher Tc ∼ 177 K than that found in orthorhombic SrRuO3 can be obtained from the curve at a bond length 〈A–O〉, which makes the geometric factor t = 〈A–O〉/(√2〈Ru–O〉) ∼ 1. This result reveals not only that the ferromagnetism in the ruthenates is extremely sensitive to the lattice strain, but also that it has an important implication for exploring the structure–property relationship in a broad range of oxides with perovskite or a perovskite-related structure. PMID:23904477

Magnetic State of Quasiordered Fe-Al Alloys Doped with Ga and B: Magnetic Phase Separation and Spin Order

NASA Astrophysics Data System (ADS)

Voronina, E. V.; Ivanova, A. G.; Arzhnikov, A. K.; Chumakov, A. I.; Chistyakova, N. I.; Pyataev, A. V.; Korolev, A. V.

2018-04-01

Results of structural, magnetic, and Mössbauer studies of quasi ordered alloys Fe65Al35 - x M x ( M x = Ga, B; x = 0, 5 at %) are presented. The magnetic state of examined structurally-single-phase alloys at low temperatures is interpreted from the viewpoint of magnetic phase separation. An explanation is proposed for the observed behavior of magnetic characteristics of Fe65Al35 and Fe65Al30Ga5 in the framework of the model of two magnetic phases, a ferromagnetic-type one and a spin density wave. The boron-doped alloy Fe65Al30B5 is shown to demonstrate behavior that is typical of materials with the ferromagnetic type of ordering.

Dynamical current-induced ferromagnetic and antiferromagnetic resonances

NASA Astrophysics Data System (ADS)

Guimarães, F. S. M.; Lounis, S.; Costa, A. T.; Muniz, R. B.

2015-12-01

We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain.

Phonon Softening due to Melting of the Ferromagnetic Order in Elemental Iron

NASA Astrophysics Data System (ADS)

Han, Qiang; Birol, Turan; Haule, Kristjan

2018-05-01

We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long-range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. We use the ab initio density functional theory + embedded dynamical mean-field theory functional approach to address the dynamic stability of iron polymorphs and the phonon softening with an increased temperature. We show that the nonharmonic and inhomogeneous phonon softening measured in iron is a result of the melting of the long-range ferromagnetic order and is unrelated to the first-order structural transition from the bcc to the fcc phase, as is usually assumed. We predict that the bcc structure is dynamically stable at all temperatures at normal pressure and is thermodynamically unstable only between the bcc-α and the bcc-δ phases of iron.

Controlling Thermodynamic Properties of Ferromagnetic Group-IV Graphene-Like Nanosheets by Dilute Charged Impurity

NASA Astrophysics Data System (ADS)

Yarmohammadi, Mohsen; Mirabbaszadeh, Kavoos

2017-05-01

Using the Kane-Mele Hamiltonian, Dirac theory and self-consistent Born approximation, we investigate the effect of dilute charged impurity on the electronic heat capacity and magnetic susceptibility of two-dimensional ferromagnetic honeycomb structure of group-IV elements including silicene, germanene and stanene within the Green’s function approach. We also find these quantities in the presence of applied external electric field. Our results show that the silicene (stanene) has the maximum (minimum) heat capacity and magnetic susceptibility at uniform electric fields. From the behavior of theses quantities, the band gap has been changed with impurity concentration, impurity scattering strength and electric field. The analysis on the impurity-dependent magnetic susceptibility curves shows a phase transition from ferromagnetic to paramagnetic and antiferromagnetic phases. Interestingly, electronic heat capacity increases (decreases) with impurity concentration in silicene (germanene and stanene) structure.

Antisite disorder induced spin glass and exchange bias effect in Nd2NiMnO6 epitaxial thin film

NASA Astrophysics Data System (ADS)

Singh, Amit Kumar; Chauhan, Samta; Chandra, Ramesh

2017-03-01

We report the observation of the exchange bias effect and spin glass behaviour at low temperature in a ferromagnetic Nd2NiMnO6 epitaxial thin film. Along with the ferromagnetic transition at ˜194 K, an additional transition is observed at lower temperature (˜55 K) as seen from M-T curves of the sample. A shift in the ac susceptibility peak with frequency has been observed at low temperature, which is a signature of a glassy phase within the sample. The detailed investigation of the memory effect and time dependent magnetic relaxation measurements reveals the presence of a spin glass phase in the Nd2NiMnO6 thin film. The exchange bias effect observed at low temperature in the sample has been associated with an antisite disorder induced spin glass phase, which results in a ferromagnetic/spin glass interface at low temperature. The exchange bias behaviour has been further confirmed by performing cooling field and temperature dependence of exchange bias along with training effect measurements.

Pressure dependence of the magnetic ground states in MnP

DOE PAGES

Matsuda, Masaaki; Ye, Feng; Dissanayake, Sachith E.; ...

2016-03-17

MnP, a superconductor under pressure, exhibits a ferromagnetic order below TC~290 K followed by a helical order with the spins lying in the ab plane and the helical rotation propagating along the c axis below Ts~50 K at ambient pressure. We performed single-crystal neutron diffraction experiments to determine the magnetic ground states under pressure. Both TC and Ts are gradually suppressed with increasing pressure and the helical order disappears at ~1.2 GPa. At intermediate pressures of 1.8 and 2.0 GPa, the ferromagnetic order first develops and changes to a conical or two-phase (ferromagnetic and helical) structure with the propagation alongmore » the b axis below a characteristic temperature. At 3.8 GPa, a helical magnetic order appears below 208 K, which hosts the spins in the ac plane and the propagation along the b axis. The period of this b axis modulation is shorter than that at 1.8 GPa. Here, our results indicate that the magnetic phase in the vicinity of the superconducting phase may have a helical magnetic correlation along the b axis.« less

Magnetic properties of manganites doped with gallium, iron, and chromium ions

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

Troyanchuk, I. O., E-mail: troyan@physics.by; Bushinsky, M. V.; Tereshko, N. V.

The magnetization and the crystal structure of the La{sub 0.7}Sr{sub 0.3}Mn{sub 1−x}M{sub x}O{sub 3} (M = Ga, Fe, Cr; x ≤ 0.3) systems are studied. The substitution of gallium and chromium is shown to cause phase separation into antiferromagnetic and ferromagnetic phases, whereas the substitution of iron for manganese stabilizes a spinglass state. The ferromagnetic phase in the chromium-substituted compositions is much more stable than that in the case of substitution by iron ions or diamagnetic gallium ions. The magnetic properties are explained in terms of the model of superexchange interactions and the localization of most e{sub g} electrons ofmore » manganese. The stabilization of ferromagnetism in the chromium-substituted compositions can be caused by the fact that the positive and negative contributions to the superexchange interaction between Mn{sup 3+} and Cr{sup 3+} ions are close to each other but the antiferromagnetic part of the exchange is predominant. Moreover, some chromium ions are in the tetravalent state, which maintains the optimum doping conditions.« less

Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO3

NASA Astrophysics Data System (ADS)

Rosenberg, Aaron J.; Katmis, Ferhat; Kirtley, John R.; Gedik, Nuh; Moodera, Jagadeesh S.; Moler, Kathryn A.

2017-12-01

The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-bandgap ferromagnetic insulator with a Curie temperature around 16 K, and SrTiO3 (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K, indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. We speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.

Giant Tunnel Magnetoresistance with a Single Magnetic Phase-Transition Electrode

NASA Astrophysics Data System (ADS)

Zhang, Jia; Chen, X. Z.; Song, C.; Feng, J. F.; Wei, H. X.; Lü, Jing-Tao

2018-04-01

The magnetic phase-transition tunnel-magnetoresistance (MPT-TMR) effect with a single magnetic electrode is investigated by first-principles calculations. The calculations show that the MPT-TMR of an α'-FeRh /MgO /Cu tunnel junction can be as high as hundreds of percent when the magnetic structure of α'-FeRh changes from G -type antiferromagnetic (G -AFM ) to ferromagnetic order. This type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance because of its huge magnetoresistance effect and similar structural simplicity. The main mechanism for the giant MPT-TMR can be attributed to the formation of interface resonant states at the G -AFM FeRh /MgO interface. A direct FeRh /MgO interface is found to be necessary for achieving a high MPT-TMR experimentally. Moreover, we find the α'-FeRh /MgO interface with FeRh in the ferromagnetic phase has nearly full spin polarization due to the negligible majority transmission and significantly different Fermi surface of two spin channels. Thus, it may act as a highly efficient and tunable spin injector. In addition, the electric-field-driven MPT of FeRh-based heteromagnetic nanostructures can be utilized to design various energy-efficient tunnel-junction structures and the corresponding lower-power-consumption devices. We also discuss the consequence of various junction defects on MPT-TMR. The interface oxygen layer is found to be detrimental to MPT-TMR. The sign of MPT-TMR is reversed with Rh termination due to the lack of contribution from the interface resonance states. However, the MPT-TMR may be robust against the oxygen vacancy inside of MgO and the shift of the Fermi energy. Our results will stimulate further experimental investigations of MPT-TMR and other fascinating phenomenon of FeRh-based tunnel junctions that may be promising in antiferromagnetic spintronics.

Fabrication and ferromagnetism of Si-SiGe/MnGe core-shell nanopillars.

PubMed

Wang, Liming; Liu, Tao; Wang, Shuguang; Zhong, Zhenyang; Jia, Quanjie; Jiang, Zuimin

2016-10-07

Si-Si0.5Ge0.5/Mn0.08Ge0.92 core-shell nanopillar samples were fabricated on ordered Si nanopillar patterned substrates by molecular beam epitaxy at low temperatures. The magnetic properties of the samples are found to depend heavily on the growth temperature of the MnGe layer. The sample grown at a moderate temperature of 300 °C has the highest Curie temperature of 240 K as well as the strongest ferromagnetic signals. On the basis of the microstructural results, the ferromagnetic properties of the samples are believed to come from the intrinsic Mn-doped amorphous or crystalline Ge ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. After being annealed at a temperature of 500 °C, all the samples exhibit the same Curie temperature of 220 K, which is in sharp contrast to the different Curie temperature for the as-grown samples, and the ferromagnetism for the annealed samples comes from Mn5GeSi2 compounds which are formed during the annealing.

Doping concentration dependence of microstructure and magnetic behaviours in Co-doped TiO2 nanorods

PubMed Central

2014-01-01

Co-doped titanium dioxide (TiO2) nanorods with different doping concentrations were fabricated by a molten salt method. It is found that the morphology of TiO2 changes from nanorods to nanoparticles with increasing doping concentration. The mechanism for the structure and phase evolution is investigated in detail. Undoped TiO2 nanorods show strong ferromagnetism at room temperature, whereas incorporating of Co deteriorates the ferromagnetic ordering. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) results demonstrate that the ferromagnetism is associated with Ti vacancy. PMID:25593558

Theory of disordered Heisenberg ferromagnets

NASA Technical Reports Server (NTRS)

Stubbs, R. M.

1973-01-01

A Green's function technique is used to calculate the magnetic properties of Heisenberg ferromagnets in which the exchange interactions deviate randomly in strength from the mean interaction. Systems of sc, bcc, and fcc topologies and of general spin values are treated. Disorder produces marked effects in the density of spin wave states, in the form of enhancement of the low-energy density and extension of the energy band to higher values. The spontaneous magnetization and the Curie temperature decrease with increasing disorder. The effects of disorder are shown to be more pronounced in the ferromagnetic than in the paramagnetic phase.

Skyrmion motion induced by plane stress waves

NASA Astrophysics Data System (ADS)

Gungordu, Utkan; Kovalev, Alexey A.

Skyrmions are typically driven by currents and magnetic fields. We propose an alternative method of driving skyrmions using plane stress waves in a chiral ferromagnetic nanotrack. We find that the effective force due to surface acoustic waves couples both to the helicity and the topological charge of the skyrmion. This coupling can be used to probe the helicity of the skyrmion as well as the nature of the Dzyaloshinskii-Moriya interaction. This is particularly important when a ferromagnet lacks both surface- and bulk-inversion symmetry. Plane stress waves can be generated using a pair of interdigital transducers (IDTs). As the nanowire is subject to half-open space boundary conditions, the skyrmion is driven by normal stress in this setup. We find that skyrmions get pinned at the antinodes of the stress wave, much similar to domain walls, which enables skyrmion motion by detuned IDTs. We also consider a nanotrack sandwiched between a piezoelectric layer and a substrate, with electrical contacts placed on top, which results in shear stress in addition to normal stress in nanotrack. We find that unlike domain walls, skyrmions can be driven using shear component of a standing stress wave. This work was supported primarily by the DOE Early Career Award DE-SC0014189, and in part by the NSF under Grants Nos. Phy-1415600, and DMR-1420645 (UG).

Hollandites as a new class of multiferroics

PubMed Central

Liu, Shuangyi; Akbashev, Andrew R.; Yang, Xiaohao; Liu, Xiaohua; Li, Wanlu; Zhao, Lukas; Li, Xue; Couzis, Alexander; Han, Myung-Geun; Zhu, Yimei; Krusin-Elbaum, Lia; Li, Jackie; Huang, Limin; Billinge, Simon J. L.; Spanier, Jonathan E.; O'Brien, Stephen

2014-01-01

Discovery of new complex oxides that exhibit both magnetic and ferroelectric properties is of great interest for the design of functional magnetoelectrics, in which research is driven by the technologically exciting prospect of controlling charges by magnetic fields and spins by applied voltages, for sensors, 4-state logic, and spintronics. Motivated by the notion of a tool-kit for complex oxide design, we developed a chemical synthesis strategy for single-phase multifunctional lattices. Here, we introduce a new class of multiferroic hollandite Ba-Mn-Ti oxides not apparent in nature. BaMn3Ti4O14.25, designated BMT-134, possesses the signature channel-like hollandite structure, contains Mn4+ and Mn3+ in a 1:1 ratio, exhibits an antiferromagnetic phase transition (TN ~ 120 K) with a weak ferromagnetic ordering at lower temperatures, ferroelectricity, a giant dielectric constant at low frequency and a stable intrinsic dielectric constant of ~200 (1-100 MHz). With evidence of correlated antiferromagnetic and ferroelectric order, the findings point to an unexplored family of structures belonging to the hollandite supergroup with multifunctional properties, and high potential for developing new magnetoelectric materials. PMID:25160888

Quantum Monte Carlo analysis of a charge ordered insulating antiferromagnet: The Ti 4O 7 Magneli phase

DOE PAGES

Benali, Anouar; Shulenburger, Luke; Krogel, Jaron T.; ...

2016-06-07

The Magneli phase Ti 4O 7 is an important transition metal oxide with a wide range of applications because of its interplay between charge, spin, and lattice degrees of freedom. At low temperatures, it has non-trivial magnetic states very close in energy, driven by electronic exchange and correlation interactions. We have examined three low- lying states, one ferromagnetic and two antiferromagnetic, and calculated their energies as well as Ti spin moment distributions using highly accurate Quantum Monte Carlo methods. We compare our results to those obtained from density functional theory- based methods that include approximate corrections for exchange and correlation.more » Our results confirm the nature of the states and their ordering in energy, as compared with density-functional theory methods. However, the energy differences and spin distributions differ. Here, a detailed analysis suggests that non-local exchange-correlation functionals, in addition to other approximations such as LDA+U to account for correlations, are needed to simultaneously obtain better estimates for spin moments, distributions, energy differences and energy gaps.« less

The influence of further-neighbor spin-spin interaction on a ground state of 2D coupled spin-electron model in a magnetic field

NASA Astrophysics Data System (ADS)

Čenčariková, Hana; Strečka, Jozef; Gendiar, Andrej; Tomašovičová, Natália

2018-05-01

An exhaustive ground-state analysis of extended two-dimensional (2D) correlated spin-electron model consisting of the Ising spins localized on nodal lattice sites and mobile electrons delocalized over pairs of decorating sites is performed within the framework of rigorous analytical calculations. The investigated model, defined on an arbitrary 2D doubly decorated lattice, takes into account the kinetic energy of mobile electrons, the nearest-neighbor Ising coupling between the localized spins and mobile electrons, the further-neighbor Ising coupling between the localized spins and the Zeeman energy. The ground-state phase diagrams are examined for a wide range of model parameters for both ferromagnetic as well as antiferromagnetic interaction between the nodal Ising spins and non-zero value of external magnetic field. It is found that non-zero values of further-neighbor interaction leads to a formation of new quantum states as a consequence of competition between all considered interaction terms. Moreover, the new quantum states are accompanied with different magnetic features and thus, several kinds of field-driven phase transitions are observed.

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

Ahmed, Asar; Gajbhiye, Namdeo S., E-mail: nsg@iitk.ac.i

Cuprous oxide (Cu{sub 2}O) flower-like nanostructures doped with various metal ions i.e. Fe, Co, Ni and Mn have been synthesized by an organic phase solution method. The powder X-ray diffraction study clearly reveals them as single phase simple cubic cuprite lattice. Study of their magnetic properties have shown that these doped samples are ferromagnetic in nature; however, no such property was observed for the undoped Cu{sub 2}O sample. The magnitude of the ferromagnetic behavior was found to be dependent on the dopant metal ions amount, which increased consistently with its increase. As total magnetic moment contribution of the doped metalmore » ions calculated was insignificant, it is believed to have originated from the induced magnetic moments at cation deficiency sites in the material, created possibly due to the disturbance of the crystal lattice by the dopant ions. The existence of the defects has been supported by photoluminescence spectra of the doped samples. -- Graphical abstract: Room temperature ferromagnetic behavior was observed in the Cu{sub 2}O nanoflowers doped with Fe, Co, Ni and Mn ions. Cation deficiencies formed due to dopant ions were possibly responsible for ferromagnetism. Display Omitted« less

Room temperature ferromagnetism in Mn-doped NiO nanoparticles

NASA Astrophysics Data System (ADS)

Layek, Samar; Verma, H. C.

2016-01-01

Mn-doped NiO nanoparticles of the series Ni1-xMnxO (x=0.00, 0.02, 0.04 and 0.06) are successfully synthesized using a low temperature hydrothermal method. Samples up to 6% Mn-doping are single phase in nature as observed from powder x-ray diffraction (XRD) studies. Rietveld refinement of the XRD data shows that all the single phase samples crystallize in the NaCl like fcc structure with space group Fm-3m. Unit cell volume decreases with increasing Mn-doping. Pure NiO nanoparticles show weak ferromagnetism, may be due to nanosize nature. Introduction of Mn within NiO lattice improves the magnetic properties significantly. Room temperature ferromagnetism is found in all the doped samples whereas the magnetization is highest for 2% Mn-doping and then decreases with further doping. The ZFC and FC branches in the temperature dependent magnetization separate well above 350 K indicating transition temperature well above room temperature for 2% Mn-doped NiO Nanoparticle. The ferromagnetic Curie temperature is found to be 653 K for the same sample as measured by temperature dependent magnetization study using vibrating sample magnetometer (VSM) in high vacuum.

Field-induced States and Excitations in the Quasicritical Spin-1 /2 Chain Linarite

NASA Astrophysics Data System (ADS)

Cemal, Eron; Enderle, Mechthild; Kremer, Reinhard K.; Fâk, Björn; Ressouche, Eric; Goff, Jon P.; Gvozdikova, Mariya V.; Zhitomirsky, Mike E.; Ziman, Tim

2018-02-01

The mineral linarite, PbCuSO4(OH )2 , is a spin-1 /2 chain with frustrating nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. We show that the predicted quantum multipolar phases are fragile and actually suppressed by a tiny orthorhombic exchange anisotropy and weak interchain interactions in favor of a dipolar fan phase. Including this anisotropy in classical simulations of a nearly critical model explains the field-dependent phase sequence of the phase diagram of linarite, its strong dependence of the magnetic field direction, and the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field.

Half-metallic ferromagnetism in Sr3Ru2O7

NASA Astrophysics Data System (ADS)

Rivero, Pablo; Meunier, Vincent; Shelton, William

2017-05-01

The bilayered member of the Ruddesden-Popper family of ruthenates, Sr3Ru2O7 , has received increasing attention due to its interesting properties and phases. By using first principle calculations we find that the ground state is characterized by a ferromagnetic (FM) half-metallic state. This state strongly competes with an antiferromagnetic metallic phase, which indicates the possible presence of a particular state characterized by the existence of different magnetic domains. To drive the system towards a phase transition we studied the electronic and magnetic properties as a function of RuO6 octahedra rotations and found that the magnetic phase does not couple with the rotation angle. Our results provide accurate electronic, structure, and magnetic ground-state properties of Sr3Ru2O7 and stimulate the investigation of other types of octahedra rotations and distortions in the search of phase transitions.

Spiral magnetism in the single-band Hubbard model: the Hartree-Fock and slave-boson approaches.

PubMed

Igoshev, P A; Timirgazin, M A; Gilmutdinov, V F; Arzhnikov, A K; Irkhin, V Yu

2015-11-11

The ground-state magnetic phase diagram is investigated within the single-band Hubbard model for square and different cubic lattices. The results of employing the generalized non-correlated mean-field (Hartree-Fock) approximation and generalized slave-boson approach by Kotliar and Ruckenstein with correlation effects included are compared. We take into account commensurate ferromagnetic, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often lacking in previous investigations. It is found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistically large Hubbard U by the correlation effects if nesting is absent and van Hove singularities are well away from the paramagnetic phase Fermi level. The magnetic phase separation plays an important role in the formation of magnetic states, the corresponding phase regions being especially wide in the vicinity of half-filling. The details of non-collinear and collinear magnetic ordering for different cubic lattices are discussed.

Electromagnetic wave absorbing properties and hyperfine interactions of Fe—Cu—Nb—Si—B nanocomposites

NASA Astrophysics Data System (ADS)

Han, Man-Gui; Guo, Wei; Wu, Yan-Hui; Liu, Min; Magundappa, L. Hadimani

2014-08-01

The Fe—Cu—Nb—Si—B alloy nanocomposite containing two ferromagnetic phases (amorphous phase and nanophase phase) is obtained by properly annealing the as-prepared alloys. High resolution transmission electron microscopy (HR-TEM) images show the coexistence of these two phases. It is found that Fe—Si nanograins are surrounded by the retained amorphous ferromagnetic phase. Mössbauer spectroscopy measurements show that the nanophase is the D03-type Fe—Si phase, which is employed to find the atomic fractions of resonant 57Fe atoms in these two phases. The microwave permittivity and permeability spectra of Fe—Cu—Nb—Si—B nanocomposite are measured in the frequency range of 0.5 GHz-10 GHz. Large relative microwave permeability values are obtained. The results show that the absorber containing the nanocomposite flakes with a volume fraction of 28.59% exhibits good microwave absorption properties. The reflection loss of the absorber is less than -10 dB in a frequency band of 1.93 GHz-3.20 GHz.

Ultrafast Study of Dynamic Exchange Coupling in Ferromagnet/Oxide/Semiconductor Heterostructures

NASA Astrophysics Data System (ADS)

Ou, Yu-Sheng

Spintronics is the area of research that aims at utilizing the quantum mechanical spin degree of freedom of electrons in solid-state materials for information processing and data storage application. Since the discovery of the giant magnetoresistance, the field of spintronics has attracted lots of attention for its numerous potential advantages over contemporary electronics, such as less power consumption, high integration density and non-volatility. The realization of a spin battery, defined by the ability to create spin current without associated charge current, has been a long-standing goal in the field of spintronics. The demonstration of pure spin current in ferromagnet/nonmagnetic material hybrid structures by ferromagnetic resonance spin pumping has defined a thrilling direction for this field. As such, this dissertation targets at exploring the spin and magnetization dynamics in ferromagnet/oxide/semiconductor heterostructures (Fe/MgO/GaAs) using time-resolved optical pump-probe spectroscopy with the long-range goal of understanding the fundamentals of FMR-driven spin pumping. Fe/GaAs heterostructures are complex systems that contain multiple spin species, including paramagnetic spins (GaAs electrons), nuclear spins (Ga and As nuclei) and ferromagnetic spins (Fe). Optical pump-probe studies on their interplay have revealed a number of novel phenomena that has not been explored before. As such they will be the major focus of this dissertation. First, I will discuss the effect of interfacial exchange coupling on the GaAs free-carrier spin relaxation. Temperature- and field-dependent spin-resolved pump-probe studies reveal a strong correlation of the electron spin relaxation with carrier freeze-out, in quantitative agreement with a theoretical interpretation that at low temperatures the free-carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. Second, we investigate the impact of tunnel barrier thickness on GaAs electron spin dynamics in Fe/MgO/GaAs heterostructures. Comparison of the Larmor frequency between samples with thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the spin lifetimes argues that inhomogeneity in the local hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs. Third, we investigated the origin and dynamics of an emergent spin population by pump power and magnetic field dependent spin-resolved pump-probe studies. Power dependent study confirms its origin to be filling of electronic states in GaAs, and further field dependent studies reveal the impact of contact hyperfine coupling on the dynamics of electron spins occupying distinct electronic states. Beyond above works, we also pursue optical detection of dynamic spin pumping in Fe/MgO/GaAs heterostructures in parallel. I will discuss the development and progress that we have made toward this goal. This project can be simply divided into two phases. In the first phase, we focused on microwave excitation and optical detection of spin pumping. In the second phase, we focused on all-optical excitation and detection of spin pumping. A number of measurement strategies have been developed and executed in both stages to hunt for a spin pumping signal. I will discuss the preliminary data based upon them.

Electron transport in ferromagnetic nanostructures

NASA Astrophysics Data System (ADS)

Lee, Sungbae

As the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens within submicrometer scale samples is one of the goals of modern condensed matter physics. Electron transport phenomena drew a lot of attention over the past two decades or so, not only because quantum corrections to the classical transport theory, but also they allow us to probe deeply into the microscopic nature of the system put to test. Although a significant amount of research was done in the past and thus extended our understanding in this field, most of these works were concentrated on simpler examples. Electron transport in strongly correlated systems is still a field that needs to be explored more thoroughly. In fact, experimental works that have been done so far to characterize coherence physics in correlated systems such as ferromagnetic metals are far from conclusive. One reason ferromagnetic samples draw such attention is that there exist correlations that lead to excitations (e.g. spin waves, domain wall motions) not present in normal metals, and these new environmental degrees of freedom can have profound effects on decoherence processes. In this thesis, three different types of magnetic samples were examined: a band ferromagnetism based metallic ferromagnet, permalloy, a III-V diluted ferromagnetic semiconductor with ferromagnetism from a hole-mediated exchange interaction, and magnetite nanocrystals and films. The first observation of time-dependent universal conductance fluctuations (TD-UCF) in permalloy is presented and our observations lead to three major conclusions. First, the cooperon contribution to the conductance is suppressed in this material. This is consistent with some theoretical expectations, and implies that weak localization will be suppressed as well. Second, we see evidence that domain wall motion leads to enhanced conductance fluctuations, demonstrating experimentally that domain walls can act as coherent scatterers of electrons. Third, the temperature dependence of the fluctuations is surprisingly strong, suggesting that the dominant decoherence mechanism in these wires is different than that in similar normal metal nanostructures. The first observation of TD-UCF in diluted magnetic semiconductors (DMS) is also presented. In contrast to analogous measurements on permalloy samples, we find a surprising suppression of TD-UCF noise in this material at low temperatures, independent of field orientation. We believe this implies that the suppression is not due to an orbital effect, and therefore some of the fluctuations originate with time-varying magnetic disorder. The temperature dependence of the TD-UCF implies either an unusual fluctuator spectrum or a nonstandard dephasing mechanism. Measurements of UCF as a function of magnetic field allow an order of magnitude estimate of the coherence length at 2 K of approximately 50 nm in this material. The last samples examined were magnetite nanocrystals and films. Magnetite has been used in technologies for millennia, from compasses to magnetoelectronic devices, although its electronic structure has remained controversial for seven decades, with a low temperature insulator and a high temperature "bad metal" separated by the Verwey transition at 120 K. A new electrically driven insulator-metal transition below the Verwey temperature in both magnetite films and nanocrystals was observed. The possibility that this was a thermal effect was tested through various methods, and we have shown that the transition is in fact truly electrically driven. This electrically driven transition also showed a great deal of rigidity against external magnetic field and high gate voltages.

Study of magnetism in Ni-Cr hardface alloy deposit on 316LN stainless steel using magnetic force microscopy

NASA Astrophysics Data System (ADS)

Kishore, G. V. K.; Kumar, Anish; Chakraborty, Gopa; Albert, S. K.; Rao, B. Purna Chandra; Bhaduri, A. K.; Jayakumar, T.

2015-07-01

Nickel base Ni-Cr alloy variants are extensively used for hardfacing of austenitic stainless steel components in sodium cooled fast reactors (SFRs) to avoid self-welding and galling. Considerable difference in the compositions and melting points of the substrate and the Ni-Cr alloy results in significant dilution of the hardface deposit from the substrate. Even though, both the deposit and the substrate are non-magnetic, the diluted region exhibits ferromagnetic behavior. The present paper reports a systematic study carried out on the variations in microstructures and magnetic behavior of American Welding Society (AWS) Ni Cr-C deposited layers on 316 LN austenitic stainless steels, using atomic force microscopy (AFM) and magnetic force microscopy (MFM). The phase variations of the oscillations of a Co-Cr alloy coated magnetic field sensitive cantilever is used to quantitatively study the magnetic strength of the evolved microstructure in the diluted region as a function of the distance from the deposit/substrate interface, with the spatial resolution of about 100 nm. The acquired AFM/MFM images and the magnetic property profiles have been correlated with the variations in the chemical compositions in the diluted layers obtained by the energy dispersive spectroscopy (EDS). The study indicates that both the volume fraction of the ferromagnetic phase and its ferromagnetic strength decrease with increasing distance from the deposit/substrate interface. A distinct difference is observed in the ferromagnetic strength in the first few layers and the ferromagnetism is observed only near to the precipitates in the fifth layer. The study provides a better insight of the evolution of ferromagnetism in the diluted layers of Ni-Cr alloy deposits on stainless steel.

A comprehensive study of piezomagnetic response in CrPS4 monolayer: mechanical, electronic properties and magnetic ordering under strains

NASA Astrophysics Data System (ADS)

Joe, Minwoong; Lee, Hosik; Menderes Alyörük, M.; Lee, Jinhwan; Youb Kim, Sung; Lee, Changgu; Lee, Jun Hee

2017-10-01

We performed first-principles calculations to investigate the magnetic, mechanical and electronic properties of the tetrachalcogenide CrPS4. Although bulk CrPS4 has been shown to exhibit a low-dimensional antiferromagnetic (AFM) ground state where ferromagnetic (FM) Cr-chains are coupled antiferromagnetically, our calculations indicated that the monolayer can be transformed to an FM material by applying a uniaxial tensile strain of  ⩾4% along the FM Cr-chain direction. The AFM-to-FM transition is explained to be driven by an increase of the exchange interaction induced by a decrease in the distance between the FM Cr-chains. A huge nonlinear piezomagnetism was predicted at the strain-induced magnetic phase boundary. Our study provides insight about rational design of single-layer magnetic materials for a wide range of spintronic devices and energy applications.

Topological phase in a two-dimensional metallic heavy-fermion system

NASA Astrophysics Data System (ADS)

Yoshida, Tsuneya; Peters, Robert; Fujimoto, Satoshi; Kawakami, Norio

2013-04-01

We report on a topological insulating state in a heavy-fermion system away from half filling, which is hidden within a ferromagnetic metallic phase. In this phase, the cooperation of the RKKY interaction and the Kondo effect, together with the spin-orbit coupling, induces a spin-selective gap, bringing about topologically nontrivial properties. This topological phase is robust against a change in the chemical potential in a much wider range than the gap size. We analyze these remarkable properties by using dynamical mean field theory and the numerical renormalization group. Its topological properties support a gapless chiral edge mode, which exhibits a non-Tomonaga-Luttinger liquid behavior due to the coupling with bulk ferromagnetic spin fluctuations. We also propose that the effects of the spin fluctuations on the edge mode can be detected via the NMR relaxation time measurement.

Neutron Diffraction and Electrical Transport Studies on Magnetic Transition in Terbium at High Pressures and Low Temperatures

NASA Astrophysics Data System (ADS)

Thomas, Sarah; Montgomery, Jeffrey; Tsoi, Georgiy; Vohra, Yogesh; Weir, Samuel; Tulk, Christopher; Moreira Dos Santos, Antonio

2013-06-01

Neutron diffraction and electrical transport measurements have been carried out on the heavy rare earth metal terbium at high pressures and low temperatures in order to elucidate its transition from a helical antiferromagnetic to a ferromagnetic ordered phase as a function of pressure. The electrical resistance measurements using designer diamonds show a change in slope as the temperature is lowered through the ferromagnetic Curie temperature. The temperature of the ferromagnetic transition decreases at a rate of -16.7 K/GPa till 3.6 GPa, where terbium undergoes a structural transition from hexagonal close packed (hcp) to an α-Sm phase. Above this pressure, the electrical resistance measurements no longer exhibit a change in slope. In order to confirm the change in magnetic phase suggested by the electrical resistance measurements, neutron diffraction measurements were conducted at the SNAP beamline at the Oak Ridge National Laboratory. Measurements were made at pressures to 5.3 GPa and temperatures as low as 90 K. An abrupt increase in peak intensity in the neutron diffraction spectra signaled the onset of magnetic order below the Curie temperature. A magnetic phase diagram of rare earth metal terbium will be presented to 5.3 GPa and 90 K based on these studies.

