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Sample records for field driven ferromagnetic

  1. Finite-size effects on the magnetoelectric response of field-driven ferroelectric/ferromagnetic chains

    NASA Astrophysics Data System (ADS)

    Jia, Chenglong; Sukhov, Alexander; Horley, Paul P.; Berakdar, Jamal

    2011-07-01

    We study theoretically the coupled multiferroic dynamics of a finite one-dimensional ferroelectric/ferromagnet chain driven by harmonic magnetic and electric fields as a function of the chain length. We consider the case of a linear magnetoelectric coupling that results from the spin-polarized screening charge at the interface. We performed Monte-Carlo simulations and calculations based on the coupled Landau-Lifshitz-Gilbert and Landau-Khalatnikov equations showing that the net magnetization and the total polarization of thin heterostructures, i.e. with up to ten ferroelectric and ferromagnetic sites counted from the interface, can be completely reversed by external electric and magnetic fields, respectively. However, for larger systems merely a limited magnetoelectrical control is achievable.

  2. Field-driven Domain Wall Motion in Ferromagnetic Nanowires with Bulk Dzyaloshinskii-Moriya Interaction

    NASA Astrophysics Data System (ADS)

    Zhuo, Fengjun; Sun, Z. Z.

    2016-04-01

    Field-driven domain wall (DW) motion in ferromagnetic nanowires with easy- and hard-axis anisotropies was studied theoretically and numerically in the presence of the bulk Dzyaloshinskii-Moriya interaction (DMI) based on the Landau-Lifshitz-Gilbert equation. We propose a new trial function and offer an exact solution for DW motion along a uniaxial nanowire driven by an external magnetic field. A new strategy was suggested to speed up DW motion in a uniaxial magnetic nanowire with large DMI parameters. In the presence of hard-axis anisotropy, we find that the breakdown field and velocity of DW motion was strongly affected by the strength and sign of the DMI parameter under external fields. This work may be useful for future magnetic information storage devices based on DW motion.

  3. Field-driven Domain Wall Motion in Ferromagnetic Nanowires with Bulk Dzyaloshinskii-Moriya Interaction

    PubMed Central

    Zhuo, Fengjun; Sun, Z. Z.

    2016-01-01

    Field-driven domain wall (DW) motion in ferromagnetic nanowires with easy- and hard-axis anisotropies was studied theoretically and numerically in the presence of the bulk Dzyaloshinskii-Moriya interaction (DMI) based on the Landau-Lifshitz-Gilbert equation. We propose a new trial function and offer an exact solution for DW motion along a uniaxial nanowire driven by an external magnetic field. A new strategy was suggested to speed up DW motion in a uniaxial magnetic nanowire with large DMI parameters. In the presence of hard-axis anisotropy, we find that the breakdown field and velocity of DW motion was strongly affected by the strength and sign of the DMI parameter under external fields. This work may be useful for future magnetic information storage devices based on DW motion. PMID:27118064

  4. Field-driven Domain Wall Motion in Ferromagnetic Nanowires with Bulk Dzyaloshinskii-Moriya Interaction.

    PubMed

    Zhuo, Fengjun; Sun, Z Z

    2016-04-27

    Field-driven domain wall (DW) motion in ferromagnetic nanowires with easy- and hard-axis anisotropies was studied theoretically and numerically in the presence of the bulk Dzyaloshinskii-Moriya interaction (DMI) based on the Landau-Lifshitz-Gilbert equation. We propose a new trial function and offer an exact solution for DW motion along a uniaxial nanowire driven by an external magnetic field. A new strategy was suggested to speed up DW motion in a uniaxial magnetic nanowire with large DMI parameters. In the presence of hard-axis anisotropy, we find that the breakdown field and velocity of DW motion was strongly affected by the strength and sign of the DMI parameter under external fields. This work may be useful for future magnetic information storage devices based on DW motion.

  5. Field driven ferromagnetic phase evolution originating from the domain boundaries in antiferromagnetically coupled perpendicular anitsotropy films

    SciTech Connect

    Jones, Juanita; Hauet, Thomas; Gunther, Christian; Hovorka, Ondrej; Berger, Andreas; Im, Mi-Young; Fischer, Peter; Hellwig, Olav

    2008-05-01

    Strong perpendicular anisotropy systems consisting of Co/Pt multilayer stacks that are antiferromagnetically coupled via thin Ru or NiO layers have been used as model systems to study the competition between local interlayer exchange and long-range dipolar interactions [1,2]. Magnetic Force Microscopy (MFM) studies of such systems reveal complex magnetic configurations with a mix of antiferromagnetic (AF) and ferromagnetic (FM) phases. However, MFM allows detecting surface stray fields only and can interact strongly with the magnetic structure of the sample, thus altering the original domain configuration of interest [3,4]. In the current study they combine magnetometry and state-of-the-art soft X-ray transmission microscopy (MXTM) to investigate the external field driven FM phase evolution originating from the domain boundaries in such antiferromagnetically coupled perpendicular anisotropy films. MXTM allows directly imaging the perpendicular component of the magnetization in an external field at sub 100 nm spatial resolution without disturbing the magnetic state of the sample [5,6]. Here they compare the domain evolution for two similar [Co(4{angstrom})/Pt(7{angstrom})]x-1/{l_brace}Co(4{angstrom})/Ru(9{angstrom})/[Co(4{angstrom})/Pt(7{angstrom})]x-1{r_brace}16 samples with slightly different Co/Pt stack thickness, i.e. slightly different strength of internal dipolar fields. After demagnetization they obtain AF domains with either sharp AF domain walls for the thinner multilayer stacks or 'tiger-tail' domain walls (one dimensional FM phase) for the thicker stacks. When increasing the external field strength the sharp domain walls in the tinner stack sample transform into the one-dimensional FM phase, which then serves as nucleation site for further FM stripe domains that spread out into all directions to drive the system towards saturation. Energy calculations reveal the subtle difference between the two samples and help to understand the observed transition, when

  6. Electric field driven magnetic domain wall motion in ferromagnetic-ferroelectric heterostructures

    SciTech Connect

    Van de Wiele, Ben; Laurson, Lasse; Franke, Kévin J. A.; Dijken, Sebastiaan van

    2014-01-06

    We investigate magnetic domain wall (MDW) dynamics induced by applied electric fields in ferromagnetic-ferroelectric thin-film heterostructures. In contrast to conventional driving mechanisms where MDW motion is induced directly by magnetic fields or electric currents, MDW motion arises here as a result of strong pinning of MDWs onto ferroelectric domain walls (FDWs) via local strain coupling. By performing extensive micromagnetic simulations, we find several dynamical regimes, including instabilities such as spin wave emission and complex transformations of the MDW structure. In all cases, the time-averaged MDW velocity equals that of the FDW, indicating the absence of Walker breakdown.

  7. Rotational properties of ferromagnetic nanoparticles driven by a precessing magnetic field in a viscous fluid.

    PubMed

    Lyutyy, T V; Denisov, S I; Reva, V V; Bystrik, Yu S

    2015-10-01

    We study the deterministic and stochastic rotational dynamics of ferromagnetic nanoparticles in a precessing magnetic field. Our approach is based on the system of effective Langevin equations and on the corresponding Fokker-Planck equation. Two key characteristics of the rotational dynamics, namely the average angular frequency of precession of nanoparticles and their average magnetization, are of interest. Using the Langevin and Fokker-Planck equations, we calculate both analytically and numerically these characteristics in the deterministic and stochastic cases, determine their dependence on the model parameters, and analyze in detail the role of thermal fluctuations.

  8. Multiple dynamic transitions in an anisotropic Heisenberg ferromagnet driven by polarized magnetic field.

    PubMed

    Acharyya, Muktish

    2004-02-01

    A uniaxially (along the Z axis) anisotropic Heisenberg ferromagnet, in the presence of time-dependent (but uniform over space) magnetic field, is studied by Monte Carlo simulation. The time-dependent magnetic field was taken as elliptically polarized where the resultant field vector rotates in the X-Z plane. The system is cooled (in the presence of the elliptically polarized magnetic field) from high temperature. As the temperature decreases, it was found that in the low anisotropy limit the system undergoes three successive dynamical phase transitions. These three dynamic transitions were confirmed by studying the temperature variation of dynamic "specific heat." The temperature variation of dynamic specific heat shows three peaks indicating three dynamic transition points.

  9. Chirality-driven intrinsic spin-glass ordering and field-induced ferromagnetism in Ni3Al nanoparticle aggregates

    NASA Astrophysics Data System (ADS)

    Kaul, S. N.; Messala, Umasankar

    2016-03-01

    Weak itinerant-electron ferromagnet Ni3Al is driven to magnetic instability (quantum critical point, QCP, where the long-range ferromagnetic order of the bulk ceases to exist) by reducing the average crystallite size to d=50 nm. 'Zero-field' (H=0) linear and nonlinear ac-susceptibilities, measured on Ni3Al nanoparticle aggregates, with d=50 nm (S1) and d=5 nm (S2), provide strong evidence for two spin glass (SG)-like thermodynamic phase transitions: one at Ti(H = 0) ≃ 30 K (Ti† (H = 0) ≃ 230 K) and the other at a lower temperature Tp(H = 0) ≃ 8 K (Th(H = 0) ≃ 52 K) in S1 (S2). 'In-field' (H ≠ 0) linear ac-susceptibility and dc magnetization demonstrate that the thermodynamic nature of these transitions is preserved in finite fields. The presently determined H-T phase diagrams for the samples S1 and S2 are compared with those predicted by the Kotliar-Sompolinsky and Gabay-Toulouse mean-field models and Monte Carlo simulations, based on the chirality-driven spin glass (SG) ordering scenario, for a three-dimensional nearest-neighbor Heisenberg SG system with or without weak random anisotropy. Such a detailed comparison permits us to unambiguously identify various 'zero-field' and 'in-field' SG phase transitions as: (i) the simultaneous paramagnetic (PM)-chiral glass (CG) and PM-SG phase transitions at Ti(H), (ii) the PM-CG transition at Ti† (H), (iii) the replica symmetry-breaking SG transition at Tp(H), and (iv) the continuous spin-rotation symmetry-breaking SG transition at Th(H). In the presence of random anisotropy, magnetization fails to saturate even at 90 kOe in S1 whereas negligibly small anisotropy allows even fields as weak as 1 kOe to saturate magnetization and induce ferromagnetism in S2. Due to the proximity to CG/SG-QCP, magnetization and susceptibility both exhibit non-Fermi liquid behavior over a wide range at low temperatures.

  10. Blume-Capel ferromagnet driven by propagating and standing magnetic field wave: Dynamical modes and nonequilibrium phase transition

    NASA Astrophysics Data System (ADS)

    Acharyya, Muktish; Halder, Ajay

    2017-03-01

    The dynamical responses of Blume-Capel (S=1) ferromagnet to the plane propagating (with fixed frequency and wavelength) and standing magnetic field waves are studied separately in two dimensions by extensive Monte Carlo simulation. Depending on the values of temperature, amplitude of the propagating magnetic field and the strength of anisotropy, two different dynamical phases are observed. For a fixed value of anisotropy and the amplitude of the propagating magnetic field, the system undergoes a dynamical phase transition from a driven spin wave propagating phase to a pinned or spin frozen state as the system is cooled down. The time averaged magnetisation over a full cycle of the propagating magnetic field plays the role of the dynamic order parameter. A comprehensive phase diagram is plotted in the plane formed by the amplitude of the propagating wave and the temperature of the system. It is found that the phase boundary shrinks inward as the anisotropy increases. The phase boundary, in the plane described by the strength of the anisotropy and temperature, is also drawn. This phase boundary was observed to shrink inward as the field amplitude increases.

  11. Magnetic-field-driven electron transport in ferromagnetic/ insulator/semiconductor hybrid structures

    NASA Astrophysics Data System (ADS)

    Volkov, N. V.; Tarasov, A. S.; Rautskii, M. V.; Lukyanenko, A. V.; Varnakov, S. N.; Ovchinnikov, S. G.

    2017-10-01

    Extremely large magnetotransport phenomena were found in the simple devices fabricated on base of the Me/SiO2/p-Si hybrid structures (where Me are Mn and Fe). These effects include gigantic magnetoimpedance (MI), dc magnetoresistance (MR) and the lateral magneto-photo-voltaic effect (LMPE). The MI and MR values exceed 106% in magnetic field about 0.2 T for Mn/SiO2/p-Si Schottky diode. LMPE observed in Fe/SiO2/p-Si lateral device reaches the value of 104% in a field of 1 T. We believe that in case with the Schottky diode MR and MI effects are originate from magnetic field influence on impact ionization process by two different ways. First, the trajectory of the electron is deflected by a magnetic field, which suppresses acquisition of kinetic energy and therefore impact ionization. Second, the magnetic field gives rise to shift of the acceptor energy levels in silicon to a higher energy. As a result, the activation energy for impact ionization significantly increases and consequently threshold voltage rises. Moreover, the second mechanism (acceptor level energy shifting in magnetic field) can be responsible for giant LMPE.

  12. Elastically Driven Ferromagnetic Resonance in Nickel Thin Films

    NASA Astrophysics Data System (ADS)

    Weiler, M.; Dreher, L.; Heeg, C.; Huebl, H.; Gross, R.; Brandt, M. S.; Goennenwein, S. T. B.

    2011-03-01

    Surface acoustic waves (SAWs) in the GHz frequency range are exploited for the all-elastic excitation and detection of ferromagnetic resonance (FMR) in a ferromagnetic-ferroelectric (Ni/LiNbO3) hybrid device. We measure the SAW magnetotransmission at room temperature as a function of frequency, external magnetic field magnitude, and orientation. Our data are well described by a modified Landau-Lifshitz-Gilbert approach, in which a virtual, strain-induced tickle field drives the magnetization precession. This causes a distinct magnetic field orientation dependence of elastically driven FMR that we observe in both model and experiment.

  13. Field driven ferromagnetic phase nucleation and propagation from the domain boundaries in antiferromagnetically coupled perpendicular anisotropy films

    SciTech Connect

    Hauet, Thomas; Gunther, Christian M.; Hovorka, Ondrej; Berger, Andreas; Im, Mi-Young; Fischer, Peter; Hellwig, Olav

    2008-12-09

    We investigate the reversal process in antiferromagnetically coupled [Co/Pt]{sub X-1}/{l_brace}Co/Ru/[Co/Pt]{sub X-1}{r_brace}{sub 16} multilayer films by combining magnetometry and Magnetic soft X-ray Transmission Microscopy (MXTM). After out-of-plane demagnetization, a stable one dimensional ferromagnetic (FM) stripe domain phase (tiger-tail phase) for a thick stack sample (X=7 is obtained), while metastable sharp antiferromagnetic (AF) domain walls are observed in the remanent state for a thinner stack sample (X=6). When applying an external magnetic field the sharp domain walls of the thinner stack sample transform at a certain threshold field into the FM stripe domain wall phase. We present magnetic energy calculations that reveal the underlying energetics driving the overall reversal mechanisms.

  14. Optical-helicity-driven magnetization dynamics in metallic ferromagnets

    NASA Astrophysics Data System (ADS)

    Choi, Gyung-Min; Schleife, André; Cahill, David G.

    2017-04-01

    Recent observations of switching of magnetic domains in ferromagnetic metals by circularly polarized light, so-called all-optical helicity dependent switching, has renewed interest in the physics that governs the interactions between the angular momentum of photons and the magnetic order parameter of materials. Here we use time-resolved-vectorial measurements of magnetization dynamics of thin layers of Fe, Ni and Co driven by picosecond duration pulses of circularly polarized light. We decompose the torques that drive the magnetization into field-like and spin-transfer components that we attribute to the inverse Faraday effect and optical spin-transfer torque, respectively. The inverse Faraday effect is approximately the same in Fe, Ni and Co, but the optical spin-transfer torque is strongly enhanced by adding a Pt capping layer. Our work provides quantitative data for testing theories of light-material interactions in metallic ferromagnets and multilayers.

  15. Elastically driven ferromagnetic resonance in nickel thin films

    NASA Astrophysics Data System (ADS)

    Weiler, M.; Heeg, C.; Huebl, H.; Gross, R.; Goennenwein, S. T. B.; Dreher, L.; Brandt, M. S.

    2011-03-01

    Due to magneto-elastic coupling, magnetic degrees of freedom are influenced by elastic deformation. We here demonstrate that the magneto-elastic interaction of a radio frequency (RF) surface acoustic wave (SAW) with a ferromagnetic thin film enables an all-elastic excitation and detection of ferromagnetic resonance (FMR). We have measured the SAW magneto-transmission at room temperature in Ni/ LiNb O3 hybrid devices as a function of SAW frequency, external magnetic field magnitude and orientation. Our data are consistently described by a modified Landau-Lifshitz-Gilbert approach, in which the magnetization precession is not driven by a conventional, external RF magnetic field, but rather by a purely virtual, internal tickle field stemming from RF magneto-elastic interactions. This causes a distinct magnetic field orientation dependence of elastically driven FMR, which we observe in both simulations and experiment. This work is financially supported by the Deutsche Forschungsgemeinschaft via project GO 944/3-1, SFB 631, and the excellence cluster Nanosystems Initiative Munich (NIM).

  16. Calculation of ultrasonic fields radiated in a ferromagnetic medium by an EMAT of arbitrary bias field driven by a current of arbitrary intensity

    SciTech Connect

    Rouge, C.; Lhémery, A.; Aristégui, C.; Walaszek, H.

    2014-02-18

    ElectroMagnetic Acoustic Transducers (EMATs) are contactless transducers generating ultrasonic waves in conductive media, notably shear horizontal and torsional waves (in plates and pipes, respectively), possibly in hostile environments. In a ferromagnetic part, the elastic strain and the magnetic field couple through magnetostriction phenomena, so that a magnetostriction and magnetization forces add up to the Lorentz force created in any conductive medium. Here, a model is proposed to predict these forces for an arbitrary bias field due to the EMAT permanent magnet and whatever the current intensity in its electric circuit, whereas the usual assumption of high bias field and low intensity current leads to important model simplifications. To handle the nonlinear behavior of all the three forces when the usual assumption cannot be made, forces are expressed in the time domain. In particular, magnetostriction force generates waves at several harmonic frequencies of the driving current frequency. Forces are then transformed into equivalent surface stresses readily usable as source terms in existing models of ultrasonic radiation, under the assumption that ultrasonic wavelengths are much longer than force penetration depths, (which is generally true in NDT applications of EMATs). Force spectra computed in various EMAT configurations are compared for illustration.

  17. Spin-torque driven ferromagnetic resonance in a nonlinear regime

    NASA Astrophysics Data System (ADS)

    Chen, W.; de Loubens, G.; Beaujour, J.-M. L.; Sun, J. Z.; Kent, A. D.

    2009-10-01

    Spin-valve based nanojunctions incorporating Co form="infix">∣Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co form="infix">∣Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors.

  18. Current-driven dynamics of chiral ferromagnetic domain walls.

    PubMed

    Emori, Satoru; Bauer, Uwe; Ahn, Sung-Min; Martinez, Eduardo; Beach, Geoffrey S D

    2013-07-01

    In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically rich spin textures such as spin spirals and skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a Néel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.

  19. Heat-driven spin transport in a ferromagnetic metal

    SciTech Connect

    Xu, Yadong; Yang, Bowen; Tang, Chi; Jiang, Zilong; Shi, Jing; Schneider, Michael; Whig, Renu

    2014-12-15

    As a non-magnetic heavy metal is attached to a ferromagnet, a vertically flowing heat-driven spin current is converted to a transverse electric voltage, which is known as the longitudinal spin Seebeck effect (SSE). If the ferromagnet is a metal, this voltage is also accompanied by voltages from two other sources, i.e., the anomalous Nernst effect in both the ferromagnet and the proximity-induced ferromagnetic boundary layer. By properly identifying and carefully separating those different effects, we find that in this pure spin current circuit the additional spin current drawn by the heavy metal generates another significant voltage by the ferromagnetic metal itself which should be present in all relevant experiments.

  20. Intercalation-driven reversible control of magnetism in bulk ferromagnets.

    PubMed

    Dasgupta, Subho; Das, Bijoy; Knapp, Michael; Brand, Richard A; Ehrenberg, Helmut; Kruk, Robert; Hahn, Horst

    2014-07-16

    An extension in magnetoelectric effects is proposed to include reversible chemistry-controlled magnetization variations. This ion-intercalation-driven magnetic control can be fully reversible and pertinent to bulk material volumes. The concept is demonstrated for ferromagnetic iron oxide where the intercalated lithium ions cause valence change and partial redistribution of Fe(3+) cations yielding a large and fully reversible change in magnetization at room temperature.

  1. Dynamical mean-field theory for flat-band ferromagnetism

    NASA Astrophysics Data System (ADS)

    Nguyen, Hong-Son; Tran, Minh-Tien

    2016-09-01

    The magnetically ordered phase in the Hubbard model on the infinite-dimensional hyper-perovskite lattice is investigated within dynamical mean-field theory. It turns out for the infinite-dimensional hyper-perovskite lattice the self-consistent equations of dynamical mean-field theory are exactly solved, and this makes the Hubbard model exactly solvable. We find electron spins are aligned in the ferromagnetic or ferrimagnetic configuration at zero temperature and half filling of the edge-centered sites of the hyper-perovskite lattice. A ferromagnetic-ferrimagnetic phase transition driven by the energy level splitting is found and it occurs through a phase separation. The origin of ferromagnetism and ferrimagnetism arises from the band flatness and the virtual hybridization between macroscopically degenerate flat bands and dispersive ones. Based on the exact solution in the infinite-dimensional limit, a modified exact diagonalization as the impurity solver for dynamical mean-field theory on finite-dimensional perovskite lattices is also proposed and examined.

  2. Current-driven periodic domain wall creation in ferromagnetic nanowires

    NASA Astrophysics Data System (ADS)

    Sitte, Matthias; Everschor-Sitte, Karin; Valet, Thierry; Rodrigues, Davi R.; Sinova, Jairo; Abanov, Ar.

    2016-08-01

    We predict the electrical generation and injection of domain walls into a ferromagnetic nanowire without the need of an assisting magnetic field. Our analytical and numerical results show that above a critical current jc domain walls are injected into the nanowire with a period T ˜(j-jc) -1 /2 . Importantly, domain walls can be produced periodically even in a simple exchange ferromagnet with uniaxial anisotropy, without requiring any standard "twisting" interaction such as Dzyaloshinskii-Moriya or dipole-dipole interactions. We show analytically that this process and the period exponents are universal and do not depend on the peculiarities of the microscopic Hamiltonian. Finally we give a specific proposal for an experimental realization.

  3. Converse magnetoelectric effect via strain-driven magnetization reorientations in ultrathin ferromagnetic films on ferroelectric substrates

    NASA Astrophysics Data System (ADS)

    Pertsev, N. A.

    2015-07-01

    A phenomenological theory is developed for the strain-driven magnetization reorientations occurring in ultrathin ferromagnetic films coupled to ferroelectric substrates experiencing electric-field-induced piezoelectric deformations. The theory takes into account the surface/interface magnetic anisotropy playing an important role in the energetics of such films and first describes the thickness-driven spin reorientation transitions emerging in the presence of substrate-induced lattice strains. Then the threshold and critical intensities of the electric field created in a ferroelectric substrate are calculated, at which different magnetic states acquire the same energy or become unstable in a strained ferromagnetic overlayer. To demonstrate stability ranges of various possible magnetization orientations, we introduce magnetoelectric orientational diagrams, where the electric-field intensity and film thickness are employed as two variables. Such diagrams are constructed for ultrathin Ni, Fe, and F e60C o40 films coupled to single crystals of classical and relaxor ferroelectrics. The inspection of these diagrams shows that the use of multiferroic hybrids comprising ultrathin ferromagnetic films significantly enlarges the range of ferroic materials suitable for experimental observations of the strain-mediated converse magnetoelectric effect.

  4. Spin-torque-driven ferromagnetic resonance in point contacts

    NASA Astrophysics Data System (ADS)

    Staudacher, T.; Tsoi, M.

    2011-04-01

    We demonstrate the technique of spin-torque-driven ferromagnetic resonance (ST-FMR) in point contacts, which enables FMR studies in sample volumes as small as a few cubic nanometers. In our experiments, we use point contacts ˜10 nm in size to inject both dc and microwave currents into F/N/F/AFM exchange-biased spin valves where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer and one of the Fs is pinned by an adjacent antiferromagnetic (AFM) layer. High current densities produce the spin-transfer torque on magnetic moments in a small contact region and drive it to resonance at appropriate frequency of the applied microwaves. The resulting magnetodynamics are detected electrically via a small rectified dc voltage, which appears across the contact at resonance. The width of the resonance varies linearly with the applied dc bias as expected for spin transfer in spin valves. Potentially, the point-contact technique extends the applicability of ST-FMR to higher/lower frequencies, smaller sample volumes, and a broader range of materials.

  5. Spin orbit driven ferromagnetic resonance and torques in single ferromagnetic layers

    NASA Astrophysics Data System (ADS)

    Maciá, Ferran; Pépin, Charles; Kent, A. D.

    2012-02-01

    The coupling of spin and charge may convert electrical currents into spin currents in non-magnetic metals. In non-magnetic metals with strong spin orbit (SO) interaction in combination with magnetic metals one can also us the effect to excite magnetization dynamics; electrical currents in the non-magnetic metal transform to spin currents and the spin currents diffuse to the magnetic metal interacting with the magnetic moments. The combination of non-magnetic metals and magnetic metals has been recently used to determine spin hall angles. Here we demonstrate that spin currents in a ferromagnetic layer associated with SO interactions can excite ferromagnetic precession in the same layer. We have studied Co|Ni multilayers with both in-plane anisotropy and weak out-of-plane anisotropy. Results show that the samples have strong SO interactions. We have injected microwaves into patterned samples with several geometries and measured the mixed voltage in the same leads. Oscillatory currents drive FMR in the thin-film layer. We show that SO torques are primarily responsible for the magnetic excitations in samples with strong SO interactions, whereas samples with a weaker SO barely respond to the injected microwaves and show asymmetric components from charge current induced Oersted fields.