Control of Ferromagnetic Resonance Frequency and Frequency Linewidth by Electrical Fields in FeCo/[Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) Heterostructures

NASA Astrophysics Data System (ADS)

Phuoc, Nguyen N.; Ong, C. K.

2016-10-01

We report our detailed investigation of the electrical tuning of the ferromagnetic resonance frequency and frequency linewidth in multiferroic heterostructures consisting of FeCo thin films grown onto [Pb(Mg1/3Nb2/3) O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates with NiFe underlayers. Our study shows that the electrical tuning range of both ferromagnetic resonance frequency and frequency linewidth in this FeCo/PMN-PT heterostructure can be very large. Specifically, the resonance frequency can be tuned from 1.8 GHz to 10.3 GHz, and the frequency linewidth can be changed from 1.6 GHz to 7.3 GHz. The electrical tuning of these microwave properties is discussed in conjunction with the result from the static magnetic characterization and is explained based on the strain-driven magnetoelectric heterostructured effect.

Ferromagnetic Resonance of a Single Magnetochiral Metamolecule of Permalloy

NASA Astrophysics Data System (ADS)

Kodama, Toshiyuki; Tomita, Satoshi; Kato, Takeshi; Oshima, Daiki; Iwata, Satoshi; Okamoto, Satoshi; Kikuchi, Nobuaki; Kitakami, Osamu; Hosoito, Nobuyoshi; Yanagi, Hisao

2016-08-01

We investigate the ferromagnetic resonance (FMR) of a single chiral structure of a ferromagnetic metal—the magnetochiral (MCh) metamolecule. Using a strain-driven self-coiling technique, micrometer-sized MCh metamolecules of metallic permalloy (Py) are fabricated without any residual Py films. The magnetization curves of ten Py MCh metamolecules obtained by an alternating gradient magnetometer show soft magnetic behavior. In cavity FMR with a magnetic-field sweep and coplanar-waveguide (CPW) FMR with a frequency sweep, the Kittel-mode FMR of the single Py metamolecule is observed. The CPW-FMR results, which are consistent with the cavity-FMR results, bring about the effective g factor, effective magnetization, and Gilbert damping of the single metamolecule. Together with calculations using these parameters, the angle-resolved cavity FMR reveals that the magnetization in the Py MCh metamolecule is most likely to be the hollow-bar type of configuration when the external magnetic field is applied parallel to the chiral axis, although the expected magnetization state at remanence is the corkscrew type of configuration.

Chaotic dynamics of Heisenberg ferromagnetic spin chain with bilinear and biquadratic interactions

NASA Astrophysics Data System (ADS)

Blessy, B. S. Gnana; Latha, M. M.

2017-10-01

We investigate the chaotic dynamics of one dimensional Heisenberg ferromagnetic spin chain by constructing the Hamiltonian equations of motion. We present the trajectory and phase plots of the system with bilinear and also biquadratic interactions. The stability of the system is analysed in both cases by constructing the Jacobian matrix and by measuring the Lyapunov exponents. The results are illustrated graphically.

Charge ordering in the metal-insulator transition of V-doped CrO2 in the rutile structure.

PubMed

Biswas, Sarajit

2018-04-17

Electronic, magnetic, and structural properties of pure and V-doped CrO 2 were extensively investigated utilizing density functional theory. Usually, pure CrO 2 is a half-metallic ferromagnet with conductive spin majority species and insulating spin minority species. This system remains in its half-metallic ferromagnetic phase even at 50% V-substitution for Cr within the crystal. The V-substituted compound Cr 0.5 V 0.5 O 2 encounters metal-insulator transition upon the application of on-site Coulomb repulsion U = 7 eV preserving its ferromagnetism in the insulating phase. It is revealed in this study that Cr 3+ -V 5+ charge ordering accompanied by the transfer of the single V-3d electron to the Cr-3dt 2g orbitals triggers metal-insulator transition in Cr 0.5 V 0.5 O 2 . The ferromagnetism of Cr 0.5 V 0.5 O 2 in the insulating phase arises predominantly due to strong Hund's coupling between the occupied electrons in the Cr-t 2g states. Besides this, the ferromagnetic Curie temperature (T c ) decreases significantly due to V-substitution. Interestingly, a structural distortion is observed due to tilting of CrO 6 or VO 6 octahedra across the metal-insulator transition of Cr 0.5 V 0.5 O 2 . Graphical abstract The V-doped compound Cr 0.5 V 0.5 O 2 is found a half-metallic ferromagnet (HMF) in the absence of on-site Coulomb interaction (U). This HMF behavor maintains up to U = 6 eV. Eventually, this system encounters metal-insulator transition (MIT) upon the application of U = 7 eV with a band gap of E g ~ 0.31 eV. Nevertheless, applications of higher U widen the band gaps. In this figure, calculated total (black), Cr-3d (red), V-3d (violet), and O-2p (blue) DOS of Cr 0.5 V 0.5 O 2 for U = 8 eV are illustrated. The system is insulating with a band gap of E g ~ 0.7 eV.

Large resistivity modulation in mixed-phase metallic systems

DOE PAGES

Lee, Yeonbae; Liu, Z. Q.; Heron, J. T.; ...

2015-01-07

Giant physical responses were discovered, in numerous systems, when two phases coexist; for example, near a phase transition. An intermetallic FeRh system undergoes a first-order antiferromagnetic to ferromagnetic transition above room temperature and shows two-phase coexistence near the transition. We have investigated the effect of an electric field to FeRh/PMN-PT heterostructures and report 8% change in the electrical resistivity of FeRh films. Such a 'giant' electroresistance (GER) response is striking in metallic systems, in which external electric fields are screened, and thus only weakly influence the carrier concentrations and mobilities. We show that our FeRh films comprise coexisting ferromagnetic andmore » antiferromagnetic phases with different resistivities and the origin of the GER effect is the strain-mediated change in their relative proportions. Finally, the observed behaviour is reminiscent of colossal magnetoresistance in perovskite manganites and illustrates the role of mixed-phase coexistence in achieving large changes in physical properties with low-energy external perturbation.« less

Strong Nonvolatile Magnon-Driven Magnetoelectric Coupling in Single-Crystal Co /[PbMg1/3Nb2/3O3] 0.71[PbTiO3]0.29 Heterostructures

NASA Astrophysics Data System (ADS)

Zhou, Cai; Shen, Lvkang; Liu, Ming; Gao, Cunxu; Jia, Chenglong; Jiang, Changjun

2018-01-01

The ability to manipulate the magnetism on interfacing ferromagnetic and ferroelectric materials via electric fields to achieve an emergent multiferroic response has enormous potential for nanoscale devices with novel functionalities. Herein, a strong electric-field control of the magnetism modulation is reported for a single-crystal Co (14 nm )/(001 )Pb (Mg1/3Nb2/3) 0.7Ti0.3O3 (PMN-PT) heterostructure by fabricating an epitaxial Co layer on a PMN-PT substrate. Electric-field-tuned ferromagnetic resonance exhibits a large resonance field shift, with a 120-Oe difference between that under positive and negative remanent polarizations, which demonstrates nonvolatile electric-field control of the magnetism. Further, considering the complexity of the twofold symmetry magnetic anisotropy, the linear change of the fourfold symmetry magnetic anisotropy, relating to the single-crystal cubic magnetocrystal anisotropy of the Co thin film, is resolved and quantified to exert a magnon-driven, strong direct magnetoelectric effect on the Co /PMN -PT interface. These results are promising for future multiferroic devices.

Insulating Ferromagnetic LaCoO3-δ Films: A Phase Induced by Ordering of Oxygen Vacancies

NASA Astrophysics Data System (ADS)

Biškup, Neven; Salafranca, Juan; Mehta, Virat; Oxley, Mark P.; Suzuki, Yuri; Pennycook, Stephen J.; Pantelides, Sokrates T.; Varela, Maria

2014-02-01

The origin of ferromagnetism in strained epitaxial LaCoO3 films has been a long-standing mystery. Here, we combine atomically resolved Z-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial LaCoO3 films, oxygen-vacancy superstructures release strain, control the film's electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co d states. Although oxygen vacancies typically dope a material n-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.

Nature of the transition between a ferromagnetic metal and a spin-glass insulator in pyrochlore molybdates.

PubMed

Hanasaki, N; Watanabe, K; Ohtsuka, T; Kézsmárki, I; Iguchi, S; Miyasaka, S; Tokura, Y

2007-08-24

The metal-insulator transition has been investigated for pyrochlore molybdates R(2)Mo(2)O(7) with nonmagnetic rare-earth ions R. The dynamical scaling analysis of ac susceptibility reveals that the geometrical frustration causes the atomic spin-glass state. The reentrant spin-glass phase exists below the ferromagnetic transition. The electronic specific heat is enhanced as compared to the band calculation result, perhaps due to the orbital fluctuation in the half-metallic ferromagnetic state. The large specific heat is rather reduced upon the transition, likely because the short-range antiferromagnetic fluctuation shrinks the Fermi surface.

Magnetic skyrmion bubble motion driven by surface acoustic waves

DOE PAGES

Nepal, Rabindra; Güngördü, Utkan; Kovalev, Alexey A.

2018-03-12

Here, we study the dynamical control of a magnetic skyrmion bubble by using counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we determine the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele’s method. The force that pushes the bubble is proportional to the strain gradient for the major strain component. We then study the dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two resonant frequencies.

Magnetic skyrmion bubble motion driven by surface acoustic waves

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

Nepal, Rabindra; Güngördü, Utkan; Kovalev, Alexey A.

Here, we study the dynamical control of a magnetic skyrmion bubble by using counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we determine the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele’s method. The force that pushes the bubble is proportional to the strain gradient for the major strain component. We then study the dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two resonant frequencies.

Neutron diffraction and electrical transport studies on magnetic ordering in terbium at high pressures and low temperatures

DOE PAGES

Thomas, Sarah A.; Montgomery, Jeffrey M.; Tsoi, Georgiy M.; ...

2013-06-11

Neutron diffraction and electrical transport measurements have been carried out on the heavy rare earth metal terbium at high pressures and low temperatures in order to elucidate the onset of ferromagnetic order as a function of pressure. The electrical resistance measurements show a change in slope as the temperature is lowered through the ferromagnetic Curie temperature. The temperature of this ferromagnetic transition decreases from approximately 240 K at ambient pressure at a rate of –16.7 K/GPa up to a pressure of 3.6 GPa, at which point the onset of ferromagnetic order is suppressed. Neutron diffraction measurements as a function ofmore » pressure at temperatures ranging from 90 K to 290 K confirm that the change of slope in the resistance is associated with the ferromagnetic ordering, since this occurs at pressures similar to those determined from the resistance results at these temperatures. Furthermore, a change in ferromagnetic ordering as the pressure is increased above 3.6 GPa is correlated with the phase transition from the ambient hexagonal close packed (hcp) structure to an α-Sm type structure at high pressures.« less

Evolution of the magnetic ground state in the electron-doped antiferromagnet CaMnO3

NASA Astrophysics Data System (ADS)

Cornelius, A. L.; Light, B. E.; Neumeier, J. J.

2003-07-01

Measurements of the specific heat on the system CaxMnO3 (x⩽0.10) are reported. Particular attention is paid to the effect that doping the parent compound with electrons by substitution of La for Ca has on the magnetic ground state. The high- (T>40 K) temperature data reveal that doping decreases TN from 122 K for the undoped sample to 103 K for x=0.10. The low temperature (T<20 K) heat-capacity data are consistent with phase separation. The undoped sample displays a finite density of states and typical antiferromagnetic behavior. The addition of electrons in the x⩽0.03 samples creates local ferromagnetism as evidenced by a decreased internal field and the need to add a ferromagnetic component to the heat-capacity data for x=0.03. Further substitution enhances the ferromagnetism as evidenced by the formation of a long-range ferromagnetic component to the undoped antiferromagnetic structure. The results are consistent with a scenario involving the formation of isolated ferromagnetic droplets for small x that start to overlap for x≈0.06 giving rise to long range ferromagnetism coexisting with antiferromagnetism.

Theory of the magnetism in La2NiMnO6

NASA Astrophysics Data System (ADS)

Sanyal, Prabuddha

2017-12-01

The magnetism of ordered and disordered La2NiMnO6 is explained using a model involving double exchange and superexchange. An important feature of this model is the majority spin hybridization in the large coupling limit, which results in ferromagnetism rather than ferrimagnetism as in Sr2FeMoO6 . The ferromagnetic insulating ground state in the ordered phase is explained. The essential role played by the Ni-Mn superexchange between the Ni eg electron spins and the Mn t2 g core electron spins in realizing this ground state is outlined. In the presence of antisite disorder, the model system is found to exhibit a tendency of becoming a spin glass at low temperatures, while it continues to retain a ferromagnetic transition at higher temperatures, similar to recent experimental observations [D. Choudhury et al., Phys. Rev. Lett. 108, 127201 (2012), 10.1103/PhysRevLett.108.127201]. This reentrant spin glass or reentrant ferromagnetic behavior is explained in terms of the competition of the ferromagnetic double exchange between the Ni eg and the Mn eg electrons, and the ferromagnetic Ni-Mn superexchange, with the antiferromagnetic antisite Mn-Mn superexchange.

Controllable phase transitions and novel selection rules in Josephson junctions with inherent orthogonality

NASA Astrophysics Data System (ADS)

Cheng, Qiang; Zhang, Kunhua; Ma, Hongyang

2018-03-01

We propose a new type of Josephson junction consisting of topologically nontrivial superconductors with inherent orthogonality and a ferromagnetic interface. It is found this type of junction can host rich ground states: 0 phase, π phase, 0 + π phase, φ0 phase and φ0 ± φ phase. Phase transitions can be controlled by changing the direction of the interfacial magnetization. Phase diagrams are presented in the orientation space. Novel selection rules for the lowest order current, sin ⁡ ϕ or cos ⁡ ϕ, of this kind of junction are derived. General conditions for the formation of various ground states are established, which possess guiding significance to the experimental design of required ground states for practical applications. We construct the succinct form of a Ginzburg-Landau type of free energy from the viewpoint of the interplay between topological superconductivity and ferromagnetism, which can immediately lead to the selection rules. The constructed terms are universally available to the topological Josephson junctions with or without inherent orthogonality reported recently. The spin supercurrent, its selection rules and their relations to the constructed energy are also investigated.

Magnon Spin Hall Magnetoresistance of a Gapped Quantum Paramagnet.

PubMed

Ulloa, Camilo; Duine, R A

2018-04-27

Motivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices.

Magnon Spin Hall Magnetoresistance of a Gapped Quantum Paramagnet

NASA Astrophysics Data System (ADS)

Ulloa, Camilo; Duine, R. A.

2018-04-01

Motivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices.

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

Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO 3

DOE PAGES

Rosenberg, Aaron J.; Katmis, Ferhat; Kirtley, John R.; ...

2017-12-15

The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-bandgap ferromagnetic insulator with a Curie temperature around 16K, and SrTiO 3 (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K,more » indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. Here, we speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.« less

Room temperature ferromagnetism of nanocrystalline Nd1.90Ni0.10O3-δ

NASA Astrophysics Data System (ADS)

Sarkar, B. J.; Mandal, J.; Dalal, M.; Bandyopadhyay, A.; Chakrabarti, P. K.

2018-05-01

Nanocrystalline sample of Ni2+ doped neodymium oxide (Nd1.90Ni0.10O3-δ, NNO) is synthesized by co-precipitation method. Analysis of X-ray diffraction (XRD) pattern by Rietveld refinement method confirms the desired phase of NNO and complete substitution of Ni2+ ions in the Nd2O3 lattice. Analyses of transmission electron microscopy (TEM) and Raman spectroscopy of NNO recorded at room temperature (RT) also substantiate this fact. Besides, no traces of impurities are found in the analyses of XRD, TEM and Raman data. Room temperature hysteresis loop of NNO suggests the presence of weak ferromagnetism (FM) in low field region ( 600 mT), but in high field region paramagnetism of the host is more prominent. Magnetization vs. temperature ( M- T) curve in the entire temperature range (300-5 K) is analyzed successfully by a combined equation generated from three-dimensional (3D) spin wave model and Curie-Weiss law, which suggests the presence of mixed paramagnetic phase together with ferromagnetic phase in the doped sample. The onset of magnetic ordering is analyzed by oxygen vacancy mediated F-center exchange (FCE) coupling mechanism.

Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO 3

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

Rosenberg, Aaron J.; Katmis, Ferhat; Kirtley, John R.

The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-bandgap ferromagnetic insulator with a Curie temperature around 16K, and SrTiO 3 (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K,more » indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. Here, we speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.« less

Materials being simultaneously ferroelectric, ferromagnetic, ferrotoroidic and ferroelastic

NASA Astrophysics Data System (ADS)

Schmid, Hans

2001-03-01

For the simultaneous occurrence of ferroelectric, ferromagnetic, ferrotoroidic and ferroelastic properties in the same phase, certain stringent symmetry and structural requirements have to be met. Among the 122 Shubnikov point groups 31 are allowing a spontaneous polarization, 31 a spontaneous magnetization and 31 a spontaneous toroidal moment, but only 9 groups allow all three types of vector property at the same time(H. Schmid, Ferroelectrics, in press). In practice this number is further reduced to 6 groups which are permitting "weak ferromagnetism", the most probable type of ferromagnetism found in ferroelectrics with magnetic superexchange interaction nets. The additional occurrence of ferroelasticity is possible for certain prototype phase/ferroic phase pairs of point group (= "Aizu species") only. In addition, one of the mentioned 9 point groups excludes ferroelasticity and allows a co-elastic phase transition only. - The presence or absence of full or partial coupling between the named primary ferroic spontaneous quantities and the resulting possibilities of coupled or non-coupled ferroic domain switching and reorientation are also ruled by the type of species(H. Schmid, Ferroelectrics, 221, 9-17 (1999)). - Apart from a few exceptions, multiferroic phases being simultaneously ferroelectric, ferromagnetic, ferrotoroidic and ferroelastic, are so far well established in the crystal family of boracites M_3B_7O_13X only, where M stands for a bivalent 3d-transition metal ion and X for an ion of the halogens Cl, Br or I(H. Schmid, Ferroelectrics, 162, 317-338 (1994)). - A toroidal moment changes sign both under time reversal and space inversion and has the same symmetry as current density, anapole moment, velocity, etc. First experimental evidence of the presence of a spontaneous toroidal moment in boracites is suggested by phenomenological theory on the basis of measured anomalies of the linear magnetoelectric effect(D.G. Sannikov, Ferroelectrics, 219, 177 (1998)). - For symmetry reasons the above mentioned 9 common point groups necessarily allow all secondary ferroic crystal physical effects (e.g. the linear magnetoelectric, electrotoroidic, magnetotoroidic effects) and all tertiary ferroic crystal physical effects (e.g. the magnetobielectric, electrobimagnetic and piezomagnetoelectric effect) (H. Schmid, Ferroelectrics, in press ),(H. Schmid, in : Complex Mediums, A. Lakhtakia, W.S. Weiglhofer, R.F. Messier, Eds, Proc. of SPIE. 4097, 12-24 (2000)).

First principles study on Fe based ferromagnetic quaternary Heusler alloys

NASA Astrophysics Data System (ADS)

Amudhavalli, A.; Rajeswarapalanichamy, R.; Iyakutti, K.

2017-11-01

The study of stable half-metallic ferromagnetic materials is important from various fundamental and application points of view in condensed matter Physics. Structural phase stability, electronic structure, mechanical and magnetic properties of Fe-based quaternary Heusler alloys XX‧YZ (X = Co, Ni; X‧ = Fe; Y = Ti; Z = Si, Ge, As) for three different phases namely α, β and γ phases of LiMgPdSn crystal structure have been studied by density functional theory with generalized gradient approximation formulated by Perdew, Burke and Ernzerhof (GGA-PBE) and the Hubbard formalism (GGA-PBE + U). This work aims to identify the ferromagnetic and half-metallic properties of XX‧YZ (X = Co, Ni, X‧ = Fe; Y = Ti; Z = Si, Ge, As) quaternary Heusler alloys. The predicted phase stability shows that α-phase is found to be the lowest energy phase at ambient pressure. A pressure-induced structural phase transition is observed in CoFeTiSi, CoFeTiGe, CoFeTiAs, NiFeTiSi, NiFeTiGe and NiFeTiAs at the pressures of 151.6 GPa, 33.7 GPa, 76.4 GPa, 85.3 GPa, 87.7 GPa and 96.5 GPa respectively. The electronic structure reveals that these materials are half metals at normal pressure whereas metals at high pressure. The investigation of electronic structure and magnetic properties are performed to reveal the underlying mechanism of half metallicity. The spin polarized calculations concede that these quaternary Heusler compounds may exhibit the potential candidate in spintronics application. The magnetic moments for these quaternary Heusler alloys in all the three different phases (α, β and γ) are estimated.

Effects of aluminum substitution on the crystal structure and magnetic properties in Zn{sub 2}Y-type hexaferrites

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

Xu, Wenfei; Yang, Jing, E-mail: jyang@ee.ecnu.edu.cn, E-mail: xdtang@sist.ecnu.edu.cn; Bai, Wei

2015-05-07

Crystal structure and magnetic properties of multiferroic Y-type hexaferrites Ba{sub 0.5}Sr{sub 1.5}Zn{sub 2}(Fe{sub 1−x}Al{sub x}){sub 12}O{sub 22} (x = 0, 0.04, 0.08, and 0.12) were investigated. The Z- and M-type impurity phases decrease with increasing Al content, and the pure phase samples can be obtained by modulating Al-doping. Lattice distortion exists in Al-doped samples due to the different radius of Al ion (0.535 Å) and Fe ion (0.645 Å). The microstructural morphologies show that the hexagonal shape grains can be observed in all the samples, and grain size decreases with increasing Al content. As for magnetic properties of Ba{sub 0.5}Sr{sub 1.5}Zn{sub 2}(Fe{sub 1−x}Al{sub x}){submore » 12}O{sub 22}, there exist rich thermal- and field-driven magnetic phase transitions. Temperature dependence of zero-field cooling magnetization curves from 5 K to 800 K exhibit three magnetic phase transitions involving conical spin phase, proper-screw spin phase, ferromagnetic phase, and paramagnetic phase, which can be found in all the samples. Furthermore, the phase-transition temperatures can be modulated by varying Al content. In addition, four kinds of typical hysteresis loops are observed in pure phase sample at different temperatures, which reveal different magnetization processes of above-motioned magnetic spin structures. Typically, triple hysteresis loops in low magnetic field range from 0 to 0.5 T can be observed at 5 K, which suggests low-field driven magnetic phase transitions from conical spin order to proper-screw spin order and further to ferrimagnetic spin order occur. Furthermore, the coercive field (H{sub C}) and the saturation magnetization (M{sub S}) enhance with increasing Al content from x = 0 to 0.08, and drop rapidly at x = 0.12, which could be attribute to that in initial Al-doped process the pitch of spin helix increases and therefore magnetization enhances, but conical spin phase eventually collapses in higher-concentration Al-doping.« less

Elastic moduli of the distorted Kagome-lattice ferromagnet Nd3Ru4Al12

NASA Astrophysics Data System (ADS)

Suzuki, Takashi; Mizuno, Takuyou; Takezawa, Kohki; Kamikawa, Shuhei; Andreev, Alexander V.; Gorbunov, Denis I.; Henriques, Margarida S.; Ishii, Isao

2018-05-01

The distorted kagome-lattice compound Nd3Ru4Al12 has the hexagonal structure. This compound is reported as a ferromagnet in which spins are aligned along the c-axis with the Curie temperature TC = 39 K . The nature of localized f-electrons is expected in Nd3Ru4Al12, and magnetic anisotropy can be attributed to a crystal electric field (CEF) effect. We performed ultrasonic measurements on a Nd3Ru4Al12 single-crystalline sample in order to investigate the phase transition at TC and the CEF effect. All longitudinal and transverse elastic moduli increase monotonically with decreasing temperature, and no clear elastic softening due to a quadrupole interaction is detected under the hexagonal CEF. This result is in contrast to an isomorphic compound Dy3Ru4Al12 with a remarkable elastic softening of the transverse modulus C44. At the ferromagnetic phase transition, the moduli show obvious elastic anomalies, suggesting characteristic couplings between a strain and a magnetic order parameter.

Study of magnetoresistance in the supercooled state of Dy-Y alloys

NASA Astrophysics Data System (ADS)

Jena, Rudra Prasad; Lakhani, Archana

2018-02-01

We report the magnetoresistance studies on Dy1-xYx (x ≤ 0.05) alloys across the first order helimagnetic to ferromagnetic phase transition. These alloys exhibit multiple magnetic phases on varying the temperature and magnetic field. The magnetoresistance studies in the hysteresis region shows irreversibility in forward and reverse field cycles. The resistivity values at zero field for these alloys after zero field cooling to the measurement temperatures, are different in both forward and reverse field cycles. The path dependence of magnetoresistance suggests the presence of helimagnetic phase as the supercooled metastable state which transforms to the stable ferromagnetic state on increasing the field. At high magnetic fields negative magnetoresistance following a linear dependence with field is observed which is attributed to the magnon scattering.

Finite-size scaling analysis on the phase transition of a ferromagnetic polymer chain model

NASA Astrophysics Data System (ADS)

Luo, Meng-Bo

2006-01-01

The finite-size scaling analysis method is applied to study the phase transition of a self-avoiding walking polymer chain with spatial nearest-neighbor ferromagnetic Ising interaction on the simple cubic lattice. Assuming the scaling M2(T,n)=n-2β/ν[Φ0+Φ1n1/ν(T-Tc)+O(n2/ν(T-Tc)2)] with the square magnetization M2 as the order parameter and the chain length n as the size, we estimate the second-order phase-transition temperature Tc=1.784J/kB and critical exponents 2β/ν≈0.668 and ν ≈1.0. The self-diffusion constant and the chain dimensions ⟨R2⟩ and ⟨S2⟩ do not obey such a scaling law.

Ab initio prediction of half-metallic properties for the ferromagnetic Heusler alloys Co2MSi (M=Ti,V,Cr)

NASA Astrophysics Data System (ADS)

Chen, Xing-Qiu; Podloucky, R.; Rogl, P.

2006-12-01

By means of density functional calculations, the magnetic and electronic properties and phase stabilities of the Heusler compounds Co2MSi (with M =Ti,V,Cr,Mn,Fe,Co,Ni) were investigated. Based on the calculated results, we predict the ferromagnetic phases of the compounds Co2TiSi, Co2VSi, and Co2CrSi to be half metals. Of particular interest is Co2CrSi because of its high density of majority-spin states at Fermi energy in combination with a reasonably high estimated Curie temperature of 747K. The compounds Co2TiSi and Co2VSi are thermodynamically stable, whereas Co2CrSi is of a metastable phase which might be stabilized by suitable experimental techniques.

Annealing effects on the structural and magnetic properties of off-stoichiometric Fe-Mn-Ga ferromagnetic shape memory alloys

DOE PAGES

Chen, Yan; Bei, Hongbin; Dela Cruz, Clarina R; ...

2016-05-07

Annealing plays an important role in modifying structures and properties of ferromagnetic shape memory alloys (FSMAs). The annealing effect on the structures and magnetic properties of off-stoichiometric Fe 45Mn 26Ga 29 FSMA has been investigated at different elevated temperatures. Rietveld refinements of neutron diffraction patterns display that the formation of the γ phase in Fe 45Mn 26Ga 29 annealed at 1073 K increases the martensitic transformation temperature and reduces the thermal hysteresis in comparison to the homogenized sample. The phase segregation of a Fe-rich cubic phase and a Ga-rich cubic phase occurs at the annealing temperature of 773 K. Themore » atomic occupancies of the alloys are determined thanks to the neutron's capability of differentiating transition metals. The annealing effects at different temperatures introduce a different magnetic characteristic that is associated with distinctive structural changes in the crystal.« less

Strain-Detecting Composite Materials

NASA Technical Reports Server (NTRS)

Wallace, Terryl A. (Inventor); Smith, Stephen W. (Inventor); Piascik, Robert S. (Inventor); Horne, Michael R. (Inventor); Messick, Peter L. (Inventor); Alexa, Joel A. (Inventor); Glaessgen, Edward H. (Inventor); Hailer, Benjamin T. (Inventor)

2016-01-01

A composite material includes a structural material and a shape-memory alloy embedded in the structural material. The shape-memory alloy changes crystallographic phase from austenite to martensite in response to a predefined critical macroscopic average strain of the composite material. In a second embodiment, the composite material includes a plurality of particles of a ferromagnetic shape-memory alloy embedded in the structural material. The ferromagnetic shape-memory alloy changes crystallographic phase from austenite to martensite and changes magnetic phase in response to the predefined critical macroscopic average strain of the composite material. A method of forming a composite material for sensing the predefined critical macroscopic average strain includes providing the shape-memory alloy having an austenite crystallographic phase, changing a size and shape of the shape-memory alloy to thereby form a plurality of particles, and combining the structural material and the particles at a temperature of from about 100-700.degree. C. to form the composite material.

Chaotic Dynamical Ferromagnetic Phase Induced by Nonequilibrium Quantum Fluctuations

NASA Astrophysics Data System (ADS)

Lerose, Alessio; Marino, Jamir; Žunkovič, Bojan; Gambassi, Andrea; Silva, Alessandro

2018-03-01

We investigate the robustness of a dynamical phase transition against quantum fluctuations by studying the impact of a ferromagnetic nearest-neighbor spin interaction in one spatial dimension on the nonequilibrium dynamical phase diagram of the fully connected quantum Ising model. In particular, we focus on the transient dynamics after a quantum quench and study the prethermal state via a combination of analytic time-dependent spin wave theory and numerical methods based on matrix product states. We find that, upon increasing the strength of the quantum fluctuations, the dynamical critical point fans out into a chaotic dynamical phase within which the asymptotic ordering is characterized by strong sensitivity to the parameters and initial conditions. We argue that such a phenomenon is general, as it arises from the impact of quantum fluctuations on the mean-field out of equilibrium dynamics of any system which exhibits a broken discrete symmetry.

Chaotic Dynamical Ferromagnetic Phase Induced by Nonequilibrium Quantum Fluctuations.

PubMed

Lerose, Alessio; Marino, Jamir; Žunkovič, Bojan; Gambassi, Andrea; Silva, Alessandro

2018-03-30

We investigate the robustness of a dynamical phase transition against quantum fluctuations by studying the impact of a ferromagnetic nearest-neighbor spin interaction in one spatial dimension on the nonequilibrium dynamical phase diagram of the fully connected quantum Ising model. In particular, we focus on the transient dynamics after a quantum quench and study the prethermal state via a combination of analytic time-dependent spin wave theory and numerical methods based on matrix product states. We find that, upon increasing the strength of the quantum fluctuations, the dynamical critical point fans out into a chaotic dynamical phase within which the asymptotic ordering is characterized by strong sensitivity to the parameters and initial conditions. We argue that such a phenomenon is general, as it arises from the impact of quantum fluctuations on the mean-field out of equilibrium dynamics of any system which exhibits a broken discrete symmetry.

Magnetic Properties of Nanoparticle Matrix Composites

DTIC Science & Technology

2015-06-02

recording materials with large value of Ku are SmCo5 with Ku = 11-20 x 10 7 erg/cm 3 for the minimum stable particle size of 2.45 nm, FePt with Ku...nanoparticles and the matrix compared with the bulk behavior of the soft and hard phases and ferromagnetic coupling. 15. SUBJECT TERMS...Magnetic materials , Ab initio methods, nanoparticles, Nanocomposites, Ferromagnetics 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT

The local structure and ferromagnetism in Fe-implanted SrTiO3 single crystals

NASA Astrophysics Data System (ADS)

Lobacheva, O.; Chavarha, M.; Yiu, Y. M.; Sham, T. K.; Goncharova, L. V.

2014-07-01

We report a connection between the local structure of low-level Fe impurities and vacancies as the cause of ferromagnetic behavior observed in strontium titanate single crystals (STO), which were implanted with Fe and Si ions at different doses then annealed in oxygen. The effects of Fe doping and post-implantation annealing of STO were studied by X-ray Absorption Near Edge Structure (XANES) spectroscopy and Superconducting Quantum Interference Device magnetometry. XANES spectra for Fe and Ti K- and L-edge reveal the changes in the local environment of Fe and Ti following the implantation and annealing steps. The annealing in oxygen atmosphere partially healed implantation damages and changed the oxidation state of the implanted iron from metallic Fe0 to Fe2+/Fe3+ oxide. The STO single crystals were weak ferromagnets prior to implantation. The maximum saturation moment was obtained after our highest implantation dose of 2 × 1016 Fe atom/cm2, which could be correlated with the metallic Fe0 phases in addition to the presence of O/Ti vacancies. After recrystallization annealing, the ferromagnetic response disappears. Iron oxide phases with Fe2+ and Fe3+ corresponding to this regime were identified and confirmed by calculations using Real Space Multiple Scattering program (FEFF9).

High-Frequency Response and Voltage Noise in Magnetic Nanocomposites

NASA Astrophysics Data System (ADS)

Buznikov, N. A.; Iakubov, I. T.; Rakhmanov, A. L.; Kugel, K. I.; Sboychakov, A. O.

We study the noise spectra and high-frequency permeability of inhomogeneous magnetic materials consisting of single-domain magnetic nanoparticles embedded into an insulating matrix. Possible mechanisms of 1/f voltage noise in phase-separated manganites is analyzed. The material is modelled by a system of small ferromagnetic metallic droplets (magnetic polarons or ferrons) in insulating antiferromagnetic or paramagnetic matrix. The electron transport is related to tunnelling of charge carriers between droplets. One of the sources of the 1/f noise in such a system stems from fluctuations of the number of droplets with extra electron. In the case of strong magnetic anisotropy, the 1/f noise can arise also due to the fluctuations of the magnetic moments of ferrons. The high frequency magnetic permeability of nanocomposite film with magnetic particles in insulating non-magnetic matrix is studied in detail. The case of strong magnetic dipole interaction and strong magnetic anisotropy of ferromagnetic granules is considered. The composite is modelled by a cubic regular array of ferromagnetic particles. The high-frequency permeability tensor components are found as a functions of frequency, temperature, ferromagnetic phase content, and magnetic anisotropy. The results demonstrate that magnetic dipole interaction leads to a shift of the resonance frequencies towards higher values, and nanocomposite film could have rather high value of magnetic permeability in the microwave range.

High-Frequency Response and Voltage Noise in Magnetic Nanocomposites

NASA Astrophysics Data System (ADS)

Buznikov, N. A.; Iakubov, I. T.; Rakhmanov, A. L.; Kugel, K. I.; Sboychakov, A. O.

2010-12-01

We study the noise spectra and high-frequency permeability of inhomogeneous magnetic materials consisting of single-domain magnetic nanoparticles embedded into an insulating matrix. Possible mechanisms of 1/f voltage noise in phase-separated manganites is analyzed. The material is modelled by a system of small ferromagnetic metallic droplets (magnetic polarons or ferrons) in insulating antiferromagnetic or paramagnetic matrix. The electron transport is related to tunnelling of charge carriers between droplets. One of the sources of the 1/f noise in such a system stems from fluctuations of the number of droplets with extra electron. In the case of strong magnetic anisotropy, the 1/f noise can arise also due to the fluctuations of the magnetic moments of ferrons. The high frequency magnetic permeability of nanocomposite film with magnetic particles in insulating non-magnetic matrix is studied in detail. The case of strong magnetic dipole interaction and strong magnetic anisotropy of ferromagnetic granules is considered. The composite is modelled by a cubic regular array of ferromagnetic particles. The high-frequency permeability tensor components are found as a functions of frequency, temperature, ferromagnetic phase content, and magnetic anisotropy. The results demonstrate that magnetic dipole interaction leads to a shift of the resonance frequencies towards higher values, and nanocomposite film could have rather high value of magnetic permeability in the microwave range.

Vortex Flipping in Superconductor-Ferromagnet Spin Valve Structures

NASA Astrophysics Data System (ADS)

Patino, Edgar J.; Aprili, Marco; Blamire, Mark; Maeno, Yoshiteru

2014-03-01

We report in plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni, below the superconducting transition Tc, our results show strong evidence of vortex flipping driven by the ferromagnets magnetization. This is a direct consequence of proximity effect that leads to vortex supercurrents leakage into the ferromagnets. Here the polarized electron spins are subject to vortices magnetic field occasioning vortex flipping. Such novel mechanism has been made possible for the first time by fabrication of the F/S/F/AF multilayered spin valves with a thin-enough S layer to barely confine vortices inside as well as thin-enough F layers to align and control the magnetization within the plane. When Co is used there is no observation of vortex flipping effect. This is attributed to Co shorter coherence length. Interestingly instead a reduction in pinning field of about 400 Oe is observed when the Nb layer is in superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts any explanation must be directly related to proximity effect and thus a remarkable phenomenon that deserves further investigation. Programa Nacional de Ciencias Basicas COLCIENCIAS (No. 120452128168).

AC magnetic-field response of the ferromagnetic superconductor UGe2 with different magnetized states

NASA Astrophysics Data System (ADS)

Tanaka, Hiroyuki; Yamaguchi, Akira; Kawasaki, Ikuto; Sumiyama, Akihiko; Motoyama, Gaku; Yamamura, Tomoo

2018-01-01

We have performed parallel measurements of dc-magnetization and ac-magnetic susceptibility for a ferromagnetic superconductor, UGe2, in the ferromagnetic-superconducting phase. dc-magnetization measurements revealed that adequate demagnetizing of the sample allows for the preparation of various magnetized states with different zero-field residual magnetization. We observed that these states exhibit varying ac superconducting response at large ac-field amplitudes. The amount of ac flux penetration is less in the demagnetized state involving many domain walls. This result seems to contradict the theory that considers the domain walls as weak links. Moreover, the ferromagnetic domain walls enforce the shielding capability of superconductivity. This observation sheds light on the role of the domain walls on superconductivity, which has been a controversial issue for several decades. Two possible scenarios are presented to explain the enhancement of the shielding capability by the domain walls.

Oxygen vacancies mediated ferromagnetism in hydrogenated Zn0.9Co0.1O film

NASA Astrophysics Data System (ADS)

Zhang, Huiyun; Wang, Ji; Cao, Yanqiang; Guo, Xinli; Li, Qi; Du, Jun; Xu, Qingyu

2018-05-01

Zn0.9Co0.1O films were prepared by pulsed laser deposition and followed by annealing treatment in hydrogen atmosphere. Both samples show ferromagnetic behavior and saturated ferromagnetic magnetization was significantly increased by five times after the hydrogenation treatment. Co ions in both samples have been confirmed to be bivalent as substituents. Moreover, hydrogenation did not change the ZnO wurtzite structure and no segregation of Co, Co oxides or any other secondary phases were detected. Furthermore, the Co 2p3/2 peaks shift to lower energy level after hydrogenation, excluding the formation of Co-H-Co complexes. The structural characterizations clearly confirmed that the increment of oxygen vacancies was due to the hydrogenation treatment. These results indicate that the oxygen vacancies play a crucial role in mediating the ferromagnetism in Zn0.9Co0.1O film.

Ferromagnetic ordering and halfmetallic state in a shandite: Co3Sn2S2

NASA Astrophysics Data System (ADS)

Schnelle, Walter; Leithe-Jasper, Andreas; Rosner, Helge; Weihrich, Richard

2013-03-01

The rapid advance in spintronics challenges an improved understanding of the underlying microscopic properties. Here, we present a joint experimental and theoretical study of Co3Sn2S2 (shandite) and related compounds. From magnetic susceptibility, specific heat and magneto-transport measurements on a shandite single crystal sample we find a phase transition to a ferromagnetic metallic state at 177 K with a saturation moment of 0.92 μB/f.u. Full potential electronic structure calculations within the local spin density approximation result in a halfmetallic ferromagnetic groundstate with a moment of 1 μB/f.u. and a tiny gap in the minority spin channel. The calculated structure optimization and structure variations show that the size of the gap is rather sensitive to the lattice geometry. Possiblities to stabilize the halfmetallic ferromagnetic behavior by various substitutions have been studied theoretically and will be discussed.

Vacancy-Induced Ferromagnetism in SnO2 Nanocrystals: A Positron Annihilation Study

NASA Astrophysics Data System (ADS)

Chen, Zhi-Yuan; Chen, Zhi-Quan; Pan, Rui-Kun; Wang, Shao-Jie

2013-02-01

SnO2 nanopowders were pressed into pellets and annealed in air from 100 to 1400°C. Both XRD and Raman spectroscopy confirm that all annealed samples were single phase with a tetragonal rutile structure. Annealing induces an increase in the SnO2 grain size from 30 to 83 nm. Positron annihilation measurements reveal vacancy defects in the grain boundary region, and the interfacial defects remain stable after annealing below 400°C, then they are gradually recovered with increasing annealing temperature up to 1200°C. Room temperature ferromagnetism was observed for SnO2 nanocrystals annealed below 1200°C, and the magnetization decreases continuously with increasing annealing temperature. However, the ferromagnetism disappears at 1200°C annealing. This shows good coincidence with the recovery of interfacial defects in the nanocrystals, suggesting that the ferromagnetism is probably induced by vacancy defects in the interface region.

The ferromagnetic monolayer Fe(110) on W(110)

NASA Astrophysics Data System (ADS)

Gradmann, U.; Liu, G.; Elmers, H. J.; Przybylski, M.

1990-07-01

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperature T c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine field B hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μB could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.

Room temperature ferromagnetism in Fe-doped CeO2 nanoparticles.

PubMed

Maensiri, Santi; Phokha, Sumalin; Laokul, Paveena; Seraphin, Supapan

2009-11-01

RT ferromagnetism was observed in nanoparticles of Fe-doped CeO2 (i.e., Ce(0.97)Fe(0.03)O2) synthesized by a sol-gel method. The undoped and Fe-doped CeO2 were characterized by XRD, Raman spectroscopy, TEM, and VSM. The undoped samples and Ce(0.97)Fe(0.03)O2 precursor exhibit a diamagnetic behavior. The 673 K-calcined Ce(0.97)Fe(0.03)O2 sample is paramagnetic whereas 773 and 873 K-calcined Ce(0.97)Fe(0.03)O2 samples are ferromagnetism having the magnetizations of 4.65 x 10(-3) emu/g and 6.20 x 10(-3) emu/g at 10 kOe, respectively. Our results indicate that the ferromagnetic property is intrinsic to the Fe-doped CeO2 system and is not a result of any secondary magnetic phase or cluster formation.

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi 2 B 2 C

DOE PAGES

Wulferding, Dirk; Yang, Ilkyu; Yang, Jinho; ...

2015-07-31

We present a local probe study of the magnetic superconductor ErNi 2B 2C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi 2B 2C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth λ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. We estimate the absolutemore » pinning force of Abrikosov vortices, which shows a position dependence and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.« less

Magnetic phase diagrams of amorphous (Ni100-xFex)-metalloid alloys: The key role of the electronic density of states at the Fermi level for the onset of magnetic order

NASA Astrophysics Data System (ADS)

Kiss, L. F.; Bakonyi, I.

2017-11-01

There have been extended studies on the appearance of ferromagnetism in transition-metal-metalloid (MD) glasses. In particular, the paramagnetic (PM) to ferromagnetic (FM) transition has been investigated on numerous (Ni100-xFex)-MD alloys upon the introduction of Fe where MD can represent a combination of various metalloid elements, while keeping the metal/metalloid ratio constant. It has been reported that adding a sufficient amount of Fe to a Pauli PM Ni-MD alloy matrix first induces a spin-glass (SG) state at low temperatures which goes over to a PM state at higher temperatures. Beyond a certain Fe content, xc, the SG state transforms to a FM state upon increasing the temperature. By plotting the characteristic transition temperatures as a function of the Fe content, a magnetic phase diagram can be constructed for each Ni-Fe-MD system which has a multicritical point (MCP) at xc. By using the reported magnetic phase diagrams of various Ni-Fe-MD alloy systems, it is shown that the critical Fe content, xc scales inversely with the density of states at the Fermi level, N(EF), of the parent Ni-MD matrix. This means that the higher the N(EF), the lower the critical Fe content to induce ferromagnetism in the Ni-MD matrix. This is then discussed in terms of the Stoner enhancement factor, S, which characterizes the tendency of the matrix to become ferromagnetic.

Inverse effect of morphotropic phase boundary on the magnetostriction of ferromagnetic Tb1-xGdxCo2

NASA Astrophysics Data System (ADS)

Zhou, Chao; Ren, Shuai; Bao, Huixin; Yang, Sen; Yao, Yonggang; Ji, Yuanchao; Ren, Xiaobing; Matsushita, Yoshitaka; Katsuya, Yoshio; Tanaka, Masahiko; Kobayashi, Keisuke

2014-03-01

The morphotropic phase boundary (MPB) has been utilized extensively in ferroelectrics and recently has attracted interest in ferromagnets [S. Yang, H. Bao, C. Zhou, Y. Wang, X. Ren, Y. Matsushita, Y. Katsuya, M. Tanaka, K. Kobayashi, X. Song, and J. Gao, Phys. Rev. Lett. 104, 197201 (2010), 10.1103/PhysRevLett.104.197201; R. Bergstrom, M. Wuttig, J. Cullen, P. Zavalij, R. Briber, C. Dennis, V. O. Garlea, and M. Laver, Phys. Rev. Lett. 111, 017203 (2013), 10.1103/PhysRevLett.111.017203] for obtaining enhanced large field-induced strain. Here we report that the MPB can also lead to weakening (the inverse effect as compared to the known MPB materials) of field-induced strain, as exhibited in the Tb1-xGdxCo2 system. With synchrotron x-ray diffractometry, the structure symmetry of TbCo2-rich compositions is detected to be rhombohedral below TC and that of GdCo2-rich compositions is tetragonal. The MPB composition Tb0.1Gd0.9Co2, corresponding to the two phases (rhombohedral and tetragonal) of coexistence, shows the exotic minimum (near zero) magnetostriction as well as the largest magnetic susceptibility among all samples. Further analysis suggests that whether MPB can enhance or weaken magnetostriction is determined by the degree of magnetic ordering of two end members that form ferromagnetic MPBs, which was not considered previously. Our work not only reveals a new type of ferromagnetic MPB, but also provides a new recipe for designing functional high-susceptibility and low-strain magnetic materials.

Super-Poissonian Shot Noise of Squeezed-Magnon Mediated Spin Transport.

PubMed

Kamra, Akashdeep; Belzig, Wolfgang

2016-04-08

The magnetization of a ferromagnet (F) driven out of equilibrium injects pure spin current into an adjacent conductor (N). Such F|N bilayers have become basic building blocks in a wide variety of spin-based devices. We evaluate the shot noise of the spin current traversing the F|N interface when F is subjected to a coherent microwave drive. We find that the noise spectrum is frequency independent up to the drive frequency, and increases linearly with frequency thereafter. The low frequency noise indicates super-Poissonian spin transfer, which results from quasiparticles with effective spin ℏ^{*}=ℏ(1+δ). For typical ferromagnetic thin films, δ∼1 is related to the dipolar interaction-mediated squeezing of F eigenmodes.

Avoided ferromagnetic quantum critical point: unusual short-range ordered state in CeFePO.

PubMed

Lausberg, S; Spehling, J; Steppke, A; Jesche, A; Luetkens, H; Amato, A; Baines, C; Krellner, C; Brando, M; Geibel, C; Klauss, H-H; Steglich, F

2012-11-21

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation (μSR). Short-range static magnetism occurs below the freezing temperature T(g) ≈ 0.7 K, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the μSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

Dynamically fluctuating electric dipole moments in fullerene-based magnets.

PubMed

Kambe, Takashi; Oshima, Kokichi

2014-09-19

We report here the direct evidence of the existence of a permanent electric dipole moment in both crystal phases of a fullerene-based magnet--the ferromagnetic α-phase and the antiferromagnetic α'-phase of tetra-kis-(dimethylamino)-ethylene-C60 (TDAE-C60)--as determined by dielectric measurements. We propose that the permanent electric dipole originates from the pairing of a TDAE molecule with surrounding C60 molecules. The two polymorphs exhibit clear differences in their dielectric responses at room temperature and during the freezing process with dynamically fluctuating electric dipole moments, although no difference in their room-temperature structures has been previously observed. This result implies that two polymorphs have different local environment around the molecules. In particular, the ferromagnetism of the α-phase is founded on the homogeneous molecule displacement and orientational ordering. The formation of the different phases with respect to the different rotational states in the Jahn-Teller distorted C60s is also discussed.

Dynamically fluctuating electric dipole moments in fullerene-based magnets

PubMed Central

Kambe, Takashi; Oshima, Kokichi

2014-01-01

We report here the direct evidence of the existence of a permanent electric dipole moment in both crystal phases of a fullerene-based magnet—the ferromagnetic α-phase and the antiferromagnetic α′-phase of tetra-kis-(dimethylamino)-ethylene-C60 (TDAE-C60)—as determined by dielectric measurements. We propose that the permanent electric dipole originates from the pairing of a TDAE molecule with surrounding C60 molecules. The two polymorphs exhibit clear differences in their dielectric responses at room temperature and during the freezing process with dynamically fluctuating electric dipole moments, although no difference in their room-temperature structures has been previously observed. This result implies that two polymorphs have different local environment around the molecules. In particular, the ferromagnetism of the α-phase is founded on the homogeneous molecule displacement and orientational ordering. The formation of the different phases with respect to the different rotational states in the Jahn–Teller distorted C60s is also discussed. PMID:25236361

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.

Quantum anomalous Hall phase and half-metallic phase in ferromagnetic (111) bilayers of 4 d and 5 d transition metal perovskites

NASA Astrophysics Data System (ADS)

Chandra, Hirak Kumar; Guo, Guang-Yu

2017-04-01

Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices (ABO3) 2/(AB'O3)10 of 4 d and 5 d transition metal perovskite (B = Ru, Rh, Ag, Re, Os, Ir, Au; AB'O3=LaAlO3 , SrTiO3) , we demonstrate that due to quantum confinement, bilayers (LaBO3)2 (B = Ru, Re, Os) and (SrBO3)2 (B = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer (LaOsO3)2 is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers (LaRuO3)2 and (SrRhO3)2 are half metallic, while the bilayer (SrIrO3)2 exhibits a peculiar colossal magnetic anisotropy. Our findings thus show that 4 d and 5 d metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.

Helical undulator based on partial redistribution of uniform magnetic field

NASA Astrophysics Data System (ADS)

Balal, N.; Bandurkin, I. V.; Bratman, V. L.; Fedotov, A. E.

2017-12-01

A new type of helical undulator based on redistribution of magnetic field of a solenoid by ferromagnetic helix has been proposed and studied both in theory and experiment. Such undulators are very simple and efficient for promising sources of coherent spontaneous THz undulator radiation from dense electron bunches formed in laser-driven photo-injectors.

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

Majetich, Sara

In the proposed research program we will investigate the time- and frequency-dependent behavior of ordered nanoparticle assemblies, or nanoparticle crystals. Magnetostatic interactions are long-range and anisotropic, and this leads to complex behavior in nanoparticle assemblies, particularly in the time- and frequency-dependent properties. We hypothesize that the high frequency performance of composite materials has been limited because of the range of relaxation times; if a composite is a dipolar ferromagnet at a particular frequency, it should have the advantages of a single phase material, but without significant eddy current power losses. Arrays of surfactant-coated monodomain magnetic nanoparticles can exhibit long-range magneticmore » order that is stable over time. The magnetic domain size and location of domain walls is governed not by structural grain boundaries but by the shape of the array, due to the local interaction field. Pores or gaps within an assembly pin domain walls and limit the domain size. Measurements of the magnetic order parameter as a function of temperature showed that domains can exist at high temoerature, and that there is a collective phase transition, just as in an exchange-coupled ferromagnet. Dipolar ferromagnets are not merely of fundamental interest; they provide an interesting alternative to exchange-based ferromagnets. Dipolar ferromagnets made with high moment metallic particles in an insulating matrix could have high permeability without large eddy current losses. Such nanocomposites could someday replace the ferrites now used in phase shifters, isolators, circulators, and filters in microwave communications and radar applications. We will investigate the time- and frequency-dependent behavior of nanoparticle crystals with different magnetic core sizes and different interparticle barrier resistances, and will measure the magnetic and electrical properties in the DC, low frequency (0.1 Hz - 1 kHz), moderate frequency (10 Hz - 500 MHz), and high frequency (up to 20 GHz) regimes. Our results will demonstrate whether a DC dipolar ferromagnet shows collective frequency-dependent reponse similar to that of an exchange-based ferromagnet, and will provide data for comparison of optimal nanocomposite properties with those of ferrites used in high frequency applications. Both the magnetic and electronic response of the composites will be examined in order to determine the frequency range where hopping conductivity leads to significant eddy current power losses. In the high frequency regime we will look for evidence of spin wave quantization and the resulting decrease in non-linear spin wave processes that could affect the performance of high frequency magnetic devices.« less

Ferromagnetism observed in silicon-carbide-derived carbon

NASA Astrophysics Data System (ADS)

Peng, Bo; Zhang, Yuming; Wang, Yutian; Guo, Hui; Yuan, Lei; Jia, Renxu

2018-02-01

Carbide-derived carbon (CDC) is prepared by etching high purity 4H-SiC single crystals in a mixed atmosphere of 5% Cl2 and 95% Ar for 120 min and 240 min. The secondary ion mass spectroscopy (SIMS) bulk analysis technique excludes the possibility of ferromagnetic transition metal (TM) contamination arising during the experimental process. The paramagnetic and ferromagnetic components are separated from the measured magnetization-magnetic field curves of the samples. Through the use of the Brillouin function, paramagnetic centers carrying a magnetic moment of ˜1.3 μB are fitted. A resolvable hysteresis loop in the low magnetic field area is preserved at room temperature. The temperature dependence of the relative intensity of the Lorentzian-like electron spin resonance (ESR) line observed by electron spin spectroscopy reveals the existence of exchange interaction between the localized paramagnetic centers. First-principles calculations show the dominant configuration of defects in the graphitic CDC films. By calculating the energy difference between the antiferromagnetic and ferromagnetic phases, we deduce that the ferromagnetic coupling is sensitive to the concentration of defects.

Strong ferromagnetic proximity polarization in ferromagnetic metal MnGa/n-type GaAs quantum well junction

NASA Astrophysics Data System (ADS)

Ji, Xiaochen; Shen, Chao; Wu, Yuanjun; Lu, Jun; Zhao, Jianhua; Zheng, Houzhi

2017-11-01

By biasing a ferromagnetic metal MnGa/10 nm-thick, n-type GaAs quantum well (QW) junction from negative to positive, it is found that its spin dynamics at zero magnetic field is composed of two components with opposite signs. One is excited by a circularly polarized pump beam. The other is induced by ferromagnetic proximity polarization (FPP), which is continuously enhanced as the bias increases towards the positive direction. The time-resolved Kerr rotations have also been measured at a magnetic field of 0.9 Tesla. A phase reversion of Larmor precession is observed as the bias passes through  +0.5 V. Following simple quantum mechanics, we become aware of the fact that the transmission and reflection rates of electrons at the interface of MnGa/n-type GaAs QW are enhanced by a factor of ν , which is the attempting frequency of electron onto a ferromagnet/semiconductor interface. That gives a reasonable explanation why the FPP effect in our MnGa/n-type GaAs QW junction is greatly enhanced as biasing it into forward direction.

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).

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

NASA Astrophysics Data System (ADS)

Markovich, V.; Jung, G.; Wisniewski, A.; Mogilyansky, D.; Puzniak, R.; Kohn, A.; Wu, X. D.; Suzuki, K.; Gorodetsky, G.

2012-09-01

Magnetic properties of electron-doped La0.23Ca0.77MnO3 manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine-nitrate method, have been investigated in the temperature range 5-300 K and magnetic fields up to 90 kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T 3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T < 100 K.

Sub-micron magnetic patterns and local variations of adhesion force induced in non-ferromagnetic amorphous steel by femtosecond pulsed laser irradiation

NASA Astrophysics Data System (ADS)

Zhang, Huiyan; Feng, Yuping; Nieto, Daniel; García-Lecina, Eva; Mcdaniel, Clare; Díaz-Marcos, Jordi; Flores-Arias, María Teresa; Gerard M., O.'connor; Baró, Maria Dolors; Pellicer, Eva; Sort, Jordi

2016-05-01

Periodic ripple and nanoripple patterns are formed at the surface of amorphous steel after femtosecond pulsed laser irradiation (FSPLI). Formation of such ripples is accompanied with the emergence of a surface ferromagnetic behavior which is not initially present in the non-irradiated amorphous steel. The occurrence of ferromagnetic properties is associated with the laser-induced devitrification of the glassy structure to form ferromagnetic (α-Fe and Fe3C) and ferrimagnetic [(Fe,Mn)3O4 and Fe2CrO4] phases located in the ripples. The generation of magnetic structures by FSPLI turns out to be one of the fastest ways to induce magnetic patterning without the need of any shadow mask. Furthermore, local variations of the adhesion force, wettability and nanomechanical properties are also observed and compared to those of the as-cast amorphous alloy. These effects are of interest for applications (e.g., biological, magnetic recording, etc.) where both ferromagnetism and tribological/adhesion properties act synergistically to optimize material performance.

Evaluation of stress-induced martensite phase in ferromagnetic shape memory alloy Fe-30.2at%Pd by non-destructive Barkhausen noise

NASA Astrophysics Data System (ADS)

Furuya, Yasubumi; Okazaki, Teiko; Ueno, Takasi; Spearing, Mark; Wutting, Manfred

2005-05-01

Barkhausen noise (BHN) method seems a useful tecnique to non-destructive evaluation of martensite phase transformation of ferromagnetic shape memory alloy, which is used as the filler of our proposing "Smart Composite Board". The concept of design for "Smart Composite Board" which can combine the non-destructive magnetic inspection and shape recovery function in the material itself was formerly proposed. In the present study, we survey the possibility of Barkhausen noise (BHN) method to detect the transformation of microscopic martensite phase caused by stress-loading in Fe-30.2at%Pd thin foil, which has a stable austenite phase (fcc structure) at room temperature. The BHN voltage was measured at loading stress up to 100 MPa in temperature range of 300K to 373K. Stress-induced martensite twin was observed by laser microscope above loading stress of 25 MPa. A phase transformation caused by loading stress were analyzed also by X-ray diffraction. The signals of BHN are analyzed by the time of magnetization and the noise frequency. BHN caused by grain boundaries appears in the lower frequency range (1kHz-3kHz) and BHN by martensite twin in the higher frequency range (8kHz-10kHz). The envelope of the BHN voltage as a function of time of magnetization shows a peak due to austenite phase at weak magnetic field. The BHN envelope due to martensite twins creates additional two peaks at intermediate magnetic field. BHN method turns out to be a powerful technique for non-destructive evaluation of the phase transformation of ferromagnetic shape memory alloy.

Room temperature ferromagnetism in a phthalocyanine based carbon material

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

Honda, Z., E-mail: honda@fms.saitama-u.ac.jp; Sato, K.; Sakai, M.

2014-02-07

We report on a simple method to fabricate a magnetic carbon material that contains nitrogen-coordinated transition metals and has a large magnetic moment. Highly chlorinated iron phthalocyanine was used as building blocks and potassium as a coupling reagent to uniformly disperse nitrogen-coordinated iron atoms on the phthalocyanine based carbon material. The iron phthalocyanine based carbon material exhibits ferromagnetic properties at room temperature and the ferromagnetic phase transition occurs at T{sub c} = 490 ± 10 K. Transmission electron microscopy observation, X-ray diffraction analysis, and the temperature dependence of magnetization suggest that the phthalocyanine molecules form three-dimensional random networks in the iron phthalocyanine based carbon material.

Perovskite-based heterostructures integrating ferromagnetic-insulating La0.1Bi0.9MnO3

NASA Astrophysics Data System (ADS)

Gajek, M.; Bibes, M.; Barthélémy, A.; Varela, M.; Fontcuberta, J.

2005-05-01

We report on the growth of thin films and heterostructures of the ferromagnetic-insulating perovskite La0.1Bi0.9MnO3. We show that the La0.1Bi0.9MnO3 perovskite grows single phased, epitaxially, and with a single out-of-plane orientation either on SrTiO3 substrates or onto strained La2/3Sr1/3MnO3 and SrRuO3 ferromagnetic-metallic buffer layers. We discuss the magnetic properties of the La0.1Bi0.9MnO3 films and heterostructures in view of their possible potential as magnetoelectric or spin-dependent tunneling devices.

Nonlocal thermoelectric effects and nonlocal Onsager relations in a three-terminal proximity-coupled superconductor-ferromagnet device.

PubMed

Machon, P; Eschrig, M; Belzig, W

2013-01-25

We study thermal and charge transport in a three-terminal setup consisting of one superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to very large nonlocal thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a nonlocal version of the Onsager relations for thermoelectric currents holds in a three-terminal quantum coherent ferromagnet-superconductor heterostructure including a spin-dependent crossed Andreev reflection and coherent electron transfer processes.

Phase transition and magnetocaloric properties of Mn50Ni42−xCoxSn8 (0 ≤ x ≤ 10) melt-spun ribbons

PubMed Central

Jiang, Yiwen; Sánchez Valdés, César Fidel; Zhang, Yudong; Esling, Claude; Zhao, Xiang

2018-01-01

The characteristics of magnetostructural coupling play a crucial role in the magnetic field-driven behaviour of magnetofunctional alloys. The availability of magnetostructural coupling over a broad temperature range is of great significance for scientific and technological purposes. This work demonstrates that strong magnetostrucural coupling can be achieved over a wide temperature range (222 to 355 K) in Co-doped high Mn-content Mn50Ni42−xCoxSn8 (0 ≤ x ≤ 10) melt-spun ribbons. It is shown that, over a wide composition range with Co content from 3 to 9 at.%, the paramagnetic austenite first transforms into ferromagnetic austenite at T C on cooling, then the ferromagnetic austenite further transforms into a weakly magnetic martensite at T M. Such strong magnetostructural coupling enables the ribbons to exhibit field-induced inverse martensitic transformation behaviour and a large magnetocaloric effect. Under a field change of 5 T, a maximum magnetic entropy change ΔS M of 18.6 J kg−1 K−1 and an effective refrigerant capacity RC eff of up to 178 J kg−1 can be achieved, which are comparable with or even superior to those of Ni-rich Ni–Mn-based polycrystalline bulk alloys. The combination of high performance and low cost makes Mn–Ni–Co–Sn ribbons of great interest as potential candidates for magnetic refrigeration. PMID:29354271

Model for a Ferromagnetic Quantum Critical Point in a 1D Kondo Lattice

NASA Astrophysics Data System (ADS)

Komijani, Yashar; Coleman, Piers

2018-04-01

Motivated by recent experiments, we study a quasi-one-dimensional model of a Kondo lattice with ferromagnetic coupling between the spins. Using bosonization and dynamical large-N techniques, we establish the presence of a Fermi liquid and a magnetic phase separated by a local quantum critical point, governed by the Kondo breakdown picture. Thermodynamic properties are studied and a gapless charged mode at the quantum critical point is highlighted.

Quantum Femtosecond Magnetism: Phase Transition in Step with Light in a Strongly Correlated Manganese Oxide

NASA Astrophysics Data System (ADS)

Wang, Jigang

2014-03-01

Research of non-equilibrium phase transitions of strongly correlated electrons is built around addressing an outstanding challenge: how to achieve ultrafast manipulation of competing magnetic/electronic phases and reveal thermodynamically hidden orders at highly non-thermal, femtosecond timescales? Recently we reveal a new paradigm called quantum femtosecond magnetism-photoinduced femtosecond magnetic phase transitions driven by quantum spin flip fluctuations correlated with laser-excited inter-atomic coherent bonding. We demonstrate an antiferromagnetic (AFM) to ferromagnetic (FM) switching during about 100 fs laser pulses in a colossal magneto-resistive manganese oxide. Our results show a huge photoinduced femtosecond spin generation, measured by magnetic circular dichroism, with photo-excitation threshold behavior absent in the picosecond dynamics. This reveals an initial quantum coherent regime of magnetism, while the optical polarization/coherence still interacts with the spins to initiate local FM correlations that compete with the surrounding AFM matrix. Our results thus provide a framework that explores quantum non-equilibrium kinetics to drive phase transitions between exotic ground states in strongly correlated elecrons, and raise fundamental questions regarding some accepted rules, such as free energy and adiabatic potential surface. This work is in collaboration with Tianqi Li, Aaron Patz, Leonidas Mouchliadis, Jiaqiang Yan, Thomas A. Lograsso, Ilias E. Perakis. This work was supported by the National Science Foundation (contract no. DMR-1055352). Material synthesis at the Ames Laboratory was supported by the US Department of Energy-Basic Energy Sciences (contract no. DE-AC02-7CH11358).

Large magnetocaloric effect and near-zero thermal hysteresis in the rare earth intermetallic Tb1-x Dy x Co2 compounds

NASA Astrophysics Data System (ADS)

Zeng, Yuyang; Tian, Fanghua; Chang, Tieyan; Chen, Kaiyun; Yang, Sen; Cao, Kaiyan; Zhou, Chao; Song, Xiaoping

2017-02-01

We report the magnetocaloric effect in a Tb1-x Dy x Co2 compound which exhibits a wide working temperature window around the Curie temperature (T C) and delivers a large refrigerant capacity (RC) with near-zero thermal hysteresis. Specifically, the wide full width at half maxima ({δ\\text{WFHM}} ) can reach up to 62 K and the RC value changes from 216.5 to 274.3 J Kg-1 when the external magnetic field increases to 5 T. Such magnetocaloric effects are attributed to a magnetic and structural transition from a paramagnetic and cubic phase to a ferromagnetic (M S along [1 1 1] direction) and rhombohedral phase or ferromagnetic (M S along [0 0 1] direction) and tetragonal phase.

Nanoscale ferromagnetism in phase-separated manganites

NASA Astrophysics Data System (ADS)

Mori, S.; Horibe, Y.; Asaka, T.; Matsui, Y.; Chen, C. H.; Cheong, S. W.