  6. Interface driven states in ferromagnetic topological insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Lauter, Valeria; Katmis, Ferhat; Moodera, Jagadeesh

    The broken time reversal symmetry (TRS) states can be introduced into a topological insulator (TI) material by ferromagnetic ordering at the interface. Recently we demonstrated a fundamental step towards realization of high temperature magnetization in Bi2Se3-EuS TI-FMI heterostructures through observation of magnetic proximity-induced symmetry breaking on the Bi2Se3 surface via the exchange interaction by depositing EuS film on the top of the Bi2Se3 surface.Here we show that we can independently break the TRS on both surfaces of a TI, which brings the long-range induced magnetism on either or both surfaces of a TI in a controlled way. We provide a depth-sensitive data on details of magnetic proximity effect in hidden interfaces by Polarized Neutron Reflectometry. The proximity coupling strength and penetration depth of magnetism into TI are extracted as functions of temperature, magnetic field and magnetic history. The large neutron absorption of Eu atoms serves as the element sensitivity and enables us to identify such magnetism in TI as proximity magnetism. This provides a next step to realization of complex heterostructures of TI and FMI leading to wide applications in TI-based next generation spintronic devices. Supported by U.S. DOE, Office of Science, BES, MIT MRSEC award DMR-0819762, NSF Grant DMR-1207469, ONR Grant N00014-13-1-0301, NSF Grant DMR-1231319.

  7. Nonlinear dynamics of three-magnon process driven by ferromagnetic resonance in yttrium iron garnet

    SciTech Connect

    Cunha, R. O.; Holanda, J.; Azevedo, A.; Rezende, S. M.; Vilela-Leão, L. H.; Rodríguez-Suárez, R. L.

    2015-05-11

    We report an investigation of the dynamics of the three-magnon splitting process associated with the ferromagnetic resonance (FMR) in films of the insulating ferrimagnet yttrium iron garnet (YIG). The experiments are performed with a 6 μm thick YIG film close to a microstrip line fed by a microwave generator operating in the 2–6 GHz range. The magnetization precession is driven by the microwave rf magnetic field perpendicular to the static magnetic field, and its dynamics is observed by monitoring the amplitude of the FMR absorption peak. The time evolution of the amplitude reveals that if the frequency is lowered below a critical value of 3.3 GHz, the FMR mode pumps two magnons with opposite wave vectors that react back on the FMR, resulting in a nonlinear dynamics of the magnetization. The results are explained by a model with coupled nonlinear equations describing the time evolution of the magnon modes.

  8. Uniform magnetization dynamics of a submicron ferromagnetic disk driven by the spin-orbit coupled spin torque

    NASA Astrophysics Data System (ADS)

    Bondarenko, P. V.; Sherman, E. Ya

    2017-07-01

    A simple model of magnetization dynamics in a ferromagnet/doped semiconductor hybrid structure with Rashba spin-orbit interaction (SOI), driven by an applied pulse of the electric field, is proposed. The electric current excited by the applied field is spin-polarized, due to the SOI and therefore it induces the magnetization rotation in the ferromagnetic layer via s-d exchange coupling. Magnetization dynamics dependence on the electric pulse shape and magnitude is analyzed for realistic values of the parameters. We show that it is similar to the dynamics of a damped nonlinear oscillator with the time-dependent frequency proportional to the square root of the applied electric field. The magnetization switching properties of an elliptic magnetic element are examined as a function of the applied field magnitude and direction.

  9. 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.

  10. Asymmetric driven dynamics of Dzyaloshinskii domain walls in ultrathin ferromagnetic strips with perpendicular magnetic anisotropy

    NASA Astrophysics Data System (ADS)

    Sánchez-Tejerina, L.; Alejos, Ó.; Martínez, E.; Muñoz, J. M.

    2016-07-01

    The dynamics of domain walls in ultrathin ferromagnetic strips with perpendicular magnetic anisotropy is studied from both numerical and analytical micromagnetics. The influence of a moderate interfacial Dzyaloshinskii-Moriya interaction associated to a bi-layer strip arrangement has been considered, giving rise to the formation of Dzyaloshinskii domain walls. Such walls possess under equilibrium conditions an inner magnetization structure defined by a certain orientation angle that make them to be considered as intermediate configurations between Bloch and Néel walls. Two different dynamics are considered, a field-driven and a current-driven dynamics, in particular, the one promoted by the spin torque due to the spin-Hall effect. Results show an inherent asymmetry associated with the rotation of the domain wall magnetization orientation before reaching the stationary regime, characterized by a constant terminal speed. For a certain initial DW magnetization orientation at rest, the rotation determines whether the reorientation of the DW magnetization prior to reach stationary motion is smooth or abrupt. This asymmetry affects the DW motion, which can even reverse for a short period of time. Additionally, it is found that the terminal speed in the case of the current-driven dynamics may depend on either the initial DW magnetization orientation at rest or the sign of the longitudinally injected current.

  11. Electric Field Control of Ferromagnetism and Magnetic Devices Using Multiferroics

    NASA Astrophysics Data System (ADS)

    Heron, John Thomas

    This dissertation presents a study of a heterostructure composed of room temperature magnetoelectric multiferroic BiFeO3 and ferromagnetic Co.90Fe.10, with specific interest in understanding the interfacial coupling mechanisms in this system and establishing the electric field control of a magnetization and spintronic devices. The field of spintronics has been plagued with the problem of a large energy dissipation as a consequence of the resistive losses that come during the writing of the magnetic state (i.e. reversing the magnetization direction). The primary aim of the work presented here is to investigate and understand a novel heterostructure and materials interface that can be demonstrated as a pathway to low energy spintronics. In this dissertation, I will address the specific aspects of multiferroicity, magnetoelectricity, and interface coupling that must be addressed in order to reverse a magnetization with an electric field. Furthermore, I will demonstrate the reversal of a magnetization with an electric field in single and multilayer magnetic devices. The primary advances made as a result of the work described herein are the use of epitaxial constraints to control the nanoscale domain structure of a multiferroic which is then correlated to the domain structure of the exchange coupled ferromagnet. Additionally, the magnetization direction of the ferromagnetic layer is controlled with only an applied electric field at both macroscopic and microscopic scales. Lastly, using this electric field control of ferromagnetism, the first demonstration of a magnetoelectric memory bit is presented.

  12. Thermodynamics of the Heisenberg ferromagnet in an applied magnetic field.

    NASA Technical Reports Server (NTRS)

    Flax, L.

    1972-01-01

    The anisotropic-Heisenberg-ferromagnet formalism developed previously is examined to include an applied magnetic field for the isotropic case in the random-phase approximation. Thermodynamic quantities such as magnetization, susceptibility, and the derivative of magnetization with respect to temperature are studied near the Curie point.

  13. Magnetic field effects on the open circuit potential of ferromagnetic electrodes in corroding solutions.

    PubMed

    Dass, Amala; Counsil, Joseph A; Gao, Xuerong; Leventis, Nicholas

    2005-06-02

    Magnetic fields shift the open circuit potential (OCP) of ferromagnetic electrodes (Fe, Co, and Ni) in corroding solutions. The OCP changes we observe (a) follow the series Fe>Co>Ni; (b) increase with the magnetic flux density; (c) reach a maximum with disk electrodes approximately 1 mm in diameter; and (d) depend on the orientation of the electrode. We report that when the surface of the electrode is oriented parallel (theta = 90 degrees) or perpendicular (theta = 0 degrees) to the magnetic field, the open circuit potential moves in opposite directions (positive and negative, respectively) with the largest changes occurring when the electrode surface is parallel to the magnetic field. Nonconvective sleeve electrodes produce the same behavior. The overall experimental evidence suggests that the magnetic field changes the OCP by modifying the surface concentrations of the paramagnetic participants in the corrosion process of the ferromagnetic electrode by species in solution; this in turn is accomplished by imposing a field-gradient driven mode of mass transfer upon paramagnetic species in solution (magnetophoresis). Simulations of the magnetic field around the ferromagnetic electrode at the two extreme orientations considered here show that in one case (theta = 90 degrees) field gradients actually repel, while in the other case (theta = 0 degrees) they attract paramagnetic species in the vicinity of the electrode.

  14. Composition-driven spin glass to ferromagnetic transition in the quasicrystal approximant Au-Al-Gd

    NASA Astrophysics Data System (ADS)

    Ishikawa, A.; Hiroto, T.; Tokiwa, K.; Fujii, T.; Tamura, R.

    2016-01-01

    We investigated the composition dependence of the magnetic susceptibility of the quasicrystal approximant Au-Al-Gd. A composition-driven ferromagnetic transition is observed in a quasicrystal approximant, which is attributed to the Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation via a variation in the Fermi wave vector. The ferromagnetic transition is most simply understood as a result of the close matching of the nearest and second-nearest spin distances with the maximum positions of the RKKY potential. The present work provides an idea that allows us to tailor the magnetic order via the electron concentration in quasicrystal approximants as well as in quasicrystals.

  15. Quantum Lifshitz Field Theory of a Frustrated Ferromagnet.

    PubMed

    Balents, Leon; Starykh, Oleg A

    2016-04-29

    We propose a universal nonlinear sigma model field theory for one-dimensional frustrated ferromagnets, which applies in the vicinity of a "quantum Lifshitz point," at which the ferromagnetic state develops a spin wave instability. We investigate the phase diagram resulting from perturbations of the exchange and of magnetic field away from the Lifshitz point, and uncover a rich structure with two distinct regimes of different properties, depending upon the value of a marginal, dimensionless, parameter of the theory. In the regime relevant for one-dimensional systems with low spin, we find a metamagnetic transition line to a vector chiral phase. This line terminates in a critical end point, beyond which there is at least one multipolar or "spin nematic" phase. We show that the field theory is asymptotically exactly soluble near the Lifshitz point.

  16. Effect of non-uniform exchange field in ferromagnetic graphene

    SciTech Connect

    Chowdhury, Debashree Basu, B.

    2015-04-15

    We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin–orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic graphene. The generation of anti damping spin–orbit torque in spin chiral magnetic graphene is also briefly discussed.

  17. Parametric frequency mixing in a magnetoelastically driven linear ferromagnetic-resonance oscillator

    NASA Astrophysics Data System (ADS)

    Chang, C. L.; Lomonosov, A. M.; Janusonis, J.; Vlasov, V. S.; Temnov, V. V.; Tobey, R. I.

    2017-02-01

    We demonstrate the linear frequency conversion of ferromagnetic resonance (FMR) frequency by optically excited elastic waves in a thin metallic film on dielectric substrates. Time-resolved probing of the magnetization directly witnesses magnetoelastically driven parametric second-harmonic generation, sum- and difference-frequency mixing from two distinct frequencies, as well as excitation of parametric resonances. Starting from the Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an elastically driven (nonlinear) parametric oscillator and show that frequency mixing is dominated by the parametric modulation of the linear FMR oscillator.

  18. Ferromagnetic thickness dependence of current-driven spin-orbit torques in different ferromagnetic and heavy metal bilayers

    NASA Astrophysics Data System (ADS)

    Wu, Jun; Fan, Xin; Wang, Tao; Chen, Yunpeng; Xiao, Q. John

    The spin-orbit torques in ferromagnetic (FM) and heavy metal (HM) bilayers have attracted extensive research interests recently because of the rich physical phenomena and potential applications. We measured the effective fields of field-like torques in Ni/Pt, NiFe/Pt and CoFeB/Pt bilayer systems by the second-order planar Hall effect. When the FM layers are less than 2nm, the effective fields increase rapidly with decreasing the FM layer thickness for all three different FM layers. Among the three FMs, the effective field in Ni is largest, followed by NiFe, then CoFeB. Above 2nm, the effective fields decrease much slower with increasing the FM layer thickness and level off to the Orested field due to the current in the Pt layer. Through FM layer thickness dependence of the field-like torque study, we found that the spin dephasing length in the FM layer, which is related to the scattering in FM layer, plays an important role in determining the magnitude of field-like spin-orbit torque in FM/HM bilayers.

  19. Magnetostatic modes in ferromagnetic samples with inhomogeneous internal fields

    NASA Astrophysics Data System (ADS)

    Arias, Rodrigo

    2015-03-01

    Magnetostatic modes in ferromagnetic samples are very well characterized and understood in samples with uniform internal magnetic fields. More recently interest has shifted to the study of magnetization modes in ferromagnetic samples with inhomogeneous internal fields. The present work shows that under the magnetostatic approximation and for samples of arbitrary shape and/or arbitrary inhomogeneous internal magnetic fields the modes can be classified as elliptic or hyperbolic, and their associated frequency spectrum can be delimited. This results from the analysis of the character of the second order partial differential equation for the magnetostatic potential under these general conditions. In general, a sample with an inhomogeneous internal field and at a given frequency, may have regions of elliptic and hyperbolic character separated by a boundary. In the elliptic regions the magnetostatic modes have a smooth monotonic character (generally decaying form the surfaces (a ``tunneling'' behavior)) and in hyperbolic regions an oscillatory wave-like character. A simple local criterion distinguishes hyperbolic from elliptic regions: the sign of a susceptibility parameter. This study shows that one may control to some extent magnetostatic modes via external fields or geometry. R.E.A. acknowledges Financiamiento Basal para Centros Cientificos y Tecnologicos de Excelencia under Project No. FB 0807 (Chile), Grant No. ICM P10-061-F by Fondo de Innovacion para la Competitividad-MINECON, and Proyecto Fondecyt 1130192.

  20. Modulation of the properties of thin ferromagnetic films with an externally applied electric field in ferromagnetic/piezoelectric/ferromagnetic hybrids

    NASA Astrophysics Data System (ADS)

    Stamopoulos, D.; Zeibekis, M.; Zhang, S. J.

    2013-10-01

    In many cases, technological advances are based on artificial low-dimensional structures of heterogeneous constituents, thus called hybrids, that when come together they provide stand-alone entities that exhibit entirely different properties. Such hybrids are nowadays intensively studied since they are attractive for both basic research and oncoming practical applications. Here, we studied hybrids constituted of piezoelectric (PE) and ferromagnetic (FM) components in the form FM/PE/FM, ultimately aiming to provide means for the controlled modulation of the properties of the FM electrodes, originating from the strain imposed to them by the PE mediator when an electric field is applied. The PE component is in single crystal form, 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT), while the FM outer layers are Cobalt (Co) in thin film form. Detailed magnetization measurements performed under variation of the electric field applied to PMN-PT demonstrated the efficient modulation of the properties of the Co electrodes at low temperature (coercive field modulation up to 27% and saturation magnetization absolute modulation up to 4% at T = 10 K for electric field not exceeding 6 kV/cm). The modulation degree faints upon increase of the temperature, evidencing that the thermal energy eventually dominates all other relevant energy scales. Candidate mechanisms are discussed for the explanation of these experimental observations. The results presented here demonstrate that commercially available materials can result in quantitatively noticeable effects. Thus, such elemental Co/PMN-PT/Co units can be used as a solid basis for the development of devices.

  1. Field tuning of ferromagnetic domain walls on elastically coupled ferroelectric domain boundaries

    NASA Astrophysics Data System (ADS)

    Franke, Kévin J. A.; Lahtinen, Tuomas H. E.; van Dijken, Sebastiaan

    2012-03-01

    We report on the evolution of ferromagnetic domain walls during magnetization reversal in elastically coupled ferromagnetic-ferroelectric heterostructures. Using optical polarization microscopy and micromagnetic simulations, we demonstrate that the spin rotation and width of ferromagnetic domain walls can be accurately controlled by the strength of the applied magnetic field if the ferromagnetic walls are pinned onto 90∘ ferroelectric domain boundaries. Moreover, reversible switching between magnetically charged and uncharged domain walls is initiated by magnetic field rotation. Switching between both wall types reverses the wall chirality and abruptly changes the width of the ferromagnetic domain walls by up to 1000%.

  2. Spin torque ferromagnetic resonance with magnetic field modulation

    NASA Astrophysics Data System (ADS)

    Gonçalves, A. M.; Barsukov, I.; Chen, Y.-J.; Yang, L.; Katine, J. A.; Krivorotov, I. N.

    2013-10-01

    We demonstrate a technique of broadband spin torque ferromagnetic resonance (ST-FMR) with magnetic field modulation for measurements of spin wave properties in magnetic nanostructures. This technique gives great improvement in sensitivity over the conventional ST-FMR measurements, and application of this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR measurements with micromagnetic simulations of the spin wave spectrum allows us to explain the character of low-frequency magnetic excitations in nanoscale MTJs.

  3. Thermally driven transverse transports and magnetic dynamics on a topological surface capped with a ferromagnet strip

    NASA Astrophysics Data System (ADS)

    Deng, Ming-Xun; Zhong, Ming; Zheng, Shi-Han; Qiu, Jian-Ming; Yang, Mou; Wang, Rui-Qiang

    2016-02-01

    We theoretically study thermally driven transport of the Dirac fermions on the surface of a topological insulator capped with a ferromagnet strip. The generation and manipulation of anomalous Hall and Nernst effects are analyzed, in which the in-plane magnetization of the ferromagnet film is found to take a decisive role. This scenario is distinct from that modulated by Berry phase where the in-plane magnetization is independent. We further discuss the thermal spin-transfer torque as a backaction of the thermoelectric transports on the magnetization and calculate the dynamics of the anomalous Hall and Nernst effects self-consistently. It is found that the magnitude of the long-time steady Hall and Nernst conductance is determined by competition between the magnetic anisotropy and current-induced effective anisotropy. These results open up a possibility of magnetically controlling the transverse thermoelectric transports or thermally manipulating the magnet switching.

  4. Ferromagnetism controlled by electric field in tilted phosphorene nanoribbon

    PubMed Central

    Farooq, M. Umar; Hashmi, Arqum; Hong, Jisang

    2016-01-01

    Study on phosphorene nanoribbon was mostly focused on zigzag and armchair structures and no ferromagnetic ground state was observed in these systems. Here, we investigated the magnetic property of tilted black phosphorene nanoribbons (TPNRs) affected by an external electric field. We also studied the edge passivation effect on the magnetism and thermal stability of the nanoribbons. The pure TPNR displayed an edge magnetic state, but it disappeared in the edge reconstructed TPNR due to the self-passivation. In addition, we found that the bare TPNR was mechanically unstable because an imaginary vibration mode was obtained. However, the imaginary vibration mode disappeared in the edge passivated TPNRs. No edge magnetism was observed in hydrogen and fluorine passivated TPRNs. In contrast, the oxygen passivated TPNR was more stable than the pure TPNR and the edge-to-edge antiferromagntic (AFM) ground state was obtained. We found that the magnetic ground state could be tuned by the electric field from antiferromagnetic (AFM) to ferromagnetic (FM) ground state. Interestingly, the oxygen passivated TPNR displayed a half-metallic state at a proper electric field in both FM and AFM states. This finding may provoke an intriguing issue for potential spintronics application using the phosphorene nanoribbons. PMID:27189417

  5. Ferromagnetism controlled by electric field in tilted phosphorene nanoribbon.

    PubMed

    Farooq, M Umar; Hashmi, Arqum; Hong, Jisang

    2016-05-18

    Study on phosphorene nanoribbon was mostly focused on zigzag and armchair structures and no ferromagnetic ground state was observed in these systems. Here, we investigated the magnetic property of tilted black phosphorene nanoribbons (TPNRs) affected by an external electric field. We also studied the edge passivation effect on the magnetism and thermal stability of the nanoribbons. The pure TPNR displayed an edge magnetic state, but it disappeared in the edge reconstructed TPNR due to the self-passivation. In addition, we found that the bare TPNR was mechanically unstable because an imaginary vibration mode was obtained. However, the imaginary vibration mode disappeared in the edge passivated TPNRs. No edge magnetism was observed in hydrogen and fluorine passivated TPRNs. In contrast, the oxygen passivated TPNR was more stable than the pure TPNR and the edge-to-edge antiferromagntic (AFM) ground state was obtained. We found that the magnetic ground state could be tuned by the electric field from antiferromagnetic (AFM) to ferromagnetic (FM) ground state. Interestingly, the oxygen passivated TPNR displayed a half-metallic state at a proper electric field in both FM and AFM states. This finding may provoke an intriguing issue for potential spintronics application using the phosphorene nanoribbons.

  6. Cubic ideal ferromagnets at low temperature and weak magnetic field

    NASA Astrophysics Data System (ADS)

    Hofmann, Christoph P.

    2017-04-01

    The low-temperature series for the free energy density, pressure, magnetization and susceptibility of cubic ideal ferromagnets in weak external magnetic fields are discussed within the effective Lagrangian framework up to three loops. The structure of the simple, body-centered, and face-centered cubic lattice is taken into account explicitly. The expansion involves integer and half-integer powers of the temperature. The corresponding coefficients depend on the magnetic field and on low-energy effective constants that can be expressed in terms of microscopic quantities. Our formulas may also serve as efficiency or consistency check for other techniques like Green's function methods, where spurious terms in the low-temperature expansion have appeared. We explore the sign and magnitude of the spin-wave interaction in the pressure, magnetization and susceptibility, and emphasize that our effective field theory approach is fully systematic and rigorous.

  7. Emerging Diluted Ferromagnetism in High-Tc Superconductors Driven by Point Defect Clusters.

    PubMed

    Gazquez, Jaume; Guzman, Roger; Mishra, Rohan; Bartolomé, Elena; Salafranca, Juan; Magén, Cesar; Varela, Maria; Coll, Mariona; Palau, Anna; Valvidares, S Manuel; Gargiani, Pierluigi; Pellegrin, Eric; Herrero-Martin, Javier; Pennycook, Stephen J; Pantelides, Sokrates T; Puig, Teresa; Obradors, Xavier

    2016-06-01

    Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high-temperature superconductor YBa2Cu3O7-δ (Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration-corrected scanning transmission microscopy has been used for exploring, on a single unit-cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect-driven ferromagnetism, and X-ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects.

  8. Emerging Diluted Ferromagnetism in High‐T c Superconductors Driven by Point Defect Clusters

    PubMed Central

    Guzman, Roger.; Mishra, Rohan; Bartolomé, Elena; Salafranca, Juan; Magén, Cesar; Varela, Maria; Coll, Mariona; Palau, Anna; Valvidares, S. Manuel; Gargiani, Pierluigi; Pellegrin, Eric; Herrero‐Martin, Javier.; Pennycook, Stephen J.; Pantelides, Sokrates T.; Puig, Teresa; Obradors, Xavier

    2016-01-01

    Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high‐temperature superconductor YBa2Cu3O7‐δ (Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration‐corrected scanning transmission microscopy has been used for exploring, on a single unit‐cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect‐driven ferromagnetism, and X‐ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects. PMID:27812469

  9. 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.

  10. Barkhausen-like antiferromagnetic to ferromagnetic phase transition driven by spin polarized current

    NASA Astrophysics Data System (ADS)

    Suzuki, Ippei; Naito, Tomoyuki; Itoh, Mitsuru; Taniyama, Tomoyasu

    2015-08-01

    We provide clear evidence for the effect of a spin polarized current on the antiferromagnetic to ferromagnetic phase transition of an FeRh wire at Co/FeRh wire junctions, where the antiferromagnetic ground state of FeRh is suppressed by injecting a spin polarized current. We find a discrete change in the current-voltage characteristics with increasing current density, which we attribute to the Barkhausen-like motion of antiferromagnetic/ferromagnetic interfaces within the FeRh wire. The effect can be understood via spin transfer, which exerts a torque to the antiferromagnetic moments of FeRh, together with non-equilibrium magnetic effective field at the interface. The conclusion is reinforced by the fact that spin unpolarized current injection from a nonmagnetic Cu electrode has no effects on the antiferromagnetic state of FeRh.

  11. Barkhausen-like antiferromagnetic to ferromagnetic phase transition driven by spin polarized current

    SciTech Connect

    Suzuki, Ippei; Naito, Tomoyuki; Itoh, Mitsuru; Taniyama, Tomoyasu

    2015-08-24

    We provide clear evidence for the effect of a spin polarized current on the antiferromagnetic to ferromagnetic phase transition of an FeRh wire at Co/FeRh wire junctions, where the antiferromagnetic ground state of FeRh is suppressed by injecting a spin polarized current. We find a discrete change in the current-voltage characteristics with increasing current density, which we attribute to the Barkhausen-like motion of antiferromagnetic/ferromagnetic interfaces within the FeRh wire. The effect can be understood via spin transfer, which exerts a torque to the antiferromagnetic moments of FeRh, together with non-equilibrium magnetic effective field at the interface. The conclusion is reinforced by the fact that spin unpolarized current injection from a nonmagnetic Cu electrode has no effects on the antiferromagnetic state of FeRh.

  12. Pressure-Temperature-Field Phase Diagram in the Ferromagnet U3P4

    NASA Astrophysics Data System (ADS)

    Araki, Shingo; Hayashida, Minami; Nishiumi, Naoto; Manabe, Hiroki; Ikeda, Yoichi; Kobayashi, Tatsuo C.; Murata, Keizo; Inada, Yoshihiko; Wiśniewski, Piotr; Aoki, Dai; Ōnuki, Yoshichika; Yamamoto, Etsuji; Haga, Yoshinori

    2015-02-01

    The pressure-temperature-field phase diagram and quantum fluctuation effect were investigated in the itinerant ferromagnet U3P4 by resistivity, ac susceptibility, and Hall effect measurements under high pressure. The zero-temperature ferromagnetic-to-paramagnetic transition is located at Pc ˜ 4.0 GPa. The tricritical point exists at P* = 3.8 GPa and T* = 32 K, where the ferromagnetic transition changes from second- to first-order. As a quantum fluctuation effect, the low-temperature resistivity at Pc follows T5/3 dependence, which was theoretically described in the case of a three-dimensional ferromagnet. This result indicates that the ferromagnetic-to-paramagnetic transition at Pc may be considered as weakly first-order. Another critical behavior, the huge enhancement of ρ0 observed at Pc, cannot be interpreted as the ferromagnetic fluctuation effect. The Hall effect measurements suggest that the magnetic structure in the ordered state changes under high pressure.

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

    SciTech Connect

    Bhoomeeswaran, H.; Sabareesan, P.; 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°, 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}.

  14. Instabilities of spin torque driven auto-oscillations of a ferromagnetic disk magnetized in plane

    NASA Astrophysics Data System (ADS)

    Mancilla-Almonacid, D.; Arias, R. E.