2007-03-01

Magnetic domain structures in phase-separated manganites were investigated by low-temperature Lorentz electron microscopy, in order to understand some unusual physical properties such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator transition. In particular, we examined a spatial distribution of the charge/orbital-ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic one in phase-separated manganites; Cr-doped Nd0.5Ca0.5MnO3 and ( La1-xPrx)CaMnO3 with x=0.375, by obtaining both the dark-field images and Lorentz electron microscopic ones. It is found that an unusual coexistence of the CO/OO and FM metallic states below a FM transition temperature in the two compounds. The present experimental results clearly demonstrated the coexisting state of the two distinct ground states in manganites.

Experimental evidence of stress-field-induced selection of variants in Ni-Mn-Ga ferromagnetic shape-memory alloys

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

Wang, Y. D.; Key Laboratory for Anisotropy and Texture of Materials; Brown, D. W.

2007-05-01

The in situ time-of-flight neutron-diffraction measurements captured well the martensitic transformation behavior of the Ni-Mn-Ga ferromagnetic shape-memory alloys under uniaxial stress fields. We found that a small uniaxial stress applied during phase transformation dramatically disturbed the distribution of variants in the product phase. The observed changes in the distributions of variants may be explained by considering the role of the minimum distortion energy of the Bain transformation in the effective partition among the variants belonging to the same orientation of parent phase. It was also found that transformation kinetics under various stress fields follows the scale law. The present investigationsmore » provide the fundamental approach for scaling the evolution of microstructures in martensitic transitions, which is of general interest to the condensed matter community.« less

Magnetism-Structure Correlations during the epsilon ->tau Transformation in Rapidly-Solidified MnAl Nanostructured Alloys

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

Jimenez-Villacorta, F; Marion, JL; Oldham, JT

2014-01-21

Magnetic and structural aspects of the annealing-induced transformation of rapidly-solidified Mn55Al45 ribbons from the as-quenched metastable antiferromagnetic (AF) epsilon-phase to the target ferromagnetic (FM) L1(0) tau-phase are investigated. The as-solidified material exhibits a majority hexagonal epsilon-MnAl phase revealing a large exchange bias shift below a magnetic blocking temperature T-B similar to 95 K (H-ex similar to 13 kOe at 10 K), ascribed to the presence of compositional fluctuations in this antiferromagnetic phase. Heat treatment at a relatively low annealing temperature T-anneal approximate to 568 K (295 degrees C) promotes the nucleation of the metastable L1(0) tau-MnAl phase at the expensemore » of the parent epsilon-phase, donating an increasingly hard ferromagnetic character. The onset of the epsilon ->tau transformation occurs at a temperature that is similar to 100 K lower than that reported in the literature, highlighting the benefits of applying rapid solidification for synthesis of the rapidly-solidified parent alloy.« less

Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes

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

Uhlir, V.; Arregi, J. A.; Fullerton, E. E.

Coupled order parameters in phase-transition materials can be controlled using various driving forces such as temperature, magnetic and electric field, strain, spin-polarized currents and optical pulses. Tuning the material properties to achieve efficient transitions would enable fast and low-power electronic devices. Here we show that the first-order metamagnetic phase transition in FeRh films becomes strongly asymmetric in mesoscale structures. In patterned FeRh stripes we observed pronounced supercooling and an avalanche-like abrupt transition from the ferromagnetic to the antiferromagnetic phase, while the reverse transition remains nearly continuous over a broad temperature range. Although modest asymmetry signatures have been found in FeRhmore » films, the effect is dramatically enhanced at the mesoscale. The activation volume of the antiferromagnetic phase is more than two orders of magnitude larger than typical magnetic heterogeneities observed in films. Finally, the collective behaviour upon cooling results from the role of long-range ferromagnetic exchange correlations that become important at the mesoscale and should be a general property of first-order metamagnetic phase transitions.« less

Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes

DOE PAGES

Uhlir, V.; Arregi, J. A.; Fullerton, E. E.

2016-10-11

Coupled order parameters in phase-transition materials can be controlled using various driving forces such as temperature, magnetic and electric field, strain, spin-polarized currents and optical pulses. Tuning the material properties to achieve efficient transitions would enable fast and low-power electronic devices. Here we show that the first-order metamagnetic phase transition in FeRh films becomes strongly asymmetric in mesoscale structures. In patterned FeRh stripes we observed pronounced supercooling and an avalanche-like abrupt transition from the ferromagnetic to the antiferromagnetic phase, while the reverse transition remains nearly continuous over a broad temperature range. Although modest asymmetry signatures have been found in FeRhmore » films, the effect is dramatically enhanced at the mesoscale. The activation volume of the antiferromagnetic phase is more than two orders of magnitude larger than typical magnetic heterogeneities observed in films. Finally, the collective behaviour upon cooling results from the role of long-range ferromagnetic exchange correlations that become important at the mesoscale and should be a general property of first-order metamagnetic phase transitions.« less

Beyond the Quantum Hall Effect: New Phases of 2D Electrons at High Magnetic Field

NASA Astrophysics Data System (ADS)

Eisenstein, James

2007-03-01

In this talk I will discuss recent experiments on high mobility single and double layer 2D electron systems in which collective phases lying outside the usual quantum Hall effect paradigm have been detected and studied. For example, in single layer 2D systems near half-filling of highly excited Landau levels new states characterized by a massive anisotropy in the electrical resistivity of the sample are observed at very low temperature. The anisotropy has been widely interpreted as the signature of a new class of correlated electron phases which incorporate a stripe-like charge density modulation. Orientational ordering of small striped domains at low temperatures accounts for the resistive anisotropy and is reminiscent of the isotropic-to-nematic phase transition in classical liquid crystals. Double layer 2D electron systems possess collective phases not present in single layer systems. In particular, when the total number of electrons in the bilayer equals the degeneracy of a single Landau level, an unusual phase appears at small layer separation. This phase possesses a novel broken symmetry, spontaneous interlayer phase coherence, which has a number of dramatic experimental signatures. The interlayer tunneling conductance develops a strong and very sharp resonance around zero bias resembling the dc Josephson effect. At the same time, both the longitudinal and Hall resistances of the sample vanish at low temperatures when currents are driven in opposite directions through the two layers. These, and other observations are broadly consistent with theories in which the broken symmetry phase can equivalently be described as a pseudospin ferromagnet or an (imperfect) excitonic superfluid. This work reflects a collaboration with M.P. Lilly, K.B. Cooper, I.B. Spielman, M. Kellogg, L.A. Tracy, L.N. Pfeiffer, and K.W. West.

Voltage-Controlled On/Off Switching of Ferromagnetism in Manganite Supercapacitors.

PubMed

Molinari, Alan; Hahn, Horst; Kruk, Robert

2018-01-01

The ever-growing technological demand for more advanced microelectronic and spintronic devices keeps catalyzing the idea of controlling magnetism with an electric field. Although voltage-driven on/off switching of magnetization is already established in some magnetoelectric (ME) systems, often the coupling between magnetic and electric order parameters lacks an adequate reversibility, energy efficiency, working temperature, or switching speed. Here, the ME performance of a manganite supercapacitor composed of a ferromagnetic, spin-polarized ultrathin film of La 0.74 Sr 0.26 MnO 3 (LSMO) electrically charged with an ionic liquid electrolyte is investigated. Fully reversible, rapid, on/off switching of ferromagnetism in LSMO is demonstrated in combination with a shift in Curie temperature of up to 26 K and a giant ME coupling coefficient of ≈226 Oe V -1 . The application of voltages of only ≈2 V results in ultralow energy consumptions of about 90 µJ cm -2 . This work provides a step forward toward low-power, high-endurance electrical switching of magnetism for the development of high-performance ME spintronics. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Spin-orbit torque in a three-dimensional topological insulator-ferromagnet heterostructure: Crossover between bulk and surface transport

NASA Astrophysics Data System (ADS)

Ghosh, S.; Manchon, A.

2018-04-01

Current-driven spin-orbit torques are investigated in a heterostructure composed of a ferromagnet deposited on top of a three-dimensional topological insulator using the linear response formalism. We develop a tight-binding model of the heterostructure adopting a minimal interfacial hybridization scheme that promotes induced magnetic exchange on the topological surface states, as well as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore our model accounts for the spin Hall effect from bulk states together with inverse spin galvanic and magnetoelectric effects at the interface on equal footing. By varying the transport energy across the band structure, we uncover a crossover from surface-dominated to bulk-dominated transport regimes. We show that the spin density profile and the nature of the spin-orbit torques differ substantially in both regimes. Our results, which compare favorably with experimental observations, demonstrate that the large dampinglike torque reported recently is more likely attributed to the Berry curvature of interfacial states, while spin Hall torque remains small even in the bulk-dominated regime.

Defect mediated ferromagnetism in Ni-doped ZnO nanocrystals evidenced by positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Chen, Zhi-Yuan; Chen, Z. Q.; Zou, B.; Zhao, X. G.; Tang, Z.; Wang, S. J.

2012-10-01

NiO/ZnO nanocomposites with NiO content of 4 at. % and 20 at. % were annealed up to 1200 °C to get Ni doped ZnO nanocrystals. Raman scattering spectra illustrate a broad and strong band at 500-600cm-1 in all nanocomposites after annealing above 700 °C, which suggests incorporation of Ni in the ZnO lattice. However, x-ray diffraction measurements show that NiO phase can be still observed in all nanocomposites after annealing, which indicates that Ni is partially doped into the ZnO structure. Positron annihilation measurements reveal large number of vacancy defects in the interface region of all nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C. Room temperature ferromagnetism can be observed in the NiO/ZnO nanocomposites, which is stronger in the 20 at. % NiO/ZnO nanocomposites, and the magnetization decreases continuously with increasing annealing temperature. This indicates that the ferromagnetism at low annealing temperatures originates from the NiO nanograins, and they become antiferromanetic after subsequent higher temperature annealing which leads to the weakening of ferromagnetism. After annealing up to 1000 °C, the ferromagnetism in both the two samples becomes nearly invisible. The disappearance of ferromagnetism shows good coincidence with the recovery of vacancy defects in NiO/ZnO nanocomposites. It can be inferred that the ferromagnetism is mediated by vacancy defects which are distributed in the interface region.

Effect of natural homointerfaces on the magnetic properties of pseudomorphic La0.7Sr0.3MnO3 thin film: Phase separation vs split domain structure

NASA Astrophysics Data System (ADS)

Congiu, Francesco; Sanna, Carla; Maritato, Luigi; Orgiani, Pasquale; Geddo Lehmann, Alessandra

2016-12-01

We studied the effect of naturally formed homointerfaces on the magnetic and electric transport behavior of a heavily twinned, 40 nm thick, pseudomorphic epitaxial film of La0.7Sr0.3MnO3 deposited by molecular beam epitaxy on ferroelastic LaAlO3(001) substrate. As proved by high resolution X-ray diffraction analysis, the lamellar twin structure of the substrate is imprinted in La0.7Sr0.3MnO3. In spite of the pronounced thermomagnetic irreversibility in the DC low field magnetization, spin-glass-like character, possibly related to the structural complexity, was ruled out, on the base of AC susceptibility results. The magnetic characterization indicates anisotropic ferromagnetism, with a saturation magnetization Ms = 3.2 μB/Mn, slightly reduced with respect to the fully polarized value of 3.7 μB/Mn. The low field DC magnetization vs temperature is non bulklike, with a two step increase in the field cooled MFC(T) branch and a two peak structure in the zero field cooled MZFC(T) one. Correspondingly, two peaks are present in the resistivity vs temperature ρ(T) curve. With reference to the behavior of epitaxial manganites deposited on bicrystal substrates, results are discussed in terms of a two phase model, in which each couple of adjacent ferromagnetic twin cores, with bulklike TC = 370 K, is separated by a twin boundary with lower Curie point TC = 150 K, acting as barrier for spin polarized transport. The two phase scenario is compared with the alternative one based on a single ferromagnetic phase with the peculiar ferromagnetic domains structure inherent to twinned manganites films, reported to be split into interconnected and spatially separated regions with in-plane and out-of-plane magnetization, coinciding with twin cores and twin boundaries respectively.

Room-temperature ferromagnetic transitions and the temperature dependence of magnetic behaviors in FeCoNiCr-based high-entropy alloys

NASA Astrophysics Data System (ADS)

Na, Suok-Min; Yoo, Jin-Hyeong; Lambert, Paul K.; Jones, Nicholas J.

2018-05-01

High-entropy alloys (HEAs) containing multiple principle alloying elements exhibit unique properties so they are currently receiving great attention for developing innovative alloy designs. In FeCoNi-based HEAs, magnetic behaviors strongly depend on the addition of alloying elements, usually accompanied by structural changes. In this work, the effect of non-magnetic components on the ferromagnetic transition and magnetic behaviors in equiatomic FeCoNiCrX (X=Al, Ga, Mn and Sn) HEAs was investigated. Alloy ingots of nominal compositions of HEAs were prepared by arc melting and the button ingots were cut into discs for magnetic measurements as functions of magnetic field and temperature. The HEAs of FeCoNiCrMn and FeCoNiCrSn show typical paramagnetic behaviors, composed of solid solution FCC matrix, while the additions of Ga and Al in FeCoNiCr exhibit ferromagnetic behaviors, along with the coexistence of FCC and BCC phases due to spinodal decomposition. The partial phase transition in both HEAs with the additions of Ga and Al would enhance ferromagnetic properties due to the addition of the BCC phase. The saturation magnetization for the base alloy FeCoNiCr is 0.5 emu/g at the applied field of 20 kOe (TC = 104 K). For the HEAs of FeCoNiCrGa and FeCoNiCrAl, the saturation magnetization significantly increased to 38 emu/g (TC = 703 K) and 25 emu/g (TC = 277 K), respectively. To evaluate the possibility of solid solution FCC and BCC phases in FeCoNiCr-type HEAs, we introduced a parameter of valence electron concentration (VEC). The proposed rule for solid solution formation by the VEC was matched with FeCoNiCr-type HEAs.

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

Quasistatic remanence in Dzyaloshinskii-Moriya interaction driven weak ferromagnets and piezomagnets

NASA Astrophysics Data System (ADS)

Pattanayak, Namrata; Bhattacharyya, Arpan; Nigam, A. K.; Cheong, Sang-Wook; Bajpai, Ashna

2017-09-01

We explore remanent magnetization (μ ) as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFMs) which are also weak ferromagnets (WFMs) and piezomagnets (PzMs). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasistatic in nature and exhibits a counterintuitive magnetic field dependence. These observations unravel an ultraslow magnetization relaxation phenomenon related to this quasistatic remanence. This feature is also observed in a defect-free single crystal of α -Fe2O3 , which is a canonical WFM and PzM. Notably, α -Fe2O3 is not a typical geometrically frustrated AFM, and in single crystal form it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFMs which either are symmetry allowed WFMs, driven by Dzyaloshinskii-Moriya interaction, or can generate this trait by tuning of size and interface. The qualitative features of the quasistatic remanence indicate that such WFMs are potential piezomagnets, in which magnetization can be tuned by stress alone.

Fast domain wall motion in the vicinity of the angular momentum compensation temperature of ferrimagnets

NASA Astrophysics Data System (ADS)

Kim, Kab-Jin; Kim, Se Kwon; Hirata, Yuushou; Oh, Se-Hyeok; Tono, Takayuki; Kim, Duck-Ho; Okuno, Takaya; Ham, Woo Seung; Kim, Sanghoon; Go, Gyoungchoon; Tserkovnyak, Yaroslav; Tsukamoto, Arata; Moriyama, Takahiro; Lee, Kyung-Jin; Ono, Teruo

2017-12-01

Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation towards this direction is that antiferromagnetic spin dynamics is expected to be much faster than its ferromagnetic counterpart. Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs. However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored, mainly because of the magnetic field immunity of antiferromagnets. Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare earth-3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility is remarkably enhanced up to 20 km s-1 T-1. The collective coordinate approach generalized for ferrimagnets and atomistic spin model simulations show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.

Magnetic refrigeration capabilities of magnetocaloric Ni2Mn:75Cu:25Ga

NASA Astrophysics Data System (ADS)

Mishra, S. K.; Jenkins, C. A.; Dubenko, I.; Samanta, T.; Ali, N.; Roy, S.

2013-03-01

Doping-driven competition between energetically similar ground states leads to many exciting materials phenomena such as the emergence of high-Tc superconductivity, diluted magnetic semiconductors, and colossal magnetoresistance. Doped Ni2MnGa Heusler alloy, which is a multifunctional ferromagnetic alloy with various exotic physical properties demonstrates this notion of rich phenomenology via modified ground spin states. Adopting this generic concept, here we will present a novel doped Ni2Mn.75Cu.25Ga alloy that offers unprecedented co-existence of the magnetocaloric effect and fully controlled ferromagnetism at room temperature. Application of site engineering enables us to manipulate the ground spin state that leads to the decrease in magnetic transition temperature and also increases the delocalization of the Mn magnetism. SQUID magnetometery suggests that Cu doping enhances the saturation magnetization, coercive field and clarity of magnetic hysteresis loops. By exploiting x-ray absorption techniques and measuring element specific magnetic hysteresis loops, here we will describe the microscopic origin of enhnaced magnetocaloric properties and d-d interaction driven charge transfer effects in Ni2Mn.75Cu.25Ga This work was supported by DOE Grant No. DE-FG02-06ER46291

Pressure-induced transition from a spin glass to an itinerant ferromagnet in the half-doped manganite L0.5Ba0.5MnO3 (L=Sm and Nd) with quenched disorder

NASA Astrophysics Data System (ADS)

Takeshita, N.; Terakura, C.; Akahoshi, D.; Tokura, Y.; Takagi, H.

2004-05-01

The effect of quenched disorder on the multiphase competition has been investigated by examining the pressure phase diagram of the half-doped manganite L0.5Ba0.5MnO3 (L=Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin-glass insulator at ambient pressure, switches to a ferromagnetic metal through an intermediate state with increasing pressure, followed by a rapid increase of the ferromagnetic transition temperature TC. The rapid increase of TC was also confirmed for Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the multicritical phase boundary in the A-site disordered system, previously observed as a function of the averaged A-site ionic radius, is essentially controlled by the pressure and hence the bandwidth. The effect of quenched disorder is therefore much more enhanced with approaching the multicritical region.

Magnetic and transport signatures of Rashba spin-orbit coupling on the ferromagnetic Kondo lattice model in two dimensions

NASA Astrophysics Data System (ADS)

Meza, Giovany A.; Riera, José A.

2014-08-01

Motivated by emergent phenomena at oxide surfaces and interfaces, particularly those involving transition metal oxides with perovskite crystal structure such as LaTiO3/SrTiO3, we examine the ferromagnetic Kondo lattice model (FKLM) in the presence of a Rashba spin-orbit coupling (RSOC). Using numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin, and transport properties on square clusters at zero temperature. We find that the main effect of the RSOC is the destruction of the ferromagnetic state present in the FKLM at low electron fillings, with the consequent suppression of conductivity. In addition, near half filling the RSOC leads to a departure of the antiferromagnetic state of the FKLM with a consequent reduction to the intrinsic tendency to electronic phase separation. The interplay between phase separation on one side, and magnetic and transport properties on the other, is carefully analyzed as a function of the RSOC/hopping ratio.

Long-Range Anti-ferromagnetic Order in Sm2Ti2O7

NASA Astrophysics Data System (ADS)

Mauws, Cole; Sarte, Paul; Hallas, Alannah; Wildes, Andrew; Quilliam, Jeffrey; Luke, Graeme; Gaulin, Bruce; Wiebe, Christopher

The spin ice state has been a key topic in frustrated magnetism for decades. Largely due to the presence of monopole-like excitations, leading to interesting physics. There has been a consistent effort in the field at synthesising new spin ice phases that possess smaller moments in the hopes of increasing the density of magnetic monopoles. As well as investigating the phase when quantum fluctuations dominate over dipolar interactions. Initially Sm2Ti2O7 was thought to be a candidate for a quantum spin ice, possessing a low moment of 1.5 μB in the high-spin case and crystal fields may reduce it to a true spin-1/2 system. However anti-ferromagnetic interactions as well as a lambda-like heat capacity anomaly pointed towards long-range antiferromagnetic order. An isotopically enriched samarium-154 single crystal was taken to the D7 polarized diffuse scattering spectrometer at the ILL. Long-range antiferromagnetic order was observed and indexed onto the all-in all-out structure. This agrees with theoretical predictions of Ising pyrochlore systems with sufficiently large anti-ferromagnetic coupling. NSERC, CFI, CIFAR, CRC.

Low temperature transport anomaly in Cr substituted (La0.67Sr0.33)MnO3 manganites

NASA Astrophysics Data System (ADS)

Tank, Tejas M.; Shelke, Vilas; Das, Sarmistha; Rana, D. S.; Thaker, C. M.; Samatham, S. S.; Ganesan, V.; Sanyal, S. P.

2017-06-01

The structural, electrical, and magnetic properties of La0.67Sr0.33Mn1-xCrxO3 (0 ≤ x ≤ 0.10) manganites have been studied by substitution of antiferromagnetic trivalent Cr ion at Mn-site. Systematic efforts have been carried out to understand the electrical resistivity behavior in the ferromagnetic metallic and paramagnetic semi-conducting phases of Cr substituted La0.67Sr0.33Mn1-xCrxO3 manganites. Polycrystalline samples show a resistivity minimum at a temperature (Tmin) of <40 K in the ferromagnetic metallic phase. Tmin shifts to higher temperatures on application of magnetic fields. The appearance of this resistivity minimum was analyzed by fittings the data according to the model that considers e-e scattering caused by enhanced Coulombic interactions. The electrical resistivity data has been best fitted in the metallic and semiconducting regime using various models. Present results suggest that intrinsic magnetic inhomogeneity like Cr3+ ions in these strongly electron-correlated manganite systems is originating due to the existence of the ferromagnetic interactions.

Magnetic interactions in La0.7Sr0.3Mn1-xMexO3 (Me=Ga, Fe, Cr) manganites

NASA Astrophysics Data System (ADS)

Troyanchuk, I. O.; Bushinsky, M. V.; Karpinsky, D. V.; Tereshko, N. V.; Dobryansky, V. M.; Többens, D. M.; Sikolenko, V.; Efimov, V.

2015-11-01

Magnetic properties and crystal structure of La0.7Sr0.3Mn1-xMexO3 (Me=Ga, Fe, Cr; x≤0.3) have been studied by neutron powder diffraction and magnetization measurements. It is shown that substitution of manganese ions by chromium or gallium ions (x=0.3) leads to phase separation into antiferromagnetic and ferromagnetic phases whereas replacement by Fe ions stabilizes spin glass state (x=0.3). Ferromagnetic interactions in Cr-substituted compounds are much more pronounced than in Fe- and Ga-doped ones. Magnetic properties are discussed in the model assuming a dominance of superexchange interactions. It is considered that ferromagnetism in the Cr-substituted compositions is associated with nearly equal contributions from positive and negative components of the superexchange interaction between Mn3+ and Cr3+ ions as well as to mixed valence of chromium ions. The spin glass state observed for the Fe-doped sample (x=0.3) is associated with strong antiferromagnetic superexchange between Fe3+-O-Fe3+ and Fe3+-O-(Mn3+, Mn4+).

Alteration of magnetic and optical properties of ultrafine dilute magnetic semiconductor ZnO:Co2+ nanoparticles.

PubMed

Sharma, Prashant K; Dutta, Ranu K; Pandey, Avinash C

2010-05-15

Single-phase ZnO:Co(2+) nanoparticles of mean size 2-8 nm were synthesized by a simple co-precipitation technique. X-ray diffraction analysis reveals that the Co-doped ZnO nanoparticles crystallize in wurtzite structure without any impurity phase. The wurtzite structure (lattice constants) of ZnO nanoparticles decrease slightly with increasing Co doping concentration. Optical absorption spectra show an increase in the band gap with increasing Co content and also give an evidence of the presence of Co(2+) ions at tetrahedral sites of ZnO and substituted for the Zn site with no evidence of metallic Co. Initially these nanoparticles showed strong ferromagnetic behavior at room temperature, however at higher doping percentage of Co(2+), the ferromagnetic behavior was suppressed, and antiferromagnetic nature was enhanced. The enhanced antiferromagnetic interaction between neighboring Co-Co ions suppressed the ferromagnetism at higher doping concentrations of Co(2+). Photoluminescence intensity owing to the vacancies varies with the Co concentration because of the increment of oxygen vacancies. Copyright © 2010 Elsevier Inc. All rights reserved.

Magnetic and critical properties of Pr0.6Sr0.4MnO3 nanocrystals prepared by a combination of the solid state reaction and the mechanical ball milling methods

NASA Astrophysics Data System (ADS)

Dung, Nguyen Thi; Linh, Dinh Chi; Huyen Yen, Pham Duc; Yu, Seong Cho; Van Dang, Nguyen; Dang Thanh, Tran

2018-06-01

Influence of the crystallite size on the magnetic and critical properties of nanocrystals has been investigated. The results show that Curie temperature and magnetization slightly decrease with decreasing average crystallite size . Based on the mean-field theory and the magnetic-field dependences of magnetization at different temperatures , we pointed out that the ferromagnetic-paramagnetic phase transition in the samples undergoes the second-order phase transition with the critical exponents (, , and ) close to those of the mean-field theory. However, there is a small deviation from those expected for the mean-field theory of the values of , and obtained for the samples. It means that short-range ferromagnetic interactions appear in the smaller particles. In other words, nanocrystals become more magnetically inhomogeneous with smaller crystallite sizes that could be explained by the presence of surface-related effects, lattice strain and distortions, which lead the strength of ferromagnetic interaction is decreased in the small crystallite sizes.

Sensitive imaging of magnetization structure and dynamics using picosecond laser heating

NASA Astrophysics Data System (ADS)

Bartell, Jason; Jermain, Colin; Aradhya, Sriharsha; Brangham, Jack; Yang, Fengyuan; Ralph, Daniel; Fuchs, Gregory

We demonstrate the time-resolved longitudinal spin Seebeck effect (TRLSSE) as the basis for an ultrafast, high-resolution, and sensitive microscope for imaging ferromagnetic insulator/normal metal spintronic devices. By focusing a picosecond laser to 0.7 μm, we generate a sub-100 ps electrical signal from the combination of the TRLSSE and the inverse spin Hall effect in yittrium iron garnet (YIG)/platinum (Pt) bilayers. This signal is a spatiotemporal measurement of the local, in-plane magnetic orientation of YIG with outstanding sensitivity better than 0.3° /√{ Hz } in samples with 20 nm of YIG. Static imaging of YIG/Pt devices reveals variations in the local magnetic anisotropy on a few micron scale. Phase-sensitive ferromagnetic resonance imaging reveals corresponding variations in the resonance field, amplitude, phase, and linewidth. These results show the TRLSSE is a powerful tool for static and dynamic studies of spintronic devices made with ferromagnetic insulators. This research was supported by the AFOSR (FA9550-14-1-0243) and by NSF (DMR-1406333, DMR-1507274, and DMR-1120296).

Equilibrium, metastability, and hysteresis in a model spin-crossover material with nearest-neighbor antiferromagnetic-like and long-range ferromagnetic-like interactions

NASA Astrophysics Data System (ADS)

Rikvold, Per Arne; Brown, Gregory; Miyashita, Seiji; Omand, Conor; Nishino, Masamichi

2016-02-01

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice S =1 /2 Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical end points and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

Magnetic structure and phase stability of the van der Waals bonded ferromagnet Fe 3-xGeTe 2

DOE PAGES

May, Andrew F.; Calder, Stuart A.; Cantoni, Claudia; ...

2016-01-08

The magnetic structure and phase diagram of the layered ferromagnetic compound Fe 3GeTe 2 have been investigated by a combination of synthesis, x-ray and neutron diffraction, high-resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single-crystal neutron diffraction finds ferromagnetic order with moments of 1.11(5)μ B/Fe aligned along the c axis at 4 K. These flux-grown crystals have a lower Curie temperature T c ≈ 150 K than crystals previously grown by vapor transport (T c = 220 K). The difference is a reduced Fe content in the flux-grown crystals, as illustrated by the behavior observedmore » in a series of polycrystalline samples. As Fe content decreases, so do the Curie temperature, magnetic anisotropy, and net magnetization. Furthermore, Hall-effect and thermoelectric measurements on flux-grown crystals suggest that multiple carrier types contribute to electrical transport in Fe 3–xGeTe 2 and structurally similar Ni 3–xGeTe 2.« less

Anomalous current in diffusive ferromagnetic Josephson junctions

NASA Astrophysics Data System (ADS)

Silaev, M. A.; Tokatly, I. V.; Bergeret, F. S.

2017-05-01

We demonstrate that in diffusive superconductor/ferromagnet/superconductor (S/F/S) junctions a finite, anomalous Josephson current can flow even at zero phase difference between the S electrodes. The conditions for the observation of this effect are noncoplanar magnetization distribution and a broken magnetization inversion symmetry of the superconducting current. The latter symmetry is intrinsic for the widely used quasiclassical approximation and prevented previous works based on this approximation from obtaining the Josephson anomalous current. We show that this symmetry can be removed by introducing spin-dependent boundary conditions for the quasiclassical equations at the superconducting/ferromagnet interfaces in diffusive systems. Using this recipe, we consider generic multilayer magnetic systems and determine the ideal experimental conditions in order to maximize the anomalous current.

Irreversible magnetization switching at the onset of superconductivity in a superconductor ferromagnet hybrid

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

Curran, P. J.; Bending, S. J.; Kim, J.

2015-12-28

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well asmore » magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices.« less

The coprime quantum chain

NASA Astrophysics Data System (ADS)

Mussardo, G.; Giudici, G.; Viti, J.

2017-03-01

In this paper we introduce and study the coprime quantum chain, i.e. a strongly correlated quantum system defined in terms of the integer eigenvalues n i of the occupation number operators at each site of a chain of length M. The n i ’s take value in the interval [2,q] and may be regarded as S z eigenvalues in the spin representation j  =  (q  -  2)/2. The distinctive interaction of the model is based on the coprimality matrix \\boldsymbolΦ : for the ferromagnetic case, this matrix assigns lower energy to configurations where occupation numbers n i and n i+1 of neighbouring sites share a common divisor, while for the anti-ferromagnetic case it assigns a lower energy to configurations where n i and n i+1 are coprime. The coprime chain, both in the ferro and anti-ferromagnetic cases, may present an exponential number of ground states whose values can be exactly computed by means of graph theoretical tools. In the ferromagnetic case there are generally also frustration phenomena. A fine tuning of local operators may lift the exponential ground state degeneracy and, according to which operators are switched on, the system may be driven into different classes of universality, among which the Ising or Potts universality class. The paper also contains an appendix by Don Zagier on the exact eigenvalues and eigenvectors of the coprimality matrix in the limit q\\to ∞ .

Magnetic and transport properties of Co2Mn1-xCrxSi Heusler alloy thin films

NASA Astrophysics Data System (ADS)

Aftab, M.; Hassnain Jaffari, G.; Hasanain, S. K.; Ali Abbas, Turab; Ismat Shah, S.

2013-09-01

Magnetic, transport, and magnetotransport properties of Co2Mn1-xCrxSi (0 ≤ x ≤ 1) DC sputter grown thin films have been investigated. In films with x > 0.2 saturation magnetization values are seen to deviate from the Slater-Pauling rule due to the enhancement of Co-Cr antisite disorder. The increasing structural disorder eventually results in a sign change of the temperature coefficient of resistivity (at x > 0.6), while a resistivity minimum is observed for the metallic compositions. From resistivity measurements, we conclude that there is a phase transition from a half-metallic ferromagnetic phase to a normal ferromagnetic phase at T ˜ 68 K in composition with x ≤ 0.2. Both the onset temperature and the temperature range for half metallic phase were found to decrease with increasing x among the metallic compositions. Magnetotransport measurements performed on metallic compositions at temperatures below and above the resistivity minimum suggest the presence of both the metallic as well as semiconducting/localized states.

The Blume-Capel model on hierarchical lattices: Exact local properties

NASA Astrophysics Data System (ADS)

Rocha-Neto, Mário J. G.; Camelo-Neto, G.; Nogueira, E., Jr.; Coutinho, S.

2018-03-01

The local properties of the spin one ferromagnetic Blume-Capel model defined on hierarchical lattices with dimension two and three are obtained by a numerical recursion procedure and studied as functions of the temperature and the reduced crystal-field parameter. The magnetization and the density of sites in the configuration S = 0 state are carefully investigated at low temperature in the region of the phase diagram that presents the phenomenon of phase reentrance. Both order parameters undergo transitions from the ferromagnetic to the ordered paramagnetic phase with abrupt discontinuities that decrease along the phase boundary at low temperatures. The distribution of magnetization in a typical profile was determined on the transition line presenting a broad multifractal spectrum that narrows towards the fractal limit (single point) as the discontinuities of the order parameters grow towards a maximum. The amplitude of the order-parameter discontinuities and the narrowing of the multifractal spectra were used to delimit the low temperature interval for the possible locus of the tricritical point.

Investigating phase transition temperatures of size separated gadolinium silicide magnetic nanoparticles

DOE PAGES

Hunagund, Shivakumar G.; Harstad, Shane M.; El-Gendy, Ahmed A.; ...