    2016-06-01

    The stability of the magnetization auto-oscillations of the ferromagnetic free layer of a cylindrical nanopillar structure is studied theoretically using a classical Hamiltonian formalism for weakly interacting nonlinear waves, in a weakly dissipative system. The free layer corresponds to a very thin circular disk, made of a soft ferromagnetic material like Permalloy, and it is magnetized in plane by an externally applied magnetic field. There is a dc electric current that traverses the structure, becomes spin polarized by a fixed layer, and excites the modes of the free layer through the transfer of spin angular momentum. If this current exceeds a critical value, it is possible to generate a large amplitude periodic auto-oscillation of a dynamic mode of the magnetization. We separate our theoretical study into two parts. First, we consider an approximate expression for the demagnetizing field in the disk, i.e., H⃗D=-4 π Mzz ̂ or a very thin film approximation, and secondly we consider the effect of the full demagnetizing field, where one sees important effects due to the edges of the disk. In both cases, as the applied current density is increased, we determine the modes that will first auto-oscillate and when these become unstable to the growth of other modes, i.e., their ranges of "isolated" auto-oscillation.

  15. Using Ferromagnetic Material to Extend and Shield the Magnetic Field of a Coil

    DTIC Science & Technology

    2017-06-14

    ARL-MR-0954 ● Jun 2017 US Army Research Laboratory Using Ferromagnetic Material to Extend and Shield the Magnetic Field of a...longer needed. Do not return it to the originator. ARL-MR-0954 ● Jun 2017 US Army Research Laboratory Using Ferromagnetic Material ...to Extend and Shield the Magnetic Field of a Coil by W Casey Uhlig Weapons and Materials Research Directorate, ARL

  16. THz-Driven Ultrafast Spin-Lattice Scattering in Amorphous Metallic Ferromagnets

    NASA Astrophysics Data System (ADS)

    Bonetti, S.; Hoffmann, M. C.; Sher, M.-J.; Chen, Z.; Yang, S.-H.; Samant, M. G.; Parkin, S. S. P.; Dürr, H. A.

    2016-08-01

    We use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (˜30 fs ). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering.

  17. Interface-Driven Ferromagnetism within the Quantum Wells of a Rare Earth Titanate Superlattice.

    PubMed

    Need, R F; Isaac, B J; Kirby, B J; Borchers, J A; Stemmer, S; Wilson, Stephen D

    2016-07-15

    Here we present polarized neutron reflectometry measurements exploring thin film heterostructures composed of a strongly correlated Mott state, GdTiO_{3}, embedded with SrTiO_{3} quantum wells. Our results reveal that the net ferromagnetism inherent to the Mott GdTiO_{3} matrix propagates into the nominally nonmagnetic SrTiO_{3} quantum wells and tracks the magnetic order parameter of the host Mott insulating matrix. Beyond a well thickness of 5 SrO layers, the magnetic moment within the wells is dramatically suppressed, suggesting that quenched well magnetism comprises the likely origin of quantum critical magnetotransport in this thin film architecture. Our data demonstrate that the interplay between proximate exchange fields and polarity-induced carrier densities can stabilize extended magnetic states within SrTiO_{3} quantum wells.

  18. Current-Driven Instability of the Quantum Anomalous Hall Effect in Ferromagnetic Topological Insulators

    NASA Astrophysics Data System (ADS)

    Kawamura, Minoru; Yoshimi, Ryutaro; Tsukazaki, Atsushi; Takahashi, Kei S.; Kawasaki, Masashi; Tokura, Yoshinori

    2017-07-01

    The instability of the quantum anomalous Hall (QAH) effect has been studied as a function of the electric current and temperature in ferromagnetic topological insulator thin films. We find that a characteristic current for the breakdown of the QAH effect is roughly proportional to the Hall-bar width, indicating that the Hall electric field is relevant to the breakdown. We also find that electron transport is dominated by variable range hopping (VRH) at low temperatures. Combining the current and temperature dependences of the conductivity in the VRH regime, the localization length of the QAH state is evaluated to be about 5 μ m . The long localization length suggests a marginally insulating nature of the QAH state due to a large number of in-gap states.

  19. Field-driven magnetization dynamics of nanoparticles and nanowires

    NASA Astrophysics Data System (ADS)

    Lu, Jie

    This thesis is about micromagnetism in confined magnetic microstructures. The field-driven magnetization dynamics of nanoparticles and nanowires is systematically discussed following a clear thread of thought: from "macrospin" to "microspin". At the same time, four topics are raised and investigated. First, inspired by the traditional ferromagnetic resonance technique, two strategies for measuring the Gilbert damping coefficient using the magnetic circular dichroism effect are presented and discussed. The investigation is performed within a framework of the linear response of the macrospin in 2-D magnetic films to external time-dependent fields. The object of the study then turns to Stoner particles, which are single-domain magnetic nanoparticles, that are quasi 0-D systems and still assumed to be macrospins. The field-driven magnetization reversal in multi-axial Stoner particles is investigated and the corresponding Eular equations are presented. The Eular equations provide a unified framework for research of this kind. After that, the macrospin assumption itself is examined. The study of when and how it fails results in the famous "nucleation problem" in micromagnetism, thus the discussion then moves into the microspin category. The nucleation problem of single-domain cuboid permalloy nanowires, which are quasi 1-D systems, is investigated and a magnetization reversal mode named "domain formation and domain wall propagation" is revealed. Field-driven magnetic domain wall propagation is an excellent example of microspin behavior, and has been a hot issue in recent spintronic research. The effects of transverse magnetic anisotropies on field-driven transverse wall propagation in narrow magnetic nanowires are systematically investigated. These results should not only deepen the understanding of the domain wall dynamics in magnetic nanowires, but also offer inspiration for further developments of ultrafast nano-devices with higher integration levels.

  20. 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.

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

    PubMed

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

    2016-07-29

    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.

  2. FMR Study of the Field Dependence of the Ferromagnetic Transition in an Organic Magnet

    NASA Astrophysics Data System (ADS)

    Kovalev, Alexey; Winter, Stephen; Hill, Stephen; Oakley, Richard

    2012-02-01

    Organic heterocyclic thia/selenazyl radicals have unique magnetic properties. First and foremost, in their crystalline form, they experience a transition to a ferromagnetic state at temperatures that are the highest for any material containing only non-metallic elements. Second, their low temperature uniaxial anisotropy field is the highest among purely organic ferromagnets [Winter et al., JACS 133, 8126 (2011)]. To investigate the effect of a magnetic field on the transition in the mixed Se-S compound (Tc= 12.5 K) at zero field, we employ ferromagnetic resonance (FMR) absorption as a measure of the anisotropy field for a single crystal. We also focus on the temperature and field dependence of the FMR linewidth. Our main finding is that the application of a field significantly broadens the ferromagnetic transition, with a noticeable FMR signal observed to as high as 2Tc in fields of a few tesla. Meanwhile, the FMR linewidth is relatively insensitive to frequency/field, though it becomes narrower upon decreasing the temperature and saturates below Tc. We will discuss the broadening of the ferromagnetic transition within the framework of scaling theory.

  3. Influence of an electric field on the ferromagnetic resonance in a plane-layered magnetic system

    NASA Astrophysics Data System (ADS)

    Karashtin, E. A.; Fraerman, A. A.

    2016-11-01

    The influence of an electric field on the ferromagnetic resonance (FMR) in a multilayer magnetic system consisting of two magnetic layers separated by a thin nonmagnetic interlayer has been investigated. It has been shown that, upon the excitation of magnetization oscillations by a microwave magnetic field, the eigenfrequencies of the ferromagnetic resonance depend on the stationary electric field applied in the plane of the layers. It has also been demonstrated that, in this system, high-frequency magnetization oscillations can be excited by an electric microwave field. The results of the investigation of the polarization properties of the excitation mechanism indicate that this effect can be observed experimentally.

  4. Hematological and toxicogenomic effects of ferromagnetic screening of natural electromagnetic fields.

    PubMed

    Ivanov, S D; Nikitina, V N; Yamshanov, V A; Kovanko, E G; Lyashko, G G; Monakhov, A S; Koshelevsky, V K

    2011-05-01

    Aftereffects of ferromagnetic screening on the hematological and toxicogenomic parameters in rats were traced over 45 days. Two-day ferromagnetic screening of male and female rats (reducing permanent constituent of magnetic field induction by 4-10 μT) led to leukopenia observed on day 14 of the experiment. Life-time evaluation of the toxicogenomic effects was carried out by rapid method for measurement of blood nucleotide DNA by fluorescent indication. In male blood leukocytes, increased aneuploidy and polyploidy were observed after 48 h of ferromagnetic screening and remained high on days 12 and 28 after screen removal. In contrast to males, leukocyte apoptosis in females was increased only 48 h after the start of ferromagnetic screening.

  5. Magnetic anisotropy control by applying an electric field to the side surface of ferromagnetic films.

    PubMed

    Terada, Hiroshi; Ohya, Shinobu; Anh, Le Duc; Iwasa, Yoshihiro; Tanaka, Masaaki

    2017-07-17

    Reducing the power consumption necessary for magnetization reversal is one of the most crucial issues facing spintronics devices. Electric field control of the magnetic anisotropy of ferromagnetic thin films is a promising method to solve this problem. However, the electric field is believed to be effective only within several nanometres of the surface in ferromagnetic metals because of its short Thomas-Fermi screening length, which prevents its practical application to devices. Herein, we successfully modulate the magnetic anisotropy of the entire region of the ferromagnetic layers in the elongated mesas of vertical spin field-effect transistors with widths as large as ~500 nm by applying an electric field to the side surface of the metallic GaMnAs-based mesas through an electric double layer. Our results will open up a new pathway for spintronics devices with ultra-low power consumption.

  6. Theory of light-induced effective magnetic field in Rashba ferromagnets

    NASA Astrophysics Data System (ADS)

    Qaiumzadeh, Alireza; Titov, Mikhail

    2016-07-01

    Motivated by recent experiments on all-optical magnetization reversal in conductive ferromagnetic thin films we use nonequilibrium formalism to calculate the effective magnetic field induced in a Rashba ferromagnet by a short laser pulse. The main contribution to the effect originates in the direct optical transitions between spin-split subbands. The resulting effective magnetic field is inversely proportional to the impurity scattering rate and can reach the amplitude of a few Tesla in the systems like Co/Pt bilayers. We show that the total light-induced effective magnetic field in ferromagnetic systems is the sum of two contributions: a helicity dependent term, which is an even function of magnetization, and a helicity independent term, which is an odd function of magnetization. The primary role of the spin-orbit interaction is to widen the frequency range for direct optical transitions.

  7. Inverse problem of pulsed eddy current field of ferromagnetic plates

    NASA Astrophysics Data System (ADS)

    Chen, Xing-Le; Lei, Yin-Zhao

    2015-03-01

    To determine the wall thickness, conductivity and permeability of a ferromagnetic plate, an inverse problem is established with measured values and calculated values of time-domain induced voltage in pulsed eddy current testing on the plate. From time-domain analytical expressions of the partial derivatives of induced voltage with respect to parameters, it is deduced that the partial derivatives are approximately linearly dependent. Then the constraints of these parameters are obtained by solving a partial linear differential equation. It is indicated that only the product of conductivity and wall thickness, and the product of relative permeability and wall thickness can be determined accurately through the inverse problem with time-domain induced voltage. In the practical testing, supposing the conductivity of the ferromagnetic plate under test is a fixed value, and then the relative variation of wall thickness between two testing points can be calculated via the ratio of the corresponding inversion results of the product of conductivity and wall thickness. Finally, this method for wall thickness measurement is verified by the experiment results of a carbon steel plate. Project supported by the National Defense Basic Technology Research Program of China (Grant No. Z132013T001).

  8. Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

    PubMed Central

    Xiu, Faxian; Wang, Yong; Zou, Jin; Wang, Kang L.

    2011-01-01

    Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism. PMID:22110869

  9. Electric-field controlled ferromagnetism in MnGe magnetic quantum dots.

    PubMed

    Xiu, Faxian; Wang, Yong; Zou, Jin; Wang, Kang L

    2011-01-01

    Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (T(c)>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn(0.05)Ge(0.95) quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn(0.05)Ge(0.95) quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn(0.05)Ge(0.95) quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism.

  10. Ferromagnetic resonance and high field ESR in a TDAE-C60 single crystal

    NASA Astrophysics Data System (ADS)

    Arčon, D.; Cevc, P.; Omerzu, A.; Blinc, R.; Mehring, M.; Knorr, S.; Grupp, A.; Barra, A.-L.; Chouteau, G.

    1998-08-01

    Frequency variable ESR measurements have been performed on well annealed TDAE-C60 single crystals between 40 MHz and 245 GHz. A non-linear variation of the electron resonance frequency with the magnetic field has been observed below TC=16 K in the radio-frequency region. The observed ferromagnetic resonance data are characteristic for a three-dimensional Heisenberg ferromagnet with a small positive uniaxial anisotropy field. The easy axis coincides with the crystal c-direction which is the direction of closest approach of the C60- ions.

  11. Light-field-driven currents in graphene.

    PubMed

    Higuchi, Takuya; Heide, Christian; Ullmann, Konrad; Weber, Heiko B; Hommelhoff, Peter

    2017-09-25

    The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond (less than 10(-15) seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation, sub-optical-cycle interband population transfer and the non-perturbative change of the transient polarizability. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields. Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material, because of its broadband and ultrafast optical response, weak screening and high damage threshold. Here we show that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond (10(-18) seconds) timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light-matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions. We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau-Zener-Stückelberg interference, composed of coherent repeated Landau-Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in

  12. Field-dependent spin-wave damping in ferromagnet/antiferromagnet bilayers

    NASA Astrophysics Data System (ADS)

    Moradi, H.

    2006-04-01

    Damon Eshbach mode is used to explain the dependence of spin-wave frequency of ferromagnetic layer in thin ferromagnetic (FM)/antiferromagnetic bilayer (FM/AFM) (Fe/FeF2) as a function of external field. We show that the exchange bias changes as a function of external field. Field-dependent spin-wave damping is calculated by Born approximation. A random field, due to roughness at interface, is invoked to explain the large broadening of the spin-wave mode when the AFM layer is present. In this calculation, the broadening of the spin-wave mode depends on the square of linewidth random fields. To explain the broadening of the spin-wave mode with observed results, the linewidth random fields should decrease with external field. For a magnitude of external field, we have a peak in broadening that can also be observed in experimental results.

  13. Apparatus for efficient sidewall containment of molten metal with horizontal alternating magnetic fields utilizing a ferromagnetic dam

    DOEpatents

    Praeg, Walter F.

    1997-01-01

    An apparatus for casting sheets of metal from molten metal. The apparatus includes a containment structure having an open side, a horizontal alternating magnetic field generating structure and a ferromagnetic dam. The magnetic field and the ferromagnetic dam contain the molten metal from leaking out side portions of the open side of the containment structure.

  14. Apparatus for efficient sidewall containment of molten metal with horizontal alternating magnetic fields utilizing a ferromagnetic dam

    DOEpatents

    Praeg, W.F.

    1997-02-11

    An apparatus is disclosed for casting sheets of metal from molten metal. The apparatus includes a containment structure having an open side, a horizontal alternating magnetic field generating structure and a ferromagnetic dam. The magnetic field and the ferromagnetic dam contain the molten metal from leaking out side portions of the open side of the containment structure. 25 figs.

  15. Effect of electric field on the magnetic characteristics of a ferromagnetic nanosemiconductor

    SciTech Connect

    Kozhushner, M. A. Lidskii, B. V.; Posvyanskii, V. S.; Trakhtenberg, L. I.

    2016-12-15

    A theory is developed to describe the effect of an electric field on the magnetization of a thin ferromagnetic semiconductor plate. It is shown that the magnetic moment density is nonuniform under these conditions and that the total magnetic moment and its density depend on the electric field and the temperature. An electric field is found to increase the Curie temperature, and an inflection point is detected in the temperature dependence of the derivative of the total magnetic moment with respect to temperature.

  16. Calculation of the surface effect in the ferromagnetic conductor with the harmonic electromagnetic field

    NASA Astrophysics Data System (ADS)

    Nosov, G. V.; Kuleshova, E. O.; Vassilyeva, Yu Z.; Elizarov, A. I.

    2016-04-01

    The authors of the paper have obtained formulas for analytical calculation of the constants with the harmonic electromagnetic field, which characterize the surface layer (a skin layer) of the ferromagnetic conductors in case of heating and nonlinear magnetic properties, which can be used for practical calculation of the electromagnetic screens, rotors of the electrical machines and inductive heating installations. A nonlinear dependence of the magnetic induction on the magnetic tension of the ferromagnetic conductor is replaced by one or two linear sections. It is considered that the skin layer of the conductor has constant quantities of the specific conductivity and averaged temperature. Linear electrodynamics equations are solved for the conductive half-space. Parameters of the ferromagnetic conductor's surface layer are calculated: magnetic permeability, the thickness of the skin layer and its averaged temperature, exposure time of the electromagnetic field on the conductor with the established maximum temperature on the conductor's surface, pressure of the field on the conductor and its resistance, inductivity of the internal magnetic field in the conductor, the thermal energy capacity. The methods credibility is confirmed with the concurrence of the resistance and inductiviry of the ferromagnetic conductor with analogous quantities from other methods.

  17. Light and electric field control of ferromagnetism in magnetic quantum structures.

    PubMed

    Boukari, H; Kossacki, P; Bertolini, M; Ferrand, D; Cibert, J; Tatarenko, S; Wasiela, A; Gaj, J A; Dietl, T

    2002-05-20

    A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd(0.96)Mn(0.04)Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible crossover between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.

  18. Single Ferromagnetic Nanocontact Based Devices as Magnetic Field Sensors

    DTIC Science & Technology

    2008-03-01

    the defined  nanohole  dimensions of  the mask for etching of the  nanoholes  and for electrodeposition of ferromagnetic  nanocontacts. The initial trials... nanoholes  produced  in 100 nm PMMA resist using does of 70, 140 and 250 μC. (D) Array of ~40 nm diameter Ni  nanodots electrodeposited into a  nanohole ...success  of  the  of  the  process  and  cleanness of  the  nanoholes   after  developer  is  applied  to  remove  the  exposed  resist. The  typical

  19. Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media

    NASA Astrophysics Data System (ADS)

    Oezelt, Harald; Kovacs, Alexander; Fischbacher, Johann; Matthes, Patrick; Kirk, Eugenie; Wohlhüter, Phillip; Heyderman, Laura Jane; Albrecht, Manfred; Schrefl, Thomas

    2016-09-01

    We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling 1.4 Tb/in2 bit patterned media. Both contributions are computed for different thicknesses of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd( 5 nm )/FePt( 5 nm ) shows a bit error rate of 10-4 with a write field of 1.16 T .

  20. Magnetic Field-Induced Superconductivity in the Ferromagnet URhGe

    NASA Astrophysics Data System (ADS)

    Lévy, F.; Sheikin, I.; Grenier, B.; Huxley, A. D.

    2005-08-01

    In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.

  1. Magnetic field-induced superconductivity in the ferromagnet URhGe.

    PubMed

    Lévy, F; Sheikin, I; Grenier, B; Huxley, A D

    2005-08-26

    In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.

  2. Magnetic memory signals variation induced by applied magnetic field and static tensile stress in ferromagnetic steel

    NASA Astrophysics Data System (ADS)

    Huang, Haihong; Yang, Cheng; Qian, Zhengchun; Han, Gang; Liu, Zhifeng

    2016-10-01

    Stress can induce a spontaneous magnetic field in ferromagnetic steel under the excitation of geomagnetic field. In order to investigate the impact of applied magnetic field and tensile stress on variation of the residual magnetic signals on the surface of ferromagnetic materials, static tensile tests of Q235 structural steel were carried out, with the normal component of the residual magnetic signals, Hp(y), induced by applied magnetic fields with different intensities measured through the tensile tests. The Hp(y), its slope coefficient KS and maximum gradient Kmax changing with the applied magnetic field H and tensile stress were observed. Results show that the magnitude of Hp(y) and its slope coefficient KS increase linearly with the increase of stress in the elastic deformation stage. Under yield stress, Hp(y) and KS reach its maximum, and then decrease slightly with further increase of stress. Applied magnetic field affects the magnitude of Hp(y) instead of changing the signal curve‧s profile; and the magnitude of Hp(y), KS, Kmax and the change rate of KS increase with the increase of applied magnetic field. The phenomenon is also discussed from the viewpoint of magnetic charge in ferromagnetic materials.

  3. Micromagnetic analysis of the magnetization dynamics driven by the Oersted field in permalloy nanorings

    NASA Astrophysics Data System (ADS)

    Martinez, Eduardo

    2012-04-01

    The magnetization dynamics in a thin ferromagnet ring driven by the Oersted field created by injecting a current along an infinite conductive wire is analyzed by means of micromagnetic simulations. Two different processes are studied: (i) the transition from an onion state to a vortex configuration when the conducting wire was placed in the center of the ring and (ii) the switching of the vortex circulation when the conducting wire is displaced from the center of the ring. The results are in good agreement with recent experimental observations, where the Oersted field was produced by an atomic force microscope tip, and the analysis provides a preliminary study for future designs of memory devices-based thin ferromagnetic rings.

  4. Skyrmion motion driven by oscillating magnetic field

    PubMed Central

    Moon, Kyoung-Woong; Kim, Duck-Ho; Je, Soong-Geun; Chun, Byong Sun; Kim, Wondong; Qiu, Z.Q.; Choe, Sug-Bong; Hwang, Chanyong

    2016-01-01

    The one-dimensional magnetic skyrmion motion induced by an electric current has attracted much interest because of its application potential in next-generation magnetic memory devices. Recently, the unidirectional motion of large (20 μm in diameter) magnetic bubbles with two-dimensional skyrmion topology, driven by an oscillating magnetic field, has also been demonstrated. For application in high-density memory devices, it is preferable to reduce the size of skyrmion. Here we show by numerical simulation that a skyrmion of a few tens of nanometres can also be driven by high-frequency field oscillations, but with a different direction of motion from the in-plane component of the tilted oscillating field. We found that a high-frequency field for small skyrmions can excite skyrmion resonant modes and that a combination of different modes results in a final skyrmion motion with a helical trajectory. Because this helical motion depends on the frequency of the field, we can control both the speed and the direction of the skyrmion motion, which is a distinguishable characteristic compared with other methods. PMID:26847334

  5. Fluctuation-Driven Magnetic Hard-Axis Ordering in Metallic Ferromagnets

    NASA Astrophysics Data System (ADS)

    Krüger, F.; Pedder, C. J.; Green, A. G.

    2014-10-01

    We demonstrate that the interplay between soft electronic particle-hole fluctuations and magnetic anisotropies can drive ferromagnetic moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic two-band model with local Coulomb and Hund's interactions and a spin-orbit-induced easy plane anisotropy. The phase diagram is calculated within the fermionic quantum order-by-disorder approach, which is based on a self-consistent free-energy expansion around a magnetically ordered state with unspecified orientation. Quantum fluctuations render the transition of the easy-plane ferromagnet first order below a tricritical point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy, and the moments flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering along the magnetic hard direction.

  6. Biaxial-stress-driven full spin polarization in ferromagnetic hexagonal chromium telluride

    NASA Astrophysics Data System (ADS)

    Xiao, Xiang-Bo; Li, Jun; Liu, Bang-Gui

    2017-03-01

    It is important to spintronics to achieve fully-spin-polarized magnetic materials that are stable and can be easily fabricated. Here, through systematical density-functional-theory investigations, we achieve high and even full spin polarization for carriers in the ground-state phase of CrTe by applying tensile biaxial stress. The resulting strain is tensile in the xy plane and compressive in the z axis. With the in-plane tensile strain increasing, the ferromagnetic order is stable against antiferromagnetic fluctuations, and a half-metallic ferromagnetism is achieved at an in-plane strain of 4.8%. With the spin-orbit coupling taken into account, the spin polarization is equivalent to 97% at the electronic transition point, and then becomes 100.0% at the in-plane strain of 6.0%. These make us believe that the full-spin-polarized ferromagnetism in this stable and easily-realizable hexagonal phase could be realized soon, and applied in spintronics.

  7. Observation of thermally driven field-like spin torque in magnetic tunnel junctions

    NASA Astrophysics Data System (ADS)

    Bose, Arnab; Shukla, Amit Kumar; Konishi, Katsunori; Jain, Sourabh; Asam, Nagarjuna; Bhuktare, Swapnil; Singh, Hanuman; Lam, Duc Duong; Fujii, Yuya; Miwa, Shinji; Suzuki, Yoshishige; Tulapurkar, Ashwin A.

    2016-07-01

    We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 109 A/m2 can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgO tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.

  8. Observation of thermally driven field-like spin torque in magnetic tunnel junctions

    SciTech Connect

    Bose, Arnab Jain, Sourabh; Asam, Nagarjuna; Bhuktare, Swapnil; Singh, Hanuman; Tulapurkar, Ashwin A.; Shukla, Amit Kumar; Konishi, Katsunori; Lam, Duc Duong; Fujii, Yuya; Miwa, Shinji; Suzuki, Yoshishige

    2016-07-18

    We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 10{sup 9 }A/m{sup 2} can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgO tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.

  9. Electric-field control of ferromagnetism in Mn-doped ZnO nanowires.

    PubMed

    Chang, Li-Te; Wang, Chiu-Yen; Tang, Jianshi; Nie, Tianxiao; Jiang, Wanjun; Chu, Chia-Pu; Arafin, Shamsul; He, Liang; Afsal, Manekkathodi; Chen, Lih-Juann; Wang, Kang L

    2014-01-01

    In this Letter, the electric-field control of ferromagnetism was demonstrated in a back-gated Mn-doped ZnO (Mn-ZnO) nanowire (NW) field-effect transistor (FET). The ZnO NWs were synthesized by a thermal evaporation method, and the Mn doping of 1 atom % was subsequently carried out in a MBE system using a gas-phase surface diffusion process. Detailed structural analysis confirmed the single crystallinity of Mn-ZnO NWs and excluded the presence of any precipitates or secondary phases. For the transistor, the field-effect mobility and n-type carrier concentration were estimated to be 0.65 cm(2)/V·s and 6.82 × 10(18) cm(-3), respectively. The magnetic hysteresis curves measured under different temperatures (T = 10-350 K) clearly demonstrate the presence of ferromagnetism above room temperature. It suggests that the effect of quantum confinements in NWs improves Tc, and meanwhile minimizes crystalline defects. The magnetoresistace (MR) of a single Mn-ZnO NW was observed up to 50 K. Most importantly, the gate modulation of the MR ratio was up to 2.5 % at 1.9 K, which implies the electric-field control of ferromagnetism in a single Mn-ZnO NW.

  10. Study of Ferromagnetic and Field Effect Properties of Zinc Oxide Thin Films

    NASA Astrophysics Data System (ADS)

    Xia, Daxue

    Spintronics (spin transport electronics), in which both spm and charge of carriers are utilized for information processing, is perceived to be a candidate to extend and possibly to become the next-generation electronics. Its advantages include nonvolatility (data retention without electrical power), lower energy consumption, faster processing speed, and higher integration densities in comparison with the current semiconductor devices relying solely on electron charge. To realize a spin-field effect transistor, two respects are prerequisite. On the one hand, the mechanism of ferromagnetism should be addressed before one could prepare magnetic semiconductor films in a controllable way. On the other hand, excellent field effect properties should be sought through a convenient and low-cost strategy for manufacturing future nano-scale spintronic devices. This thesis is comprised of two parts. Firstly, it deals with the synthesis, characterization, and magnetism of transition-metal-doped or un-doped zinc oxide (ZnO) thin films. Secondly, it focuses on the field effect properties of solution processable ZnO thin films, which are not only of great interest for current charge-carrier based thin film transistors, but also of fundamental importance in future spin-based transistors. A facile spin-coating technique has been developed to fabricate ZnO thin films. Even without magnetic element doping, the film is found to show room temperature ferromagnetism. A broad series of advanced microscopic and spectroscopic techniques are utilized to characterize the thin films properties. Oxygen vacancy defects are tentatively attributed to the observed ferromagnetism. Following the similar method, Ga doped or Ga, Co co-doped ZnO thin films are prepared. The ferromagnetism is enhanced with Ga doping, providing more carriers. It is discovered that room temperature ferromagnetism can exist in both highly conductive regime and the less conductive or near insulating regime. Transition metal

  11. 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, Ni80Fe20) 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

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

    SciTech Connect

    Jungfleisch, M. B.; Zhang, W.; Sklenar, J.; Jiang, W.; Pearson, J. E.; Ketterson, J. B.; Hoffmann, A.

    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 direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni80Fe20) 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.

  13. Effect of magnetoelastic film thickness on power absorption in acoustically driven ferromagnetic resonance

    NASA Astrophysics Data System (ADS)

    Labanowski, D.; Jung, A.; Salahuddin, S.

    2017-09-01

    Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric substrate are capable of exciting magnetoelastic ferromagnets into resonance. In this work, we explore the effects of magnetoelastic film thickness on the coupling of SAWs into such magnetic thin films. We find that power absorption as a function of film thickness begins to saturate above thicknesses of ≈20 nm. This is contrary to current models that predict an exponential increase of the absorption as a function of increasing film thickness. We show that the saturation happens due to an increase in the damping of the film beyond this thickness range.

  14. Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

    NASA Astrophysics Data System (ADS)

    Woo, Seonghoon; Litzius, Kai; Krüger, Benjamin; Im, Mi-Young; Caretta, Lucas; Richter, Kornel; Mann, Maxwell; Krone, Andrea; Reeve, Robert M.; Weigand, Markus; Agrawal, Parnika; Lemesh, Ivan; Mawass, Mohamad-Assaad; Fischer, Peter; Kläui, Mathias; Beach, Geoffrey S. D.

    2016-05-01

    Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s-1 as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.

  15. Control of current-induced spin-orbit effects in a ferromagnetic heterostructure by electric field

    NASA Astrophysics Data System (ADS)

    Liu, R. H.; Lim, W. L.; Urazhdin, S.

    2014-06-01

    We study the effects of electrostatic gating on the current-induced phenomena in ultrathin ferromagnet/heavy metal heterostructures. We utilize heterodyne detection and analysis of symmetry with respect to the direction of the magnetic field to separate electric field contributions to the magnetic anisotropy, current-induced fieldlike torque, and damping torque. Analysis of the electric field effects allows us to estimate the Rashba and the spin Hall contributions to the current-induced phenomena. Electrostatic gating can provide insight into the spin-orbit phenomena, and enable new functionalities in spintronic devices.

  16. Thickness dependence study of current-driven ferromagnetic resonance in Y3Fe5O12/heavy metal bilayers

    NASA Astrophysics Data System (ADS)

    Fang, Z.; Mitra, A.; Westerman, A. L.; Ali, M.; Ciccarelli, C.; Cespedes, O.; Hickey, B. J.; Ferguson, A. J.

    2017-02-01

    We use ferromagnetic resonance to study the current-induced torques in YIG/heavy metal bilayers. YIG samples with thickness varying from 14.8 nm to 80 nm, with the Pt or Ta thin film on top, are measured by applying a microwave current into the heavy metals and measuring the longitudinal DC voltage generated by both spin rectification and spin pumping. From a symmetry analysis of the FMR lineshape and its dependence on YIG thickness, we deduce that the Oersted field dominates over spin-transfer torque in driving magnetization dynamics.

  17. Thermalization of field driven quantum systems

    PubMed Central

    Fotso, H.; Mikelsons, K.; Freericks, J. K.

    2014-01-01

    There is much interest in how quantum systems thermalize after a sudden change, because unitary evolution should preclude thermalization. The eigenstate thermalization hypothesis resolves this because all observables for quantum states in a small energy window have essentially the same value; it is violated for integrable systems due to the infinite number of conserved quantities. Here, we show that when a system is driven by a DC electric field there are five generic behaviors: (i) monotonic or (ii) oscillatory approach to an infinite-temperature steady state; (iii) monotonic or (iv) oscillatory approach to a nonthermal steady state; or (v) evolution to an oscillatory state. Examining the Hubbard model (which thermalizes under a quench) and the Falicov-Kimball model (which does not), we find both exhibit scenarios (i–iv), while only Hubbard shows scenario (v). This shows richer behavior than in interaction quenches and integrability in the absence of a field plays no role. PMID:24736404

  18. Thermalization of field driven quantum systems

    NASA Astrophysics Data System (ADS)

    Fotso, H.; Mikelsons, K.; Freericks, J. K.

    2014-04-01

    There is much interest in how quantum systems thermalize after a sudden change, because unitary evolution should preclude thermalization. The eigenstate thermalization hypothesis resolves this because all observables for quantum states in a small energy window have essentially the same value; it is violated for integrable systems due to the infinite number of conserved quantities. Here, we show that when a system is driven by a DC electric field there are five generic behaviors: (i) monotonic or (ii) oscillatory approach to an infinite-temperature steady state; (iii) monotonic or (iv) oscillatory approach to a nonthermal steady state; or (v) evolution to an oscillatory state. Examining the Hubbard model (which thermalizes under a quench) and the Falicov-Kimball model (which does not), we find both exhibit scenarios (i-iv), while only Hubbard shows scenario (v). This shows richer behavior than in interaction quenches and integrability in the absence of a field plays no role.

  19. A Study of Applying Pulsed Remote Field Eddy Current in Ferromagnetic Pipes Testing.

    PubMed

    Luo, Qingwang; Shi, Yibing; Wang, Zhigang; Zhang, Wei; Li, Yanjun

    2017-05-05

    Pulsed Remote Field Eddy Current Testing (PRFECT) attracts the attention in the testing of ferromagnetic pipes because of its continuous spectrum. This paper simulated the practical PRFECT of pipes by using ANSYS software and employed Least Squares Support Vector Regression (LSSVR) to extract the zero-crossing time to analyze the pipe thickness. As a result, a secondary peak is found in zero-crossing time when transmitter passed by a defect. The secondary peak will lead to wrong quantification and the localization of defects, especially when defects are found only at the transmitter location. Aiming to eliminate the secondary peaks, double sensing coils are set in the transition zone and Wiener deconvolution filter is applied. In the proposed method, position dependent response of the differential signals from the double sensing coils is calibrated by employing zero-mean normalization. The methods proposed in this paper are validated by analyzing the simulation signals and can improve the practicality of PRFECT of ferromagnetic pipes.

  20. Field-induced ferromagnetic structure in Er(2)Ni(2)Pb.

    PubMed

    Prokeš, K; Mydosh, J A

    2009-05-27

    We have studied the effect of magnetic fields up to 4.5 T on the ground-state structure in Er(2)Ni(2)Pb using powder neutron diffraction measurements at low temperatures. The zero-field magnetic state that itself is not uniform and consists of different magnetic phases is rather unstable against the magnetic field. As the field is increased, the magnetic reflections of the zero-field structure disappear and a new magnetic phase with commensurate propagation vector is clearly observed in a field of 0.5 T. At higher fields a ferromagnetic state is established in Er(2)Ni(2)Pb, which can be fully described only by a model that combines at least two irreducible representations. The refined Er magnetic moment magnitude of 9.10 ± 0.07 µ(B) is very close to the Er(3+) free ion value of 9.0 µ(B).

  1. Current driven spin–orbit torque oscillator: ferromagnetic and antiferromagnetic coupling

    PubMed Central

    Johansen, Øyvind; Linder, Jacob

    2016-01-01

    We consider theoretically the impact of Rashba spin–orbit coupling on spin torque oscillators (STOs) in synthetic ferromagnets and antiferromagnets that have either a bulk multilayer or a thin film structure. The synthetic magnets consist of a fixed polarizing layer and two free magnetic layers that interact through the Ruderman-Kittel-Kasuya-Yosida interaction. We determine analytically which collinear states along the easy axis that are stable, and establish numerically the phase diagram for when the system is in the STO mode and when collinear configurations are stable, respectively. It is found that the Rashba spin–orbit coupling can induce anti-damping in the vicinity of the collinear states, which assists the spin transfer torque in generating self-sustained oscillations, and that it can substantially increase the STO part of the phase diagram. Moreover, we find that the STO phase can extend deep into the antiferromagnetic regime in the presence of spin–orbit torques. PMID:27653357

  2. Self-consistent magnetization dynamics of a ferromagnetic quantum dot driven by a spin bias

    NASA Astrophysics Data System (ADS)

    Siu, Z. B.; Jalil, M. B. A.; Tan, S. G.

    2012-04-01

    We present an iterative scheme which combines the non-equilibrium Green's function (NEGF) for evaluating the quantum spin transport in a ferromagnetic quantum dot device and the Landau-Lifshitz (LL) equation for modeling the magnetization dynamics of the dot. For a given initial magnetization, the spin polarization of current and the resulting spin torque in the dot are calculated using the NEGF formalism. The torque acts on the magnetic moment of the dot, and the resultant magnetization dynamics is obtained from the LL equation. The new value of the dot's magnetization is then used as an input for the next round of NEGF calculation, and the whole process is repeated iteratively. The spin torque is thus calculated self-consistently with the dynamics of the magnetic moment of the dot. We apply this self-consistent iterative scheme to study the magnetization dynamics in an exemplary quantum dot system with an induced spin bias in the leads under varying damping conditions.

  3. Electric-Field Modulation of Damping Constant in a Ferromagnetic Semiconductor (Ga,Mn)As.

    PubMed

    Chen, Lin; Matsukura, Fumihiro; Ohno, Hideo

    2015-07-31

    The modulation of the Gilbert damping constant α in (Ga,Mn)As by the application of an electric field is detected by ferromagnetic resonance measurements, where α increases with decreasing hole concentration. The smaller modulation of other magnetic parameters, such as magnetic anisotropy fields and Landé g factor, suggests that the modulation of α is governed by other effects rather than the spin-orbit coupling. Comparison of the conductivity dependence of α with that of the magnetization indicates that the magnetic disorder induced by carrier localization plays a major role in determining the magnitude of α in (Ga,Mn)As.

  4. Observation of Thermoelectric Currents in High-Field Superconductor-Ferromagnet Tunnel Junctions

    NASA Astrophysics Data System (ADS)

    Kolenda, S.; Wolf, M. J.; Beckmann, D.

    2016-03-01

    We report on the experimental observation of spin-dependent thermoelectric currents in superconductor-ferromagnet tunnel junctions in high magnetic fields. The thermoelectric signals are due to a spin-dependent lifting of the particle-hole symmetry, and are found to be in excellent agreement with recent theoretical predictions. The maximum Seebeck coefficient inferred from the data is about -100 μ V /K , much larger than commonly found in metallic structures. Our results directly prove the coupling of spin and heat transport in high-field superconductors.

  5. Anisotropy and pressure dependence of the upper critical field of the ferromagnetic superconductor UGe2

    NASA Astrophysics Data System (ADS)

    Sheikin, I.; Huxley, A.; Braithwaite, D.; Brison, J. P.; Watanabe, S.; Miyake, K.; Flouquet, J.

    2001-12-01

    The temperature and pressure dependence of the upper critical field, Hc2, of the ferromagnetic superconductor UGe2 is reported for fields applied along all three crystallographic axes. For fields parallel to the easy magnetic a axis, the relationship between an unusual reentrant behavior of Hc2 and a field-induced transition associated with a change in the electronic density of states is reviewed. For transverse field directions a significant evolution in the behavior of Hc2 with pressure is found. As the pressure is decreased the dependence of Hc2 on temperature for fields along the crystal's c axis acquires a positive curvature that extends from the critical temperature, Tc, down to almost the lowest temperature measured (Tc/10) where Hc2 exceeds the usual weak coupling paramagnetic and orbital limits.

  6. Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field.

    PubMed

    Ben Dor, Oren; Yochelis, Shira; Radko, Anna; Vankayala, Kiran; Capua, Eyal; Capua, Amir; Yang, See-Hun; Baczewski, Lech Tomasz; Parkin, Stuart Stephen Papworth; Naaman, Ron; Paltiel, Yossi

    2017-02-23

    Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 10(6) A·cm(-2), or about 1 × 10(25) electrons s(-1) cm(-2). This relatively high current density significantly affects the devices' structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 10(13) electrons per cm(2) are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magnetic anisotropy. These results present a simple low-power magnetization mechanism when operating at ambient conditions.

  7. Reversible switching of ferromagnetism in ZnCuO nanorods by electric field

    SciTech Connect

    Zou, Changwei Wang, Hongjun; Liang, Feng; Shao, Lexi

    2015-04-06

    The reproducible switching of ferromagnetism in ZnCuO nanorods by applying a reversible electric field has been realized. High-resolution transmission electron microscopy images showed a hexagonal wurtzite structure with no detectable trace of secondary phase or precipitation of Cu impurity in the ZnCuO nanorods. The Cu concentrations in the ZnCuO nanorods were tested by energy dispersive spectroscopy and x-ray photoelectron spectroscopy and found to be about 2.7 at. %. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies (V{sub O}) localized mainly on surface of the ZnCuO nanorods. Subsequently, the variation of V{sub O} concentration during the resistive switching process modulates the ferromagnetism of the ZnCuO nanorods. The saturation magnetization at low resistance state is apparently 6.4 times larger than that at high resistance state for an Au/ZnCuO/ITO structure. An indirect double-exchange model has been used to explain the ferromagnetism in ZnCuO nanorods.

  8. Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field

    PubMed Central

    Ben Dor, Oren; Yochelis, Shira; Radko, Anna; Vankayala, Kiran; Capua, Eyal; Capua, Amir; Yang, See-Hun; Baczewski, Lech Tomasz; Parkin, Stuart Stephen Papworth; Naaman, Ron; Paltiel, Yossi

    2017-01-01

    Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 106 A·cm−2, or about 1 × 1025 electrons s−1 cm−2. This relatively high current density significantly affects the devices' structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 1013 electrons per cm2 are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magnetic anisotropy. These results present a simple low-power magnetization mechanism when operating at ambient conditions. PMID:28230054

  9. Reversible switching of ferromagnetism in ZnCuO nanorods by electric field

    NASA Astrophysics Data System (ADS)

    Zou, Changwei; Wang, Hongjun; Liang, Feng; Shao, Lexi

    2015-04-01

    The reproducible switching of ferromagnetism in ZnCuO nanorods by applying a reversible electric field has been realized. High-resolution transmission electron microscopy images showed a hexagonal wurtzite structure with no detectable trace of secondary phase or precipitation of Cu impurity in the ZnCuO nanorods. The Cu concentrations in the ZnCuO nanorods were tested by energy dispersive spectroscopy and x-ray photoelectron spectroscopy and found to be about 2.7 at. %. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies (VO) localized mainly on surface of the ZnCuO nanorods. Subsequently, the variation of VO concentration during the resistive switching process modulates the ferromagnetism of the ZnCuO nanorods. The saturation magnetization at low resistance state is apparently 6.4 times larger than that at high resistance state for an Au/ZnCuO/ITO structure. An indirect double-exchange model has been used to explain the ferromagnetism in ZnCuO nanorods.

  10. Tailoring magnetostriction sign of ferromagnetic composite by increasing magnetic field strength

    NASA Astrophysics Data System (ADS)

    Gou, Junming; Liu, Xiaolian; Wu, Kaiyun; Wang, Yue; Hu, Shanshan; Zhao, Hui; Xiao, Andong; Ma, Tianyu; Yan, Mi

    2016-08-01

    The unitary deformation of single-phase ferromagnets by a magnetic field, i.e., either positive or negative linear magnetostriction, allows only monotonous control. Here we report a proof-of-principle ferromagnetic composite Fe73Ga27, for which the magnetostriction sign changes from positive to negative by increasing the magnetic field strength. Based on the transformation from body-centered-cubic (BCC) to face-centered-cubic (FCC) phases in this binary system, Fe73Ga27 composite is prepared by aging the BCC averaged precursor for 3 days at 803 K. Magnetic measurements indicate that the BCC phase exhibits smaller magnetocrystalline anisotropy constant than the FCC phase. The offset effect between BCC and FCC phases produces positive net magnetostriction at low magnetic fields but negative net magnetostriction at high magnetic fields. By tuning the field strength, such composites can mediate compressive and tensile strains to other functional materials, e.g., piezoelectric material or optic-fibers, which is beneficial to design multifunctional devices.

  11. Field-driven dynamics of nematic microcapillaries

    NASA Astrophysics Data System (ADS)

    Khayyatzadeh, Pouya; Fu, Fred; Abukhdeir, Nasser Mohieddin

    2015-12-01

    Polymer-dispersed liquid-crystal (PDLC) composites long have been a focus of study for their unique electro-optical properties which have resulted in various applications such as switchable (transparent or translucent) windows. These composites are manufactured using desirable "bottom-up" techniques, such as phase separation of a liquid-crystal-polymer mixture, which enable production of PDLC films at very large scales. LC domains within PDLCs are typically spheroidal, as opposed to rectangular for an LCD panel, and thus exhibit substantially different behavior in the presence of an external field. The fundamental difference between spheroidal and rectangular nematic domains is that the former results in the presence of nanoscale orientational defects in LC order while the latter does not. Progress in the development and optimization of PDLC electro-optical properties has progressed at a relatively slow pace due to this increased complexity. In this work, continuum simulations are performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. Using a simplified elliptic cylinder (microcapillary) geometry as an approximation of spheroidal PDLC domains, the effects of geometry (aspect ratio), surface anchoring, and external field strength are studied through the use of the Landau-de Gennes model of the nematic LC phase.

  12. Enhancing electric-field control of ferromagnetism through nanoscale engineering of high-Tc MnxGe1-x nanomesh

    NASA Astrophysics Data System (ADS)

    Nie, Tianxiao; Tang, Jianshi; Kou, Xufeng; Gen, Yin; Lee, Shengwei; Zhu, Xiaodan; He, Qinglin; Chang, Li-Te; Murata, Koichi; Fan, Yabin; Wang, Kang L.

    2016-10-01

    Voltage control of magnetism in ferromagnetic semiconductor has emerged as an appealing solution to significantly reduce the power dissipation and variability beyond current CMOS technology. However, it has been proven to be very challenging to achieve a candidate with high Curie temperature (Tc), controllable ferromagnetism and easy integration with current Si technology. Here we report the effective electric-field control of both ferromagnetism and magnetoresistance in unique MnxGe1-x nanomeshes fabricated by nanosphere lithography, in which a Tc above 400 K is demonstrated as a result of size/quantum confinement. Furthermore, by adjusting Mn doping concentration, extremely giant magnetoresistance is realized from ~8,000% at 30 K to 75% at 300 K at 4 T, which arises from a geometrically enhanced magnetoresistance effect of the unique mesh structure. Our results may provide a paradigm for fundamentally understanding the high Tc in ferromagnetic semiconductor nanostructure and realizing electric-field control of magnetoresistance for future spintronic applications.

  13. High-field moment polarization in the ferromagnetic superconductor UCoGe

    NASA Astrophysics Data System (ADS)

    Knafo, W.; Matsuda, T. D.; Aoki, D.; Hardy, F.; Scheerer, G. W.; Ballon, G.; Nardone, M.; Zitouni, A.; Meingast, C.; Flouquet, J.

    2012-11-01

    We report magnetization and magnetoresistivity measurements on the isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields up to 60 T and temperatures from 1.5 to 80 K. At low temperature, a moment polarization in UCoGe in a field μ0H∥b of around 50 T leads to well-defined anomalies in both magnetization and magnetoresistivity. These anomalies vanish in temperatures higher than 30-40 K, where maxima in the magnetic susceptibility and the field-induced variation of the magnetoresistivity are found. A comparison is made between UCoGe and URhGe, where a moment reorientation in a magnetic field μ0H∥b of 12 T leads to field-induced reentrant superconductivity.

  14. Separation of two attractive ferromagnetic ellipsoidal particles by hydrodynamic interactions under alternating magnetic field

    NASA Astrophysics Data System (ADS)

    Abbas, Micheline; Bossis, Georges

    2017-06-01

    In applications where magnetic particles are used to detect and dose targeted molecules, it is of major importance to prevent particle clustering and aggregation during the capture stage in order to maximize the capture rate. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection by optical measurement of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a range of low frequencies fc 1field to particle magnetization strength, whereas fc 1 tends to zero when this ratio increases.

  15. A constraint-free phase field model for ferromagnetic domain evolution

    PubMed Central

    Yi, Min; Xu, Bai-Xiang

    2014-01-01

    A continuum constraint-free phase field model is proposed to simulate the magnetic domain evolution in ferromagnetic materials. The model takes the polar and azimuthal angles (ϑ1,ϑ2), instead of the magnetization unit vector m(m1,m2,m3), as the order parameters. In this way, the constraint on the magnetization magnitude can be exactly satisfied automatically, and no special numerical treatment on the phase field evolution is needed. The phase field model is developed from a thermodynamic framework which involves a configurational force system for ϑ1 and ϑ2. A combination of the configurational force balance and the second law of thermodynamics leads to thermodynamically consistent constitutive relations and a generalized evolution equation for the order parameters (ϑ1,ϑ2). Beneficial from the constraint-free model, the three-dimensional finite-element implementation is straightforward, and the degrees of freedom are reduced by one. The model is shown to be capable of reproducing the damping-dependent switching dynamics, and the formation and evolution of domains and vortices in ferromagnetic materials under the external magnetic or mechanical loading. Particularly, the calculated out-of-plane component of magnetization in a vortex is verified by the corresponding experimental results, as well as the motion of the vortex under a magnetic field. PMID:25383036

  16. A constraint-free phase field model for ferromagnetic domain evolution.

    PubMed

    Yi, Min; Xu, Bai-Xiang

    2014-11-08

    A continuum constraint-free phase field model is proposed to simulate the magnetic domain evolution in ferromagnetic materials. The model takes the polar and azimuthal angles (ϑ1,ϑ2), instead of the magnetization unit vector m(m1,m2,m3), as the order parameters. In this way, the constraint on the magnetization magnitude can be exactly satisfied automatically, and no special numerical treatment on the phase field evolution is needed. The phase field model is developed from a thermodynamic framework which involves a configurational force system for ϑ1 and ϑ2. A combination of the configurational force balance and the second law of thermodynamics leads to thermodynamically consistent constitutive relations and a generalized evolution equation for the order parameters (ϑ1,ϑ2). Beneficial from the constraint-free model, the three-dimensional finite-element implementation is straightforward, and the degrees of freedom are reduced by one. The model is shown to be capable of reproducing the damping-dependent switching dynamics, and the formation and evolution of domains and vortices in ferromagnetic materials under the external magnetic or mechanical loading. Particularly, the calculated out-of-plane component of magnetization in a vortex is verified by the corresponding experimental results, as well as the motion of the vortex under a magnetic field.

  17. Ferromagnetic microswimmer

    NASA Astrophysics Data System (ADS)

    Belovs, M.; Cěbers, A.

    2009-05-01

    The self-propelling motion of the flexible ferromagnetic swimmer is described. Necessary symmetry breaking is achieved by the buckling instability at field inversion. The characteristics of self-propulsion are in good agreement with the numerical calculations of the Floquet multipliers for the ferromagnetic filament under the action of ac magnetic field. In the low frequency range the power stroke of self-propelling motion is similar to that used by the unicellular green algae chlamydomonas and in the high frequency region the self-propulsion is due to the undulation waves propagating from the free ends perpendicularly to ac magnetic field.

  18. Ferromagnetic microswimmer.

    PubMed

    Belovs, M; Cēbers, A

    2009-05-01

    The self-propelling motion of the flexible ferromagnetic swimmer is described. Necessary symmetry breaking is achieved by the buckling instability at field inversion. The characteristics of self-propulsion are in good agreement with the numerical calculations of the Floquet multipliers for the ferromagnetic filament under the action of ac magnetic field. In the low frequency range the power stroke of self-propelling motion is similar to that used by the unicellular green algae chlamydomonas and in the high frequency region the self-propulsion is due to the undulation waves propagating from the free ends perpendicularly to ac magnetic field.

  19. Investigation of charge injection and transport behavior in multilayer structure consisted of ferromagnetic metal and organic polymer under external fields

    NASA Astrophysics Data System (ADS)

    Zhao, Hua; Meng, Wei-Feng

    2017-10-01

    In this paper a five layer organic electronic device with alternately placed ferromagnetic metals and organic polymers: ferromagnetic metal/organic layer/ferromagnetic metal/organic layer/ferromagnetic metal, which is injected a spin-polarized electron from outsides, is studied theoretically using one-dimensional tight binding model Hamiltonian. We calculated equilibrium state behavior after an electron with spin is injected into the organic layer of this structure, charge density distribution and spin polarization density distribution of this injected spin-polarized electron, and mainly studied possible transport behavior of the injected spin polarized electron in this multilayer structure under different external electric fields. We analyze the physical process of the injected electron in this multilayer system. It is found by our calculation that the injected spin polarized electron exists as an electron-polaron state with spin polarization in the organic layer and it can pass through the middle ferromagnetic layer from the right-hand organic layer to the left-hand organic layer by the action of increasing external electric fields, which indicates that this structure may be used as a possible spin-polarized charge electronic device and also may provide a theoretical base for the organic electronic devices and it is also found that in the boundaries between the ferromagnetic layer and the organic layer there exist induced interface local dipoles due to the external electric fields.