2018-01-11

Gadolinium silicide (Gd 5Si 4) nanoparticles (NPs) exhibit different properties compared to their parent bulk materials due to finite size, shape, and surface effects. NPs were prepared by high energy ball-milling of the as-cast Gd 5Si 4 ingot and size separated into eight fractions using time sensitive sedimentation in an applied dc magnetic field with average particle sizes ranging from 700 nm to 82 nm. The largest Gd 5Si 4 NPs order ferromagnetically at 316 K. A second anomaly observed at 110 K can be ascribed to a Gd 5Si 3 impurity. Here as the particle sizes decrease, the volumemore » fraction of Gd 5Si 3 phase increases at the expense of the Gd 5Si 4 phase, and the ferromagnetic transition temperature of Gd 5Si 4 is reduced from 316 K to 310 K, while the ordering of the minor phase is independent of the particle size, remaining at 110 K.« less

Anisotropy of the ferromagnetic L10 phase in the Mn-Al-C alloys induced by high-pressure spark plasma sintering

NASA Astrophysics Data System (ADS)

Tyrman, Muriel; Ahmim, Smail; Pasko, Alexandre; Etgens, Victor; Mazaleyrat, Frédéric; Quetel-Weben, Simon; Perrière, Loïc; Guillot, Ivan

2018-05-01

The metastable τ-phase of MnAl equi-atomic compound belongs to a family of ferromagnetic alloys with L10 crystal structure. Stabilization of the phase by adding 2 at. % using manganese carbide (Mn23C6) enhances the magnetization in relation with the increase in lattice volume. It is thus a promising candidate for rare-earth-free permanent magnets. Coercivity of Mn-Al-C alloys being still weak, there is an interest to see to which extend sintering/transformation of the ɛ-phase by Spark Plasma Sintering (SPS) can increase the coercivity and the anisotropy. The structural and the magnetic properties were studied for samples sintered at 550 °C under uniaxial pressure of 100, 200, 300 and 400 MPa. Coercivity, remanence and anistotropy appears with the sintering pressure. The high pressure applied while sintering produces preferential orientation of the flake-shaped grains which influences the remanence.

Structural, electronic, mechanical and magnetic properties of rare earth nitrides REN (RE= Pm, Eu and Yb)

NASA Astrophysics Data System (ADS)

Murugan, A.; Rajeswarapalanichamy, R.; Santhosh, M.; Iyakutti, K.

2015-07-01

The structural, electronic and mechanical properties of rare earth nitrides REN (RE=Pm, Eu and Yb) are investigated in NaCl and CsCl, and zinc blende structures using first principles calculations based on density functional theory. The calculated lattice parameters are in good agreement with the available results. Among the considered structures, these nitrides are most stable in NaCl structure. A pressure induced structural phase transition from NaCl to CsCl phase is observed in all these nitrides. The electronic structure reveals that these rare earth nitrides are half metallic at normal pressure. These nitrides are found to be covalent and ionic in the stable phase. The computed elastic constants indicate that these nitrides are mechanically stable and elastically anisotropic. Our results confirm that these nitrides are ferromagnetic in nature. A ferromagnetic to non-magnetic phase transition is observed at the pressures of 21.5 GPa and 46.1 GPa in PmN and YbN respectively.

Field induced ferromagnetic fraction enlargement in phase separated La_0.5Ca_0.5MnO_3

NASA Astrophysics Data System (ADS)

Ghivelder, Luis; Freitas, R. S.; Sacanel, J.; Parisi, F.; Levy, P.

2001-03-01

A systematic study of the magnetic and transport properties of a series of phase separated La_0.5Ca_0.5MnO3 compounds is reported. The investigated samples all have the same composition but different grain sizes, which modifies the volume fraction of the coexisting ferromagnetic (FM) and antiferromagnetic charge-ordered (AFM-CO) phases. Magnetoresistance and magnetization measurements were performed with two different experimental procedures: a standard field-cooled cooling (FC) mode, and a second method in which the field is turned on only while measuring each data point, and switched off while cooling the samples. Magnetization and magnetoresistance measurements display big differences when comparing the data obtained with the different procedures. The overall results are interpret in terms of a field induced FM fraction enlargement. In transport measurements this effect yield a percolative transition. Magnetization data shows evidence for the formation of AFM-CO regions within the FM phase. * e-mail: luisghiv@if.ufrj.br

Investigating phase transition temperatures of size separated gadolinium silicide magnetic nanoparticles

NASA Astrophysics Data System (ADS)

Hunagund, Shivakumar G.; Harstad, Shane M.; El-Gendy, Ahmed A.; Gupta, Shalabh; Pecharsky, Vitalij K.; Hadimani, Ravi L.

2018-05-01

Gadolinium silicide (Gd5Si4) nanoparticles (NPs) exhibit different properties compared to their parent bulk materials due to finite size, shape, and surface effects. NPs were prepared by high energy ball-milling of the as-cast Gd5Si4 ingot and size separated into eight fractions using time sensitive sedimentation in an applied dc magnetic field with average particle sizes ranging from 700 nm to 82 nm. The largest Gd5Si4 NPs order ferromagnetically at 316 K. A second anomaly observed at 110 K can be ascribed to a Gd5Si3 impurity. As the particle sizes decrease, the volume fraction of Gd5Si3 phase increases at the expense of the Gd5Si4 phase, and the ferromagnetic transition temperature of Gd5Si4 is reduced from 316 K to 310 K, while the ordering of the minor phase is independent of the particle size, remaining at 110 K.

Investigating phase transition temperatures of size separated gadolinium silicide magnetic nanoparticles

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

Hunagund, Shivakumar G.; Harstad, Shane M.; El-Gendy, Ahmed A.

Gadolinium silicide (Gd 5Si 4) nanoparticles (NPs) exhibit different properties compared to their parent bulk materials due to finite size, shape, and surface effects. NPs were prepared by high energy ball-milling of the as-cast Gd 5Si 4 ingot and size separated into eight fractions using time sensitive sedimentation in an applied dc magnetic field with average particle sizes ranging from 700 nm to 82 nm. The largest Gd 5Si 4 NPs order ferromagnetically at 316 K. A second anomaly observed at 110 K can be ascribed to a Gd 5Si 3 impurity. Here as the particle sizes decrease, the volumemore » fraction of Gd 5Si 3 phase increases at the expense of the Gd 5Si 4 phase, and the ferromagnetic transition temperature of Gd 5Si 4 is reduced from 316 K to 310 K, while the ordering of the minor phase is independent of the particle size, remaining at 110 K.« less

Effect of sol temperature on structural, morphological and magnetic properties of PZT thin films on alumina substrate

NASA Astrophysics Data System (ADS)

Sreelalitha, K.; Thyagarajan, K.

2016-01-01

In the present study, we investigate the structural, morphological and magnetic properties of sol-gel spin-coated PZT thin films on alumina substrate. The morphotropic phase boundary (MPB) of PZT [Pb (Zr1-xTix)03] between the tetragonal and rhombohedral phases occurs at the Zr/Ti ratio of 52/48. At the MPB the physical properties of PZT are of far-reaching importance due to their possible crystalline phases. In this study Pb(Zr0.52Ti0.48)03 sols are prepared at room temperature and at 125 °C. The gels are coated onto alumina substrate using a spin-coating unit as two and three layers. The structural studies using XRD confirm the perovskite phase formation at an annealing temperature of 660 °C for both films. The structural parameter grain size, dislocation density, lattice parameters and strain were dependent on the sol temperature. The SEM morphology of the samples represents well-developed dense grain structure and thickness in micrometer ranges. The VSM analysis shows diamagnetic and ferromagnetic hysteresis loop. The ferromagnetism at low fields in PZT films is confirmed by studying the magnetic properties of powder made of the same gel. The effect of heat treatment on the gel preparation is observed on structural, morphological and magnetic properties of PZT thin films. The ferromagnetism in PZT can be attributed to oxygen vacancies. The squareness ratio of the films shows the application of the films as a high-density recording medium.

Magnetic phase transitions and ferromagnetic short-range correlations in single-crystal Tb5Si2.2Ge1.8

NASA Astrophysics Data System (ADS)

Zou, M.; Pecharsky, V. K.; Gschneidner, K. A., Jr.; Schlagel, D. L.; Lograsso, T. A.

2008-07-01

Magnetic phase transitions in a Tb5Si2.2Ge1.8 single crystal have been studied as a function of temperature and magnetic field. Magnetic-field dependencies of the critical temperatures are highly anisotropic for both the main magnetic ordering process occurring around 120 K and a spin reorientation transition at ˜70K . Magnetic-field-induced phase transitions occur with the magnetic field applied isothermally along the a and b axes (but not along the c axis) between 1.8 and 70 K in fields below 70 kOe. Strong anisotropic thermal irreversibility is observed in the Griffiths phase regime between 120 and 200 K with applied fields ranging from 10 to 1000 Oe. Our data (1) show that the magnetic and structural phase transitions around 120 K are narrowly decoupled; (2) uncover the anisotropy of ferromagnetic short-range order in the Griffiths phase; and (3) reveal some unusual magnetic domain effects in the long-range ordered state of the Tb5Si2.2Ge1.8 compound. The temperature-magnetic field phase diagrams with field applied along the three major crystallographic directions have been constructed.

Quasi-two-dimensional Bose-Einstein condensation of lattice bosons in the spin-1/2 XXZ ferromagnet K2CuF4

NASA Astrophysics Data System (ADS)

Hirata, Satoshi; Kurita, Nobuyuki; Yamada, Motoki; Tanaka, Hidekazu

2017-05-01

K2CuF4 is magnetically described as a spin-1/2 , quasi-two-dimensional (2D), square-lattice XXZ ferromagnet with weak easy-plane anisotropy. The magnetic ordering for an applied magnetic field H parallel to the c axis is equivalent to the Bose-Einstein condensation (BEC) of lattice bosons, as discussed by Matsubara and Matsuda [T. Matsubara and H. Matsuda, Prog. Theor. Phys. 16, 569 (1956), 10.1143/PTP.16.569]. Magnetization and specific-heat measurements were performed to obtain the temperature versus magnetic field phase diagram for H ∥c . The phase boundary between polarized and ordered phases was found to be expressed by the power law Hc(T ) -Hc(0 ) ∝Tϕ with exponent ϕ ≈1.0 in a wide temperature range, in agreement with the theory of quasi-2D BEC.

Varying Eu2+ magnetic order by chemical pressure in EuFe2(As1-xPx)2

NASA Astrophysics Data System (ADS)

Zapf, S.; Wu, D.; Bogani, L.; Jeevan, H. S.; Gegenwart, P.; Dressel, M.

2011-10-01

Based on low-field magnetization measurements on a series of single crystals, we present a scheme of the Eu2+ spin alignment in EuFe2(As1-xPx)2. We explain observations of the Eu2+ ordering previously reported, reconciling different existing phase diagrams. The magnetic moments of the Eu2+ ions are slightly canted, yielding a ferromagnetic contribution along the c direction that becomes stronger with pressure, until superconductivity sets in. The spin-density wave as well as the superconducting phase coexist with an antiferromagnetic interlayer coupling of the canted spins. Reducing the interlayer distance finally leads to a ferromagnetic Eu2+ interlayer coupling and to the suppression of superconductivity.

Induced Ferromagnetism at BiFeO 3/YBa 2Cu 3O 7 Interfaces

DOE PAGES

Zhu, Jian-Xin; Wen, Xiao-Dong; Haraldsen, J. T.; ...

2014-06-20

We report that transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO 3/YBa 2Cu 3O 7 interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO 3, we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interactionmore » strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. Lastly, the emergence of interfacial ferromagnetism should have implications to electronic and transport properties.« less

Deep melting reveals liquid structural memory and anomalous ferromagnetism in bismuth.

PubMed

Shu, Yu; Yu, Dongli; Hu, Wentao; Wang, Yanbin; Shen, Guoyin; Kono, Yoshio; Xu, Bo; He, Julong; Liu, Zhongyuan; Tian, Yongjun

2017-03-28

As an archetypal semimetal with complex and anisotropic Fermi surface and unusual electric properties (e.g., high electrical resistance, large magnetoresistance, and giant Hall effect), bismuth (Bi) has played a critical role in metal physics. In general, Bi displays diamagnetism with a high volumetric susceptibility ([Formula: see text]10 -4 ). Here, we report unusual ferromagnetism in bulk Bi samples recovered from a molten state at pressures of 1.4-2.5 GPa and temperatures above [Formula: see text]1,250 K. The ferromagnetism is associated with a surprising structural memory effect in the molten state. On heating, low-temperature Bi liquid (L) transforms to a more randomly disordered high-temperature liquid (L') around 1,250 K. By cooling from above 1,250 K, certain structural characteristics of liquid L' are preserved in L. Bi clusters with characteristics of the liquid L' motifs are further preserved through solidification into the Bi-II phase across the pressure-independent melting curve, which may be responsible for the observed ferromagnetism.

Modelling copper-phthalocyanine/cobalt-phthalocyanine chains: towards magnetic quantum metamaterials.

PubMed

Wu, Wei

2014-07-23

The magnetic properties of a theoretically designed molecular chain structure CuCoPc2, in which copper-phthalocyanine (CuPc) and cobalt-phthalocyanine (CoPc) alternate, have been investigated across a range of chain structures. The computed exchange interaction for the α-phase CuCoPc2 is ∼ 5 K (ferromagnetic), in strong contrast to the anti-ferromagnetic interaction recently observed in CuPc and CoPc. The computed exchange interactions are strongly dependent on the stacking angle but weakly on the sliding angle, and peak at 20 K (ferromagnetic). These ferromagnetic interactions are expected to arise from direct exchange with the strong suppression of super-exchange interaction. These first-principles calculations show that π-conjugated molecules, such as phthalocyanine, could be used as building blocks for the design of magnetic materials. This therefore extends the concept of quantum metamaterials further into magnetism. The resulting new magnetic materials could find applications in the studies such as organic spintronics.

Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb₂Ti₂O₇.

PubMed

Chang, Lieh-Jeng; Onoda, Shigeki; Su, Yixi; Kao, Ying-Jer; Tsuei, Ku-Ding; Yasui, Yukio; Kakurai, Kazuhisa; Lees, Martin Richard

2012-01-01

In a class of frustrated magnets known as spin ice, magnetic monopoles emerge as classical defects and interact via the magnetic Coulomb law. With quantum-mechanical interactions, these magnetic charges are carried by fractionalized bosonic quasi-particles, spinons, which can undergo Bose-Einstein condensation through a first-order transition via the Higgs mechanism. Here, we report evidence of a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb(2)Ti(2)O(7). Polarized neutron scattering experiments show that the diffuse [111]-rod scattering and pinch-point features, which develop on cooling are suddenly suppressed below T(C)~0.21 K, where magnetic Bragg peaks and a full depolarization of the neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained on the basis of a quantum spin-ice model, whose high-temperature phase is effectively described as a magnetic Coulomb liquid, whereas the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.

Electric Field Controlled Magnetism in BiFeO3/Ferromagnet Films

NASA Astrophysics Data System (ADS)

Holcomb, M. B.; Chu, Y. H.; Martin, L. W.; Gajek, M.; Seidel, J.; Ramesh, R.; Scholl, A.; Fraile-Rodriguez, A.

2008-03-01

Electric field control of magnetism is a hot technological topic at the moment due to its potential to revolutionize today's devices. Magnetoelectric materials, those having both electric and magnetic order and the potential for coupling between the two, are a promising avenue to approach electric control. BiFeO3, both a ferroelectric and an antiferromagnet, is the only single phase room temperature magnetoelectric that is currently known. In addition to other possibilities, its multiferroic nature has potential in the very active field of exchange bias, where an antiferromagnetic thin film pins the magnetic direction of an adjoining ferromagnetic layer. Since this antiferromagnet is electrically tunable, this coupling could allow electric-field control of the ferromagnetic magnetization. Direction determination of antiferromagnetic domains in BFO has recently been shown using linear and circular dichroism studies. Recently, this technique has been extended to look at the magnetic domains of a ferromagnetic grown on top of BFO. The clear magnetic changes induced by application of electric fields reveal the possibility of electric control.

Deep melting reveals liquid structural memory and anomalous ferromagnetism in bismuth

PubMed Central

Shu, Yu; Yu, Dongli; Hu, Wentao; Wang, Yanbin; Shen, Guoyin; Kono, Yoshio; Xu, Bo; He, Julong; Liu, Zhongyuan; Tian, Yongjun

2017-01-01

As an archetypal semimetal with complex and anisotropic Fermi surface and unusual electric properties (e.g., high electrical resistance, large magnetoresistance, and giant Hall effect), bismuth (Bi) has played a critical role in metal physics. In general, Bi displays diamagnetism with a high volumetric susceptibility (∼10−4). Here, we report unusual ferromagnetism in bulk Bi samples recovered from a molten state at pressures of 1.4–2.5 GPa and temperatures above ∼1,250 K. The ferromagnetism is associated with a surprising structural memory effect in the molten state. On heating, low-temperature Bi liquid (L) transforms to a more randomly disordered high-temperature liquid (L′) around 1,250 K. By cooling from above 1,250 K, certain structural characteristics of liquid L′ are preserved in L. Bi clusters with characteristics of the liquid L′ motifs are further preserved through solidification into the Bi-II phase across the pressure-independent melting curve, which may be responsible for the observed ferromagnetism. PMID:28289195

One-dimensional ferromagnetic array compound [Co3(SBA)2(OH)2(H2O)2]n, (SBA = 4-sulfobenzoate)

NASA Astrophysics Data System (ADS)

Honda, Zentaro; Nomoto, Naoyuki; Fujihara, Takashi; Hagiwara, Masayuki; Kida, Takanori; Sawada, Yuya; Fukuda, Takeshi; Kamata, Norihiko

2018-06-01

We report on the syntheses, crystal structure, and magnetic properties of the transition metal coordination polymer [Co3(SBA)2(OH)2(H2O)2]n, (SBA = 4-sulfobenzoate) in which CoO6 octahedra are linked through their edges, forming one-dimensional (1D) Co(II) arrays running along the crystal a-axis. These arrays are further perpendicularly bridged by SBA ligand to construct a three-dimensional framework. Its magnetic properties have been investigated, and ferromagnetic interactions within the arrays have been found. From heat capacity measurements, we have found that this compound exhibits a three-dimensional ferromagnetic phase transition at TC = 1.54 K, and the specific heat just above TC shows a Schottky anomaly which originates from an energy gap caused by uniaxial magnetic anisotropy. These results suggest that [Co3(SBA)2(OH)2(H2O)2]n consists of weakly coupled 1D ferromagnetic Ising arrays.

Nonlinear Magnetic Dynamics and The Switching Phase Diagrams in Spintronic Devices

NASA Astrophysics Data System (ADS)

Yan, Shu

Spin-transfer torque induced magnetic switching, by which the spin-polarized current transfers its magnetic moment to the ferromagnetic layer and changes its magnetization, holds great promise towards faster and smaller magnetic bits in data-storage applications due to the lower power consumption and better scalability. We propose an analytic approach which can be used to calculate the switching phase diagram of a nanomagnetic system in the presence of both magnetic field and spin-transfer torque in an exact fashion. This method is applied to the study of switching conditions for the uniaxial, single domain magnetic layers in different spin-transfer devices. In a spin valve with spin polarization collinear with the easy axis, we get a modified Stoner-Wohlfarth astroid which represents many of the features that have been found in experiment. It also shows a self-crossing boundary and demonstrates a region with three stable equilibria. We demonstrate that the region of stable equilibria with energy near the maximum can be reached only through a narrow bottleneck in the field space, which sets a stringent requirement for magnetic field alignment in the experiments. Switching diagrams are then calculated for the setups with magnetic field not perfectly aligned with the easy axis. In a ferromagnet-heavy-metal bilayer device with strong spin Hall effect, the in plane current becomes spin-polarized and transfers its magnetic moment to the ferromagnetic layer by diffusion. The three-dimensional asymmetric phase diagram is calculated. In the case that the external field is confined in the vertical plane defined by the direction of the current and the easy axis, the spin-transfer torque shifts the conventional in-plane (IP) equilibria within the same plane, and also creates two out-of-plane (OOP) equilibria, one of which can be stable. The threshold switching currents for IP switching and OOP switching are discussed. We also address the magnetic switching processes. Damping switching and precessional switching are two different switching types that are typically considered in recent studies. In the damping mode the switching is slow and heavily depends on the initial deviation, while in the precessional mode the accurate manipulation of the field or current pulse is required. We propose a switching scenario for a fast and reliable switching by taking advantage of the out-of-plane stable equilibrium in the SHE induced magnetic switching. The magnetization is first driven by a pulse of field and current towards the OOP equilibrium without precession. Since it is in the lower half of the unit sphere, no backwards pulse is required for a complete switching. This indicates a potentially feasible method of reliable ultra-fast magnetic control.

A proposed method for electronic feedback compensation of damping in ferromagnetic resonance

DOE PAGES

Zohar, S.; Sterbinsky, G. E.

2017-07-10

Here, we propose an experimental technique for extending feedback compensation of dissipative radiation used in nuclear magnetic resonance (NMR) to encompass ferromagnetic resonance (FMR). This method uses a balanced microwave power detector whose output is phase shifted π/2, amplified, and fed back to drive precession. Using classical control theory, we predict an electronically controllable narrowing of field swept FMR line-widths. This technique is predicted to compensate other sources of spin dissipation in addition to radiative loss.

A proposed method for electronic feedback compensation of damping in ferromagnetic resonance

NASA Astrophysics Data System (ADS)

Zohar, S.; Sterbinsky, G. E.

2017-12-01

We propose an experimental technique for extending feedback compensation of dissipative radiation used in nuclear magnetic resonance (NMR) to encompass ferromagnetic resonance (FMR). This method uses a balanced microwave power detector whose output is phase shifted π / 2 , amplified, and fed back to drive precession. Using classical control theory, we predict an electronically controllable narrowing of field swept FMR line-widths. This technique is predicted to compensate other sources of spin dissipation in addition to radiative loss.

Transition from the diamagnetic insulator to ferromagnetic metal in La1-xSrxCoO3

NASA Astrophysics Data System (ADS)

Knížek, Karel; Jirák, Zdeněk; Hejtmánek, Jiří; Novák, Pavel

2010-05-01

We have analyzed, using the theoretical GGA+U calculations, different configurations of spin states (low-spin, LS; intermediate-spin, IS and high-spin, HS Co) and proposed a model that accounts for magnetic and electric transport properties of perovskite cobaltites upon doping by charge carriers. In particular, it appears that the compositional transition from the diamagnetic LS phase of LaCoO3 to the ferromagnetic metallic IS phase in La1-xSrxCoO3 ( x>0.2) involves the same mechanisms as the high-temperature transition in pure LaCoO3. The process occurs gradually via a phase-separated state, where metallic IS domains stabilized through a charge transfer between Co and Co neighbors coexist with the Co poor regions in the LS ground state (or at higher temperatures, in mixed LS/HS state). This phase separation vanishes when doping in La1-xSrxCoO3 reaches x˜0.2, and a uniform IS phase, analogous to that in pure LaCoO3 in the high-temperature limit, is established.

Reentrant behaviors in the phase diagram of spin-1 planar ferromagnet with single-ion anisotropy

NASA Astrophysics Data System (ADS)

Rabuffo, I.; De Cesare, L.; Caramico D'Auria, A.; Mercaldo, M. T.

2018-05-01

We used the two-time Green function framework to investigate the role played by the easy-axis single-ion anisotropy on the phase diagram of (d > 2)-dimensional spin-1planar ferromagnets, which exhibit a magnetic field induced quantum phase transition. We tackled the problem using two different kind of approximations: the Anderson-Callen decoupling scheme and the Devlin approach. In the latter scheme, the exchange anisotropy terms in the equations of motion are treated at the Tyablikov decoupling level while the crystal field anisotropy contribution is handled exactly. The emerging key result is a reentrant structure of the phase diagram close to the quantum critical point, for certain values of the single-ion anisotropy parameter. We compare the results obtained within the two approximation schemes. In particular, we recover the same qualitative behavior. We show the phase diagram, close to the field-induced quantum critical point and the behavior of the susceptibility for different values of the single-ion anisotropy parameter, enhancing the differences between the two different scenarios (i.e. with and without reentrant behavior).

Towards a new class of heavy ion doped magnetic semiconductors for room temperature applications

PubMed Central

Lee, Juwon; Subramaniam, Nagarajan Ganapathi; Agnieszka Kowalik, Iwona; Nisar, Jawad; Lee, Jaechul; Kwon, Younghae; Lee, Jaechoon; Kang, Taewon; Peng, Xiangyang; Arvanitis, Dimitri; Ahuja, Rajeev

2015-01-01

The article presents, using Bi doped ZnO, an example of a heavy ion doped oxide semiconductor, highlighting a novel p-symmetry interaction of the electronic states to stabilize ferromagnetism. The study includes both ab initio theory and experiments, which yield clear evidence for above room temperature ferromagnetism. ZnBixO1−x thin films are grown using the pulsed laser deposition technique. The room temperature ferromagnetism finds its origin in the holes introduced by the Bi doping and the p-p coupling between Bi and the host atoms. A sizeable magnetic moment is measured by means of x-ray magnetic circular dichroism at the O K-edge, probing directly the spin polarization of the O(2p) states. This result is in agreement with the theoretical predictions and inductive magnetometry measurements. Ab initio calculations of the electronic and magnetic structure of ZnBixO1−x at various doping levels allow to trace the origin of the ferromagnetic character of this material. It appears, that the spin-orbit energy of the heavy ion Bi stabilizes the ferromagnetic phase. Thus, ZnBixO1−x doped with a heavy non-ferromagnetic element, such as Bi, is a credible example of a candidate material for a new class of compounds for spintronics applications, based on the spin polarization of the p states. PMID:26592564

Microstructure and phase composition characterization in a Co{sub 38}Ni{sub 33}Al{sub 29} ferromagnetic shape memory alloy

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

Lu, J.B.

2016-08-15

Transmission electron microscopy was performed to investigate the microstructures of a secondary phase and its surrounding matrix in a Co{sub 38}Ni{sub 33}Al{sub 29} ferromagnetic shape memory alloy. The secondary phase shows a γ′ L1{sub 2} structure exhibiting a dendritic morphology with enclosed B2 austenite regions while the matrix shows the L1{sub 0} martensitic structure. A secondary phase-austenite-martensite sandwich structure with residual austenite ranging from several hundred nanometers to several micrometers wide is observed at the secondary phase-martensite interface due to the depletion of Co and enrichment of Al in the chemical gradient zone and the effect of the strong martensiticmore » start temperature dependency of the element concentrations. The crystallographic orientation relationship of the secondary phase and the B2 austenite fits the Kurdjumov-Sachs relationship. - Highlights: •The secondary phase has a γ′ L1{sub 2} structure exhibiting a dendritic morphology. •A secondary phase-austenite-martensite sandwich structure is observed. •The structural sandwich structure is due to elemental composition variation. •The secondary phase and the B2 austenite fit the Kurdjumov-Sachs relationship.« less

Magnetization, resistivity, specific heat and ab initio calculations of Gd₅Sb₃.

PubMed

Samatham, S Shanmukharao; Patel, Akhilesh Kumar; Lukoyanov, Alexey V; Suresh, K G

2018-06-07

We report on the combined results of structural, magnetic, transport and calorimetric properties of Mn₅Si₃-type hexagonal Gd₅Sb₃, together with <i>ab-initio</i> calculations. It exhibits a ferromagnetic (FM)-like transition at 265 K, antiferromagnetic (AFM) Néel transition at 95.5 K followed by a spin-orientation transition at 62 K. The system is found to be in AFM state down to 2 K in a field of 70 kOe. The FM-AFM phase coexistence is not noticeable despite large positive Curie-Weiss temperature (θ_CW = 223.5 ± 0.2 K). Instead, low-temperature AFM and high-temperature FM-like phases are separated in large temperatures. Temperature-magnetic field (<i>H</i>-<i>T</i>) phase diagram reveals field-driven complex magnetic phases. Within the AFM phase, the system is observed to undergo field-driven spin-orientation transitions. Field-induced tricritical and quantum critical points appear to be absent due to strong AFM nature and by the intervention of FM-like state between paramagnetic and AFM states, respectively. The metallic behavior of the compound is inferred from resistivity along with large Sommerfeld parameter. However, no sign of strong electron-correlations is reasoned from the Kadowaki-Wood's ratio <i>A</i>/γ² ∼ 1.9×10^-6 μΩ.cm.(mol.K)²(mJ)^-2, despite heavy γ. Essentially, <i>ab initio</i> calculations accounting for electronic correlations confirm AFM nature of low-temperature magnetic state in Gd₅Sb₃ and attainable FM ordering in agreement with experimental data. © 2018 IOP Publishing Ltd.

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

Liu, Yu; Petrovic, C.

CrI 3 is a promising candidate for the van der Waals bonded ferromagnetic devices since its ferromagnetism can be maintained upon exfoliating of bulk crystals down to single layer. In this work we studied critical properties of bulk CrI 3 single crystals around the paramagnetic to ferromagnetic phase transition. Critical exponents β= 0.260(4) with a critical temperature T c= 60.05(13) K and γ= 1.136(6) with T c= 60.43(4) K are obtained by the Kouvel-Fisher method, whereas δ= 5.32(2) is obtained by a critical isotherm analysis at T c= 60 K. In conclusion, the critical exponents determined in bulk CrI 3more » single crystals suggest a three-dimensional long-range magnetic coupling with the exchange distance decaying as J(r)≈r -4:69« less

Phase diagram of the chiral magnet Cr1 /3NbS2 in a magnetic field

NASA Astrophysics Data System (ADS)

Tsuruta, K.; Mito, M.; Deguchi, H.; Kishine, J.; Kousaka, Y.; Akimitsu, J.; Inoue, K.

2016-03-01

We construct the phase diagram of the chiral magnet Cr1 /3NbS2 in a dc magnetic field (Hdc) using ac magnetic susceptibility measurements. At Hdc=0 , the ac response at the transition from the helical magnetic (HM) state to the paramagnetic (PM) state consists of a giant third-order harmonic component (M3 ω) and a first-order harmonic component (M1 ω). By applying Hdc perpendicular to the c axis, the HM state is transformed to the chiral soliton lattice (CSL) state, which is a superlattice tuned by Hdc. The above giant M3 ω is markedly suppressed at small Hdc. The CSL state is found to consist of CSL-1, with dominant helical texture and a poor ferromagnetic array, and CSL-2, with a large ferromagnetic array. The transition between CSL-1 and the PM state causes a linear magnetic response, the dominant component of which is the in-phase M1 ω. With increasing temperature, CSL-2 is transformed into the forced ferromagnetic (FFM) state, and ultimately the PM state is reached. The transition between CSL-2 and the FFM state consists of a large M3 ω and large out-of-phase M1 ω as well as in-phase M1 ω. The transition between the FMM and PM states also yields a linear magnetic response, like the CSL-1-PM-state transition. Five typical magnetic dynamics in the transitions among the HM state, CSL-1, CSL-2, FFM state, and PM state were identified according to the equivalent dynamical motion equation of a nonlinear spring model.

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} ferroelectrics

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

Zhou, Wenliang; Yang, Pingxiong, E-mail: pxyang@ee.ecnu.edu.cn; Chu, Junhao

2014-09-15

Structural phase transition, narrow band gap (E{sub g}), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO{sub 3}]{sub 1−x}[BaNi{sub 1/2}Nb{sub 1/2}O{sub 3−δ}]{sub x} (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it ismore » noted that KBNNO with compositions x = 0.1–0.3 have quite narrow E{sub g} of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO{sub 3} (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetism of the samples develops from diamagnetism to ferromagnetism and paramagnetism, originating from the ferromagnetic–antiferromagnetic competition. These results are helpful in the deeper understanding of phase transitions, band gap tunability, and magnetism variations in perovskite oxides and show the potential role, such materials can play, in perovskite solar cells and multiferroic applications.« less

Evolving microstructure, magnetic properties and phase transition in a mechanically alloyed Ni0.5Zn0.5Fe2O4 single sample

NASA Astrophysics Data System (ADS)

Ismail, Ismayadi; Hashim, Mansor; Kanagesan, Samikannu; Ibrahim, Idza Riati; Nazlan, Rodziah; Wan Ab Rahman, Wan Norailiana; Abdullah, Nor Hapishah; Mohd Idris, Fadzidah; Bahmanrokh, Ghazaleh; Shafie, Mohd Shamsul Ezzad; Manap, Masni

2014-02-01

We report on an investigation to unravel the dependence of magnetic properties on microstructure while they evolve in parallel under the influence of sintering temperature of a single sample of Ni0.5Zn0.5Fe2O4 synthesized via mechanical alloying. A single sample, instead of the normally practiced approach of using multiple samples, was sintered at various sintering temperatures from 500 °C to 1400 °C. The morphology of the samples was studied by means of scanning electron microscopy (SEM) equipped with EDX; density measurement was conducted using the Archimedes principle; and hysteresis measurement was carried out using a B-H hysteresisgraph system. XRD data showed that the first appearance of a single phase was at 800 °C and an amorphous phase was traced at lower sintering temperatures. We correlated the microstructure and the magnetic properties and showed that the important grain-size threshold for the appearance of significant ordered magnetism (mainly ferromagnetism) was about ≥0.3 µm. We found that there were three stages of magnetic phase evolution produced via the sintering process with increasing temperatures. The first stage was dominated by paramagnetic states with some superparamagnetic behavior; the second stage was influenced by moderately ferromagnetic states and some paramagnetic states; and the third stage consisted of strongly ferromagnetic states with negligible paramagnetic states. We found that three factors sensitively influenced the sample's content of ordered magnetism—the ferrite-phase crystallinity degree, the number of grains above the critical grain size and the number of large enough grains for domain wall accommodation.

Magnetism in nanoparticle LaCoO3

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

Durand, A. M.; Belanger, D. P.; Booth, C. H.

2014-06-24

Neutron scattering and magnetometry measurements have been used to study phase transitions in LaCoO3 (LCO). For H <= 100 Oe, evidence for a ferromagnetic (FM) transition is observed at T-c approximate to 87 K. For 1 kOe <= H <= 60 kOe, no transition is apparent. For all H, Curie-Weiss analysis shows predominantly antiferromagnetic (AFM) interactions for T > T-c, but the lack of long-range AFM order indicates magnetic frustration. We argue that the weak ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and nanoparticles. The lattice strain is present at themore » bulk surfaces and at the interfaces between the LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly affected by the Co-O-Co angle (gamma), in agreement with recent band calculations which predict that ferromagnetic long-range order can only take place above a critical value, gamma C. Consistent with recent thin film estimations, we find gamma C = 162.8 degrees. For gamma > gamma C, we observe power-law behavior in the structural parameters. gamma decreases with T until the critical temperature, T-o approximate to 37 K; below T-o the rate of change becomes very small. For T < T-o, FM order appears to be confined to regions close to the surfaces, likely due to the lattice strain keeping the local Co-O-Co angle above gamma C.« less

Influences of P doping on magnetic phase transition and structure in MnCoSi ribbon

NASA Astrophysics Data System (ADS)

Du, Qian-Heng; Chen, Guo-Fu; Yang, Wen-Yun; Hua, Mu-Xin; Du, Hong-Lin; Wang, Chang-Sheng; Liu, Shun-Quan; Hang, Jing-Zhi; Zhou, Dong; Zhang, Yan; Yan, Jin-Bo

2015-06-01

The structure and magnetic properties of MnCoSi1- x Px (x = 0.05-0.50) are systematically investigated. With P content increasing, the lattice parameter a increases monotonically while both b and c decrease. At the same time, the temperature of metamagnetic transition from a low-temperature non-collinear ferromagnetic state to a high-temperature ferromagnetic state decreases and a new magnetic transition from a higher-magnetization ferromagnetic state to a lower-magnetization ferromagnetic state is observed in each of these compounds for the first time. This is explained by the changes of crystal structure and distance between Mn and Si atoms with the increase of temperature according to the high-temperature XRD result. The metamagnetic transition is found to be a second-order magnetic transition accompanied by a low inversed magnetocaloric effect (1.0 J·kg-1·K-1 at 5 T) with a large temperature span (190 K at 5 T) compared with the scenario of MnCoSi. The changes in the order of metamagnetic transition and structure make P-doped MoCoSi compounds good candidates for the study of magnetoelastic coupling and the modulation of magnetic phase transition. Project supported by the National Natural Science Foundation of China (Grant No. 11275013), the Fund from the National Physics Laboratory, China Academy of Engineering Physics (Grant No. 2013DB01), and the National Key Basic Research Program of China (Grant No. 2010CB833104).

Defect evolution and its impact on the ferromagnetism of Cu-doped ZnO nanocrystals upon thermal treatment: A positron annihilation study

NASA Astrophysics Data System (ADS)

Chen, Zhi-Yuan; Chen, Yuqian; Zhang, Q. K.; Qi, N.; Chen, Z. Q.; Wang, S. J.; Li, P. H.; Mascher, P.

2017-01-01

CuO/ZnO nanocomposites with 4 at. % CuO were annealed in air at various temperatures between 100 and 1200 °C to produce Cu-doped ZnO nanocrystals. X-ray diffraction shows that a CuO phase can be observed in the CuO/ZnO nanocomposites annealed at different temperatures, and the Cu-doped ZnO nanocrystals are identified to be of wurtzite structure. The main peak (101) appears at slightly lower diffraction angles with increasing annealing temperature from 400 up to 1200 °C, which confirms the successful doping of Cu into the ZnO lattice above 400 °C. Scanning electron microscopy indicates that most particles in the CuO/ZnO nanocomposites are isolated when annealing at 100-400 °C, but these particles have a tendency to form clusters or aggregates as the annealing temperature increases from 700 to 1000 °C. Positron annihilation measurements reveal a large number of vacancy defects in the interface region of the nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C. Room-temperature ferromagnetism can be observed in the CuO/ZnO nanocomposites, and the magnetization decreases continuously with increasing annealing temperature. However, there may be several different origins of ferromagnetism in the CuO/ZnO nanocomposites. At low annealing temperatures, the ferromagnetism originates from the CuO nanograins, and the ferromagnetism of CuO nanograins decreases with an increase in the grain size after subsequent higher temperature annealing, which leads to the weakening of ferromagnetism in the CuO/ZnO nanocomposites. After annealing from 400 to 1000 °C, the ferromagnetism gradually vanishes. The ferromagnetism is probably induced by Cu substitution but is mediated by vacancy defects in the CuO/ZnO nanocomposites. The disappearance of ferromagnetism coincides well with the recovery of vacancy defects. It can be inferred that the ferromagnetism is mediated by vacancy defects that are distributed in the interface region.

Effects of phase transformation on the microstructures and magnetostriction of Fe-Ga and Fe-Ga-Zn ferromagnetic shape memory alloys

NASA Astrophysics Data System (ADS)

Lin, Yin-Chih; Lin, Chien-Feng

2015-05-01

The phase transformation and magnetostriction of bulk Fe73Ga27 and Fe73Ga18Zn9 (at. %) ferromagnetic shape memory alloys (FSMs) were investigated by transmission electron microscopy (TEM), x-ray diffraction (XRD), and a magnetostrictive-meter setup. For the Fe73Ga27 FSM alloy solution treated at 1100 °C for 4 h and quenched in ice brine, the antiphase boundary segments of the D03 domain were observed in the A2 (disordered) matrix, and the Fe73Ga27 FSM alloy had an optimal magnetostriction (λ‖s = 71 × 10-6 and λ⊥s = -31 × 10-6). In Fe73Ga27 FSM alloy as-quenched, aged at 700 °C for 24 h, and furnace cooled, D03 nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., L10-like martensite) via Bain distortion, and finally L12 (Fe3Ga) structures precipitated, as observed by TEM and XRD. The L10-like martensite and L12 phases in the aged Fe73Ga27 FSM alloy drastically decreased the magnetostriction from positive to negative (λ‖s = -20 × 10-6 and λ⊥s = -8 × 10-6). However, in Fe73Ga18Zn9 FSM alloy as-quenched and aged, the phase transformation of D03 to an intermediate tetragonal martensite phase and precipitation of L12 structures were not found. The results indicate that the aged Fe73Ga18Zn9 FSM alloy maintained stable magnetostriction (λ‖s = 36 × 10-6 and λ⊥s = -31 × 10-6). Adding Zn can improve the ferromagnetic shape memory effect of aged Fe73Ga18Zn9 alloy, which may be useful in application of the alloy in high temperature environments.

Dynamics of chiral domain wall under the spin-orbit torques in heavy metal/ferromagnet bilayers with in-plane anisotropy

NASA Astrophysics Data System (ADS)

Yan, Han; He, Peng-Bin; Cai, Meng-Qiu; Li, Zai-Dong

2017-11-01

The dynamics of domain wall driven by the spin-orbit torques is theoretically studied in the heavy metal/ferromagnet bilayer with Dzyaloshinskii-Moriya interaction (DMI) and in-plane magnetic anisotropy. Based on the Walker profile, we infer that DMI has a selectivity for the chirality of head-to-head (tail-to-tail) static wall. By analyzing the dynamic equations obtained from the collective coordinates methods, we find that there exists a switching or a hysteresis of the polarity of wall in the low-current regime. In the presence of DMI, the wall can keep sustained propagation which velocity saturates for high current and is proportional to the strength of DMI. Furthermore, the DMI makes the adjacent walls possess the same chirality and move in the same direction.

Controllable transport of a skyrmion in a ferromagnetic narrow channel with voltage-controlled magnetic anisotropy

NASA Astrophysics Data System (ADS)

Wang, Junlin; Xia, Jing; Zhang, Xichao; Zhao, G. P.; Ye, Lei; Wu, Jing; Xu, Yongbing; Zhao, Weisheng; Zou, Zhigang; Zhou, Yan

2018-05-01

Magnetic skyrmions have potential applications in next-generation spintronic devices with ultralow energy consumption. In this work, the current-driven skyrmion motion in a narrow ferromagnetic nanotrack with voltage-controlled magnetic anisotropy (VCMA) is studied numerically. By utilizing the VCMA effect, the transport of skyrmion can be unidirectional in the nanotrack, leading to a one-way information channel. The trajectory of the skyrmion can also be modulated by periodically located VCMA gates, which protects the skyrmion from destruction by touching the track edge. In addition, the location of the skyrmion can be controlled by adjusting the driving pulse length in the presence of the VCMA effect. Our results provide guidelines for practical realization of the skyrmion-based information channel, diode, and skyrmion-based electronic devices such as racetrack memory.

6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni-Mn-Ga

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

Murray, S. J.; Marioni, M.; Allen, S. M.

2000-08-07

Field-induced strains of 6% are reported in ferromagnetic Ni-Mn-Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependencemore » of strain on magnetic field and external stress using as input parameters only measured quantities. (c) 2000 American Institute of Physics.« less

Anomalous finite-size effect due to quasidegenerate phases in triangular antiferromagnets with long-range interactions and mapping to the generalized six-state clock model

NASA Astrophysics Data System (ADS)

Nishino, Masamichi; Miyashita, Seiji

2016-11-01

The effect of long-range (LR) interactions on frustrated-spin models is an interesting problem, which provides rich ordering processes. We study the effect of LR interactions on triangular Ising antiferromagnets with the next-nearest-neighbor ferromagnetic interaction (TIAFF). In the thermodynamic limit, the LRTIAFF model should reproduce the corresponding mean-field results, in which successive phase transitions occur among various phases, i.e., the disordered paramagnetic phase, so-called partially disordered phase, three-sublattice ferrimagnetic phase, and two-sublattice ferrimagnetic phase. In the present paper we focus on the magnetic susceptibility at the transition point between the two-sublattice ferrimagnetic and the disordered paramagnetic phases at relatively large ferromagnetic interactions. In the mean-field analysis, the magnetic susceptibility shows no divergence at the transition point. In contrast, a divergencelike enhancement of the susceptibility is observed in Monte Carlo simulations in finite-size systems. We investigate the origin of this difference and find that it is attributed to a virtual degeneracy of the free energies of the partially disordered and 2-FR phases. We also exploit a generalized six-state clock model with an LR interaction, which is a more general system with Z6 symmetry. We discuss the phase diagram of this model and find that it exhibits richer transition patterns and contains the physics of the LRTIAFF model.

Simultaneous occurrence of multiferroism and short-range magnetic order in DyFeO 3

DOE PAGES

Wang, Jinchen; Liu, Juanjuan; Sheng, Jieming; ...

2016-04-06

In this paper, we present a combined neutron scattering and magnetization study on the multiferroic DyFeO 3, which shows a very strong magnetoelectric effect. Applying magnetic field along the c axis, the weak ferromagnetic order of the Fe ions is quickly recovered from a spin reorientation transition, and the long-range antiferromagnetic order of Dy becomes a short-range one. We found that the short-range order concurs with the multiferroic phase and is responsible for its sizable hysteresis. In conclusion, our H-T phase diagram suggests that the strong magnetoelectric effect in DyFeO 3 has to be understood with not only the weakmore » ferromagnetism of Fe but also the short-range antiferromagnetic order of Dy.« less

The unusual magnetism of nanoparticle LaCoO3.

PubMed

Durand, A M; Belanger, D P; Hamil, T J; Ye, F; Chi, S; Fernandez-Baca, J A; Booth, C H; Abdollahian, Y; Bhat, M

2015-05-08

Bulk and nanoparticle powders of LaCoO3 (LCO) were synthesized and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T ≈ 85 K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To≈40 K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co3O4 impurity phase, which induces tensile strain on the LCO lattice. A core-interface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.

The unusual magnetism of nanoparticle LaCoO 3

DOE PAGES

Durand, A. M.; Belanger, D. P.; Hamil, T. J.; ...

2015-04-15

Bulk and nanoparticle powders of LaCoO 3 (LCO) were synthesized and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T≈85K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To ≈ 40 K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co 3O 4 impurity phase, which induces tensile strain on the LCO lattice. A core-interfacemore » model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.« less

The unusual magnetism of nanoparticle LaCoO3

NASA Astrophysics Data System (ADS)

Durand, A. M.; Belanger, D. P.; Hamil, T. J.; Ye, F.; Chi, S.; Fernandez-Baca, J. A.; Booth, C. H.; Abdollahian, Y.; Bhat, M.

2015-05-01

Bulk and nanoparticle powders of LaCoO3 (LCO) were synthesized and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T ≈ 85 K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To≈40 K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co3O4 impurity phase, which induces tensile strain on the LCO lattice. A core-interface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.

Investigation of the difference between spin Hall magnetoresistance rectification and spin pumping from the viewpoint of magnetization dynamics

NASA Astrophysics Data System (ADS)

Zhang, Qihan; Fan, Xiaolong; Zhou, Hengan; Kong, Wenwen; Zhou, Shiming; Gui, Y. S.; Hu, C.-M.; Xue, Desheng

2018-02-01

Spin pumping (SP) and spin rectification due to spin Hall magnetoresistance (SMR) can result in a dc resonant voltage signal, when magnetization in ferromagnetic insulator/nonmagnetic structures experiences ferromagnetic resonance. Since the two effects are often interrelated, quantitative identification of them is important for studying the dynamic nonlocal spin transport through an interface. In this letter, the key difference between SP and SMR rectification was investigated from the viewpoint of spin dynamics. The phase-dependent nature of SMR rectification, which is the fundamental characteristic distinguishing it from SP, was tested by a well-designed experiment. In this experiment, two identical yttrium iron garnet/Pt strips with a π phase difference in dynamic magnetization show the same SP signals and inverse SMR signals.

Low temperature nucleation of Griffiths Phase in Co doped LaMnO3 nanostructures

NASA Astrophysics Data System (ADS)

Adeela, N.; Khan, U.; Naz, S.; Iqbal, M.; Irfan, M.; Cheng, Y.

2017-11-01

We have reported magnetic properties of La1-xCoxMnO3 nanostructures synthesized by hydrothermal route. The crystal structure has been characterized by X-ray diffraction (XRD) technique, which shows rhombohedral perovskite structure at room temperature. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to analyse morphology and chemical composition of prepared nanoparticles. Magnetic hysteresis loops of all the samples exhibit ferromagnetic behaviour at 10 K. Inverse susceptibility graphs as a function of temperature represent deviation from Curie Weiss law. The indication for short range ferromagnetic clusters well above Curie temperature is observed due to the Griffiths Phase (GP). It is proposed that the presence of GP arises from induced size effects of La and Co ions.

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.

Final report: Mapping Interactions in Hybrid Systems with Active Scanning Probes

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

Berezovsky, Jesse

2017-09-29

This project aimed to study and map interactions between components of hybrid nanodevices using a novel scanning probe approach. To enable this work, we initially constructed a flexible experimental apparatus allowing for simultaneous scanning probe and confocal optical microscopy measurements. This setup was first used for all-optical measurements of nanostructures, with the focus then shifting to hybrid devices in which single coherent electron spins are coupled to micron-scale ferromagnetic elements, which may prove useful for addressing single spins, enhanced sensing, or spin-wave-mediated coupling of spins for quantum information applications. A significant breakthrough was the realization that it is not necessarymore » to fabricate a magnetic structure on a scanning probe – instead a ferromagnetic vortex core can act as an integrated, solid state, scanning probe. The core of the vortex produces a very strong, localized fringe field which can be used analogously to an MFM tip. Unlike a traditional MFM tip, however, the vortex core is scanned within an integrated device (eliminating drift), and can be moved on vastly faster timescales. This approach allows the detailed investigation of interactions between single spins and complex driven ferromagnetic dynamics.« less

Measurements of ultrafast spin-profiles and spin-diffusion properties in the domain wall area at a metal/ferromagnetic film interface.

PubMed

Sant, T; Ksenzov, D; Capotondi, F; Pedersoli, E; Manfredda, M; Kiskinova, M; Zabel, H; Kläui, M; Lüning, J; Pietsch, U; Gutt, C

2017-11-08

Exciting a ferromagnetic material with an ultrashort IR laser pulse is known to induce spin dynamics by heating the spin system and by ultrafast spin diffusion processes. Here, we report on measurements of spin-profiles and spin diffusion properties in the vicinity of domain walls in the interface region between a metallic Al layer and a ferromagnetic Co/Pd thin film upon IR excitation. We followed the ultrafast temporal evolution by means of an ultrafast resonant magnetic scattering experiment in surface scattering geometry, which enables us to exploit the evolution of the domain network within a 1/e distance of 3 nm to 5 nm from the Al/FM film interface. We observe a magnetization-reversal close to the domain wall boundaries that becomes more pronounced closer to the Al/FM film interface. This magnetization-reversal is driven by the different transport properties of majority and minority carriers through a magnetically disordered domain network. Its finite lateral extension has allowed us to measure the ultrafast spin-diffusion coefficients and ultrafast spin velocities for majority and minority carriers upon IR excitation.

Transport and Magnetization in Bad Metals Itinerant Ferromagnets

NASA Astrophysics Data System (ADS)

Klein, Lior

1997-03-01

While much attention has been given to the study of itinerant ferromagnets that are good metals (k_Fl >> 1), very little is known about the transport properties of itinerant ferromagnets in the badly metallic limit (k_Fl= \\cal O ). Here we present our study of the pseudo-cubic perovskite SrRuO3 which is in the limit of k_Fl= \\cal O (1) in its purest form (e.g. single crystals) and is also an itinerant ferromagnet with Tc ~ 160 K. We findfootnote L. Klein, J. S. Dodge, C. H. Ahn, G. J. Snyder, T. H. Geballe, M. R. Beasley, and A. Kapitulnik, Phys. Rev. Lett. 77, 2774 (1996); L. Klein, J. S. Dodge, C. H. Ahn, J. W. Reiner, L. Mieville, T. H.Geballe, M. R. Beasley, and A. Kapitulnik, J. Phys. Condens. Matter 8, 10111 (1996). that while the magnetic properties of SrRuO3 in the paramagnetic phase, near the ferromagnetic phase transition and at low temperatures are normal and similar to those of iron or nickel, the transport properties sharply deviate from those of good metallic ferromagnets: a) As Tarrow T_c^+ the temperature derivative of the magnetic part of the resistivity, dρ _m/dT, diverges with an exponent on the order of 1, an order of magnitude larger than the expected specific heat exponent of ~ 0.1. b) While the critical behavior of dρ _m/dT around Tc is usually found to be symmetric, very weak divergence of dρ _m/dT is observed as Tarrow T_c^-. c) At low temperatures ρ rapidly increases in correlation with the magnetization instead of the usually observed T^2 dependence. d) At T < 4 K for low-residual-resistivity films, and at higher temperatures for high-residual-resistivity films, Kondo-like resistivity minima are observed. We conjecture that the distinct transport behavior of SrRuO3 is related to its being a 'bad metal' in the k_Fl= \\cal O (1) limit, and discuss the possible relevance of our results to the unusual transport properties of other 'bad metals' such as high-temperature superconductors, fullerenes and organic conductors.

Development of engineering components having dual functionality

NASA Astrophysics Data System (ADS)

Thompson, L. D.; Waldbusser, R.; Pratt, E.

2000-05-01

This paper describes the engineering research and development of a smart aircraft bolt designed for high-tension wing attachment applications on USAF C-130 cargo planes. The bolt is a load-carrying structural component as well as serving as a damage sensor. The bolt material choice is TRIP (Transformation Induced Plasticity) steel; high-strength, metastable austenitic steel that gradually and irreversibly transforms to martensite during deformation. The martensite is ferromagnetic whereas the parent austenite phase is paramagnetic, having no significant ferromagnetic response. The ferromagnetic signature of the bolt can be correlated to the peak deformation strain and further correlated with the peak damage state. Stress-assisted phase transformations occur within the elastic regime while strain-induced transformations occur in the post-yield regime. Both transformation variants produce easily detected signals that can be readily monitored. A review of the nature of TRIP steel materials, their properties and behavior, and the smart bolt design approach is presented with examples of the quantitative output obtained during simulated laboratory testing. A brief discussion of the detection electronics and interrogation system is provided to familiarize the audience with the technical issues encompassing this technology. Preliminary results from field testing and service experience are reviewed.

Ce3 - xMgxCo9 : Transformation of a Pauli Paramagnet into a Strong Permanent Magnet

NASA Astrophysics Data System (ADS)

Lamichhane, Tej N.; Taufour, Valentin; Palasyuk, Andriy; Lin, Qisheng; Bud'ko, Sergey L.; Canfield, Paul C.

2018-02-01

We report on the synthesis of single-crystal and polycrystalline samples of Ce3 -xMgxCo9 solid solution (0 ≤x ≲1.4 ) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6 c site of the CeCo3 structure. Ferromagnetism is induced by Mg substitutions starting as low as x =0.18 and reaching a Curie temperature as high as 450 K for x =1.35 . Measurements on single crystals with x =1.34 and TC=440 K indicate an axial magnetic anisotropy with an anisotropy field of 6 T and a magnetization of 6 μB/f .u . at 300 K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests that other nonmagnetic phases, similar to CeCo3 , may also conceal nearby ferromagnetic phases.

Ferroelectric-ferromagnetic coupling in hexagonal YMnO3 film

NASA Astrophysics Data System (ADS)

Cheng, Shaobo; Li, Menglei; Deng, Shiqing; Bao, Shanyong; Tang, Peizhe; Duan, Wenhui; Ma, Jing; Nan, Cewen; Zhu, Jing

Simultaneously achieving ferroelectricity and ferromagnetism in a single phase material is an important research topic in recent decades. Here, we demonstrate that with the modulation of oxygen vacancies, the ferroelectric-ferromagnetic coupling can be realized in the typical hexagonal manganite: YMnO3. The first-principal calculations are used to reveal the importance of oxygen vacancies on the alterations of magnetic behaviors for YMnO3. In order to obtain net magnetic moments, the on-top oxygen vacancies of MnO5 clusters should be created, thus the initial 2D spin frustration structure of Mn ions will be broken. By growing YMnO3 film on Al2O3 substrate, large in-plane compressive strain is induced, thus we can experimentally realize the on-top oxygen vacancies. With the help of SQUID and spherical aberration corrected TEM, the magnetic moments are experimentally measured and the correlations between the crystal structures and magnetic properties can be clearly understood. Our findings may pave a way for future applications of single phase multiferroic materials. National 973 Project of China (2015CB654902, 2011CB606405) and Chinese National Natural Science Foundation (11374174, 51390471).

Ce 3 - x Mg x Co 9 : Transformation of a Pauli Paramagnet into a Strong Permanent Magnet

DOE PAGES

Lamichhane, Tej N.; Taufour, Valentin; Palasyuk, Andriy; ...

2018-02-23

In this article we report on the synthesis of single-crystal and polycrystalline samples of Ce 3-xMg xCo 9 solid solution (0 ≤ x ≲ 1.4) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6c site of the CeCo 3 structure. Ferromagnetism is induced by Mg substitutions starting as low as x = 0.18 and reaching a Curie temperature as high as 450 K for x = 1.35 . Measurements on single crystals with x = 1.34 and T C = 440 K indicatemore » an axial magnetic anisotropy with an anisotropy field of 6 T and a magnetization of 6 μ B/f.u. at 300 K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests that other nonmagnetic phases, similar to CeCo 3, may also conceal nearby ferromagnetic phases.« less

Equilibrium, metastability, and hysteresis in a model spin-crossover material with nearest-neighbor antiferromagnetic-like and long-range ferromagnetic-like interactions

DOE PAGES

Rikvold, Per Arne; Brown, Gregory; Miyashita, Seiji; ...

2016-02-16

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean- field method for a simplified model of a spin-crossovermaterialwith a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice S = 1/2 Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley ( equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical end points and mean-field critical points surrounded by horn-shapedmore » regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. As a result, we believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.« less

Ce 3 - x Mg x Co 9 : Transformation of a Pauli Paramagnet into a Strong Permanent Magnet

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

Lamichhane, Tej N.; Taufour, Valentin; Palasyuk, Andriy

In this article we report on the synthesis of single-crystal and polycrystalline samples of Ce 3-xMg xCo 9 solid solution (0 ≤ x ≲ 1.4) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6c site of the CeCo 3 structure. Ferromagnetism is induced by Mg substitutions starting as low as x = 0.18 and reaching a Curie temperature as high as 450 K for x = 1.35 . Measurements on single crystals with x = 1.34 and T C = 440 K indicatemore » an axial magnetic anisotropy with an anisotropy field of 6 T and a magnetization of 6 μ B/f.u. at 300 K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests that other nonmagnetic phases, similar to CeCo 3, may also conceal nearby ferromagnetic phases.« less

Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures

NASA Astrophysics Data System (ADS)

Hajiali, M. R.; Mohseni, S. Morteza; Jamilpanah, L.; Hamdi, M.; Roozmeh, S. E.; Mohseni, S. Majid

2017-11-01

When a flow of electrons passes through a paramagnetic layer with strong spin-orbit-coupling such as platinum (Pt), a net spin current is produced via the spin Hall effect (SHE). This spin current can exert a torque on the magnetization of an adjacent ferromagnetic layer which can be probed via magnetization dynamic responses, e.g., spin-torque ferromagnetic resonance. Nevertheless, that effect in the lower frequency magnetization dynamic regime where the skin effect occurs in high permeability ferromagnetic conductors, namely, the magneto-impedance (MI) effect, can be fundamentally important, and has not been studied so far. Here, by utilizing the MI effect in the magnetic-ribbon/Pt heterostructure with high transvers magnetic permeability that allows the ac current effectively confined at the skin depth of ˜100 nm thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via the MI measurement is investigated. We observed a systematic MI frequency shift that increases by increasing the applied current amplitude and thickness of the Pt layer (varying from 0 nm to 20 nm). In addition, the role of the Pt layer in the ribbon/Pt heterostructure is evaluated with the ferromagnetic resonance effect representing a standard Gilbert damping increase as a result of the presence of the SHE. Our results unveil the role of SOT in dynamic control of the transverse magnetic permeability probed by impedance spectroscopy as a useful and valuable technique for detection of future SHE devices.

Room temperature ferromagnetism in non-magnetic doped TiO2 nanoparticles

NASA Astrophysics Data System (ADS)

Gómez-Polo, C.; Larumbe, S.; Pastor, J. M.

2013-05-01

Room-temperature ferromagnetism in non-magnetic doped TiO2 semiconductor nanoparticles is analyzed in the present work. Undoped and N-doped TiO2 nanoparticles were obtained employing sol-gel procedure using urea as the nitrogen source. The obtained gels were first dried at 70 °C and afterwards calcined in air at 300 °C. A residual carbon concentration was retained in the samples as a consequence of the organic decomposition process. Post-annealing treatments at 300 °C under air and vacuum conditions were also performed. The crystallographic structure of nanoparticles was analyzed by X-ray diffraction, obtaining a single anatase crystalline phase after the calcinations (mean nanoparticle diameters around 5-8 nm). SQUID magnetometry was employed to analyze the magnetic response of the samples. Whereas for the undoped samples synthesized with hydrolysis rate h = 6, paramagnetic like behavior is observed at room temperature, the N-doped nanoparticles (h = 3) show a weak ferromagnetic response (saturation magnetization ≈10-3 emu/g). Moreover, a clear reinforcement of the room-temperature ferromagnetism response is found with the post-annealing treatments, in particular that performed in vacuum. Thus, the results indicate the dominant role of the oxygen stoichiometry and the oxygen vacancies in the room temperature ferromagnetic response of these TiO2 nanoparticles.

Size- and pressure-controlled ferromagnetism in LaCoO3 nanoparticles

NASA Astrophysics Data System (ADS)

Fita, I.; Markovich, V.; Mogilyansky, D.; Puzniak, R.; Wisniewski, A.; Titelman, L.; Vradman, L.; Herskowitz, M.; Varyukhin, V. N.; Gorodetsky, G.

2008-06-01

Magnetic properties of nanocrystalline LaCoO3 with particle size of 25, 30, 32, and 38 nm, prepared by the citrate method, were investigated in temperature range 2-320 K, magnetic field up to 50 kOe, and under hydrostatic pressure up to 11 kbar. All nanoparticles exhibit weak ferromagnetism below TC≈85K , in agreement with recent observation on LaCoO3 particles and tensile thin films. It was found that with decreasing particle size, i.e., with increasing the surface to volume ratio, the unit-cell volume increases monotonically due to the surface effect. The ferromagnetic moment increases as well, simultaneously with lattice expansion, whereas TC remains nearly unchanged. On the other hand, an applied hydrostatic pressure suppresses strongly the ferromagnetic phase leading to its full disappearance at 10 kbar, while the TC does not change visibly under pressure. It appears that the ferromagnetism in LaCoO3 nanoparticles is controlled by the unit-cell volume. This clear correlation suggests that the nature of ferromagnetic ground state of LaCoO3 is likely related to orbitally ordered Jahn-Teller active Co3+ ions with intermediate-spin (IS) state, which may persist in the expanded lattice at low temperatures. A robust orbital order presumed among the IS Co3+ species can explain the very stable TC observed for LaCoO3 samples prepared under different conditions: single crystal powders, nanoparticles, and thin films.

Magnetic moment distribution modeling in non stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

NASA Astrophysics Data System (ADS)

Lázpita, P.; Gutiérrez, J.; Barandiarán, J. M.; Chernenko, V. A.; Mondelli, C.; Chapon, L.

2014-11-01

Neutron polarized diffraction technique has been used to elucidate the magnetic moment distribution density in non stoichiometric Ni—Mn—Ga single crystals. These experiments allow us to determine a localized magnetic moment in the Mn position in the austenitic phase, and to validity qualitatively previous models of magnetic distributions where there are antiferromagnetic and ferromagnetic coupling for Mn atoms that are sited out of their properly positions. This measurements show the deep dependence of the magnetic moment with the composition and the atomic order.

Development of spontaneous magnetism and half-metallicity in monolayer MoS2

NASA Astrophysics Data System (ADS)

Rahman, Altaf Ur; Rahman, Gul; García-Suárez, Víctor M.

2017-12-01

Half-metallic behavior and ferromagnetism are predicted in strained MoS2 with different light elements adsorbed using density functional theory. We find that strain increases the density of states at the Fermi energy for Y doping (Y = H, Li, and F) at the S sites and strain-driven magnetism develops in agreement with the Stoner mean field model. Strain-driven magnetism requires less strain (∼3%) for H doping as compared with F and Li doping. No saturation of the spin-magnetic moment is observed in Li-doped MoS2 due to less charge transfer from the Mo d electrons and the added atoms do not significantly increase the Spin-orbit coupling. Half-metallic ferromagnetism is predicted in H and F-doped MoS2. Fixed magnetic moments calculations are also performed, and the DFT computed data is fitted with the Landau mean field theory to investigate the emergence of spontaneous magnetism in Y-doped MoS2. We predict spontaneous magnetism in systems with large (small) mag netic moments for H/F (Li) atoms. The large (small) magnetic moments are atttributed to the electronegativity difference between S and Y atoms. These results suggest that H and F adsorbed monolayer MoS2 is a good candidate for spin-based electronic devices.

Ferroelectric-ferromagnetic multilayers: A magnetoelectric heterostructure with high output charge signal

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

Prokhorenko, S.; Kohlstedt, H.; Pertsev, N. A., E-mail: pertsev.domain@mail.ioffe.ru

2014-09-21

Multiferroic composites and heterostructures comprising ferroelectric and ferromagnetic materials exhibit room-temperature magnetoelectric (ME) effects greatly exceeding those of single-phase magnetoelectrics known to date. Since these effects are mediated by the interfacial coupling between ferroic constituents, the ME responses may be enhanced by increasing the density of interfaces and improving their quality. A promising material system providing these features is a ferroelectric-ferromagnetic multilayer with epitaxial interfaces. In this paper, we describe theoretically the strain-mediated direct ME effect exhibited by free-standing multilayers composed of single-crystalline ferroelectric nanolayers interleaved by conducting ferromagnetic slabs. Using a nonlinear thermodynamic approach allowing for specific mechanical boundarymore » conditions of the problem, we first calculate the polarization states and dielectric properties of ferroelectric nanolayers in dependence on the lattice mismatch between ferroic constituents and their volume fractions. In these calculations, the ferromagnetic component is described by a model which combines linear elastic behavior with magnetic-field-dependent lattice parameters. Then the quasistatic ME polarization and voltage coefficients are evaluated using the theoretical strain sensitivity of ferroelectric polarization and measured effective piezomagnetic coefficients of ferromagnets. For Pb(Zr₀.₅Ti₀.₅)O₃-FeGaB and BaTiO₃-FeGaB multilayers, the ME coefficients are calculated numerically as a function of the FeGaB volume fraction and used to evaluate the output charge and voltage signals. It is shown that the multilayer geometry of a ferroelectric-ferromagnetic nanocomposite opens the way for a drastic enhancement of the output charge signal. This feature makes biferroic multilayers advantageous for the development of ultrasensitive magnetic-field sensors for technical and biomedical applications.« less

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

Barber, M.N.; Derrida, B.