  20. Magnetic-field-dependent small-angle neutron scattering on random anisotropy ferromagnets

    NASA Astrophysics Data System (ADS)

    Michels, Andreas; Weissmüller, Jörg

    2008-06-01

    We report on the recently developed technique of magnetic-field-dependent small-angle neutron scattering (SANS), with attention to bulk ferromagnets exhibiting random magnetic anisotropy. In these materials, the various magnetic anisotropy fields (magnetocrystalline, magnetoelastic, and/or magnetostatic in origin) perturb the perfectly parallel spin alignment of the idealized ferromagnetic state. By varying the applied magnetic field, one can control one of the ordering terms which competes with the above-mentioned perturbing fields. Experiments which explore the ensuing reaction of the magnetization will therefore provide information not only on the field-dependent spin structure but, importantly, on the underlying magnetic interaction terms. This strategy, which underlies conventional studies of hysteresis loops in magnetometry, is here combined with magnetic SANS. While magnetometry generally records only a single scalar quantity, the integral magnetization, SANS provides access to a vastly richer data set, the Fourier spectrum of the response of the spin system as a function of the magnitude and orientation of the wave vector. The required data-analysis procedures have recently been established, and experiments on a number of magnetic materials, mostly nanocrystalline or nanocomposite metals, have been reported. Here, we summarize the theory of magnetic-field-dependent SANS along with the underlying description of random anisotropy magnets by micromagnetic theory. We review experiments which have explored the magnetic interaction parameters, the value of the exchange-stiffness constant as well as the Fourier components of the magnetic anisotropy field and of the magnetostatic stray field. A model-independent approach, based on the experimental autocorrelation function of the spin misalignment, provides access to the characteristic length of the spin misalignment. The field dependence of this quantity is in quantitative agreement with the predictions of

  1. Domain wall pinning controlled by the magnetic field of four nanoparticles in a ferromagnetic nanowire

    NASA Astrophysics Data System (ADS)

    Ermolaeva, O. L.; Skorokhodov, E. V.; Mironov, V. L.

    2016-11-01

    This paper presents the results of theoretical and experimental investigations of the domain wall pinning in a planar ferromagnetic system consisting of a nanowire and four rectangular uniformly magnetized nanoparticles located at an angle to the nanowire axis. Based on the calculations of the interaction energy of the domain wall with stray fields of nanoparticles and the micromagnetic simulation, it has been demonstrated that, in this system, there are different variants of the domain wall pinning, which are determined by the relative orientation of the magnetic moments of nanoparticles and the magnetization of the nanowire. The possibility of the creation of magnetic logic cells based on the structures under consideration has been discussed.

  2. Field effect controlled ferromagnetism in transition metal doped ZnO

    NASA Astrophysics Data System (ADS)

    Bellingeri, E.; Pellegrino, L.; Biasotti, M.; Pallecchi, I.; Canu, G.; Gerbi, A.; Vignolo, M.; Siri, A. S.; Marré, D.; Rusponi, S.; Lehnert, A.; Nolting, F.

    2008-02-01

    The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their potential application in spintronics. In this work, we focused on transition metal (TM) (Co, Mn and Cu) doped Zinc oxide (ZnO) because room temperature ferromagnetism was both theoretically predicted and experimentally observed. However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or clustering is still under debate. Measuring the dependence of the magnetic properties on the carrier concentration can clarify the underlying physics. The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density by electric field. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO 3) heterostructures by pulsed laser deposition. These devices offers the capability to modulate the carrier density of a probe accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ~10 15 to ~10 20 e -/cm 3, estimated by Hall effect measurements under FE). The Co and Mn films measured by DC SQUID magnetometer result ferromagnetic and anomalous Hall effect was observed at low temperature but nor ferromagnetic nor antiferromagnetic signal was detectable in the XMD spectra. Cu doped films are insulating and nonmagnetic. Photo Emission Electron Microscopy (x-PEEM) and magnetic force microscopy (MFM) showed that the sample are homogeneus and no clustering of TM were detected

  3. Magnetic-field-induced quantum criticality in a planar ferromagnet with single-ion anisotropy

    NASA Astrophysics Data System (ADS)

    Mercaldo, M. T.; Rabuffo, I.; De Cesare, L.; Caramico D'Auria, A.

    2014-08-01

    We analyze the effects induced by single-ion anisotropy on quantum criticality in a d-dimensional spin-3/2 planar ferromagnet. To tackle this problem we employ the two-time Green's function method, using the Tyablikov decoupling for exchange interactions and the Anderson-Callen decoupling for single-ion anisotropy. In our analysis the role of non-thermal control parameter which drives the quantum phase transition is played by a longitudinal external magnetic field. We find that the single-ion anisotropy has substantial effects on the structure of the phase diagram close to the quantum critical point.

  4. High-field magnetization measurements on a ferromagnetic amorphous alloy from 295 to 5K

    SciTech Connect

    Szymczak, P. ); Graham, C.D. Jr. ); Gibbs, M.R.J. )

    1994-11-01

    Magnetization measurements on an amorphous ferromagnetic alloy Fe[sub 78](SiB)[sub 22] have been made over the temperature range from 5 to 295K and in fields to 5T, using a SQUID magnetometer and a superconducting magnet. As-received and field-annealed samples were measured. Having data over a range of temperatures allows the spin-wave contribution to the magnetization to be determined, and then subtracted. When the spin-wave contribution is removed, a substantial high-field susceptibility remains, which is independent of temperature. Attempts to fit the corrected curves to one of two theoretical equations were not conclusive, but the best fit seems to be to M = M[sub 0] + aH[sup [minus]0.5] + bH. The annealing treatment has no significant effect on the high-field magnetization.

  5. Shell-ferromagnetism of nano-Heuslers generated by segregation under magnetic field

    PubMed Central

    Çakır, A.; Acet, M.; Farle, M.

    2016-01-01

    We report on a new functional property in an AF martensitic Heusler Ni50Mn45In5, which when annealed at high temperatures under a magnetic field, segregates and forms Ni50Mn25In25 Heusler precipitates embedded in a Ni50Mn50 matrix. The precipitates are paramagnetic whereas the matrix is antiferromagnetic. However, the spins at the interface with the Ni50Mn50 matrix align with the field during their nucleation and growth and become strongly pinned in the direction of the applied field during annealing, whereas the core spins become paramagnetic. This shell-ferromagnetism persists up to 600 K and is so strongly pinned that the remanent magnetization remains unchanged, even when the field is reversed or when the temperature is cycled between low temperatures and close to the annealing temperature. PMID:27412644

  6. Hydrogel Actuation by Electric Field Driven Effects

    NASA Astrophysics Data System (ADS)

    Morales, Daniel Humphrey

    Hydrogels are networks of crosslinked, hydrophilic polymers capable of absorbing and releasing large amounts of water while maintaining their structural integrity. Polyelectrolyte hydrogels are a subset of hydrogels that contain ionizable moieties, which render the network sensitive to the pH and the ionic strength of the media and provide mobile counterions, which impart conductivity. These networks are part of a class of "smart" material systems that can sense and adjust their shape in response to the external environment. Hence, the ability to program and modulate hydrogel shape change has great potential for novel biomaterial and soft robotics applications. We utilized electric field driven effects to manipulate the interaction of ions within polyelectrolyte hydrogels in order to induce controlled deformation and patterning. Additionally, electric fields can be used to promote the interactions of separate gel networks, as modular components, and particle assemblies within gel networks to develop new types of soft composite systems. First, we present and analyze a walking gel actuator comprised of cationic and anionic gel legs attached by electric field-promoted polyion complexation. We characterize the electro-osmotic response of the hydrogels as a function of charge density and external salt concentration. The gel walkers achieve unidirectional motion on flat elastomer substrates and exemplify a simple way to move and manipulate soft matter devices in aqueous solutions. An 'ionoprinting' technique is presented with the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions induced by electric fields. The bound charges change the local mechanical properties of the gel to induce relief patterns and evoke localized stress, causing rapid folding in air. The ionically patterned hydrogels exhibit programmable temporal and spatial shape transitions which can be tuned by the duration and/or strength of

  7. Influence of constant and ac electric fields on ferromagnetic resonance in magnetoelectric composites

    NASA Astrophysics Data System (ADS)

    Tatarenko, A. S.; Bichurin, M. I.; Petrov, V. M.; Fillipov, D. A.; Srinivasan, G.

    2004-03-01

    A composite of ferromagnetic and ferroelectric phases is expected to show magnetoelectric coupling that is mediated by mechanical deformation. For such composites, we proposed a model to treat the magnetoelectric (ME) coupling at frequencies corresponding to ferromagnetic resonance (FMR) [1,2]. The effect manifests as a shift in the resonance field when subjected to a constant electric field. Here we discuss a theory for the influence of both dc and high frequency electric fields on FMR in the composites. The model predicts a significant increase in the strength of ME coupling when the electric field is tuned to the electromechanical resonance (EMR) frequency. We assume the composite to be a homogeneous medium. By solving combined elastostatics, electrostatics and magnetostatics equations, we estimate the ME constants using effective parameters. The calculations are for 3-0, 0-3 and 2-2 connectivities. Expressions for ME coefficients are obtained as a function of interface coupling and the volume fraction for the piezoelectric phase. Under the influence of a constant electric field E, our model predicts a shift in the ferromagnetic resonance field that is proportional to ME constants. In the presence of an ac electric field, we estimate a strong ME coupling when the frequency is tuned to EMR. As an example, the FMR field shift at 9.3 GHz due an ac electrical field tuned to EMR at 350 kHz is determined for multilayer and bulk composites of nickel ferrite - lead zirconate titanate. It is shown that ME interactions are enhanced by several orders of magnitude compared to off resonance values. 1. M.I. Bichurin, I. A. Kornev, V. M. Petrov, A. S. Tatarenko, Yu. V. Kiliba, and G. Srinivasan. Phys. Rev. B 64, 094409 (2001). 2. M.I. Bichurin, V. M. Petrov, Yu. V. Kiliba, and G. Srinivasan. Phys. Rev. B 66, 134404 (2002). - supported by grants from the Russian Ministry of Education (Å02-3.4-278), the Universities of Russia Foundation (UNR 01.01.007), and the National Science

  8. Electric field driven switching of individual magnetic skyrmions (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Hsu, Pin-Jui

    2016-10-01

    An interesting class of interface-driven non-collinear spin structures, i.e., chiral domain walls, cycloidal spin spirals and Néel-type skyrmions, have been observed in ultrathin transition metal films grown on heavy-element substrates making use of spin-polarized scanning tunneling microscopy (SP-STM) [1]. Due to a lack of structural inversion symmetry at interfaces, they exhibit a unique rotational sense as a consequence of interfacial Dzyaloshinskii-Moriya (DM) interactions. In this talk, I will present our results based on the investigations of such chiral spin textures under the influence of strain relief and the effect of local electric fields. While a nanoskyrmion lattice was revealed for Fe monolayers (ML) grown on Ir(111), a cycloidal spin spiral ground state has been resolved on Fe double-layers (DL) by employing SP-STM with vectorial magnetic field. As a result of a large lattice mismatch between the epitaxially grown Fe-DL film and the underlying Ir(111) substrate, local uniaxial strain relief occurs, leading to dislocation line patterns. Interestingly, the wavevector of spin spirals is strictly guided along the dislocation lines, while the spin spiral's wavefront exhibits a zigzag deformation [2]. By further increasing the Fe coverage to triple-layers (TL), the zigzag spin spiral remains the magnetic ground state, but with an enhanced periodicity as compared to that of Fe-DL. A magnetic phase transition from the spin spiral to a skyrmionic state, and finally to a saturated ferromagnetic state occurs for Fe-TL by applying an external magnetic field. STM-induced writing and deleting of individual skyrmions is demonstrated with a pronounced bias-polarity dependence, suggesting the decisive role of the local electric field between STM tip and Fe film for the switching mechanism [3]. [1] K. von Bergmann, A. Kubetzka, O. Pietzsch, and R. Wiesendanger, J. Phys.: Condens. Matter 26, 394002 (2014) [2] P.-J. Hsu, A. Finco, L. Schmidt, A. Kubetzka, K. von

  9. Effect of tilted magnetic fields on bistable nanomagnets in hybrid semiconductor/ferromagnet devices

    NASA Astrophysics Data System (ADS)

    Meier, G.; Grundler, D.; Broocks, K.-B.; Heyn, Ch; Heitmann, D.

    2000-02-01

    Using the anisotropic Hall effect in a sub-micron lithographically well-defined two-dimensional electron system we measure the stray field of individual ferromagnetic nanoparticles in tilted magnetic fields. Our model calculations and experimental data show that one can map out the particle's hysteresis loop in great detail even if the field Happ is tilted away from the specimen's easy-axis by an angle φapp around 90°. The investigated Ni nanomagnets exhibit a well-defined remanent `up'- and `down'-state. For the angular-dependent switching we find two different regimes: below a critical angle φc, the hysteresis loop is irreversible and squared, for φc< φapp⩽90° it becomes partly reversible, but discontinuous jumps are still resolved. This characteristic switching behavior is found to depend on the nanomagnet's diameter.

  10. Training effects induced by cycling of magnetic field in ferromagnetic rich phase-separated nanocomposite manganites

    NASA Astrophysics Data System (ADS)

    Das, Kalipada; Das, I.

    2015-12-01

    We have carried out an experimental investigation of magneto-transport and magnetic properties of charge-ordered Pr0.67Ca0.33MnO3 (PCMO) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles along with a nanocomposite consisting of those two types of nanoparticles. From the magneto-transport measurements, clear irreversibility is observed in the field dependence of resistance due to magnetic field cycling in the case of PCMO nanoparticles. The value of resistance increases during such a field cycling. However such an irreversibility is absent in the case of LSMO nanoparticles as well as nanocomposites. On the other hand, the magnetic measurements indicate the gradual growth of antiferromagnetic phases in all samples leading to a decrease in magnetization. These inconsistencies between magneto-transport and magnetic behaviors are attributed to the magnetic training effects.

  11. Anomalous Magnetic Field Dependence of Charge Carrier Density in Ferromagnetic Semiconductors

    NASA Astrophysics Data System (ADS)

    Kuivalainen, P.; Sinkkonen, J.; Stubb, T.

    1980-01-01

    This paper reports calculations of temperature and magnetic field dependent thermal and optical activation energies of a shallow donor state and the energy of the conduction band edge in a ferromagnetic semiconductor. The formation of the bound magnetic polaron (BMP), i.e., a magnetically polarized cluster associated with the donor electron, is taken into account. The solution of a set of coupled equations for the energy of a donor electron and for the local non-uniform magnetization around the donor center indicates that the activation energies have their maxima near the Curie temperature and decrease with the application of a magnetic field. This decrease leads to a strong magnetic field dependence of the charge carrier density nc explains well the giant negative magnetoresistance of EuSe observed experimentally at low temperatures.

  12. On Cluster Properties of Classical Ferromagnets in an External Magnetic Field

    NASA Astrophysics Data System (ADS)

    Fröhlich, Jürg; Rodríguez, Pierre-François

    2017-02-01

    Correlation functions of ferromagnetic spin systems satisfying a Lee-Yang property are studied. It is shown that, for classical systems in a non-vanishing uniform external magnetic field h, the connected correlation functions decay exponentially in the distances between the spins, i.e., the inverse correlation length ("mass gap"), m( h), is strictly positive. Our proof is very short and transparent and is valid for complex values of the external magnetic field h, provided that Re h not = 0. It implies a mean-field lower bound on m( h), as h searrow 0, first established by Lebowitz and Penrose for the Ising model. Our arguments also apply to some quantum spin systems.

  13. Singlet-to-triplet interconversion using hyperfine as well as ferromagnetic fringe fields

    PubMed Central

    Wohlgenannt, M.; Flatté, M. E.; Harmon, N. J.; Wang, F.; Kent, A. D.; Macià, F.

    2015-01-01

    Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several per cent and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity/electroluminescence values in the absence of an applied field. We briefly review a model based on fringe-field-induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance and magnetoelectroluminescence. PMID:25987575

  14. Spin-to-charge conversion in lateral and vertical topological-insulator/ferromagnet heterostructures with microwave-driven precessing magnetization

    NASA Astrophysics Data System (ADS)

    Mahfouzi, Farzad; Nagaosa, Naoto; Nikolić, Branislav K.

    2014-09-01

    Using the charge-conserving Floquet-Green function approach to open quantum systems driven by an external time-periodic potential, we analyze how spin current pumped by the precessing magnetization of a ferromagnetic (F) layer is injected laterally into the interface with strong spin-orbit coupling (SOC) and converted into charge current flowing in the same direction. In the case of a metallic interface with the Rashba SOC used in recent experiments [J. C. R. Sánchez, L. Vila, G. Desfonds, S. Gambarelli, J. P. Attané, J. M. De Teresa, C. Magén, and A. Fert, Nat. Commun. 4, 2944 (2013), 10.1038/ncomms3944], both spin ISα and charge I current flow within the interface where I /ISα≃ 2-8% (depending on the precession cone angle), while for a F/topological-insulator (F/TI) interface employed in related experiments [Y. Shiomi, K. Nomura, Y. Kajiwara, K. Eto, M. Novak, K. Segawa, Y. Ando, and E. Saitoh, arXiv:1312.7091] the conversion efficiency is greatly enhanced (I /ISα≃ 40-60%) due to perfect spin-momentum locking on the surface of a TI. The spin-to-charge conversion occurs also when spin current is pumped vertically through the F/TI interface with smaller efficiency (I /ISα˜0.001%), but with the charge current signal being sensitive to whether the Dirac fermions at the interface are massive or massless.

  15. Induced magnetization and power loss for a periodically driven system of ferromagnetic nanoparticles with randomly oriented easy axes

    NASA Astrophysics Data System (ADS)

    Denisov, S. I.; Lyutyy, T. V.; Pedchenko, B. O.; Hryshko, O. M.

    2016-07-01

    We study the effect of an elliptically polarized magnetic field on a system of noninteracting, single-domain ferromagnetic nanoparticles characterized by a uniform distribution of easy axis directions. Our main goal is to determine the average magnetization of this system and the power loss in it. In order to calculate these quantities analytically, we develop a general perturbation theory for the Landau-Lifshitz-Gilbert (LLG) equation and find its steady-state solution for small magnetic field amplitudes. On this basis, we derive the second-order expressions for the average magnetization and power loss, investigate their dependence on the magnetic field frequency, and analyze the role of subharmonic resonances resulting from the nonlinear nature of the LLG equation. For arbitrary amplitudes, the frequency dependence of these quantities is obtained from the numerical solution of this equation. The impact of transitions between different regimes of regular and chaotic dynamics of magnetization, which can be induced in nanoparticles by changing the magnetic field frequency, is examined in detail.

  16. Proximity-driven enhanced magnetic order at ferromagnetic-insulator-magnetic-topological-insulator interface

    SciTech Connect

    Li, Mingda; Zhu, Yimei; Chang, Cui -Zu; Kirby, B. J.; Jamer, Michelle E.; Cui, Wenping; Wu, Lijun; Wei, Peng; Heiman, Don; Li, Ju; Moodera, Jagadeesh S.; Katmis, Ferhat

    2015-08-17

    Magnetic exchange driven proximity effect at a magnetic-insulator–topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. In this study, we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb2–xVxTe3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

  17. Proximity-driven enhanced magnetic order at ferromagnetic-insulator-magnetic-topological-insulator interface

    DOE PAGES

    Li, Mingda; Zhu, Yimei; Chang, Cui -Zu; ...

    2015-08-17

    Magnetic exchange driven proximity effect at a magnetic-insulator–topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. In this study, we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb2–xVxTe3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

  18. Electric field-free gas breakdown in explosively driven generators

    SciTech Connect

    Shkuratov, Sergey I.; Baird, Jason; Talantsev, Evgueni F.; Altgilbers, Larry L.

    2010-07-15

    All known types of gas discharges require an electric field to initiate them. We are reporting on a unique type of gas breakdown in explosively driven generators that does not require an electric field.

  19. Perfect spin-valley filter controlled by electric field in ferromagnetic silicene

    SciTech Connect

    Soodchomshom, Bumned E-mail: fscibns@ku.ac.th

    2014-01-14

    The spin-valley currents in silicene-based normal/sublattice-dependent ferromagnetic/normal junction are investigated. Unlike that in graphene, the pseudo Dirac mass in silicene is generated by spin-orbit interaction and tunable by applying electric or exchange fields into it. This is due to silicon-based honeycomb lattice having buckled structure. As a result, it is found that the junction leads to currents perfectly split into four groups, spin up (down) in k- and k{sup ′}-valleys, when applying different values of the electric field, considered as a perfect spin-valley polarization (PSVP) for electronic application. The PSVP is due to the interplay of spin-valley-dependent Dirac mass and chemical potential in the barrier. The PSVP also occurs only for the energy comparable to the spin-orbit energy gap. This work reveals potential of silicene for spinvalleytronics applications.

  20. A ferromagnetic model for the action of electric and magnetic fields in cryopreservation.

    PubMed

    Kobayashi, Atsuko; Kirschvink, Joseph L

    2014-04-01

    Recent discussions in the literature have questioned the ability of electromagnetic exposure to inhibit ice crystal formation in supercooled water. Here we note that strong electric fields are able to disrupt the surface boundary layer of inert air on the surface of materials, promoting higher rates of heat transport. We also note that most biological tissues contain ferromagnetic materials, both biologically precipitated magnetite (Fe3O4) as well as environmental contaminants that get accidentally incorporated into living systems. Although present at trace levels, the number density of these particulates is high, and they have extraordinarily strong interactions with weak, low-frequency magnetic fields of the sort involved in claims of electromagnetic cryopreservation. Magnetically-induced mechanical oscillation of these particles provides a plausible mechanism for the disruption of ice-crystal nucleation in supercooled water.

  1. Strain measurement in ferromagnetic crystals using dark-field electron holography

    NASA Astrophysics Data System (ADS)

    Murakami, Yasukazu; Niitsu, Kodai; Kaneko, Syuhei; Tanigaki, Toshiaki; Sasaki, Taisuke; Akase, Zentaro; Shindo, Daisuke; Ohkubo, Tadakatsu; Hono, Kazuhiro

    2016-11-01

    This study proposes a method to separate the geometric phase shift due to lattice strain from the undesired phase information, resulting from magnetic fields that are superposed in the dark-field electron holography (DFEH) observations. Choosing a distinct wave vector for the Bragg reflection reversed the sense of the geometric phase shift, while the sense of the magnetic information remained unchanged. In the case of an Nd-Fe-B permanent magnet, once the unwanted signal was removed by data processing, the residual phase image revealed a strain map. Even though the applications of DFEH have thus far been limited to non-magnetic systems, the method proposed in this work is also applicable to strain measurements in various ferromagnetic systems.

  2. Magnetic-field-modulated resonant tunneling in ferromagnetic-insulator-nonmagnetic junctions.

    PubMed

    Song, Yang; Dery, Hanan

    2014-07-25

    We present a theory for resonance-tunneling magnetoresistance (MR) in ferromagnetic-insulator-nonmagnetic junctions. The theory sheds light on many of the recent electrical spin injection experiments, suggesting that this MR effect rather than spin accumulation in the nonmagnetic channel corresponds to the electrically detected signal. We quantify the dependence of the tunnel current on the magnetic field by quantum rate equations derived from the Anderson impurity model, with the important addition of impurity spin interactions. Considering the on-site Coulomb correlation, the MR effect is caused by competition between the field, spin interactions, and coupling to the magnetic lead. By extending the theory, we present a basis for operation of novel nanometer-size memories.

  3. Reversal of Magnetisation in Ising Ferromagnet by the Field Having Gradient

    NASA Astrophysics Data System (ADS)

    Dhar, Abyaya; Acharyya, Muktish

    2016-11-01

    We have studied the reversal of magnetisation in Ising ferromagnet by the field having gradient along a particular direction. We employed the Monte Carlo simulation with Metropolis single spin flip algorithm. The average lifetime of the metastable state was observed to increase with the magnitude of the gradient of applied field. In the high gradient regime, the system was observed to show two distinct region of up and down spins. The interface or the domain wall was observed to move as one increases the gradient. The displacement of the mean position of the interface was observed to increase with the gradient as hyperbolic tangent function. The roughness of the interface was observed to decay exponentially as the gradient increases. The number of spin flip per site was observed to show a discontinuity in the vicinity of the domain wall. The amount of the discontinuity was found to diverge with the system size as a power law fashion with an exponent 5/3.

  4. Accurate mean-field modeling of the Barkhausen noise power in ferromagnetic materials, using a positive-feedback theory of ferromagnetism

    NASA Astrophysics Data System (ADS)

    Harrison, R. G.

    2015-07-01

    A mean-field positive-feedback (PFB) theory of ferromagnetism is used to explain the origin of Barkhausen noise (BN) and to show why it is most pronounced in the irreversible regions of the hysteresis loop. By incorporating the ABBM-Sablik model of BN into the PFB theory, we obtain analytical solutions that simultaneously describe both the major hysteresis loop and, by calculating separate expressions for the differential susceptibility in the irreversible and reversible regions, the BN power response at all points of the loop. The PFB theory depends on summing components of the applied field, in particular, the non-monotonic field-magnetization relationship characterizing hysteresis, associated with physical processes occurring in the material. The resulting physical model is then validated by detailed comparisons with measured single-peak BN data in three different steels. It also agrees with the well-known influence of a demagnetizing field on the position and shape of these peaks. The results could form the basis of a physics-based method for modeling and understanding the significance of the observed single-peak (and in multi-constituent materials, multi-peak) BN envelope responses seen in contemporary applications of BN, such as quality control in manufacturing, non-destructive testing, and monitoring the microstructural state of ferromagnetic materials.

  5. Zero field conductance singularity in two terminal ferromagnet-topological insulator device

    NASA Astrophysics Data System (ADS)

    Duan, Xiaopeng; Semenov, Yuriy G.; Kim, Ki Wook

    2014-03-01

    Spin-momentum interlocking of surface electronic states on 3D topological insulator (TI) grants the unique opportunity to generate electric current directed according to the spin polarization of injected electrons instead of the applied electric field. Such asymmetry in momentum distribution of injected electrons takes place in the vicinity of ferromagnetic contact but vanishes on the length of few mean free passes. We propose to use this property in two terminal devices consisting of two parallel ferromagnetic contacts deposited on the surface of 3D TI. When the injected spin polarization leads to electron momentum pointing towards the other electrode, it facilitate the direct transmission, resulting in a lower resistance; in contrast with a reversed bias, the spin-determined momentum points away from the other electrode, because of which the electrons could gain the right momentum only after multiple scatterings to approach the second electrode, thus resulting in a higher resistance. We stress that this asymmetry in the resistance keeps up to arbitrarily small applied voltage since it does not need the formation of space charge region that is essential in conventional diodes. The rectification ratio near zero voltage are estimated and potential application are discussed. This work was supported, in part, by the US Army Research Office and FAME (one of six centers of STARnet, a SRC program sponsored by MARCO and DARPA).

  6. Electric field effect in multilayer Cr2Ge2Te6: a ferromagnetic 2D material

    NASA Astrophysics Data System (ADS)

    Xing, Wenyu; Chen, Yangyang; Odenthal, Patrick M.; Zhang, Xiao; Yuan, Wei; Su, Tang; Song, Qi; Wang, Tianyu; Zhong, Jiangnan; Jia, Shuang; Xie, X. C.; Li, Yan; Han, Wei

    2017-06-01

    The emergence of two-dimensional (2D) materials has attracted a great deal of attention due to their fascinating physical properties and potential applications for future nano-electronic devices. Since the first isolation of graphene, a Dirac material, a large family of new functional 2D materials have been discovered and characterized, including insulating 2D boron nitride, semiconducting 2D transition metal dichalcogenides and black phosphorus, and superconducting 2D bismuth strontium calcium copper oxide, molybdenum disulphide and niobium selenide, etc. Here, we report the identification of ferromagnetic thin flakes of Cr2Ge2Te6 (CGT) with thickness down to a few nanometers, which provides a very important piece to the van der Waals structures consisting of various 2D materials. We further demonstrate the giant modulation of the channel resistance of 2D CGT devices via electric field effect. Our results illustrate the gate voltage tunability of 2D CGT and the potential of CGT, a ferromagnetic 2D material, as a new functional quantum material for applications in future nanoelectronics and spintronics.

  7. Fermi Surface Manipulation by External Magnetic Field Demonstrated for a Prototypical Ferromagnet

    NASA Astrophysics Data System (ADS)

    Młyńczak, E.; Eschbach, M.; Borek, S.; Minár, J.; Braun, J.; Aguilera, I.; Bihlmayer, G.; Döring, S.; Gehlmann, M.; Gospodarič, P.; Suga, S.; Plucinski, L.; Blügel, S.; Ebert, H.; Schneider, C. M.

    2016-10-01

    We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of Δ E =100 meV and Δ k =0.1 Å-1 . We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spin-orbit interaction even for elements as light as 3 d ferromagnets. These results set a new paradigm for the investigations of spin-orbit effects in the spintronic materials. The same methodology could be used in the bottom-up design of the devices based on the switching of spin-orbit gaps such as electric-field control of magnetic anisotropy or tunneling anisotropic magnetoresistance.

  8. Induced Anisotropy in FeCo-Based Nanocrystalline Ferromagnetic Alloys (HITPERM) by Very High Field Annealing

    NASA Technical Reports Server (NTRS)

    Johnson, F.; Garmestani, H.; Chu, S.-Y.; McHenry, M. E.; Laughlin, D. E.

    2004-01-01

    Very high magnetic field annealing is shown to affect the magnetic anisotropy in FeCo-base nanocrystalline soft ferromagnetic alloys. Alloys of composition Fe(44.5)Co(44.5)Zr(7)B(4) were prepared by melt spinning into amorphous ribbons, then wound to form toroidal bobbin cores. One set of cores was crystallized in a zero field at 600 deg. C for 1 h, then, field annealed at 17 tesla (T) at 480 deg. C for 1 h. Another set was crystallized in a 17-T field at 480 deg. C for 1 h. Field orientation was transverse to the magnetic path of the toroidal cores. An induced anisotropy is indicated by a sheared hysteresis loop. Sensitive torque magnetometry measurements with a Si cantilever sensor indicated a strong, uniaxial, longitudinal easy axis in the zero-field-crystallized sample. The source is most likely magnetoelastic anisotropy, caused by the residual stress from nanocrystallization and the nonzero magnetostriction coefficient for this material. The magnetostrictive coefficient lambda(5) is measured to be 36 ppm by a strain gage technique. Field annealing reduces the magnitude of the induced anisotropy. Core loss measurements were made in the zero-field-crystallized, zero-field-crystallized- than-field-annealed, and field-crystallized states. Core loss is reduced 30%-50% (depending on frequency) by field annealing. X-ray diffraction reveals no evidence of crystalline texture or orientation that would cause the induced anisotropy. Diffusional pair ordering is thought to be the cause of the induced anisotropy. However, reannealing the samples in the absence of a magnetic field at 480 deg. C does not completely remove the induced anisotropy.

  9. Electric field control of magnetization direction across the antiferromagnetic to ferromagnetic transition.

    PubMed

    Zheng, Guohui; Ke, San-Huang; Miao, Maosheng; Kim, Jinwoong; Ramesh, R; Kioussis, Nicholas

    2017-07-14

    Electric-field-induced magnetic switching can lead to a new paradigm of ultra-low power nonvolatile magnetoelectric random access memory (MeRAM). To date the realization of MeRAM relies primarily on ferromagnetic (FM) based heterostructures which exhibit low voltage-controlled magnetic anisotropy (VCMA) efficiency. On the other hand, manipulation of magnetism in antiferromagnetic (AFM) based nanojunctions by purely electric field means (rather than E-field induced strain) remains unexplored thus far. Ab initio electronic structure calculations reveal that the VCMA of ultrathin FeRh/MgO bilayers exhibits distinct linear or nonlinear behavior across the AFM to FM metamagnetic transition depending on the Fe- or Rh-interface termination. We predict that the AFM Fe-terminated phase undergoes an E-field magnetization switching with large VCMA efficiency and a spin reorientation across the metamagnetic transition. In sharp contrast, while the Rh-terminated interface exhibits large out-of-plane (in-plane) MA in the FM (AFM) phase, its magnetization is more rigid to external E-field. These findings demonstrate that manipulation of the AFM Néel-order magnetization direction via purely E-field means can pave the way toward ultra-low energy AFM-based MeRAM devices.

  10. Three-dimensional finite element analysis of residual magnetic field for ferromagnets under early damage

    NASA Astrophysics Data System (ADS)

    Yao, Kai; Shen, Kai; Wang, Zheng-Dao; Wang, Yue-Sheng

    2014-03-01

    In this study, 3D finite element analysis is presented by calculating the residual magnetic field signals of ferromagnets under the plastic deformation. The contour maps of tangential and normal RMF gradients are given, and the 3D effect is discussed. The results show that the tangential peak-peak amplitude and normal peak-vale amplitude are remarkably different in 2D and 3D simulations, but the tangential peak-peak width and normal peak-vale width are similar. Moreover, some key points are capable of capturing the plastic-zone shape, especially when the lift-off is small enough. The present study suggests an effective defect identification method with Metal magnetic memory (MMM) technique.

  11. Energetics of variant conversion in ferromagnetic shape memory alloys by external magnetic fields

    NASA Astrophysics Data System (ADS)

    Steuwer, Axel; Mori, Tsutomu; Kato, Hiroyuki; Wada, Taishi

    2003-08-01

    Using energetics, we examine the occurrence of large strains, so-called giant magnetostriction, in ferromagnetic shape memory alloys by the application of an external magnetic field. It is claimed that these strains originate from the conversion of one martensite variant to another. In this article, we attempt to show that magnetic work cannot supply the work required for the conversion of martensite variants in most cases. It is also pointed out that the latter work dissipates, while most of the magnetic work is conserved, as indicated by almost hysteresis-free magnetization curves. Therefore, simple energy conservation arguments rule out the suggested variant conversion mechanism not only quantitatively, but also qualitatively. A possible explanation for the occurrence of large strains is offered.

  12. Electric-field control of magnetism via strain transfer across ferromagnetic/ferroelectric interfaces.

    PubMed

    Taniyama, Tomoyasu

    2015-12-23

    By taking advantage of the coupling between magnetism and ferroelectricity, ferromagnetic (FM)/ferroelectric (FE) multiferroic interfaces play a pivotal role in manipulating magnetism by electric fields. Integrating the multiferroic heterostructures into spintronic devices significantly reduces energy dissipation from Joule heating because only an electric field is required to switch the magnetic element. New concepts of storage and processing of information thus can be envisioned when the electric-field control of magnetism is a viable alternative to the traditional current based means of controlling magnetism. This article reviews some salient aspects of the electric-field effects on magnetism, providing a short overview of the mechanisms of magneto-electric (ME) coupling at the FM/FE interfaces. A particular emphasis is placed on the ME effect via interfacial magneto-elastic coupling arising from strain transfer from the FE to FM layer. Recent results that demonstrate the electric-field control of magnetic anisotropy, magnetic order, magnetic domain wall motion, and etc are described. Obstacles that need to be overcome are also discussed for making this a reality for future device applications.

  13. Pulsed field actuation of Ni-Mn-Ga ferromagnetic shape memory alloy single crystal

    NASA Astrophysics Data System (ADS)

    Marioni, M.; Bono, D.; Banful, A. B.; del Rosario, M.; Rodriguez, E.; Peterson, B. W.; Allen, S. M.; O'Handley, R. C.

    2003-10-01

    Ferromagnetic Shape Memory Alloy Ni-Mn-Ga has twin boundaries in the martensitic phase that move when a suitable magnetic field is applied. In this fashion strains of up to 6% have been observed for static fields in single crystals [1]. Recently 2.5% strain has been demonstrated [2] in Ni-Mn-Ga single crystals for oscillating fields up to frequencies of 75 Hz (150 Hz actuation). This work studies the actuation of single crystals when pulsed fields are applied. Fields in the 0.4-1.5MA/m-range were generated in an air coil with rise times of the order of 1ms and below. The elongation of the samples is measured with a light beam reflected off the tip of the crystal. Single twin boundaries have been observed to advance 0.16 mm during 600 μsec-ong pulses. Actuation has been shown to be possible at least up to frequencies of 1700 Hz.

  14. Exploring 360 domain walls in ferromagnetic nanostructures using circular magnetic fields

    NASA Astrophysics Data System (ADS)

    Sarella, Anandakumar; Kaya, F. I.; Aidala, K. E.

    Ferromagnetic nanostructures can exhibit intriguing magnetic states, such as the metastable 360 domain wall (DW), in which two 180 DWs combine to form a nearly flux closed state in sufficiently thin structures. These composite structures have potential to maximize storage densities due to their minimal stray fields. We study a straightforward method to nucleate 360 DWs in nanorings, nanowires, using in-plane circular fields, as if from a current carrying wire passing through the substrate in close proximity to the nanostructures. Our simulations, using OOMMF, predict that the vortex state of a ring with appropriate geometry will reverse from CW to CCW through an intermediate state consisting of pairs of 360 DWs. We examine the dependence of the switching field and intermediate states on geometric properties such as the diameter, thickness, and width of the ring. Using the local circular field, we can also nucleate 360 DWs in nanowires, pinning the location of the DWs at notches spaced as close as 100 nm apart, suggesting high density storage. We are currently studying these structures experimentally using AFM/MFM. We generate the circular field by passing current through AFM tip and image the resulting magnetic states with MFM. NSF Grants No. DMR 1208042 and 1207924. Simulations were run on the Odyssey cluster, Research Computing Group at Harvard.

  15. High-field magnetization of band ferromagnets Co2 YAl ( Y = Ti, V, Cr, Mn, Fe, Ni)

    NASA Astrophysics Data System (ADS)

    Kourov, N. I.; Marchenkov, V. V.; Perevozchikova, Yu. A.; Korolev, A. V.; Weber, H. W.

    2016-12-01

    The temperature dependences of the magnetization of ferromagnetic Heusler alloys Co2 YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at H = 50 kOe in the range 2 K < T < 1100 K. It is shown that the high-field ( H ≥ 20 kOe) magnetization is described within the Stoner model.

  16. Classification of magnons in rotated ferromagnetic Heisenberg model and their competing responses in transverse fields

    NASA Astrophysics Data System (ADS)

    Sun, Fadi; Ye, Jinwu; Liu, Wu-Ming

    2016-07-01

    In this paper, we study the rotated ferromagnetic Heisenberg model (RFHM) in two different transverse fields, hx and hz, which can be intuitively visualized as studying spin-orbit coupling (SOC) effects in two-dimensional (2D) Ising or anisotropic X Y model in a transverse field. At a special SOC class, it was found in our previous work [Phys. Rev. A 92, 043609 (2015), 10.1103/PhysRevA.92.043609] that the RFHM at a zero field owns an exact spin-orbit coupled ground state called the Y -x state. It supports not only the commensurate magnons (called C -C0 and C -Cπ ), but also the incommensurate magnons (called C-IC). These magnons are nonrelativistic, not embedded in the exact ground state, so need to be thermally excited or generated by various external probes. Their dramatic response under a longitudinal hy field was recently worked out by Sun et al. [arXiv:1502.05338]. Here we find they respond very differently under the two transverse fields. Any hx (hz) introduces quantum fluctuations to the ground state and changes the collinear Y -x state to a canted coplanar Y X -x (Y Z -x ) state. The C -C0,C -Cπ , and C-IC magnons become relativistic and sneak into the quantum ground state. We determine the competing boundaries among the C -C0,C -Cπ , and C-IC magnons, especially the detailed dispersions of the C-IC magnons inside the canted phases, which can be mapped out by the transverse spin structure factors. As hx (hz) increases further, the C -C0 magnons always win the competition and emerge as the seeds to drive a transition from the Y X -x (or Y Z -x ) to the ferromagnetic along the X (orZ ) direction called the X -FM (or Z -FM) phase. We show that the transition is in the 3D Ising universality class and it becomes the 3D X Y transition at the two Abelian points. We evaluate these magnons' contributions to magnetization and specific heat at low temperatures which can be measured by various established experimental techniques. The nature of the finite

  17. Influence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs

    NASA Astrophysics Data System (ADS)

    Philippe, M. P.; Fagnard, J. F.; Wéra, L.; Morita, M.; Nariki, S.; Teshima, H.; Caps, H.; Vanderheyden, B.; Vanderbemden, P.

    2016-03-01

    Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) “trapped field magnets” are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a “crossed-field” configuration. Experimental results show that the usual “crossed-field demagnetization” caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet.

  18. Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure

    NASA Astrophysics Data System (ADS)

    Cai, Kaiming; Yang, Meiyin; Ju, Hailang; Wang, Sumei; Ji, Yang; Li, Baohe; Edmonds, Kevin William; Sheng, Yu; Zhang, Bao; Zhang, Nan; Liu, Shuai; Zheng, Houzhi; Wang, Kaiyou

    2017-07-01

    All-electrical and programmable manipulations of ferromagnetic bits are highly pursued for the aim of high integration and low energy consumption in modern information technology. Methods based on the spin-orbit torque switching in heavy metal/ferromagnet structures have been proposed with magnetic field, and are heading toward deterministic switching without external magnetic field. Here we demonstrate that an in-plane effective magnetic field can be induced by an electric field without breaking the symmetry of the structure of the thin film, and realize the deterministic magnetization switching in a hybrid ferromagnetic/ferroelectric structure with Pt/Co/Ni/Co/Pt layers on PMN-PT substrate. The effective magnetic field can be reversed by changing the direction of the applied electric field on the PMN-PT substrate, which fully replaces the controllability function of the external magnetic field. The electric field is found to generate an additional spin-orbit torque on the CoNiCo magnets, which is confirmed by macrospin calculations and micromagnetic simulations.

  19. Phase Diagram Of UGe2: The Magnetic Transition within the Ferromagnetic Phase and the Superconducting Transition; the Effect of Magnetic Field on the Ambient-Pressure Ferromagnetic Phase

    NASA Astrophysics Data System (ADS)

    Phillips, N. E.; Bouquet, F.; Fisher, R. A.; Hardy, F.; Oeschler, N.; Lashley, J. C.; Flouquet, J.; Huxley, A.

    2007-03-01

    Superconductivity in UGe2 occurs near 1.2 GPa at the 0-K termination of the phase boundary (Tx, Px) of a magnetic transition that occurs within the ferromagnetic phase. Ambient-pressure specific-heat measurements show a hysteretic transition at Tx(0) ˜ 22 K, reminiscent of the CDW/SDW transition in α-U, and consistent with the suggestion that the transition in UGe2 is also a CDW/SDW transition. The magnetic field dependence of the specific heat, at ambient pressure, demonstrates the presence of structure in the electron density of states and an unusual nature of the ferromagnetic ordering at the Curie temperature. Specific-heat measurements to 1.8 GPa give an estimate of the latent heat of the transition and determine the phase boundary for 1 <= T <= 11 K. Contrary to expectations, the onset temperature of the superconducting transition is independent of pressure in the region in which it was observed, 1.08 <= P <= 1.35 GPa.

  20. Aging and random-field magnetism in ferromagnet/antiferromagnet bilayers

    NASA Astrophysics Data System (ADS)

    Ma, Tianyu; Freeman, Ryan; Cheng, Xiang; Boettcher, Stefan; Urazhdin, Sergei

    Exchange interaction at the interface between a ferromagnet (F) and an antiferromagnet (AF) results in a random effective exchange field acting on both F and AF, which can produce complex equilibrium and dynamical states. We utilized anisotropic magnetoresistance to look for signatures of such states in epitaxial Py =Permalloy/Fe50Mn50 and polycrystalline CoO/Py bilayers. For thin AF layers, both systems exhibit slow cooperative aging indicative of a complex glassy state. Aging follows the same small power-law or logarithmic dependence and is observed over a wide range of temperatures and fields, suggesting a universal aging mechanism. Glassy relaxation is not observed at any temperature for AF thickness above 3.5nm. We argue that these observations are inconsistent with the usual ``granular'' and ``domain-state'' models of F/AF systems. We discuss the implications of our results for the random field magnetism, and the relationship between the dimensionality and the topological properties of magnetic systems. Supported by NSF DMR-1504449.

  1. Hysteretic ac loss in a coated superconductor subjected to oscillating magnetic fields: ferromagnetic effect and frequency dependence

    NASA Astrophysics Data System (ADS)

    Ma, Guang-Tong

    2014-06-01

    Numerical simulations of the hysteretic ac loss in a coated superconductor with a more realistic version of the architecture were performed via the finite-element technique in the presence of an oscillating magnetic field. The coated superconductor was electromagnetically modeled by resorting to the quasistatic approximation of a vector potential approach in conjunction with nonlinear descriptions of the superconducting layer and the ferromagnetic substrate therein by a power-law model and the Langevin equation, respectively. A diverse effect of the ferromagnetic substrate on the hysteretic ac loss, depending on the strength of the applied magnetic field, was displayed, and its underlying cause was identified. The dependence of the hysteretic ac loss on the applied frequency is found to be related to a critical amplitude of the applied magnetic field, and the eddy-current loss dissipated in the metal coatings becomes prominent as the frequency increases only at high applied magnetic fields.

  2. Magnetic moment and local magnetic induction of superconducting/ferromagnetic structures subjected to crossed fields: experiments on GdBCO and modelling

    NASA Astrophysics Data System (ADS)

    Fagnard, J. F.; Morita, M.; Nariki, S.; Teshima, H.; Caps, H.; Vanderheyden, B.; Vanderbemden, P.

    2016-12-01

    Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed.

  3. Electric field control of ferromagnetism at room temperature in GaCrN (p-i-n) device structures

    NASA Astrophysics Data System (ADS)

    El-Masry, N. A.; Zavada, J. M.; Reynolds, J. G.; Reynolds, C. L.; Liu, Z.; Bedair, S. M.

    2017-08-01

    We have demonstrated a room temperature dilute magnetic semiconductor based on GaCrN epitaxial layers grown by metalorganic chemical vapor deposition. Saturation magnetization Ms increased when the GaCrN film is incorporated into a (p-GaN/i-GaCrN/n-GaN) device structure, due to the proximity of mediated holes present in the p-GaN layer. Zero field cooling and field cooling were measured to ascertain the absence of superparamagnetic behavior in the films. A (p-GaN/i-GaCrN/n-GaN) device structure with room temperature ferromagnetic (FM) properties that can be controlled by an external applied voltage has been fabricated. In this work, we show that the applied voltage controls the ferromagnetic properties, by biasing the (p-i-n) structure. With forward bias, ferromagnetism in the GaCrN layer was increased nearly 4 fold of the original value. Such an enhancement is due to carrier injection of holes into the Cr deep level present in the i-GaCrN layer. A "memory effect" for the FM behavior of the (p-i-n) GaCrN device structure persisted for 42 h after the voltage bias was turned off. These measurements also support that the observed ferromagnetism in the GaCrN film is not due to superparamagnetic clusters but instead is a hole-mediated phenomenon.

  4. Enhancing electric-field control of ferromagnetism through nanoscale engineering of high-Tc MnxGe1-x nanomesh.

    PubMed

    Nie, Tianxiao; Tang, Jianshi; Kou, Xufeng; Gen, Yin; Lee, Shengwei; Zhu, Xiaodan; He, Qinglin; Chang, Li-Te; Murata, Koichi; Fan, Yabin; Wang, Kang L

    2016-10-20

    Voltage control of magnetism in ferromagnetic semiconductor has emerged as an appealing solution to significantly reduce the power dissipation and variability beyond current CMOS technology. However, it has been proven to be very challenging to achieve a candidate with high Curie temperature (Tc), controllable ferromagnetism and easy integration with current Si technology. Here we report the effective electric-field control of both ferromagnetism and magnetoresistance in unique MnxGe1-x nanomeshes fabricated by nanosphere lithography, in which a Tc above 400 K is demonstrated as a result of size/quantum confinement. Furthermore, by adjusting Mn doping concentration, extremely giant magnetoresistance is realized from ∼8,000% at 30 K to 75% at 300 K at 4 T, which arises from a geometrically enhanced magnetoresistance effect of the unique mesh structure. Our results may provide a paradigm for fundamentally understanding the high Tc in ferromagnetic semiconductor nanostructure and realizing electric-field control of magnetoresistance for future spintronic applications.

  5. Cooling field and temperature dependent exchange bias in spin glass/ferromagnet bilayers

    PubMed Central

    Rui, W. B.; Hu, Y.; Du, A.; You, B.; Xiao, M. W.; Zhang, W.; Zhou, S. M.; Du, J.

    2015-01-01

    We report on the experimental and theoretical studies of cooling field (HFC) and temperature (T) dependent exchange bias (EB) in FexAu1 − x/Fe19Ni81 spin glass (SG)/ferromagnet (FM) bilayers. When x varies from 8% to 14% in the FexAu1 − x SG alloys, with increasing T, a sign-changeable exchange bias field (HE) together with a unimodal distribution of coercivity (HC) are observed. Significantly, increasing in the magnitude of HFC reduces (increases) the value of HE in the negative (positive) region, resulting in the entire HE ∼ T curve to move leftwards and upwards. In the meanwhile, HFC variation has weak effects on HC. By Monte Carlo simulation using a SG/FM vector model, we are able to reproduce such HE dependences on T and HFC for the SG/FM system. Thus this work reveals that the SG/FM bilayer system containing intimately coupled interface, instead of a single SG layer, is responsible for the novel EB properties. PMID:26348277

  6. Quantum spin-1 anisotropic ferromagnetic Heisenberg model in a crystal field: a variational approach.

    PubMed

    Carvalho, D C; Plascak, J A; Castro, L M

    2013-09-01

    A variational approach based on Bogoliubov inequality for the free energy is employed in order to treat the quantum spin-1 anisotropic ferromagnetic Heisenberg model in the presence of a crystal field. Within the Bogoliubov scheme an improved pair approximation has been used. The temperature-dependent thermodynamic functions have been obtained and provide much better results than the previous simple mean-field scheme. In one dimension, which is still nonintegrable for quantum spin-1, we get the exact results in the classical limit, or near-exact results in the quantum case, for the free energy, magnetization, and quadrupole moment, as well for the transition temperature. In two and three dimensions the corresponding global phase diagrams have been obtained as a function of the parameters of the Hamiltonian. First-order transition lines, second-order transition lines, tricritical and tetracritical points, and critical endpoints have been located through the analysis of the minimum of the Helmholtz free energy and a Landau-like expansion in the approximated free energy. Only first-order quantum transitions have been found at zero temperature. Limiting cases, such as isotropic Heisenberg, Blume-Capel, and Ising models, have been analyzed and compared to previous results obtained from other analytical approaches as well as from Monte Carlo simulations.

  7. Approach for removing ghost-images in remote field eddy current testing of ferromagnetic pipes

    NASA Astrophysics Data System (ADS)

    Luo, Q. W.; Shi, Y. B.; Wang, Z. G.; Zhang, W.; Zhang, Y.

    2016-10-01

    In the non-destructive testing of ferromagnetic pipes based on remote field eddy currents, an array of sensing coils is often used to detect local defects. While testing, the image that is obtained by sensing coils exhibits a ghost-image, which originates from both the transmitter and sensing coils passing over the same defects in pipes. Ghost-images are caused by transmitters and lead to undesirable assessments of defects. In order to remove ghost-images, two pickup coils are coaxially set to each other in remote field. Due to the time delay between differential signals tested by the two pickup coils, a Wiener deconvolution filter is used to identify the artificial peaks that lead to ghost-images. Because the sensing coils and two pickup coils all receive the same signal from one transmitter, they all contain the same artificial peaks. By subtracting the artificial peak values obtained by the two pickup coils from the imaging data, the ghost-image caused by the transmitter is eliminated. Finally, a relatively highly accurate image of local defects is obtained by these sensing coils. With proposed method, there is no need to subtract the average value of the sensing coils, and it is sensitive to ringed defects.

  8. Approach for removing ghost-images in remote field eddy current testing of ferromagnetic pipes.

    PubMed

    Luo, Q W; Shi, Y B; Wang, Z G; Zhang, W; Zhang, Y

    2016-10-01

    In the non-destructive testing of ferromagnetic pipes based on remote field eddy currents, an array of sensing coils is often used to detect local defects. While testing, the image that is obtained by sensing coils exhibits a ghost-image, which originates from both the transmitter and sensing coils passing over the same defects in pipes. Ghost-images are caused by transmitters and lead to undesirable assessments of defects. In order to remove ghost-images, two pickup coils are coaxially set to each other in remote field. Due to the time delay between differential signals tested by the two pickup coils, a Wiener deconvolution filter is used to identify the artificial peaks that lead to ghost-images. Because the sensing coils and two pickup coils all receive the same signal from one transmitter, they all contain the same artificial peaks. By subtracting the artificial peak values obtained by the two pickup coils from the imaging data, the ghost-image caused by the transmitter is eliminated. Finally, a relatively highly accurate image of local defects is obtained by these sensing coils. With proposed method, there is no need to subtract the average value of the sensing coils, and it is sensitive to ringed defects.