We study the phase diagram of the two-dimensional anisotropic next-nearest neighbor Ising (ANNNI) model by comparing the time evolution of two distinct spin configurations submitted to the same thermal noise. We clearly se several dynamical transitions between ferromagnetic, paramagnetic, antiphase, and floating phases. These dynamical transitions seem to occur rather close to the transition lines determined previously in the literature.

Ground-state magnetic phase diagram of bow-tie graphene nanoflakes in external magnetic field

NASA Astrophysics Data System (ADS)

Szałowski, Karol

2013-12-01

The magnetic phase diagram of a ground state is studied theoretically for graphene nanoflakes of bow-tie shape and various sizes in external in-plane magnetic field. The tight-binding Hamiltonian supplemented with Hubbard term is used to model the electronic structure of the systems in question. The existence of the antiferromagnetic phase with magnetic moments localized at the sides of the bow-tie is found for low field and a field-induced spin-flip transition to ferromagnetic state is predicted to occur in charge-undoped structures. For small nanoflake doped with a single charge carrier, the low-field phase is ferrimagnetic and a metamagnetic transition to ferromagnetic ordering can be forced by the field. The critical field is found to decrease with increasing size of the nanoflake. The influence of diagonal and off-diagonal disorder on the mentioned magnetic properties is studied. The effect of off-diagonal disorder is found to be more important than that of diagonal disorder, leading to significantly widened distribution of critical fields for disordered population of nanoflakes.

In-depth study of the H - T phase diagram of Sr 4 Ru 3 O 10 by magnetization experiments

DOE PAGES

Weickert, F.; Civale, L.; Maiorov, B.; ...

2017-09-28

Here, we present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10 below the ferromagnetic transition TC as a double step in the magnetization at two critical fields Hc1 and Hc2. We map the H-T phase diagram with special focus on the temperature range 50 K ≤T≤TC. We find that the critical field Hc1(T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8 T for the lowest temperatures and ends in a criticalmore » endpoint at (1 T; 80 K). We also conclude from the temperature dependence of the ratio Hc1Hc2(T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.« less

In-depth study of the H - T phase diagram of Sr4Ru3O10 by magnetization experiments

NASA Astrophysics Data System (ADS)

Weickert, F.; Civale, L.; Maiorov, B.; Jaime, M.; Salamon, M. B.; Carleschi, E.; Strydom, A. M.; Fittipaldi, R.; Granata, V.; Vecchione, A.

2018-05-01

We present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10 below the ferromagnetic transition TC as a double step in the magnetization at two critical fields Hc1 and Hc2. We map the H - T phase diagram with special focus on the temperature range 50 K ≤ T ≤TC . We find that the critical field Hc1 (T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8 T for the lowest temperatures and ends in a critical endpoint at (1 T; 80 K). We conclude from the temperature dependence of the ratio Hc 1/Hc 2 (T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

Spin Currents and Spin Orbit Torques in Ferromagnets and Antiferromagnets

NASA Astrophysics Data System (ADS)

Hung, Yu-Ming

This thesis focuses on the interactions of spin currents and materials with magnetic order, e.g., ferromagnetic and antiferromagnetic thin films. The spin current is generated in two ways. First by spin-polarized conduction-electrons associated with the spin Hall effect in heavy metals (HMs) and, second, by exciting spin-waves in ferrimagnetic insulators using a microwave frequency magnetic field. A conduction-electron spin current can be generated by spin-orbit coupling in a heavy non-magnetic metal and transfer its spin angular momentum to a ferromagnet, providing a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. The torques on the magnetization are known as spin-orbit torques (SOT). In the first part of my thesis project I investigated and contrasted the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<1 nm) perpendicularly magnetized CoFeB layers on beta-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, pulses with short duration (≤10 ns) and larger amplitude (≃10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. The partial reversal is associated with the limited time for reversed domain expansion during the pulse. The second part of my thesis project studies and considers applications of SOT-driven domain wall (DW) motion in a perpendicularly magnetized ultrathin ferromagnet sandwiched between a heavy metal and an oxide. My experiment results demonstrate that the DW motion can be explained by a combination of the spin Hall effect, which generates a SOT, and Dzyaloshinskii-Moriya interaction, which stabilizes chiral Neel-type DW. Based on SOT-driven DW motion and magnetic coupling between electrically isolated ferromagnetic elements, I proposed a new type of spin logic devices. I then demonstrate the device operation by using micromagnetic modeling which involves studying the magnetic coupling induced by fringe fields from chiral DWs in perpendicularly magnetized nanowires. The last part of my thesis project reports spin transport and spin-Hall magnetoresistance (SMR) in yttrium iron garnet Y3Fe5O 12 (YIG)/NiO/Pt trilayers with varied NiO thickness. To characterize the spin transport through NiO we excite ferromagnetic resonance in YIG with a microwave frequency magnetic field and detect the voltage associated with the inverse spin-Hall effect (ISHE) in the Pt layer. The ISHE signal is found to decay exponentially with the NiO thickness with a characteristic decay length of 3.9 nm. However, in contrast to the ISHE response, as the NiO thickness increases the SMR signal goes towards zero abruptly at a NiO thickness of 4 nm, highlighting the different length scales associated with the spin-transport in NiO and SMR in such trilayers.

Strong electronic interaction and multiple quantum Hall ferromagnetic phases in trilayer graphene

NASA Astrophysics Data System (ADS)

Datta, Biswajit; Dey, Santanu; Samanta, Abhisek; Agarwal, Hitesh; Borah, Abhinandan; Watanabe, Kenji; Taniguchi, Takashi; Sensarma, Rajdeep; Deshmukh, Mandar M.

2017-02-01

Quantum Hall effect provides a simple way to study the competition between single particle physics and electronic interaction. However, electronic interaction becomes important only in very clean graphene samples and so far the trilayer graphene experiments are understood within non-interacting electron picture. Here, we report evidence of strong electronic interactions and quantum Hall ferromagnetism seen in Bernal-stacked trilayer graphene. Due to high mobility ~500,000 cm2 V-1 s-1 in our device compared to previous studies, we find all symmetry broken states and that Landau-level gaps are enhanced by interactions; an aspect explained by our self-consistent Hartree-Fock calculations. Moreover, we observe hysteresis as a function of filling factor and spikes in the longitudinal resistance which, together, signal the formation of quantum Hall ferromagnetic states at low magnetic field.

Three-dimensional magnetic critical behavior in CrI 3

DOE PAGES

Liu, Yu; Petrovic, C.

2018-01-18

CrI 3 is a promising candidate for the van der Waals bonded ferromagnetic devices since its ferromagnetism can be maintained upon exfoliating of bulk crystals down to single layer. In this work we studied critical properties of bulk CrI 3 single crystals around the paramagnetic to ferromagnetic phase transition. Critical exponents β= 0.260(4) with a critical temperature T c= 60.05(13) K and γ= 1.136(6) with T c= 60.43(4) K are obtained by the Kouvel-Fisher method, whereas δ= 5.32(2) is obtained by a critical isotherm analysis at T c= 60 K. In conclusion, the critical exponents determined in bulk CrI 3more » single crystals suggest a three-dimensional long-range magnetic coupling with the exchange distance decaying as J(r)≈r -4:69« less

Ferromagnetic Interactions in the Surface State of LaCoO3

NASA Astrophysics Data System (ADS)

Yan, J.-Q.; Zhou, J.-S.; Goodenough, J. B.

2004-03-01

The spin-state degree of freedom is a peculiar property of LaCoO3 and has been the subject of continuing interest since the 1950s.The thermal excitation from low-spin state to higher-spin state induces a sharp increase of magnetic susceptibility,c(T), above 35 K. A Curie-like paramagnetism below 35 K has been attributed to impurities, oxygen nonstoichiometry, or localized spins of the surface layer. The minimum at 35 K of c(T) varies in magnitude in different reports and single crystals exhibit a smaller c(T) than polycrystals. A ferromagnetic component in LaCoO3 has been found and attributed to CoIV or isolated regions of magnetic phase in a nonmagnetic matrix. We report magnetic measurements of both single crystal and cold-pressed single-crystal powders. Cold-pressing significantly reduces the grain size and the increased surface area gives a strong ferromagnetism with Tc 85 K. The magnitude of c(T) at 35 K depends on the contribution of both the low-T paramagnetism and the ferromagnetic component.

Local electronic structure and ferromagnetic interaction in La(Co,Ni)O3

NASA Astrophysics Data System (ADS)

Schuppler, S.; Nagel, P.; Fuchs, D.; Löhneysen, H. V.; Merz, M.; Huang, M.-J.

Perovskite-related transition-metal oxides exhibit properties ranging from insulating to superconducting as well as unusual magnetic phases, and cobaltates, in particular, have been known for their propensity for spin-state transitions. Nonmagnetic LaCoO3 and paramagnetic LaNiO3 are parent compounds for the La(Co1-xNix) O3 (LCNO) family, which, for intermediate Ni content x, exhibits ferromagnetism. The local electronic structure and the ferromagnetic interaction in LCNO have been studied by x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD). XAS indicates a mixed-valence state for both Co and Ni, with both valences changing systematically with increasing x. Simultaneously, a spin-state redistribution towards HS (Co site) and LS (Ni site) occurs, and temperature-dependent spin-state transitions are increasingly suppressed. XMCD identifies the element-specific contributions to the magnetic moment and interactions. A simple model based on a double-exchange-like mechanism between Co3+ HS and Ni3+HS can qualitatively account for the evolution of ferromagnetism in the LCNO series.

Ferromagnetic ordering and halfmetallic state in a shandite: Co3Sn2S2

NASA Astrophysics Data System (ADS)

Rosner, Helge; Weihrich, Richard; Schnelle, Walter

2005-03-01

The recent rapid development in spintronics challenges the search for new magnetic half metals with high Curie temperatures as well as an improved understanding of the underlying microscopic properties. Here, we present a joint experimental and theoretical study of the recently reinvestigated shandite Co3Sn2S2 [1]. From magnetic susceptibility, specific heat and resistivity measurements on powder samples we find a phase transition to a ferromagnetic metallic state at 177 K with a saturation moment of 0.87 μB/f.u. Full potential electronic structure calculations within the local spin density approximation result in a halfmetallic ferromagnetic groundstate with a moment of 1 μB/f.u. and a tiny gap in the minority spin channel. The calculated structure optimization and structure variations show that the size of the gap is rather sensitive to the lattice geometry. Possibilities to stabilize the halfmetallic ferromagnetic behaviour by various substitutions have been studied theoretically and will be discussed in detail.[1]R. Weihrich et. al. Z. Anorg. Allg. Chem. 630, 1767, (2004)

Self-Assembled Layered Supercell Structure of Bi2AlMnO6 with Strong Room-Temperature Multiferroic Properties.

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

Li, Leigang; Boullay, Philippe; Lu, Ping

2017-02-01

Room-temperature (RT) multiferroics, possessing ferroelectricity and ferromagnetism simultaneously at RT, hold great promise in miniaturized devices including sensors, actuators, transducers, and multi-state memories. In this work, we report a novel 2D layered RT multiferroic system with self-assembled layered supercell structure consisting of two mismatch-layered sub-lattices of [Bi 3O 3+δ] and [MO 2] 1.84 (M=Al/Mn, simply named as BAMO), i.e., alternative layered stacking of two mutually incommensurate sublattices made of a three-layer-thick Bi-O slab and a one-layer-thick Al/Mn-O octahedra slab along the out-of-plane direction. Strong room-temperature multiferroic responses, e.g., ferromagnetic and ferroelectric properties, have been demonstrated and attributed to the highlymore » anisotropic 2D nature of the non-ferromagnetic and ferromagnetic sublattices which are highly mismatched. The work demonstrates an alternative design approach for new 2D layered oxide materials that hold promises as single-phase multiferroics, 2D oxides with tunable bandgaps, and beyond.« less

Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb2Ti2O7

PubMed Central

Chang, Lieh-Jeng; Onoda, Shigeki; Su, Yixi; Kao, Ying-Jer; Tsuei, Ku-Ding; Yasui, Yukio; Kakurai, Kazuhisa; Lees, Martin Richard

2012-01-01

In a class of frustrated magnets known as spin ice, magnetic monopoles emerge as classical defects and interact via the magnetic Coulomb law. With quantum-mechanical interactions, these magnetic charges are carried by fractionalized bosonic quasi-particles, spinons, which can undergo Bose–Einstein condensation through a first-order transition via the Higgs mechanism. Here, we report evidence of a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb2Ti2O7. Polarized neutron scattering experiments show that the diffuse [111]-rod scattering and pinch-point features, which develop on cooling are suddenly suppressed below TC~0.21 K, where magnetic Bragg peaks and a full depolarization of the neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained on the basis of a quantum spin-ice model, whose high-temperature phase is effectively described as a magnetic Coulomb liquid, whereas the ground state shows a nearly collinear ferromagnetism with gapped spin excitations. PMID:22871811

Crystal structure and magnetic properties of cyclohexylammonium trichlorocuprate(II): A quasi 1d Heisenberg S = {1}/{2} ferromagnet

NASA Astrophysics Data System (ADS)

Groenendijk, H. A.; Blöte, H. W. J.; van Duyneveldt, A. J.; Gaura, R. M.; Landee, C. P.; Willett, R. D.

1981-06-01

The crystal structure of [C 6H 11NH 3] CuCl 3, cyclohexylammonium trichlorocuprate(II) (CHAC), is orthorhombic, space group P2 12 12 1 with a = 19.441(5), b = 8.549(2) and c = 6.190(1) Å. The salt contains chains of CuCl -3 ions along the c axis. From magnetization and susceptibility measurements it is found that the compound behaves as a one-dimensional S = {1}/{2} Heisenberg ferromagnet with J1/ k = 70(2) K. Antiferromagnetic ordering with a weak ferromagnetic moment along the a axis occurs below T c = 2.18(2) K. From the metamagnetic phase diagram the interchain interactions are derived using mean field theory: z2J2/ z1J1 = 1.1 × 10 -3 and z3J3/ z1J1 = -1.0 × 10 -4. Also a small anisotropy ( J|/ J⊥ ≈ 0.01) is found in the intrachain interaction. The measurements indicate that CHAC is one of the best approximations to the 1d Heisenberg ferromagnet known to date.

Quantum Phase Transitions in Conventional Matrix Product Systems

NASA Astrophysics Data System (ADS)

Zhu, Jing-Min; Huang, Fei; Chang, Yan

2017-02-01

For matrix product states(MPSs) of one-dimensional spin-1/2 chains, we investigate a new kind of conventional quantum phase transition(QPT). We find that the system has two different ferromagnetic phases; on the line of the two ferromagnetic phases coexisting equally, the system in the thermodynamic limit is in an isolated mediate-coupling state described by a paramagnetic state and is in the same state as the renormalization group fixed point state, the expectation values of the physical quantities are discontinuous, and any two spin blocks of the system have the same geometry quantum discord(GQD) within the range of open interval (0,0.25) and the same classical correlation(CC) within the range of open interval (0,0.75) compared to any phase having no any kind of correlation. We not only realize the control of QPTs but also realize the control of quantum correlation of quantum many-body systems on the critical line by adjusting the environment parameters, which may have potential application in quantum information fields and is helpful to comprehensively and deeply understand the quantum correlation, and the organization and structure of quantum correlation especially for long-range quantum correlation of quantum many-body systems.

Spin-Driven Emergent Antiferromagnetism and Metal-Insulator Transition in Nanoscale p-Si

NASA Astrophysics Data System (ADS)

Lou, Paul C.; Kumar, Sandeep

2018-04-01

The entanglement of the charge, spin and orbital degrees of freedom can give rise to emergent behavior especially in thin films, surfaces and interfaces. Often, materials that exhibit those properties require large spin orbit coupling. We hypothesize that the emergent behavior can also occur due to spin, electron and phonon interactions in widely studied simple materials such as Si. That is, large intrinsic spin-orbit coupling is not an essential requirement for emergent behavior. The central hypothesis is that when one of the specimen dimensions is of the same order (or smaller) as the spin diffusion length, then non-equilibrium spin accumulation due to spin injection or spin-Hall effect (SHE) will lead to emergent phase transformations in the non-ferromagnetic semiconductors. In this experimental work, we report spin mediated emergent antiferromagnetism and metal insulator transition in a Pd (1 nm)/Ni81Fe19 (25 nm)/MgO (1 nm)/p-Si (~400 nm) thin film specimen. The spin-Hall effect in p-Si, observed through Rashba spin-orbit coupling mediated spin-Hall magnetoresistance behavior, is proposed to cause the spin accumulation and resulting emergent behavior. The phase transition is discovered from the diverging behavior in longitudinal third harmonic voltage, which is related to the thermal conductivity and heat capacity.

Quantum phase transition and non-Fermi liquid behavior in Fe1-x Co x Si (x ⩾ 0.7)

NASA Astrophysics Data System (ADS)

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

2018-04-01

We report on the nature of electron correlations in Fe1-x Co x Si (0.7 ≤slant x < 1 ) using combined results of magnetization, specific heat and transport properties. Doping driven quantum critical point is observed to occur at x˜ 0.75 . The magnetically unstable regime is identified to be centered around x\\in [0.75, 0.95 ]. The emergence of non-Fermi liquid behaviors in x  =  0.8 (near to ferromagnetic quantum critical point) and x  =  0.9 (disorder-induced) compositions are discussed on the basis of the power-law dependence of susceptibility χ ˜ T-g (g ˜ 1.07 for x  =  0.8 and 0.55 for x  =  0.9), specific heat C/T ˜ T-1+λ (λ ˜ 1.52 for x  =  0.8 and 0.9) and resistivity Δρ ˜ Td (d ˜ 1.56 for x  =  0.8 and 1.38 for x  =  0.9). Further, a comprehensive classification of doping dependent physical properties of Fe1-x Co x Si is presented in the revisited temperature-composition (T-x) phase diagram.

Hypervelocity impacts and magnetization of small bodies in the Solar System

NASA Technical Reports Server (NTRS)

Chen, Guangqing; Ahrens, Thomas J.; Hide, Raymond

1995-01-01

The observed magnetism of asteroids such as Gaspra and Ida (and other small bodies in the solar system including the Moon and meteorites) may have resulted from an impact-induced shock wave producing a thermodynamic state in which iron-nickel alloy, dispersed in a silicate matrix, is driven from the usual low-temperature, low-pressure, alpha, kaemacite, phase to the paramagnetic, epsilon (hcp), phase. The magnetization was acquired upon rarefaction and reentry into the ferromagnetic, alpha, structure. The degree of remagnetization depends on the strength of the ambient field, which may have been associated with a Solar-System-wide magnetic field. A transient field induced by the impact event itself may have resulted in a significant, or possibly, even a dominant contribution, as well. The scaling law of Housen et al. (Housen, K. R., R. M. Schmidt, and K. A. Holsapple 1991) for catastrophic asteroid impact disaggregation imposes a constraint on the degree to which small planetary bodies may be magnetized and yet survive fragmentation by the same event. Our modeling results show it is possible that Ida was magnetized when a large impact fractured a 125 +/- 22-km-radius protoasteroid to form the Koronis family. Similarly, we calculate that Gaspra could be a magnetized fragment of a 45 +/- 15 km-radius protoasteroid.

Fabrication of Optimized Superconducting Phase Inverters Based on Superconductor-Ferromagnet-Superconductor pi π -Junctions

NASA Astrophysics Data System (ADS)

Bolginov, V. V.; Rossolenko, A. N.; Shkarin, A. B.; Oboznov, V. A.; Ryazanov, V. V.

2018-03-01

We have implemented a trilayer technological approach to fabricate Nb-Cu_{0.47} Ni_{0.53}-Nb superconducting phase inverters (π -junctions) with enhanced critical current. Within this technique, all three layers of the superconductor-ferromagnet-superconductor junction deposited in a single vacuum cycle that have allowed us to obtain π -junctions with critical current density up to 20 kA/cm^2. The value achieved is a factor of 10 higher than for the step-by-step method used in earlier works. Our additional experiments have shown that this difference is related to a bilayered CuNi/Cu barrier used in the case of the step-by-step technique and interlayer diffusion at the CuNi/Cu interface. We show that the interlayer diffusion can be utilized for fine tuning of the 0{-}π transition temperature of already fabricated junctions. The results obtained open new opportunities for the CuNi-based phase inverters in digital and quantum Josephson electronics.

Quantum fluctuations and gapped Goldstone modes in spinor Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Beekman, Aron

2015-03-01

The classical Heisenberg ferromagnet is an exact eigenstate of the quantum Hamiltonian and therefore has no quantum fluctuations. Furthermore it has a reduced number of Goldstone modes, an order parameter that is itself a symmetry generator, is a highest-weight state for the spin algebra, and has no tower of states of vanishing energy. We derive the connection between all these properties and provide general criteria for their presence in other spontaneously-broken symmetry states. The phletora of groundstates in spinor Bose-Einstein condensates is an ideal testing ground for these predictions. In particular the phases with non-maximal polarization (e.g. the F-phase in spin-3 condensates) have an additional gapped mode that is a partner to the quadratically dispersing Goldstone mode, as compared to the maximally polarized, ferromagnetic phase. Furthermore there is a fundamental limit to the coherence time of superpositions in the non-maximally polarized state, which should manifest itself for small-size systems.

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

Weickert, F.; Civale, L.; Maiorov, B.

Here, we present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10 below the ferromagnetic transition TC as a double step in the magnetization at two critical fields Hc1 and Hc2. We map the H-T phase diagram with special focus on the temperature range 50 K ≤T≤TC. We find that the critical field Hc1(T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8 T for the lowest temperatures and ends in a criticalmore » endpoint at (1 T; 80 K). We also conclude from the temperature dependence of the ratio Hc1Hc2(T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.« less

Changes in the electronic structure and spin dynamics across the metal-insulator transition in LaLa 1-xSr xCoO 3

DOE PAGES

Smith, R. X.; Hoch, M. J. R.; Moulton, W. G.; ...

2016-01-25

The magnetoelectronic properties of La 1-xSr xCoO 3, which include giant magnetoresistance, are strongly dependent on the level of hole doping. The system evolves, with increasing x, from a spin glass insulator to a metallic ferromagnet with a metal-insulator (MI) transition at x C ~ 0.18. Nanoscale phase separation occurs in the insulating phase and persists, to some extent, into the just-metallic phase. The present experiments at 4.2 K have used 139La NMR to investigate the transition from hopping dynamics for x < x C to Korringa-like ferromagnetic metal behavior for x > x C. A marked decrease in themore » spin-lattice relaxation rate is found in the vicinity of x C as the MI transition is crossed. Lastly, this behavior is accounted for in terms of the evolution of the electronic structure and dynamics with cluster size.« less

Avalanche of entanglement and correlations at quantum phase transitions.

PubMed

Krutitsky, Konstantin V; Osterloh, Andreas; Schützhold, Ralf

2017-06-16

We study the ground-state entanglement in the quantum Ising model with nearest neighbor ferromagnetic coupling J and find a sequential increase of entanglement depth d with growing J. This entanglement avalanche starts with two-point entanglement, as measured by the concurrence, and continues via the three-tangle and four-tangle, until finally, deep in the ferromagnetic phase for J = ∞, arriving at a pure L-partite (GHZ type) entanglement of all L spins. Comparison with the two, three, and four-point correlations reveals a similar sequence and shows strong ties to the above entanglement measures for small J. However, we also find a partial inversion of the hierarchy, where the four-point correlation exceeds the three- and two-point correlations, well before the critical point is reached. Qualitatively similar behavior is also found for the Bose-Hubbard model, suggesting that this is a general feature of a quantum phase transition. This should be taken into account in the approximations starting from a mean-field limit.

The phase diagrams of a spin 1/2 core and a spin 1 shell nanoparticle with a disordered interface

NASA Astrophysics Data System (ADS)

Zaim, N.; Zaim, A.; Kerouad, M.

2016-12-01

The critical and compensation behaviors, of a spherical ferrimagnetic nanoparticle, consisting of a ferromagnetic core of spin-1/2 A atoms, a ferromagnetic shell of spin-1 B atoms and a disordered interface in between that is characterized by a random arrangement of A and B atoms of ApB1-p type and a negative A - B coupling, are studied. The ground state phase diagrams of the system have been determined in the (JAB, D/jA) and (JB, D/jA) planes. Monte Carlo simulation based on Metropolis algorithm has been used to study the effects of the concentration parameter p, the crystal field, the coupling between B - B atoms jB and the antiferromagnetic interface coupling jAB on the phase diagrams and the magnetic properties of the system. It has been found that one, two or even three compensation point(s) can appear for appropriate values of the system parameters.

Spin dynamics of antiferromagnets in the presence of a homogeneous magnetization

NASA Astrophysics Data System (ADS)

Kirkpatrick, T. R.; Belitz, D.

2017-06-01

We use general hydrodynamic equations to determine the long-wavelength spin excitations in isotropic antiferromagnets in the presence of a homogeneous magnetization. The latter may be induced, such as in antiferromagnets in an external magnetic field, or spontaneous, such as in ferrimagnetic or canted phases that are characterized by the coexistence of antiferromagnetic and ferromagnetic order. Depending on the physical situation, we find propagating spin waves that are gapped in some cases and gapless in others, diffusive modes, or relaxational modes. The excitation spectra turn out to be qualitatively different depending on whether or not the homogeneous magnetization is a conserved quantity. The results lay the foundation for a description of a variety of quantum phase transitions, including the transition from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop transitions that are observed in some antiferromagnets. They also are crucial for incorporating weak localization and Altshuler-Aronov effects into the descriptions of quantum phases in both clean and disordered magnetic metals.

Composition induced metal-insulator quantum phase transition in the Heusler type Fe2VAl.

PubMed

Naka, Takashi; Nikitin, Artem M; Pan, Yu; de Visser, Anne; Nakane, Takayuki; Ishikawa, Fumihiro; Yamada, Yuh; Imai, Motoharu; Matsushita, Akiyuki

2016-07-20

We report the magnetism and transport properties of the Heusler compound Fe2+x V1-x Al at  -0.10  ⩽  x  ⩽  0.20 under pressure and a magnetic field. A metal-insulator quantum phase transition occurred at x  ≈  -0.05. Application of pressure or a magnetic field facilitated the emergence of finite zero-temperature conductivity σ 0 around the critical point, which scaled approximately according to the power law (P  -  P c ) (γ) . At x  ⩽  -0.05, a localized paramagnetic spin appeared, whereas above the ferromagnetic quantum critical point at x  ≈  0.05, itinerant ferromagnetism was established. At the quantum critical points at x  =  -0.05 and 0.05, the resistivity and specific heat exhibited singularities characteristic of a Griffiths phase appearing as an inhomogeneous electronic state.

Self-learning kinetic Monte Carlo simulations of diffusion in ferromagnetic α -Fe–Si alloys

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

Nandipati, Giridhar; Jiang, Xiujuan; Vemuri, Rama S.

Diffusion in α-Fe-Si alloys is studied using AKSOME, an on-lattice self-learning KMC code, in the ferromagnetic state. Si diffusivity in the α-Fe matrix were obtained with and without the magnetic disorder in various temperature ranges. In addition we studied vacancy diffusivity in ferromagnetic α-Fe at various Si concentrations up to 12.5at.% in the temperature range of 350–550 K. The results were compared with available experimental and theoretical values in the literature. Local Si-atom dependent activation energies for vacancy hops were calculated using a broken-model and were stored in a database. The migration barrier and prefactors for Si-diffusivity were found tomore » be in reasonable agreement with available modeling results in the literature. Magnetic disorder has a larger effect on the prefactor than on the migration barrier. Prefactor was approximately an order of magnitude and the migration barrier a tenth of an electron-volt higher with magnetic disorder when compared to a fully ferromagnetic ordered state. In addition, the correlation between various have a larger effect on the Si-diffusivity extracted in various temperature range than the magnetic disorder. In the case of vacancy diffusivity, the migration barrier more or less remained constant while the prefactor decreased with increasing Si concentration in the disordered or A2-phase of Fe-Si alloy. Important vacancy-Si/Fe atom exchange processes and their activation barriers were also identified and discuss the effect of energetics on the formation of ordered phases in Fe-Si alloys.« less

The road to superconducting spintronics

NASA Astrophysics Data System (ADS)

Eschrig, Matthias

Energy efficient computing has become a major challenge, with the increasing importance of large data centres across the world, which already today have a power consumption comparable to that of Spain, with steeply increasing trend. Superconducting computing is progressively becoming an alternative for large-scale applications, with the costs for cooling being largely outweighed by the gain in energy efficiency. The combination of superconductivity and spintronics - ``superspintronics'' - has the potential and flexibility to develop into such a green technology. This young field is based on the observation that new phenomena emerge at interfaces between superconducting and other, competing, phases. The past 15 years have seen a series of pivotal predictions and experimental discoveries relating to the interplay between superconductivity and ferromagnetism. The building blocks of superspintronics are equal-spin Cooper pairs, which are generated at the interface between superconducting and a ferromagnetic materials in the presence of non-collinear magnetism. Such novel, spin-polarised Cooper pairs carry spin-supercurrents in ferromagnets and thus contribute to spin-transport and spin-control. Geometric Berry phases appear during the singlet-triplet conversion process in structures with non-coplanar magnetisation, enhancing functionality of devices, and non-locality introduced by superconducting order leads to long-range effects. With the successful generation and control of equal-spin Cooper pairs the hitherto notorious incompatibility of superconductivity and ferromagnetism has been not only overcome, but turned synergistic. I will discuss these developments and their extraordinary potential. I also will present open questions posed by recent experiments and point out implications for theory. This work is supported by the Engineering and Physical Science Research Council (EPSRC Grant No. EP/J010618/1).

Power loss for a periodically driven ferromagnetic nanoparticle in a viscous fluid: The finite anisotropy aspects

NASA Astrophysics Data System (ADS)

Lyutyy, T. V.; Hryshko, O. M.; Kovner, A. A.

2018-01-01

The coupled magnetic and mechanical motion of a ferromagnetic nanoparticle in a viscous fluid is considered within the dynamical approach. The equation based on the total momentum conservation law is used for the description of the mechanical rotation, while the modified Landau-Lifshitz-Gilbert equation is utilized for the description of the internal magnetic dynamics. The exact expressions for the particles trajectories and the power loss are obtained in the linear approximation. The comparison with the results of other widespread approaches, such as the model of fixed particle and the model of rigid dipole, is performed. It is established that in the small oscillations mode the damping precession of the nanoparticle magnetic moment is the main channel of energy dissipation, but the motion of the nanoparticle easy axis can significantly influence the value of the resulting power loss.

Current-induced switching in a magnetic insulator

NASA Astrophysics Data System (ADS)

Avci, Can Onur; Quindeau, Andy; Pai, Chi-Feng; Mann, Maxwell; Caretta, Lucas; Tang, Astera S.; Onbasli, Mehmet C.; Ross, Caroline A.; Beach, Geoffrey S. D.

2017-03-01

The spin Hall effect in heavy metals converts charge current into pure spin current, which can be injected into an adjacent ferromagnet to exert a torque. This spin-orbit torque (SOT) has been widely used to manipulate the magnetization in metallic ferromagnets. In the case of magnetic insulators (MIs), although charge currents cannot flow, spin currents can propagate, but current-induced control of the magnetization in a MI has so far remained elusive. Here we demonstrate spin-current-induced switching of a perpendicularly magnetized thulium iron garnet film driven by charge current in a Pt overlayer. We estimate a relatively large spin-mixing conductance and damping-like SOT through spin Hall magnetoresistance and harmonic Hall measurements, respectively, indicating considerable spin transparency at the Pt/MI interface. We show that spin currents injected across this interface lead to deterministic magnetization reversal at low current densities, paving the road towards ultralow-dissipation spintronic devices based on MIs.

Adaptive microwave impedance memory effect in a ferromagnetic insulator.

PubMed

Lee, Hanju; Friedman, Barry; Lee, Kiejin

2016-12-14

Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures.

Prediction of two-dimensional electron gas mediated magnetoelectric coupling at ferroelectric PbTiO3/SrTiO3 heterostructures

NASA Astrophysics Data System (ADS)

Wei, Lan-ying; Lian, Chao; Meng, Sheng

2017-05-01

First-principles calculations predict the emergence of magnetoelectric coupling mediated by two-dimensional electron gas (2DEG) at the ferroelectric PbTiO3/SrTiO3 heterostructure. Free electrons endowed by naturally existing oxygen vacancies in SrTiO3 are driven to the heterostructure interface under the polarizing field of ferroelectric PbTiO3 to form a 2DEG. The electrons are captured by interfacial Ti atoms, which surprisingly exhibits ferromagnetism even at room temperature with a small critical density of ˜15.5 μ C /cm2 . The ferroelectricity-controlled ferromagnetism mediated by interfacial 2DEG shows strong magnetoelectric coupling strength, enabling convenient control of magnetism by electric field and vice versa. The PbTiO3/SrTiO3 heterostructure is cheap, easily grown, and controllable, promising future applications in low-cost spintronics and information storage at ambient condition.

Preisach modeling and compensation for smart material hysteresis

NASA Astrophysics Data System (ADS)

Hughes, Declan C.; Wen, John T.