  9. In-plane magnetic anisotropy and temperature dependence of switching field in (Ga, Mn) as ferromagnetic semiconductors.

    PubMed

    Kamara, S; Terki, F; Dumas, R; Dehbaoui, M; Sadowski, J; Galéra, R M; Tran, Q-H; Charar, S

    2012-06-01

    We explore the magnetic anisotropy of GaMnAs ferromagnetic semiconductor by Planar Hall Effect (PHE) measurements. Using low magnitude of applied magnetic field (i.e., when the magnitude H is smaller than both cubic Hc and uniaxial Hu anisotropy field), we have observed various shapes of applied magnetic field direction dependence of Planar Hall Resistance (PHR). In particular, in two regions of temperature. At T < Tc/2, the "square-shape" signal and at T > Tc/2 the "zigzag-shape" signal of PHR. They reflect different magnetic anisotropy and provide information about magnetization reversal process in GaMnAs ferromagnetic semiconductor. The theoretical model calculation of PHR based on the free energy density reproduces well the experimental data. We report also the temperature dependence of anisotropy constants and magnetization orientations. The transition of easy axis from biaxial to uniaxiale axes has been observed and confirmed by SQUID measurements.

  10. Interfacial spin-orbit fields in ferromagnet/normal metal (FN) and ferromagnet/superconductor (FS) systems

    NASA Astrophysics Data System (ADS)

    Hoegl, Petra; Matos-Abiague, Alex; Zutic, Igor; Fabian, Jaroslav

    Breaking of space-inversion symmetry at interfaces induces spin-orbit fields as an emergent phenomenon. Interfacial spin-orbit fields are believed to enable a wealth of new phenomena, not existent or fragile in the bulk, such as the tunneling anisotropic magnetoresistance (TAMR), interfacial spin-orbit torques, Skyrmions, or possible realization of topological superconductors. We theoretically investigate spin-polarized transport in FN and FS junctions in the presence of Rashba and Dresselhaus interfacial spin-orbit fields. The interplay of magnetism and spin-orbit fields leads to a marked magnetoanisotropy of the conductances. Remarkably, the anisotropy in FS systems--magnetoanisotropic Andreev reflection (MAAR)--is giant compared to TAMR, its normal-state counterpart in FN junctions. We further report on the dependence of spin-flip probability currents on characteristic system parameters. This work has been supported by DFG SFB 689, International Doctorate Program Topological Insulators of the Elite Network of Bavaria, DOE-BES Grant No. DE-SC0004890, and ONR N000141310754.

  11. Coherent polarization driven by external electromagnetic fields

    NASA Astrophysics Data System (ADS)

    Apostol, M.; Ganciu, M.

    2010-11-01

    The coherent interaction of the electromagnetic radiation with an ensemble of polarizable, identical particles with two energy levels is investigated in the presence of external electromagnetic fields. The coupled non-linear equations of motion are solved in the stationary regime and in the limit of small coupling constants. It is shown that an external electromagnetic field may induce a macroscopic occupation of both the energy levels of the particles and the corresponding photon states, governed by a long-range order of the quantum phases of the internal motion (polarization) of the particles. A lasing effect is thereby obtained, controlled by the external field. Its main characteristics are estimated for typical atomic matter and atomic nuclei. For atomic matter the effect may be considerable (for usual external fields), while for atomic nuclei the effect is extremely small (practically insignificant), due to the great disparity in the coupling constants. In the absence of the external field, the solution, which is non-analytic in the coupling constant, corresponds to a second-order phase transition (super-radiance), which was previously investigated.

  12. Large Field Visualization with Demand-Driven Calculation

    NASA Technical Reports Server (NTRS)

    Moran, Patrick J.; Henze, Chris

    1999-01-01

    We present a system designed for the interactive definition and visualization of fields derived from large data sets: the Demand-Driven Visualizer (DDV). The system allows the user to write arbitrary expressions to define new fields, and then apply a variety of visualization techniques to the result. Expressions can include differential operators and numerous other built-in functions, ail of which are evaluated at specific field locations completely on demand. The payoff of following a demand-driven design philosophy throughout becomes particularly evident when working with large time-series data, where the costs of eager evaluation alternatives can be prohibitive.

  13. Electric-field-driven switching of individual magnetic skyrmions

    NASA Astrophysics Data System (ADS)

    Hsu, Pin-Jui; Kubetzka, André; Finco, Aurore; Romming, Niklas; von Bergmann, Kirsten; Wiesendanger, Roland

    2017-02-01

    Controlling magnetism with electric fields is a key challenge to develop future energy-efficient devices. The present magnetic information technology is mainly based on writing processes requiring either local magnetic fields or spin torques, but it has also been demonstrated that magnetic properties can be altered on the application of electric fields. This has been ascribed to changes in magnetocrystalline anisotropy caused by spin-dependent screening and modifications of the band structure, changes in atom positions or differences in hybridization with an adjacent oxide layer. However, the switching between states related by time reversal, for example magnetization up and down as used in the present technology, is not straightforward because the electric field does not break time-reversal symmetry. Several workarounds have been applied to toggle between bistable magnetic states with electric fields, including changes of material composition as a result of electric fields. Here we demonstrate that local electric fields can be used to switch reversibly between a magnetic skyrmion and the ferromagnetic state. These two states are topologically inequivalent, and we find that the direction of the electric field directly determines the final state. This observation establishes the possibility to combine electric-field writing with the recently envisaged skyrmion racetrack-type memories.

  14. A general nonlinear magnetomechanical model for ferromagnetic materials under a constant weak magnetic field

    NASA Astrophysics Data System (ADS)

    Shi, Pengpeng; Jin, Ke; Zheng, Xiaojing

    2016-04-01

    Weak magnetic nondestructive testing (e.g., metal magnetic memory method) concerns the magnetization variation of ferromagnetic materials due to its applied load and a weak magnetic surrounding them. One key issue on these nondestructive technologies is the magnetomechanical effect for quantitative evaluation of magnetization state from stress-strain condition. A representative phenomenological model has been proposed to explain the magnetomechanical effect by Jiles in 1995. However, the Jiles' model has some deficiencies in quantification, for instance, there is a visible difference between theoretical prediction and experimental measurements on stress-magnetization curve, especially in the compression case. Based on the thermodynamic relations and the approach law of irreversible magnetization, a nonlinear coupled model is proposed to improve the quantitative evaluation of the magnetomechanical effect. Excellent agreement has been achieved between the predictions from the present model and previous experimental results. In comparison with Jiles' model, the prediction accuracy is improved greatly by the present model, particularly for the compression case. A detailed study has also been performed to reveal the effects of initial magnetization status, cyclic loading, and demagnetization factor on the magnetomechanical effect. Our theoretical model reveals that the stable weak magnetic signals of nondestructive testing after multiple cyclic loads are attributed to the first few cycles eliminating most of the irreversible magnetization. Remarkably, the existence of demagnetization field can weaken magnetomechanical effect, therefore, significantly reduces the testing capability. This theoretical model can be adopted to quantitatively analyze magnetic memory signals, and then can be applied in weak magnetic nondestructive testing.

  15. Low Schottky barrier black phosphorus field-effect devices with ferromagnetic tunnel contacts.

    PubMed

    Kamalakar, M Venkata; Madhushankar, B N; Dankert, André; Dash, Saroj P

    2015-05-13

    Black phosphorus (BP) has been recently unveiled as a promising 2D direct bandgap semiconducting material. Here, ambipolar field-effect transistor behavior of nanolayers of BP with ferromagnetic tunnel contacts is reported. Using TiO2/Co contacts, a reduced Schottky barrier <50 meV, which can be tuned further by the gate voltage, is obtained. Eminently, a good transistor performance is achieved in the devices discussed here, with drain current modulation of four to six orders of magnitude and a mobility of μh ≈ 155 cm(2) V(-1) s(-1) for hole conduction at room temperature. Magnetoresistance calculations using a spin diffusion model reveal that the source-drain contact resistances in the BP device can be tuned by gate voltage to an optimal range for injection and detection of spin-polarized holes. The results of the study demonstrate the prospect of BP nanolayers for efficient nanoelectronic and spintronic devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  16. A general nonlinear magnetomechanical model for ferromagnetic materials under a constant weak magnetic field

    SciTech Connect

    Shi, Pengpeng; Zheng, Xiaojing; Jin, Ke

    2016-04-14

    Weak magnetic nondestructive testing (e.g., metal magnetic memory method) concerns the magnetization variation of ferromagnetic materials due to its applied load and a weak magnetic surrounding them. One key issue on these nondestructive technologies is the magnetomechanical effect for quantitative evaluation of magnetization state from stress–strain condition. A representative phenomenological model has been proposed to explain the magnetomechanical effect by Jiles in 1995. However, the Jiles' model has some deficiencies in quantification, for instance, there is a visible difference between theoretical prediction and experimental measurements on stress–magnetization curve, especially in the compression case. Based on the thermodynamic relations and the approach law of irreversible magnetization, a nonlinear coupled model is proposed to improve the quantitative evaluation of the magnetomechanical effect. Excellent agreement has been achieved between the predictions from the present model and previous experimental results. In comparison with Jiles' model, the prediction accuracy is improved greatly by the present model, particularly for the compression case. A detailed study has also been performed to reveal the effects of initial magnetization status, cyclic loading, and demagnetization factor on the magnetomechanical effect. Our theoretical model reveals that the stable weak magnetic signals of nondestructive testing after multiple cyclic loads are attributed to the first few cycles eliminating most of the irreversible magnetization. Remarkably, the existence of demagnetization field can weaken magnetomechanical effect, therefore, significantly reduces the testing capability. This theoretical model can be adopted to quantitatively analyze magnetic memory signals, and then can be applied in weak magnetic nondestructive testing.

  17. Ferromagnetic superconductors

    NASA Astrophysics Data System (ADS)

    Huxley, Andrew D.

    2015-07-01

    The co-existence of superconductivity and ferromagnetism is of potential interest for spintronics and high magnetic field applications as well as a fascinating fundamental state of matter. The recent focus of research is on a family of ferromagnetic superconductors that are superconducting well below their Curie temperature, the first example of which was discovered in 2000. Although there is a 'standard' theoretical model for how magnetic pairing might bring about such a state, why it has only been seen in a few materials that at first sight appear to be very closely related has yet to be fully explained. This review covers the current state of knowledge of the magnetic and superconducting properties of these materials with emphasis on how they conform and differ from the behaviour expected from the 'standard' model and from each other.

  18. The effect of field-dependent heat capacity on the characteristics of the ferromagnetic Ericsson refrigeration cycle

    NASA Astrophysics Data System (ADS)

    Yan, Zijun; Chen, Jincan

    1992-07-01

    The characteristics of a magnetic Ericsson refrigeration cycle are investigated on the basis of the thermodynamic properties of the ferromagnetic material. The effect of field-dependent heat capacity on regeneration is discussed. The coefficients of performance of the Ericsson magnetic refrigeration cycle are derived. Finally, it is pointed out that, according to theoretical analysis, the Ericsson magnetic refrigeration cycle can be expected to reach or approach perfect regeneration by using a mixture of several magnetic materials as the working substance.

  19. Effects of guide field in driven magnetic reconnection

    NASA Astrophysics Data System (ADS)

    Cheng, C. Z.; Inoue, S.; Horiuchi, R.; Ono, Y.; Guo, X.

    2016-10-01

    Decoupling of electron and ion dynamics is the key physical process in the magnetic reconnection layer. It leads to the generation of parallel E-field and in-plane electrostatic E-field, and determines how particles gain energy. For antiparallel magnetic reconnection (zero guide field case), the electron and ion dynamics decoupling is due to meandering particle (unmagnetized) orbits in the field reversal region and particle acceleration by parallel electric field in the separatrix region. The parallel E-field is produced mainly from the driven inductive E-field due to the quadrupole out-of-plane magnetic field generation. The decoupling of electron and ion dynamics causes charge separation which produces the in-plane electrostatic E-field. If the guide field is stronger than the reconnecting magnetic field, both electrons and ions are magnetized in the entire magnetic reconnection domain, and the electron-ion dynamics decoupling process changes from the zero guide field case. Then, the structure of parallel and electrostatic E-fields, and thus how electrons/ions gain energy also changes. We will explain the physical mechanisms of electron-ion dynamics decoupling on the E-field generation, and how electron and ion are heated/accelerated based on the driven reconnection simulation results.

  20. Intermediate inflation driven by DBI scalar field

    NASA Astrophysics Data System (ADS)

    Nazavari, N.; Mohammadi, A.; Ossoulian, Z.; Saaidi, Kh.

    2016-06-01

    Picking out a DBI scalar field as inflation, the slow-rolling inflationary scenario is studied by attributing an exponential time function to scale factor, known as intermediate inflation. The perturbation parameters of the model are estimated numerically for two different cases, and the final result is compared with Planck data. The diagram of tensor-to-scalar ratio r versus scalar spectra index ns is illustrated, and it is found that they are within an acceptable range as suggested by Planck. In addition, the acquired values for amplitude of scalar perturbation reveal the ability of the model to depict a good picture of the Universe in one of its earliest stages. As a further argument, the non-Gaussianity is investigated, displaying that the model prediction stands in a 68% C.L. regime according to the latest Planck data.

  1. Motion Driven by Strain Gradient Fields

    PubMed Central

    Wang, Chao; Chen, Shaohua

    2015-01-01

    A new driving mechanism for direction-controlled motion of nano-scale objects is proposed, based on a model of stretching a graphene strip linked to a rigid base with linear springs of identical stiffness. We find that the potential energy difference induced by the strain gradient field in the graphene strip substrate can generate sufficient force to overcome the static and kinetic friction forces between the nano-flake and the strip substrate, resulting in the nanoscale flake motion in the direction of gradient reduction. The dynamics of the nano-flake can be manipulated by tuning the stiffness of linear springs, stretching velocity and the flake size. This fundamental law of directional motion induced by strain gradient could be very useful for promising designs of nanoscale manipulation, transportation and smart surfaces. PMID:26323603

  2. Electric Field Driven Torque in ATP Synthase

    PubMed Central

    Miller, John H.; Rajapakshe, Kimal I.; Infante, Hans L.; Claycomb, James R.

    2013-01-01

    FO-ATP synthase (FO) is a rotary motor that converts potential energy from ions, usually protons, moving from high- to low-potential sides of a membrane into torque and rotary motion. Here we propose a mechanism whereby electric fields emanating from the proton entry and exit channels act on asymmetric charge distributions in the c-ring, due to protonated and deprotonated sites, and drive it to rotate. The model predicts a scaling between time-averaged torque and proton motive force, which can be hindered by mutations that adversely affect the channels. The torque created by the c-ring of FO drives the γ-subunit to rotate within the ATP-producing complex (F1) overcoming, with the aid of thermal fluctuations, an opposing torque that rises and falls with angular position. Using the analogy with thermal Brownian motion of a particle in a tilted washboard potential, we compute ATP production rates vs. proton motive force. The latter shows a minimum, needed to drive ATP production, which scales inversely with the number of proton binding sites on the c-ring. PMID:24040370

  3. Electric field driven torque in ATP synthase.

    PubMed

    Miller, John H; Rajapakshe, Kimal I; Infante, Hans L; Claycomb, James R

    2013-01-01

    FO-ATP synthase (FO) is a rotary motor that converts potential energy from ions, usually protons, moving from high- to low-potential sides of a membrane into torque and rotary motion. Here we propose a mechanism whereby electric fields emanating from the proton entry and exit channels act on asymmetric charge distributions in the c-ring, due to protonated and deprotonated sites, and drive it to rotate. The model predicts a scaling between time-averaged torque and proton motive force, which can be hindered by mutations that adversely affect the channels. The torque created by the c-ring of FO drives the γ-subunit to rotate within the ATP-producing complex (F1) overcoming, with the aid of thermal fluctuations, an opposing torque that rises and falls with angular position. Using the analogy with thermal Brownian motion of a particle in a tilted washboard potential, we compute ATP production rates vs. proton motive force. The latter shows a minimum, needed to drive ATP production, which scales inversely with the number of proton binding sites on the c-ring.

  4. Measuring Energy Scaling of Laser Driven Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Williams, Jackson; Goyon, Clement; Mariscal, Derek; Pollock, Brad; Patankar, Siddharth; Moody, John

    2016-10-01

    Laser-driven magnetic fields are of interest in particle confinement, fast ignition, and ICF platforms as an alternative to pulsed power systems to achieve many times higher fields. A comprehensive model describing the mechanism responsible for creating and maintaining magnetic fields from laser-driven coils has not yet been established. Understanding the scaling of key experimental parameters such as spatial and temporal uniformity and duration are necessary to implement coil targets in practical applications yet these measurements prove difficult due to the highly transient nature of the fields. We report on direct voltage measurements of laser-driven coil targets in which the laser energy spans more than four orders of magnitude. Results suggest that at low energies, laser-driven coils can be modeled as an electric circuit; however, at higher energies plasma effects dominate and a simple circuit treatment is insufficient to describe all observed phenomenon. The favorable scaling with laser power and pulse duration, observed in the present study and others at kilojoule energies, has positive implications for sustained, large magnetic fields for applications on the NIF. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

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

  6. Alkali-vapor magnetic resonance driven by fictitious radiofrequency fields

    SciTech Connect

    Zhivun, Elena; Wickenbrock, Arne; Patton, Brian; Budker, Dmitry

    2014-11-10

    We demonstrate an all-optical {sup 133}Cs scalar magnetometer, operating in nonzero magnetic field, in which the magnetic resonance is driven by an effective oscillating magnetic field provided by the AC Stark shift of an intensity-modulated laser beam. We achieve a projected shot-noise-limited sensitivity of 1.7fT/√(Hz) and measure a technical noise floor of 40fT/√(Hz). These results are essentially identical to a coil-driven scalar magnetometer using the same setup. This all-optical scheme offers advantages over traditional coil-driven magnetometers for use in arrays and in magnetically sensitive fundamental physics experiments, e.g., searches for a permanent electric dipole moment of the neutron.

  7. Hiding objects in AC magnetic fields of power grid frequency by two-shell ferromagnetic/superconducting cloak

    NASA Astrophysics Data System (ADS)

    Šouc, J.; Solovyov, M.; Gömöry, F.

    2016-07-01

    Performance of magnetic cloak made from commercially available materials has been tested by verifying its ability to suppress the magnetic signatures of metallic and ferromagnetic objects. The range of magnetic field amplitudes from 0.1 to 10 mT and frequencies around 50-60 Hz were used. The cloak combines the inner tube from high-temperature superconductor that should be cooled by liquid nitrogen, with the outer tube made from MnZn ferrite powder mixed in plastic matter. Superconductor is in the form of tapes wound in helical manner on a round former. Such design is promising when the objects with dimensions reaching several centimeters should be cloaked. Performance of the small model manufactured following this design was demonstrated by observing ˜20 times reduction of the magnetic signature of metallic or ferromagnetic objects.

  8. Damping of confined modes in a ferromagnetic thin insulating film: angular momentum transfer across a nanoscale field-defined interface.

    PubMed

    Adur, Rohan; Du, Chunhui; Wang, Hailong; Manuilov, Sergei A; Bhallamudi, Vidya P; Zhang, Chi; Pelekhov, Denis V; Yang, Fengyuan; Hammel, P Chris

    2014-10-24

    We observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned yttrium iron garnet film by means of the intense local dipolar field of a micromagnetic tip. We find that the damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems, the measured intralayer spin-mixing conductance g_↑↓=5.3×10(19)  m(-2) demonstrates efficient intralayer angular momentum transfer.

  9. Real-space phase field investigation of evolving magnetic domains and twin structures in a ferromagnetic shape memory alloy

    NASA Astrophysics Data System (ADS)

    Wu, H. H.; Pramanick, A.; Ke, Y. B.; Wang, X.-L.

    2016-11-01

    A real-space phase field model combining Landau-Lifshitz-Gilbert equation and time-dependent Ginzburg-Landau equation is developed to investigate the evolution of ferromagnetic domains and martensitic twin structures in a ferromagnetic shape memory alloy at different lengthscales. Both domain and twin structures are obtained by simultaneously solving for minimization of magnetic, elastic, and magnetoelastic coupling energy terms via a nonlinear finite element method. The model is applied to simulate magneto-structural evolution within a nanoparticle and a bulk single-crystal of the alloy Ni2MnGa, which are subjected to mechanical strains. It is shown that a nanoparticle contains magnetic vortex structures within a single twin variant, whereas for a bulk crystal both 90° and 180° domain structures are present within multiple twin variants.

  10. Nonequilibrium condensation and coarsening of field-driven dipolar colloids.

    PubMed

    Jäger, Sebastian; Schmidle, Heiko; Klapp, Sabine H L

    2012-07-01

    In colloidal suspensions, self-organization processes can be easily fueled by external fields. Here we consider monolayers of particles with permanent dipole moments that are driven by rotating external fields. In recent experiments, it has been shown that the particles in such systems self-organize into two-dimensional clusters. Here we report results from a computer simulation study of these pattern forming systems. Specifically, we employ Langevin dynamics simulations, Brownian dynamics simulations that include hydrodynamic interactions, and Wang-Landau Monte Carlo simulations of soft spheres interacting via dipolar potentials. We investigate at which field strengths and frequencies clusters form and explore the influence of hydrodynamic interactions. We also examine the phase behavior of the equilibrium system resulting from a time average of the colloidal interactions in the rotating field. In this way we demonstrate that the clustering described in the driven system arises from a first-order phase transition between a vapor and a condensed phase.

  11. Nonequilibrium condensation and coarsening of field-driven dipolar colloids

    NASA Astrophysics Data System (ADS)

    Jäger, Sebastian; Schmidle, Heiko; Klapp, Sabine H. L.

    2012-07-01

    In colloidal suspensions, self-organization processes can be easily fueled by external fields. Here we consider monolayers of particles with permanent dipole moments that are driven by rotating external fields. In recent experiments, it has been shown that the particles in such systems self-organize into two-dimensional clusters. Here we report results from a computer simulation study of these pattern forming systems. Specifically, we employ Langevin dynamics simulations, Brownian dynamics simulations that include hydrodynamic interactions, and Wang-Landau Monte Carlo simulations of soft spheres interacting via dipolar potentials. We investigate at which field strengths and frequencies clusters form and explore the influence of hydrodynamic interactions. We also examine the phase behavior of the equilibrium system resulting from a time average of the colloidal interactions in the rotating field. In this way we demonstrate that the clustering described in the driven system arises from a first-order phase transition between a vapor and a condensed phase.

  12. Electric field-driven currents in the ionosphere of Mars

    NASA Astrophysics Data System (ADS)

    Fillingim, M. O.; Lillis, R. J.; Ma, Y.

    2011-10-01

    Mars has a complex magnetic topology. Crustal magnetic fields can interact with the solar wind magnetic field to form magnetic cusps. On the nightside, solar wind electron precipitation can produce regions of enhanced ionization at cusps while closed field regions adjacent to cusps can be devoid of significant ionization. Previously, using an electron transport model, we calculated the electron density and spatial structure of the nightside ionosphere of Mars using Mars Global Surveyor electron measurements as input. Localized regions of enhanced ionospheric density were found to occur at magnetic cusps adjacent to low density voids [1]. Additionally, we calculated the horizontal ionospheric currents driven by strong plasma gradients and by thermospheric neutral winds. In the dynamo region of the ionosphere, the collisional ions move in the direction of the applied force (the plasma gradient or neutral wind) while the magnetized electrons move perpendicular to both the applied force and ambient magnetic field. This difference in motion drives horizontal currents. Subsequently, we considered the existence of wind-driven cusp electrojets created by secondary currents arising from polarization electric fields which form in the presence of strong conductivity gradients [2]. At Earth, ionospheric currents at high latitudes are driven predominantly by externally imposed (magnetospheric) electric fields. Here, we compute the horizontal ionospheric currents in the vicinity of magnetic cusps resulting from external electric fields. In the absence of electric field observations, we use the electric field calculated from a global model of the Mars-solar wind interaction as input. We compare the magnitude of these currents with those driven by neutral winds and plasma gradients. Additionally, we estimate the magnitude of the electric field-driven electrojets, analogous to Earth's auroral electrojets. These enhanced currents can lead to localized, enhanced Joule heating

  13. Flow Field Measurement of Mixing Driven by Buoyancy

    NASA Technical Reports Server (NTRS)

    Batur, C.; Zhong, H.

    2003-01-01

    Mixing driven by buoyancy-induced flows inside a cavity consists of stretching and folding of an interface. Measurement of the flow field using particle imaging velocimetry shows that during stretching the flow field has a single elliptic point, thus dominated by a single vortex. However, global bifurcation that results in folding introduces a hyperbolic point whereby the flow field degenerates to multiple vortex interactions. The short-lived coherent structure observed during mixing which results in the Rayleigh- Taylor morphology is attributed to vortex interactions. The mixing characteristics of non-homogeneous fluids driven by buoyancy are important towards understanding transport phenomenon in a microgravity environment. Mixing consists of stretching and folding of an interface due to a flow field whose intensity depends on the body force. For miscible liquids, the characteristic of the flow field determines whether mass transport is governed by diffusion or bulk stirring which induces mixing. For technologically important processes, transport of mass is governed by the coupling of the body force to scalar gradients such as concentration and or temperature' 2 3 . In order to lend insight into these classes of problems we consider a model experimental system to study mixing driven by buoyancy-induced flows. The characteristics of mixing is addressed from detail measurements of the flow field using particle imaging velocimetry (PIV), and its corresponding interface dynamics using image processing techniques.

  14. Amplification of magnetic fields by supernova-driven turbulence

    NASA Astrophysics Data System (ADS)

    Kim, J.; Balsara, D. S.