1995-02-01

Many of the Smart materials being investigated (e.g., Shape Memory Alloys (SMAs), piezoceramics, and magnetostrictives) exhibit significant hysteresis effects, especially when driven with large control signals. In this paper the similarity between the microscopic domain kinematics that generate static hysteresis effects, or ferromagnetics, piezoceramics and SMAs is noted. The Preisach independent domain hysteresis model, and its derivatives, have been shown to be a comprehensive class of hysteresis operator that captures the major features of ferromagnetic hysteresis, and hence it is proposed here as a suitable model for piezoceramic and SMA hysteresis also. This basic Preisach model is used to model piezoceramic sheet actuators bonded to a flexible aluminum beam, and a Nitinol SMA wire muscle that applies a bending force to the end of the beam. A numerical inverse Preisach hysteresis series compensator is also proposed and applied in a real time experiment thereby reducing the apparent nonlinear hysteresis effects for the piezoceramic actuator quasi-static case.

Preisach modeling of piezoceramic and shape memory alloy hysteresis

NASA Astrophysics Data System (ADS)

Hughes, Declan C.; Wen, John T.

1996-05-01

Smart materials such as piezoceramics, magnetostrictive materials, and shape memory alloys exhibit significant hysteresis, especially when driven with large input signals. Hysteresis can lead to unwanted harmonics, inaccuracy in open loop control, and instability in closed loop control. The Preisach independent domain hysteresis model has been shown to capture the major features of hysteresis arising in ferromagnetic materials. Noting the similarity between the microscopic domain kinematics that generate static hysteresis effects in ferromagnetics, piezoceramics, and shape memory alloys, we apply the Preisach model for the hysteresis in piezoceramic and shape memory alloy materials. This paper reviews the basic properties of the Preisach model, discusses control-theoretic issues such as identification, simulation, and inversion, and presents experimental results for piezoceramic sheet actuators bonded to a flexible aluminum beam, and a Nitinol SMA wire muscle that applies a bending force to the end of a beam.

Adaptive microwave impedance memory effect in a ferromagnetic insulator

PubMed Central

Lee, Hanju; Friedman, Barry; Lee, Kiejin

2016-01-01

Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures. PMID:27966536

Characterization of lunar ferromagnetic phases by the effective linewidth method

NASA Technical Reports Server (NTRS)

Patton, C. E.; Schmidt, H.

1978-01-01

The effective line-width technique, first developed to study the physics of microwave relaxation in ferrites, has been successfully applied to lunar matter. Effective line-width measurements have been made on two selected samples containing disperse spherical metallic iron particles below 40 microns in diam. The data were obtained for fields from 7 to 12 kOe and a temperature range 125 - 300 K. The effective line width was field-independent and temperature-independent at 650 - 750 Oe. The high-field tails of the ferromagnetic resonance absorption were highly Lorentzian. From the relatively large and temperature-independent high-field effective line widths, it appears that (1) the metallic iron phases in lunar soil are rather impure; (2) the impurities are passive, in that there is no evidence for a temperature peak process; and (3) these samples contain no appreciable magnetite.

Studies of electronic and magnetic properties of LaVO3 thin film

NASA Astrophysics Data System (ADS)

Jana, Anupam; Karwal, Sharad; Choudhary, R. J.; Phase, D. M.

2018-04-01

We have investigated the electronic and magnetic properties of pulsed laser deposited Mott insulator LaVO3 (LVO) thin film. Structural characterization revels the single phase [00l] oriented LVO thin film. Enhancement of out of plane lattice parameter indicates the compressively strained LVO film. Electron spectroscopic studies demonstrate that vanadium is present in V3+ state. An energy dispersive X-ray spectroscopic study ensures the stoichiometric growth of the film. Very smooth surface is observed in scanning electron micrograph. Colour mapping for elemental distribution reflect the homogeneity of LVO film. The bifurcation between zero-field-cooled and Field-cooled curves clearly points towards the weak ferromagnetic phase presence in compressively strained LVO thin film. A finite value of coercivity at 300 K reflects the possibility of room temperature ferromagnetism of LVO thin film.

Quantum fluctuations in anisotropic triangular lattices with ferromagnetic and antiferromagnetic exchange

NASA Astrophysics Data System (ADS)

Schmidt, Burkhard; Thalmeier, Peter

2014-05-01

The Heisenberg model on a triangular lattice is a prime example of a geometrically frustrated spin system. However most experimentally accessible compounds have spatially anisotropic exchange interactions. As a function of this anisotropy, ground states with different magnetic properties can be realized. Motivated by recent experimental findings on Cs2CuCl4-xBrx, we discuss the full phase diagram of the anisotropic model with two exchange constants J1 and J2, including possible ferromagnetic exchange. Furthermore a comparison with the related square lattice model is carried out. We discuss the zero-temperature phase diagram, ordering vector, ground-state energy, and ordered moment on a classical level and investigate the effect of quantum fluctuations within the framework of spin-wave theory. The field dependence of the ordered moment is shown to be nonmonotonic with field and control parameter.

Room temperature ferromagnetism and luminescent behavior of Ni doped ZnO nanoparticles prepared by coprecipitation method

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

Arora, Deepawali; Mahajan, Aman; Kaur, Parvinder

2016-05-23

The samples of Zn{sub 1-x}Ni{sub x}O (x= 0.00 and 0.05) were prepared using coprecipitation method and annealed at different temperatures. The effect of Ni ion substitution on the structural and optical properties has been studied using X-ray Diffraction, UV-Visible, Photoluminescence and Magnetic measurements. XRD measurements demonstrate that all the prepared samples are wurtzite polycrystalline single phase in nature, ruling out the presence of any secondary phase formation. Ultraviolet visible measurements showed a decrease in band gap with the increase in annealing temperature and doping concentration. The PL data shows the red shift in all the samples and luminescence quenching withmore » Ni doping. Compared to undoped ZnO, Ni doped ZnO showed room temperature ferromagnetism.« less

Phase transition studies of BiMnO3: Mean field theory approximations

NASA Astrophysics Data System (ADS)

Priya K. B, Lakshmi; Natesan, Baskaran

2015-06-01

We studied the phase transition and magneto-electric coupling effect of BiMnO3 by employing mean field theory approximations. To capture the ferromagnetic and ferroelectric transitions of BiMnO3, we construct an extended Ising model in a 2D square lattice, wherein, the magnetic (electric) interactions are described in terms of the direct interactions between the localized magnetic (electric dipole) moments of Mn ions with their nearest neighbors. To evaluate our model, we obtain magnetization, magnetic susceptibility and electric polarization using mean field approximation calculations. Our results reproduce both the ferromagnetic and the ferroelectric transitions, matching very well with the experimental reports. Furthermore, consistent with experimental observations, our mean field results suggest that there is indeed a coupling between the magnetic and electric ordering in BiMnO3.

Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5

NASA Astrophysics Data System (ADS)

Krenke, Thorsten; ćakır, Aslı; Scheibel, Franziska; Acet, Mehmet; Farle, Michael

2016-12-01

The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50-xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR- have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media.

Effect of Sr-doping on electronic and magnetic properties of La2-xSrxCoMnO6

NASA Astrophysics Data System (ADS)

Khan, Anasua; Chatterjee, Swastika; Mandal, P. R.; Nath, T. K.

2018-04-01

In this report, La2-xSrxCoMnO6 (x=0, 1) have been synthesised using sol-gel technique. La2CoMnO6 (LCMO) takes a monoclinic phase, whereas LaSrCoMnO6 (LSCMO) appears in a mixed phase of having both monoclinic and rhombohedral symmetries. DC magnetization measurement shows that LCMO is Ferromagnetic in nature whereas LSCMO shows magnetic glassy nature. This experimental result is verified by ab-initio calculation using GGA+SO+U as implemented in WIEN2k code. Total energy calculations suggest that antisite disorder is enhanced with Sr doping at La site and LSCMO is predominantly ferromagnetic in nature. Co ions which appeared in high spin +2 charge state, converts to intermediate spin +3 charge state with Sr doping.

Random crystal field effect on the magnetic and hysteresis behaviors of a spin-1 cylindrical nanowire

NASA Astrophysics Data System (ADS)

Zaim, N.; Zaim, A.; Kerouad, M.

2017-02-01

In this work, the magnetic behavior of the cylindrical nanowire, consisting of a ferromagnetic core of spin-1 atoms surrounded by a ferromagnetic shell of spin-1 atoms is studied in the presence of a random crystal field interaction. Based on Metropolis algorithm, the Monte Carlo simulation has been used to investigate the effects of the concentration of the random crystal field p, the crystal field D and the shell exchange interaction Js on the phase diagrams and the hysteresis behavior of the system. Some characteristic behaviors have been found, such as the first and second-order phase transitions joined by tricritical point for appropriate values of the system parameters, triple and isolated critical points can be also found. Depending on the Hamiltonian parameters, single, double and para hysteresis regions are explicitly determined.

Critical behavior near the ferromagnetic phase transition in double perovskite Nd2NiMnO6

NASA Astrophysics Data System (ADS)

Ali, Anzar; Sharma, G.; Singh, Yogesh

2018-05-01

The knowledge of critical exponents plays a crucial role in trying to understand the interaction mechanism near a phase transition. In this report, we present a detailed study of the critical behaviour near the ferromagnetic (FM) transition (TC ˜ 193 K) in Nd2NiMnO6 using the temperature and magnetic field dependent isothermal magnetisation measurements. We used various analysis methods such as Arrott plot, modified Arrott plot, and Kouvel-Fisher plot to estimate the critical parameters. The magnetic critical parameters β = 0.49±0.02, γ = 1.05±0.04 and critical isothermal parameter δ = 3.05±0.02 are in excellent agreement with Widom scaling. The critical parameters analysis emphasizes that mean field interaction is the mechanism driving the FM transition in Nd2NiMnO6.

Strain-induced phase transition and electron spin-polarization in graphene spirals

PubMed Central

Zhang, Xiaoming; Zhao, Mingwen

2014-01-01

Spin-polarized triangular graphene nanoflakes (t-GNFs) serve as ideal building blocks for the long-desired ferromagnetic graphene superlattices, but they are always assembled to planar structures which reduce its mechanical properties. Here, by joining t-GNFs in a spiral way, we propose one-dimensional graphene spirals (GSs) with superior mechanical properties and tunable electronic structures. We demonstrate theoretically the unique features of electron motion in the spiral lattice by means of first-principles calculations combined with a simple Hubbard model. Within a linear elastic deformation range, the GSs are nonmagnetic metals. When the axial tensile strain exceeds an ultimate strain, however, they convert to magnetic semiconductors with stable ferromagnetic ordering along the edges. Such strain-induced phase transition and tunable electron spin-polarization revealed in the GSs open a new avenue for spintronics devices. PMID:25027550

Strain-induced phase transition and electron spin-polarization in graphene spirals.

PubMed

Zhang, Xiaoming; Zhao, Mingwen

2014-07-16

Spin-polarized triangular graphene nanoflakes (t-GNFs) serve as ideal building blocks for the long-desired ferromagnetic graphene superlattices, but they are always assembled to planar structures which reduce its mechanical properties. Here, by joining t-GNFs in a spiral way, we propose one-dimensional graphene spirals (GSs) with superior mechanical properties and tunable electronic structures. We demonstrate theoretically the unique features of electron motion in the spiral lattice by means of first-principles calculations combined with a simple Hubbard model. Within a linear elastic deformation range, the GSs are nonmagnetic metals. When the axial tensile strain exceeds an ultimate strain, however, they convert to magnetic semiconductors with stable ferromagnetic ordering along the edges. Such strain-induced phase transition and tunable electron spin-polarization revealed in the GSs open a new avenue for spintronics devices.

A new effective correlation mean-field theory for the ferromagnetic spin-1 Blume-Capel model in a transverse crystal field

NASA Astrophysics Data System (ADS)

Roberto Viana, J.; Rodriguez Salmon, Octavio D.; Neto, Minos A.; Carvalho, Diego C.

2018-02-01

A new approximation technique is developed so as to study the quantum ferromagnetic spin-1 Blume-Capel model in the presence of a transverse crystal field in the square lattice. Our proposal consists of approaching the spin system by considering islands of finite clusters whose frontiers are surrounded by noninteracting spins that are treated by the effective-field theory. The resulting phase diagram is qualitatively correct, in contrast to most effective-field treatments, in which the first-order line exhibits spurious behavior by not being perpendicular to the anisotropy axis at low-temperatures. The effect of the transverse anisotropy is also verified by the presence of quantum phase transitions. The possibility of using larger sizes constitutes an advantage to other approaches where the implementation of larger sizes is computationally costly.

Cu1–xFexO: hopping transport and ferromagnetism

PubMed Central

Nasir, Mohd.; Islam, Rakibul; Ahmed, Md. A; Ayaz, Saniya; Kumar, Gautham; Kumar, Sunil; Prajapat, C. L.; Roussel, Frederick; Biring, Sajal

2017-01-01

Single phase, sol–gel prepared Cu1–xFexO (0 ≤ x ≤ 0.125) powders are characterized in terms of structural, electronic and magnetic properties. Using dielectric and magnetic studies we investigate the coupling of electron and spin. The electrical conductivities and activation energies are studied with increasing Fe content. Modelling of experimental conductivity data emphasizes a single hopping mechanism for all samples except x = 0.125, which have two activation energies. Hole doping is confirmed by confirming a majority Fe3+ substitution of Cu2+ in CuO from X-ray photoelectron spectroscopy studies (XPS). Such a substitution results in stabilized ferromagnetism. Fe substitution introduces variation in coercivity as an intrinsic magnetic property in Fe-doped CuO, and not as a secondary impurity phase. PMID:28989741

Magnetic-field-induced irreversible antiferromagnetic-ferromagnetic phase transition around room temperature in as-cast Sm-Co based SmCo7-xSix alloys

NASA Astrophysics Data System (ADS)

Feng, D. Y.; Zhao, L. Z.; Liu, Z. W.

2016-04-01

A magnetic-field-induced irreversible metamagnetic phase transition from antiferro- to ferromagnetism, which leads to an anomalous initial-magnetization curve lying outside the magnetic hysteresis loop, is reported in arc-melted SmCo7-xSix alloys. The transition temperatures are near room temperature, much higher than other compounds with similar initial curves. Detailed investigation shows that this phenomenon is dependent on temperature, magnetic field and Si content and shows some interesting characteristics. It is suggested that varying interactions between the Sm and Co layers in the crystal are responsible for the formation of a metastable AFM structure, which induces the anomalous phenomenon in as-cast alloys. The random occupation of 3g sites by Si and Co atoms also has an effect on this phenomenon.

Temperature Dependence of the Magnetization of the Ni52Mn24Ga24 Alloy in Various Structural States

NASA Astrophysics Data System (ADS)

Musabirov, I. I.; Sharipov, I. Z.; Mulyukov, R. R.

2015-10-01

are presented of a study of the temperature dependence of the magnetization σ(Т) of the polycrystalline Ni52Mn24Ga24 alloy in various structural states: in the initial coarse-grained state, after severe plastic deformation by high pressure torsion, and after stepped annealing of the deformed specimen at temperatures from 200 to 700°С for 30 min. As a study of the σ(Т) curve shows, in an alloy possessing a coarse-grained initial structure, a martensitic phase transition and a magnetic phase transition are observed in the room temperature interval. The martensitic transformation takes place in the ferromagnetic state of the alloy. This transformation is accompanied by an abrupt lowering of the magnetization of the material, associated with a lowering of the symmetry of the crystalline lattice and a high value of the magnetocrystalline anisotropy constant of the alloy in the martensitic phase. It is shown that as a result of plastic deformation there takes place a destruction of ferromagnetic order and a suppression of the martensitic transformation. Consecutive annealing after deformation leads to a gradual recovery of ferromagnetic order and growth of the magnetization of the material. Recovery of the martensitic transformation begins to be manifested only after annealing of the alloy at a temperature of 500°C, when the mean grain size in the recrystallized structure reaches a value around 1 μm.

Short-range magnetic order, irreversibility and giant magnetoresistance near the triple points in the (x, T) magnetic phase diagram of ZrMn6Sn6-xGax

NASA Astrophysics Data System (ADS)

Mazet, T.; Ihou-Mouko, H.; Marêché, J.-F.; Malaman, B.

2010-04-01

We have studied pseudo-layered ZrMn6Sn6-xGax intermetallics (0.55 ≤ x ≤ 0.81) using magnetic, magnetoresistivity and powder neutron diffraction measurements. All the alloys studied have magnetic ordering temperatures in the 450-490 K temperature range. They present complex temperature-dependent partially disordered magnetic structures whose ferromagnetic component develops upon increasing the Ga content. ZrMn6Sn6-xGax alloys with x ≤ 0.69 are essentially collinear antiferromagnets at high-temperature and adopt antifan-like arrangements at low temperature. For x ≥ 0.75, the alloys order ferromagnetically and evolve to a fan-like structure upon cooling. The intermediate compositions (x = 0.71 and 0.73) present a canted fan-like order at high temperature and another kind of antifan-like arrangement at low temperature. The degree of short-range order tends to increase upon approaching the intermediate compositions. The (x, T) phase diagram contains two triple points (x ~ 0.70; T ~ 460 K and x ~ 0.74; T ~ 455 K), where the paramagnetic, an incommensurate and a commensurate phases meet, which possess some of the features of Lifshitz point. Irreversibilities manifest in the low-temperature magnetization curves at the antifan-fan or fan-ferromagnetic boundaries as well as inside the fan region. Giant magnetoresistance is observed, even above room temperature.

Hybridization-mediated anisotropic coupling in plutonium compounds

NASA Astrophysics Data System (ADS)

Banerjea, Amitava; Cooper, Bernard R.; Thayamballi, Pradeep

1984-09-01

The magnetic behavior of a class of cerium and light actinide compounds containing moderately delocalized f electrons has been explained on the basis of an anisotropic two-ion interaction that arises from the hybridization of band electrons and the f electrons. This theory, first developed by Siemann and Cooper for cerium compounds using the treatment of Coqblin and Schrieffer for the hybridization, was later generalized by Thayamballi and Cooper to fn systems in the L-S and j-j coupling limits. We here extend the theory to the case of intermediate intraionic coupling and further include the possibility of long-period antiferromagnetic structures. In particular, we have considered the Pu3+(f5) ion in PuSb. The theory reproduces the experimentally observed magnetic behavior of PuSb quite closely, predicting a phase transition from a low-temperature ferromagnetic phase to a long-period antiferromagnetic phase at about 75 K, for a fitting to a Néel temperature of 85 K, with ordered moments close to the experimental values. However, while the modulation in the long-period antiferromagnetic phase has been experimentally observed to be longitudinal, the theory predicts a transverse modulation with moments aligned along the cube edge. We also present the T=0 magnetic excitation spectrum in the ferromagnetic phase calculated on the basis of this theory using the random-phase approximation.

Potentially improved glasses from space environment

NASA Technical Reports Server (NTRS)

Nichols, R.

1977-01-01

The benefits of processing glasses in a low-gravity space environment are examined. Containerless processing, the absence of gravity driven convection, and lack of sedimentation are seen as potential advantages. Potential applications include the formation of glass-ceramics with a high content of active elements for ferromagnetic devices, the production of ultrapure chalcogenide glasses for laser windows and IR fiber optics, and improved glass products for use in optical systems and laser fusion targets.

Proceedings of the 8th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology

NASA Astrophysics Data System (ADS)

Ishioka, Sachio; Fujikawa, Kazuo

2006-06-01

Preface -- Committees -- Opening address / H. Fukuyama -- Welcoming address / N. Osakabe -- Special lecture. Albert Einstein: opportunity and perception / C. N. Yang -- Quantum information and entanglement. Quantum optics with single atoms and photons / H. J. Kimble. Quantum information system experiments using a single photon source / Y. Yamamoto. Quantum communication and quantum computation with entangled photons / A. Zeilinger. High-fidelity quantum teleportation and a quantum teleportation network for continuous variables / N. Takei, A. Furusawa. Long lived entangled states / H. Häffner ... [et al.]. Quantum non-locality using tripartite entanglement with non-orthogonal states / J. V. Corbett, D. Home. Quantum entanglement and wedge product / H Heydari. Analysis of the generation of photon pairs in periodically poled lithium niobate / J. Söderholm ... [et al.]. Generation of entangled photons in a semiconductor and violation of Bell's inequality / G. Oohata, R. Shimizu, K. Edamatsu -- Quantum computing. Decoherence of a Josephson junction flux qubit / Y. Nakamura ... [et al.]. Spectroscopic analysis of a candidate two-qubit silicon quantum computer in the microwave regime / J. Gorman, D. G. Hasko, D. A. Williams. Berry phase detection in charge-coupled flux-qubits and the effect of decoherence / H. Nakano ... [et al.]. Locally observable conditions for the successful implementation of entangling multi-qubit quantum gates / H. F. Hofmann, R. Okamoto, S. Takeuchi. State control in flux qubit circuits: manipulating optical selection rules of microwave-assisted transitions in three-level artificial atoms / Y.-X. Liu ... [et al.]. The effect of local structure and non-uniformity on decoherence-free states of charge qubits / T. Tanamoto, S. Fujita. Entanglement-assisted estimation of quantum channels / A. Fujiwara. Superconducting quantum bit with ferromagnetic [symbol]-Junction / T. Yamashita, S. Takahashi, S. Maekawa. Generation of macroscopic Greenberger-Horne-Zeilinger states in Josephson systems / T. Fujii, M. Nishida, N. Hatakenaka -- Quantum-dot systems. Tunable tunnel and exchange couplings in double quantum dots / S. Tarucha, T. Hatano, M. Stopa. Coherent transport through quantum dots / S. Katsumoto ... [et al.]. Electrically pumped single-photon sources towards 1.3 [symbol]m / X. Xu ... [et al.]. Aharonov-Bohm-type effects in antidot arrays and their decoherence / M. Kato ... [et al.]. Nonequilibrium Kondo dot connected to ferromagnetic leads / Y. Utsumi ... [et al.]. Full counting-statistics in a single-electron transistor in the presence of strong quantum fluctuations / Y. Utsumi -- Anomalous Hall effect and Spin-Hall effect. Geometry and the anomalous Hall effect in ferromagnets / N. P. Ong, W.-L. Lee. Control of spin chirality, Berry phase, and anomalous Hall effect / Y. Tokura, Y. Taguchi. Quantum geometry and Hall effect in ferromagnets and semiconductors / N. Nagaosa. Spin-Hall effect in a semiconductor two-dimensional hole gas with strong spin-orbit coupling / J. Wunderlich ... [et al.]. Intrinsic spin Hall effect in semiconductors / S. Murakami -- Spin related phenomena. Theory of spin transfer phenomena in magnetic metals and semiconductors / A. S. Núñez, A. H. MacDonald. Spin filters of semiconductor nanostructures / T. Dietl, G. Grabecki, J. Wróbel. Experimental study on current-driven domain wall motion / T. Ono ... [et al.]. Magnetization reversal of ferromagnetic nano-dot by non local spin injection / Y. Otani, T. Kimura. Theory of current-driven domain wall dynamics / G. Tatara ... [et al.]. Magnetic impurity states and ferromagnetic interaction in diluted magnetic semiconductors / M. Ichimura ... [et al.]. Geometrical effect on spin current in magnetic nano-structures / M. Ichimura, S. Takahashi, S. Maekawa. Ferromagnetism in anatase TiO[symbol] codoped with Co and Nb / T. Hitosugi ... [et al.] -- Superconductivity in nano-systems. Nonlinear quantum effects in nanosuperconductors / C. Carballeira ... [et al.]. Coalescence and rearrangement of vortices in mesoscopic superconductors / A. Kanda ... [et al.]. Superconductivity in topologically nontrivial spaces / M. Hayashi ... [et al.]. DC-SQUID ratchet using atomic point contact / Y. Ootuka, H. Miyazaki, A. Kanda. Superconducting wire network under spatially modulated magnetic field / H. Sano ... [et al.]. Simple and stable control of mechanical break junction for the study of superconducting atomic point contact / H. Miyazaki ... [et al.]. Critical currents in quasiperiodic pinning arrays: one-dimensional chains and Penrose lattices / V. R. Misko, S. Savel'ev, F. Nori. Macroscopic quantum tunneling in high-Tc superconductor Josephson junctions / S. Kawabata -- Novel properties of carbon nanotubes. Carbon nanotubes and unique transport properties: importance of symmetry and channel number / T. Ando. Optical processes in single-walled carbon nanotubes threaded by a magnetic flux / J. Kono ... [et al.]. Non-equilibrium transport through a single-walled carbon nanotube with highly transparent coupling to reservoirs / P. Recher, N. Y. Kim, Y. Yamamoto -- Novel properties of nano-systems. Transport properties in low dimensional artificial lattice of gold nano-particles / S. Saito ... [et al.]. First principles study of dihydride-chain structures on H-terminated Si(100) surface / Y. Suwa ... [et al.]. Electrical property of Ag nanowires fabricated on hydrogen-terminated Si(100) surface / M. Fujimori, S. Heike, T. Hashizume. Effect of environment on ionization of excited atoms embedded in a solid-state cavity / M. Ando ... [et al.]. Development of universal virtual spectroscope for optoelectronics research: first principles software replacing dielectric constant measurements / T. Hamada ... [et al.]. Quantum Nernst effect / H Nakamura, N. Hatano, R. Shirasaki -- Precise measurements. Quantum phenomena visualized using electron waves / A. Tonomura. An optical lattice clock: ultrastable atomic clock with engineered perturbation / H. Katori ... [et al.]. Development of Mach-Zehnder interferometer and "coherent beam steering" technique for cold neutron / K. Taketani ... [et al.]. Surface potential measurement by atomic force microscopy using a quartz resonator / S. Heike, T. Hashizume -- Fundamental Problems in quantum physics. Berry's phases and topological properties in the Born-Oppenheimer approximation / K. Fujikawa. Self-trapping of Bose-Einstein condensates by oscillating interactions / H. Saito, M. Ueda. Spinor solitons in Bose-Einstein condensates - atomic spin transport / J. Ieda. Spin decoherence in a gravitational field / H. Terashima, M. Ueda. Berry's phase of atoms with different sign of the g-factor in a conical rotating magnetic field observed by a time-domain atom interferometer / A. Morinaga ... [et al.] -- List of participants.

Potentials and challenges of integration for complex metal oxides in CMOS devices and beyond

NASA Astrophysics Data System (ADS)

Kim, Y.; Pham, C.; Chang, J. P.

2015-02-01

This review focuses on recent accomplishments on complex metal oxide based multifunctional materials and the potential they hold in advancing integrated circuits. It begins with metal oxide based high-κ materials to highlight the success of their integration since 45 nm complementary metal-oxide-semiconductor (CMOS) devices. By simultaneously offering a higher dielectric constant for improved capacitance as well as providing a thicker physical layer to prevent the quantum mechanical tunnelling of electrons, high-κ materials have enabled the continued down-scaling of CMOS based devices. The most recent technology driver has been the demand to lower device power consumption, which requires the design and synthesis of novel materials, such as complex metal oxides that exhibit remarkable tunability in their ferromagnetic, ferroelectric and multiferroic properties. These properties make them suitable for a wide variety of applications such as magnetoelectric random access memory, radio frequency band pass filters, antennae and magnetic sensors. Single-phase multiferroics, while rare, offer unique functionalities which have motivated much scientific and technological research to ascertain the origins of their multiferroicity and their applicability to potential devices. However, due to the weak magnetoelectric coupling for single-phase multiferroics, engineered multiferroic composites based on magnetostrictive ferromagnets interfacing piezoelectrics or ferroelectrics have shown enhanced multiferroic behaviour from effective strain coupling at the interface. In addition, nanostructuring of the ferroic phases has demonstrated further improvement in the coupling effect. Therefore, single-phase and engineered composite multiferroics consisting of complex metal oxides are reviewed in terms of magnetoelectric coupling effects and voltage controlled ferromagnetic properties, followed by a review on the integration challenges that need to be overcome to realize the materials’ full potential.

Magnetization switching in nanoscale ferromagnetic grains: MFM observables from Monte Carlo simulations

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

Richards, H.L.; Sides, S.W.; Novotny, M.A.

1996-12-31

Recently experimental techniques, such as magnetic force microscopy (MFM), have enabled the magnetic state of individual sub-micron particles to be resolved. Motivated by these experimental developments, the authors use Monte Carlo simulations of two-dimensional kinetic Ising ferromagnets to study the magnetic relaxation in a negative applied field of a grain with an initial magnetization m{sub 0} = + 1. They use classical droplet theory to predict the functional forms for some quantities which can be observed by MFM. An example is the probability that the magnetization is positive, which is a function of time, field, grain size, and grain dimensionality.more » The qualitative agreement between experiments and their simulations of switching in individual single-domain ferromagnets indicates that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.« less

Spin-wave dynamics and exchange interactions in multiferroic NdFe3(BO3)4 explored by inelastic neutron scattering

NASA Astrophysics Data System (ADS)

Golosovsky, I. V.; Ovsyanikov, A. K.; Aristov, D. N.; Matveeva, P. G.; Mukhin, A. A.; Boehm, M.; Regnault, L.-P.; Bezmaternykh, L. N.

2018-04-01

Magnetic excitations and exchange interactions in multiferroic NdFe3(BO3)4 were studied by inelastic neutron scattering in the phase with commensurate antiferromagnetic structure. The observed spectra were analyzed in the frame of the linear spin-wave theory. It was shown that only the model, which includes the exchange interactions within eight coordination spheres, describes satisfactorily all observed dispersion curves. The calculation showed that the spin-wave dynamics is governed by the strongest antiferromagnetic intra-chain interaction and three almost the same inter-chain interactions. Other interactions, including ferromagnetic exchange, appeared to be insignificant. The overall energy balance of the antiferromagnetic inter-chain exchange interactions, which couple the moments from the adjacent ferromagnetic layers as well as within a layer, stabilizes ferromagnetic arrangement in the latter. It demonstrates that the pathway geometry plays a crucial role in forming of the magnetic structure.

First-principles study on half-metallic ferromagnetic properties of Zn1- x V x Se ternary alloys

NASA Astrophysics Data System (ADS)

Khatta, Swati; Tripathi, S. K.; Prakash, Satya

2017-09-01

The spin-polarised density functional theory along with self-consistent plane-wave pseudopotential is used to investigate the half-metallic ferromagnetic properties of ternary alloys Zn1- x V x Se. The generalized gradient approximation is used for exchange-correlation potential. The equilibrium lattice constants, bulk modulus, and its derivatives are calculated. The calculated spin-polarised energy-band structures reveal that these alloys are half-metallic for x = 0.375 and 0.50 and nearly half-metallic for other values of x. The estimated direct and indirect bandgaps may be useful for the magneto-optical absorption experiments. It is found that there is strong Zn 4s, Se 4p, and V 3d orbital hybridization in the conduction bands of both the spins, while Se 4p and V 3d orbital hybridization predominates in the valence bands of both the spins. The s, p-d, and p-d orbital hybridization reduces the local magnetic moment of V atoms and small local magnetic moments are produced on Zn and Se atoms which get coupled with V atoms in ferromagnetic and antiferromagnetic phases, respectively. The conduction and valence-band-edge splittings and exchange constants predict the ferromagnetism in these alloys. The conduction band-impurity (s and p-d) exchange interaction is more significant for ferromagnetism in these alloys than the valence band-impurity (p-d) exchange interaction.

Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO3 Thin Films.

PubMed

Varga, Tamas; Droubay, Timothy C; Kovarik, Libor; Nandasiri, Manjula I; Shutthanandan, Vaithiyalingam; Hu, Dehong; Kim, Bumsoo; Jeon, Seokwoo; Hong, Seungbum; Li, Yulan; Chambers, Scott A

2017-07-05

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO 3 -type compounds, MTiO 3 (M = Fe, Mn, Ni). Although the coexistence of ferroelectric polarization and ferromagnetism has been demonstrated in this rare multiferroic family before, first in bulk FeTiO 3 , then in thin-film NiTiO 3 , the coupling of the two order parameters has not been confirmed. Here, we report the stabilization of polar, ferromagnetic NiTiO 3 by oxide epitaxy on a LiNbO 3 substrate utilizing tensile strain and demonstrate the theoretically predicted coupling between its polarization and ferromagnetism by X-ray magnetic circular dichroism under applied fields. The experimentally observed direction of ferroic ordering in the film is supported by simulations using the phase-field approach. Our work validates symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and WFM in MTiO 3 transition metal titanates crystallizing in the LiNbO 3 structure. It also demonstrates the applicability of epitaxial strain as a viable alternative to high-pressure crystal growth to stabilize metastable materials and a valuable tuning parameter to simultaneously control two ferroic order parameters to create a multiferroic. Multiferroic NiTiO 3 has potential applications in spintronics where ferroic switching is used, such as new four-stage memories and electromagnetic switches.

Room temperature luminescence and ferromagnetism of AlN:Fe

NASA Astrophysics Data System (ADS)

Li, H.; Cai, G. M.; Wang, W. J.

2016-06-01

AlN:Fe polycrystalline powders were synthesized by a modified solid state reaction (MSSR) method. Powder X-ray diffraction and transmission electron microscopy results reveal the single phase nature of the doped samples. In the doped AlN samples, Fe is in Fe2+ state. Room temperature ferromagnetic behavior is observed in AlN:Fe samples. Two photoluminescence peaks located at about 592 nm (2.09 eV) and 598 nm (2.07 eV) are observed in AlN:Fe samples. Our results suggest that AlN:Fe is a potential material for applications in spintronics and high power laser devices.