    2006-06-01

    Observations of μG magnetic fields in radio galaxies at cosmological epochs as early as around z=2 have shortened the available time for dynamo action. This fact suggests that the mean-field dynamo mechanism in a global galactic scale either is too slow to amplify a seed field generated by the Biermann battery effect to the level of the observed field strength at z˜2 or needs much stronger seed fields of an order of 10-10 G. A ``contamination'' picture that amplified magnetic fields in smaller objects, such as stars or AGNs, within a relatively shorter timescale spread out through supernova ejecta, stellar winds, and AGN jets to nearby environments is gaining momentum. In line with this picture, we demonstrate, through three-dimensional numerical experiments, that magnetic fields can be amplified by supernova-driven turbulence with two orders of magnitude smaller e-folding timescale than that of the mean-field dynamo mechanism. Therefore, supernova-driven turbulence may play an important role in amplifying small-scale B-fields in any astrophysical systems that have harbored massive stars.

  15. Magnetoanisotropic Josephson effect due to interfacial spin-orbit fields in superconductor/ferromagnet/superconductor junctions

    NASA Astrophysics Data System (ADS)

    Costa, Andreas; Högl, Petra; Fabian, Jaroslav

    2017-01-01

    We study theoretically the effects of interfacial Rashba and Dresselhaus spin-orbit coupling in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions—with allowing for tunneling barriers between the ferromagnetic and superconducting layers—by solving the Bogoljubov-de Gennes equation for realistic heterostructures and applying the Furusaki-Tsukada technique to calculate the electric current at a finite temperature. The presence of spin-orbit couplings leads to out-of-plane and in-plane magnetoanisotropies of the Josephson current, which are giant in comparison to current magnetoanisotropies in similar normal-state ferromagnet/normal metal (F/N) junctions. Especially huge anisotropies appear in the vicinity of 0 -π transitions, caused by the exchange-split bands in the ferromagnetic metal layer. We also show that the direction of the Josephson critical current can be controlled (inducing 0 -π transitions) by the strength of the spin-orbit coupling and, more crucial, by the orientation of the magnetization. Such a control can bring new functionalities into Josephson junction devices.

  16. Dynamics of Spinor Condensates Driven by an Inhomogeneous Magnetic Field

    NASA Astrophysics Data System (ADS)

    Zheng, Gong-Ping; Chang, Gao-Zhan; Li, Pin; Li, Ting; Wei, L. F.

    2017-06-01

    A variational wavefunction including the breather, dipole and scissor modes simultaneously is constructed to investigate the collective-excitation dynamics of spin-1 ^{87}Rb condensates driven by a space- and time-dependent magnetic field. When the Dirac point never enters the condensate, it is shown that the dipole, breather and scissor modes will be all excited driven by the sinusoidal oscillation of the Dirac point, due to the coupling of different collective modes from the inhomogeneity of the magnetic field. A resonance-driving phenomenon is observed. If the Dirac point passes through the condensate, our numerical results agree with most experimental observations (Ray et al. in Nature 505:657, 2014) and find that the center of mass of the condensate does not follow the zero point of the magnetic field. Hopefully, our method can be extended to study the similar dynamics for the other spinor condensates.

  17. An effective magnetic field from optically driven phonons

    NASA Astrophysics Data System (ADS)

    Nova, T. F.; Cartella, A.; Cantaluppi, A.; Först, M.; Bossini, D.; Mikhaylovskiy, R. V.; Kimel, A. V.; Merlin, R.; Cavalleri, A.

    2016-10-01

    Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator-metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research--for example, as a way to affect the topology of electronic phases.

  18. Dynamics of Spinor Condensates Driven by an Inhomogeneous Magnetic Field

    NASA Astrophysics Data System (ADS)

    Zheng, Gong-Ping; Chang, Gao-Zhan; Li, Pin; Li, Ting; Wei, L. F.

    2017-10-01

    A variational wavefunction including the breather, dipole and scissor modes simultaneously is constructed to investigate the collective-excitation dynamics of spin-1 ^{87}Rb condensates driven by a space- and time-dependent magnetic field. When the Dirac point never enters the condensate, it is shown that the dipole, breather and scissor modes will be all excited driven by the sinusoidal oscillation of the Dirac point, due to the coupling of different collective modes from the inhomogeneity of the magnetic field. A resonance-driving phenomenon is observed. If the Dirac point passes through the condensate, our numerical results agree with most experimental observations (Ray et al. in Nature 505:657, 2014) and find that the center of mass of the condensate does not follow the zero point of the magnetic field. Hopefully, our method can be extended to study the similar dynamics for the other spinor condensates.

  19. Three-dimensional stress-induced magnetic anisotropic constitutive model for ferromagnetic material in low intensity magnetic field

    NASA Astrophysics Data System (ADS)

    Sun, Le; Liu, Xin'en; Jia, Dong; Niu, Hongpan

    2016-09-01

    Metal magnetic memory (MMM) technique is a promising tool for inspecting early damage in ferromagnetic components due to its high sensitivity to stress in weak geomagnetic field. However, the quantitative analysis methods for the MMM haven't been sufficiently studied yet for absence of a reasonable constitutive model. A three-dimensional stress-induced magnetic anisotropic constitutive model is proposed in this paper to study magneto-mechanical coupling effect of the MMM. The model is developed in principal stress space and a linear relation between magnetization and magnetic field is employed for low intensity magnetic field. As a result, stress-induced magnetic anisotropy is represented by stress dependence of magnetic permeability in different directions, which is simple and convenient for applications in the MMM technique. Based on the model, the effect of stress on magnetic permeability and surface magnetic field is computed and compared with experimental data for a tensioned ferromagnetic specimen in low intensity magnetic field. The good consistency implies the validity of the proposed model.

  20. The magnetic properties of one-dimensional spin-1 ferromagnetic Heisenberg model in a magnetic field within Callen approximation

    NASA Astrophysics Data System (ADS)

    Liu, Ming-Wei; Chen, Yuan; Song, Chuang-Chuang; Wu, You; Ding, Hai-Ling

    2011-03-01

    The effect of magnetic field h on the magnetic properties of the one-dimensional spin-1 ferromagnetic Heisenberg model is studied by the double-time Green's function method. The magnetization and susceptibility are obtained within the Callen approximation. The zero-field susceptibility is as a decreasing function of the temperature T. The magnetization m increases in the whole field region, but the susceptibility maximum χ(Tm) decreases. The position Tm of the susceptibility maximum is both solved analytically and fits well to be a power law Tm∼hγ at low fields and to be linear increasing at high fields. The height χ(Tm) decreases as a power law χ(Tm)∼h with h increasing. The exponents (γ,β) obtained in our results agree with the other theoretical results. Our results are roughly in agreement with the results obtained in the experiment of Ni(OH)(NO3)H2O.

  1. Effect of Annealing in Magnetic Field on Ferromagnetic Nanoparticle Formation in Cu-Al-Mn Alloy with Induced Martensite Transformation.

    PubMed

    Titenko, Anatoliy; Demchenko, Lesya

    2016-12-01

    The paper considers the influence of aging of high-temperature phase on subsequent martensitic transformation in Cu-Al-Mn alloy. The morphology of behavior of martensitic transformation as a result of alloy aging under annealing in a constant magnetic field with different sample orientation relatively to the field direction and without field was studied for direct control of the processes of martensite induction at cooling. Temperature dependences of electrical resistance, magnetic susceptibility, and magnetization, as well as field dependences of magnetization, and phase composition were found. The tendency to the oriented growth of precipitated ferromagnetic phase nanoparticles in a direction of applied field and to an increase of their volume fraction under thermal magnetic treatment of material that favors a reversibility of induced martensitic transformation is observed.

  2. A model to predict the ultrasonic field radiated by magnetostrictive effects induced by EMAT in ferromagnetic parts

    NASA Astrophysics Data System (ADS)

    Clausse, B.; Lhémery, A.; Walaszek, H.

    2017-01-01

    An Electro-Magnetic Acoustic Transducer (EMAT) is a non-contact source used in Ultrasonic Testing (UT) which generates three types of dynamic excitations into a ferromagnetic part: Lorentz force, magnetisation force, and magnetostrictive effect. This latter excitation is a strain resulting from a magnetoelastic interaction between the external magnetic field and the mechanical part. Here, a tensor model is developed to transform this effect into an equivalent body force. It assumes weak magnetoelastic coupling and a dynamic magnetic field much smaller than the static one. This approach rigorously formulates the longitudinal Joule’s magnetostriction, and makes it possible to deal with arbitrary material geometries and EMAT configurations. Transduction processes induced by an EMAT in ferromagnetic media are then modelled as equivalent body forces. But many models developed for efficiently predicting ultrasonic field radiation in solids assume source terms given as surface distributions of stress. To use these models, a mathematical method able to accurately transform these body forces into equivalent surface stresses has been developed. By combining these formalisms, the magnetostrictive strain is transformed into equivalent surface stresses, and the ultrasonic field radiated by magnetostrictive effects induced by an EMAT can be both accurately and efficiently predicted. Numerical examples are given for illustration.

  3. Electric-field-driven resistive switching in dissipative Hubbard model

    NASA Astrophysics Data System (ADS)

    Li, Jiajun; Aron, Camille; Kotliar, Gabriel; Han, Jong

    Understanding of solids driven out of equilibrium by external fields has been one of the central goals in condensed matter physics for the past century and is relevant to nanotechnology applications such as resistive transitions. We study how strongly correlated electrons on a dissipative lattice evolve from equilibrium when driven by a constant electric field, focusing on the extent of the linear regime and hysteretic non-linear effects at higher fields. We access the non-equilibrium steady states, non-perturbatively in both the field and the electronic interactions, by means of a non-equilibrium dynamical mean-field theory in the Coulomb gauge. The linear response regime is limited by Joule heating effects and breaks down at fields orders of magnitude smaller than the quasi-particle energy scale. For large electronic interactions, strong but experimentally accessible electric fields can induce a resistive switching by driving the strongly correlated metal into a Mott insulator. Hysteretic I- V curves suggest that the non-equilibrium current is carried through a spatially inhomogeneous metal-insulator mixed state.

  4. Electric-field-driven resistive switching in dissipative Hubbard model

    NASA Astrophysics Data System (ADS)

    Li, Jiajun; Aron, Camille; Kotliar, Gabriel; Han, Jong

    2015-03-01

    Understanding of solids driven out of equilibrium by external fields has been one of the central goals in condensed matter physics for the past century and is relevant to nanotechnology applications such as resistive transitions. We study how strongly correlated electrons on a dissipative lattice evolve from equilibrium when driven by a constant electric field, focusing on the extent of the linear regime and hysteretic non-linear effects at higher fields. We access the non-equilibrium steady states, non-perturbatively in both the field and the electronic interactions, by means of a non-equilibrium dynamical mean-field theory in the Coulomb gauge. The linear response regime is limited by Joule heating effects and breaks down at fields orders of magnitude smaller than the quasi-particle energy scale. For large electronic interactions, strong but experimentally accessible electric fields can induce a resistive switching by driving the strongly correlated metal into a Mott insulator. Hysteretic I- V curves suggest that the non-equilibrium current is carried through a spatially inhomogeneous metal-insulator mixed state. Supported by NSF DMR-0907150, NSF DMR-1308141.

  5. Cathode driven high gain crossed-field amplifier

    NASA Astrophysics Data System (ADS)

    1983-07-01

    The objective of this three-phase program is to achieve the design of a cathode driven high gain re-entrant Crossed Field Amplifier capable of meeting the parameters of Raytheon Company specification No. 968838 dated 10 May 1978. The effort includes the fabrication and test of three developmental and four final configuration tubes. One final configuration tube will be life tested and two will be delivered to the Navy. The tasks discussed during this report period relate to the cold tests performed on various subassemblies of model no. 4 and on the sealed-in model no. 4 of the S-band high gain cathode driven crossed field amplifier. Based on the performance of model no. 3 certain remedial measures have been implemented in model no. 4 that have resulted in the elimination of key resonances within the tube and an improvement in the isolation between the cathode and anode circuits.

  6. Stable solutions of inflation driven by vector fields

    NASA Astrophysics Data System (ADS)

    Emami, Razieh; Mukohyama, Shinji; Namba, Ryo; Zhang, Ying-li

    2017-03-01

    Many models of inflation driven by vector fields alone have been known to be plagued by pathological behaviors, namely ghost and/or gradient instabilities. In this work, we seek a new class of vector-driven inflationary models that evade all of the mentioned instabilities. We build our analysis on the Generalized Proca Theory with an extension to three vector fields to realize isotropic expansion. We obtain the conditions required for quasi de-Sitter solutions to be an attractor analogous to the standard slow-roll one and those for their stability at the level of linearized perturbations. Identifying the remedy to the existing unstable models, we provide a simple example and explicitly show its stability. This significantly broadens our knowledge on vector inflationary scenarios, reviving potential phenomenological interests for this class of models.

  7. Field theory and anisotropy of a cubic ferromagnet near the Curie point

    NASA Astrophysics Data System (ADS)

    Kudlis, A.; Sokolov, A. I.

    2017-02-01

    It is known that critical fluctuations can change the effective anisotropy of a cubic ferromagnet near the Curie point. If the crystal undergoes a phase transition into the orthorhombic phase and the initial anisotropy is not too strong, then the effective anisotropy acquires the universal value A* = v*/ u* at T c, where u* and v* are the coordinates of the cubic fixed point of the renormalization group equations in the scaling equation of state and expressions for nonlinear susceptibilities. Using the pseudo-ɛ-expansion method, we find the numerical value of the anisotropy parameter A at the critical point. Padé resummation of the six-loop pseudo-ɛ-expansions for u*, v*, and A* leads to the estimate A* = 0.13 ± 0.01, giving evidence that observation of anisotropic critical behavior of cubic ferromagnets in physical and computer experiments is entirely possible.

  8. Predicted Very Large Thermoelectric Effect in Ferromagnet-Superconductor Junctions in the Presence of a Spin-Splitting Magnetic Field

    NASA Astrophysics Data System (ADS)

    Ozaeta, A.; Virtanen, P.; Bergeret, F. S.; Heikkilä, T. T.

    2014-02-01

    We show that a huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin split by a Zeeman field with a ferromagnet with a nonzero polarization. The resulting thermopower exceeds kB/e by a large factor, and the thermoelectric figure of merit ZT can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.

  9. Predicted very large thermoelectric effect in ferromagnet-superconductor junctions in the presence of a spin-splitting magnetic field.

    PubMed

    Ozaeta, A; Virtanen, P; Bergeret, F S; Heikkilä, T T

    2014-02-07

    We show that a huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin split by a Zeeman field with a ferromagnet with a nonzero polarization. The resulting thermopower exceeds kB/e by a large factor, and the thermoelectric figure of merit ZT can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.

  10. Effects of 50Hz Homogeneous Ferromagnetic Field on the Concentrations of Intracellular Calcium-ion of SNU Cells and Lymphma Cells In Vitro.

    PubMed

    Guo, Hongyong; Xing, Lingxiao; Zheng, Mo; Fan, Changzai; Li, Ying; Wang, Junling; Zhang, Xianghong; Yan, Weili

    2005-01-01

    The study was designed to observe the effects of 50 Hz homogeneous ferromagnetic field (0.097T) exposure on change of intracellular Calcium-ion concentration of human low differentiation gastric adenocarcinoma cell line - SNU and lymphocytes in vitro. SNU cells and human peripheral blood lymphocytes in vitro were exposured to 50 Hz homogeneous ferromagnetic field at different time periods. Cells were harvested after magnetic field exposure. The concentration of intracellular Calcium-ion of cells in control and different experimental groups were measured with confocal microscope. The concentration of intracellular Calcium-ion of SNU cells and human peripheral blood lymphocytes were all increased after magnetic field exposure. 50Hz homogeneous ferromagnetic field exposure could induce increases of the concentration of intracellular Calcium-ion of SNU cells and human peripheral blood lymphocytes in vitro.

  11. Monte Carlo study of magnetization dynamics in uniaxial ferromagnetic nanowires in the presence of oscillating and biased magnetic fields

    NASA Astrophysics Data System (ADS)

    Yüksel, Yusuf

    2015-03-01

    We examine the dynamic phase transition properties of ferromagnetic uniaxial nanowires with tunable radius r in the presence of both oscillating and biased magnetic fields. Variation of the transition temperature as a function of amplitude h0 of the oscillating field has been analyzed in the absence of magnetic bias hb and at constant oscillation period P . We find that the transition temperature as a function of h0 exhibits an exponential decay behavior. We also investigate the magnetization dynamics in terms of magnetic hysteresis loops as functions of the parameters hb,h0 , and P . Our calculations are qualitatively in agreement with the recent experimental results obtained for uniaxial cobalt films in which it was concluded that the bias field hb is the conjugate field of the dynamic order parameter.

  12. 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

  13. Electric Field Control of the Ferromagnetic CaRuO3 /CaMnO3 Interface

    NASA Astrophysics Data System (ADS)

    Grutter, Alexander; Kirby, Brian; Gray, Matthew; Flint, Charles; Suzuki, Yuri; Borchers, Julie

    2015-03-01

    Electric field control of magnetism has been recognized as one of the most important goals in nanoscale magnetics research. The most popular routes towards achieving magnetoelectric (ME) coupling have focused on heterostructures incorporating multiferroics or ferroelectrics. Such studies often rely on voltage induced distortion to induce strain in the magnetic film and alter the magnetic properties. However, successful attempts to induce ME coupling without multiferroicity or magnetoelasticity remain relatively rare. The ferromagnetic interface between the antiferromagnetic insulator CaMnO3 and the paramagnetic metal CaRuO3 is a promising candidate for direct magnetization control. This interfacial ferroagnetism is stabilized through the competition between interfacial double exchange and antiferromagnetic superexchange between adjacent Mn4+ so that the system is expected to be very sensitive to small changes in interfacial carrier density. Using polarized neutron reflectometry, we have probed the electric field dependence of the interfacial magnetization of CaRuO3/CaMnO3 bilayers deposited on SrTiO3. We find that electric fields of +/-8 kV/m are sufficient to switch the interfaces from largely ferromagnetic to completely antiferromagnetic.

  14. Instability-driven electromagnetic fields in coronal plasmas

    SciTech Connect

    Manuel, M. J.-E.; Li, C. K.; Seguin, F. H.; Sinenian, N.; Frenje, J. A.; Casey, D. T.; Petrasso, R. D.; Hager, J. D.; Betti, R.; Hu, S. X.; Delettrez, J.; Meyerhofer, D. D.

    2013-04-15

    Filamentary electromagnetic fields previously observed in the coronae of laser-driven spherical targets [F. H. S eguin et al., Phys. Plasma. 19, 012701 (2012)] have been further investigated in laser irradiated plastic foils. Face-on proton-radiography provides an axial view of these filaments and shows coherent cellular structure regardless of initial foil-surface conditions. The observed cellular fields are shown to have an approximately constant scale size of 210 lm throughout the plasma evolution. A discussion of possible field-generation mechanisms is provided and it is demonstrated that the likely source of the cellular field structure is the magnetothermal instability. Using predicted temperature and density profiles, the fastest growing modes of this instability were found to be slowly varying in time and consistent with the observed cellular size.

  15. Instability-driven electromagnetic fields in coronal plasmas

    DOE PAGES

    Manuel, M. J.-E.; Li, C. K.; Seguin, F. H.; ...

    2013-04-15

    Filamentary electromagnetic fields previously observed in the coronae of laser-driven spherical targets [F. H. S eguin et al., Phys. Plasma. 19, 012701 (2012)] have been further investigated in laser irradiated plastic foils. Face-on proton-radiography provides an axial view of these filaments and shows coherent cellular structure regardless of initial foil-surface conditions. The observed cellular fields are shown to have an approximately constant scale size of 210 lm throughout the plasma evolution. A discussion of possible field-generation mechanisms is provided and it is demonstrated that the likely source of the cellular field structure is the magnetothermal instability. Using predicted temperature andmore » density profiles, the fastest growing modes of this instability were found to be slowly varying in time and consistent with the observed cellular size.« less

  16. Electric Field-Driven Assembly of Sulfonated Polystyrene Microspheres

    PubMed Central

    Mikkelsen, Alexander; Wojciechowski, Jarosław; Rajňák, Michal; Kurimský, Juraj; Khobaib, Khobaib; Kertmen, Ahmet; Rozynek, Zbigniew

    2017-01-01

    A designed assembly of particles at liquid interfaces offers many advantages for development of materials, and can be performed by various means. Electric fields provide a flexible method for structuring particles on drops, utilizing electrohydrodynamic circulation flows, and dielectrophoretic and electrophoretic interactions. In addition to the properties of the applied electric field, the manipulation of particles often depends on the intrinsic properties of the particles to be assembled. Here, we present an easy approach for producing polystyrene microparticles with different electrical properties. These particles are used for investigations into electric field-guided particle assembly in the bulk and on surfaces of oil droplets. By sulfonating polystyrene particles, we produce a set of particles with a range of dielectric constants and electrical conductivities, related to the sulfonation reaction time. The paper presents diverse particle behavior driven by electric fields, including particle assembly at different droplet locations, particle chaining, and the formation of ribbon-like structures with anisotropic properties. PMID:28772690

  17. Domain wall motion driven by an oscillating magnetic field

    NASA Astrophysics Data System (ADS)

    Moon, Kyoung-Woong; Kim, Duck-Ho; Kim, Changsoo; Kim, Dae-Yun; Choe, Sug-Bong; Hwang, Chanyong

    2017-03-01

    The coherent unidirectional motion of magnetic domain walls (DWs) is a key technology used in memory and logic device applications, as demonstrated in magnetic strips by electric current flow as well as in films by oscillation of a tilted magnetic field. Here we introduce a coherent unidirectional motion of DWs in the strip, utilizing an oscillating field, which is described within a previous 1D model. The essential criterion for DW motion in this approach is the oscillating-field-induced modulation of the DW width, which has not been previously considered. This DW motion driven by width modulation sheds light on high frequency domain manipulation in spin devices. A comprehensive inspection of field angle dependence reveals that unidirectional DW motion in this model requires chiral DWs, followed by asymmetric deformation of the domain shape.

  18. Magnetic-field-induced quantum criticality in a spin- S planar ferromagnet with single-ion anisotropy

    NASA Astrophysics Data System (ADS)

    Mercaldo, M. T.; Rabuffo, I.; De Cesare, L.; Caramico D'Auria, A.

    2013-08-01

    The effects of single-ion anisotropy on quantum criticality in a d-dimensional spin- S planar ferromagnet is explored by means of the two-time Green's function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter D. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. However, for values of the uniaxial crystal-field parameter below a positive threshold, the conventional magnetic-field-induced quantum critical scenario remains unchanged.

  19. Observation of ferromagnetic exchange, spin crossover, reductively induced oxidation, and field-induced slow magnetic relaxation in monomeric cobalt nitroxides.

    PubMed

    Gass, Ian A; Tewary, Subrata; Nafady, Ayman; Chilton, Nicholas F; Gartshore, Christopher J; Asadi, Mousa; Lupton, David W; Moubaraki, Boujemaa; Bond, Alan M; Boas, John F; Guo, Si-Xuan; Rajaraman, Gopalan; Murray, Keith S

    2013-07-01

    The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of

  20. Electric-field-driven polymer entry into asymmetric nanoscale channels.

    PubMed

    Nikoofard, Narges; Fazli, Hossein

    2012-02-01

    The electric-field-driven entry process of flexible charged polymers such as single-stranded DNA (ssDNA) into asymmetric nanoscale channels such as the α-hemolysin protein channel is studied theoretically and using molecular dynamics simulations. Dependence of the height of the free-energy barrier on the polymer length, the strength of the applied electric field, and the channel entrance geometry is investigated. It is shown that the squeezing effect of the driving field on the polymer and the lateral confinement of the polymer before its entry to the channel crucially affect the barrier height and its dependence on the system parameters. The attempt frequency of the polymer for passing the channel is also discussed. Our theoretical and simulation results support each other and describe related data sets of polymer translocation experiments through the α-hemolysin protein channel reasonably well.

  1. Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions.

    PubMed

    Kolenda, Stefan; Machon, Peter; Beckmann, Detlef; Belzig, Wolfgang

    2016-01-01

    Background: Thermoelectric effects result from the coupling of charge and heat transport and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron-hole symmetry, which is usually quite small in metal structures. In addition, thermoelectric effects decrease towards low temperatures, which usually makes them vanishingly small in metal nanostructures in the sub-Kelvin regime. Results: We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the dependence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction. Conclusion: Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators.

  2. Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions

    PubMed Central

    Kolenda, Stefan; Machon, Peter

    2016-01-01

    Background: Thermoelectric effects result from the coupling of charge and heat transport and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron–hole symmetry, which is usually quite small in metal structures. In addition, thermoelectric effects decrease towards low temperatures, which usually makes them vanishingly small in metal nanostructures in the sub-Kelvin regime. Results: We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the dependence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction. Conclusion: Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators. PMID:28144509

  3. 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

  4. Electric field-driven water dipoles: nanoscale architecture of electroporation.

    PubMed

    Tokman, Mayya; Lee, Jane HyoJin; Levine, Zachary A; Ho, Ming-Chak; Colvin, Michael E; Vernier, P Thomas

    2013-01-01

    Electroporation is the formation of permeabilizing structures in the cell membrane under the influence of an externally imposed electric field. The resulting increased permeability of the membrane enables a wide range of biological applications, including the delivery of normally excluded substances into cells. While electroporation is used extensively in biology, biotechnology, and medicine, its molecular mechanism is not well understood. This lack of knowledge limits the ability to control and fine-tune the process. In this article we propose a novel molecular mechanism for the electroporation of a lipid bilayer based on energetics analysis. Using molecular dynamics simulations we demonstrate that pore formation is driven by the reorganization of the interfacial water molecules. Our energetics analysis and comparisons of simulations with and without the lipid bilayer show that the process of poration is driven by field-induced reorganization of water dipoles at the water-lipid or water-vacuum interfaces into more energetically favorable configurations, with their molecular dipoles oriented in the external field. Although the contributing role of water in electroporation has been noted previously, here we propose that interfacial water molecules are the main players in the process, its initiators and drivers. The role of the lipid layer, to a first-order approximation, is then reduced to a relatively passive barrier. This new view of electroporation simplifies the study of the problem, and opens up new opportunities in both theoretical modeling of the process and experimental research to better control or to use it in new, innovative ways.

  5. Keldysh field theory for driven open quantum systems.

    PubMed

    Sieberer, L M; Buchhold, M; Diehl, S

    2016-09-01

    Recent experimental developments in diverse areas-ranging from cold atomic gases to light-driven semiconductors to microcavity arrays-move systems into the focus which are located on the interface of quantum optics, many-body physics and statistical mechanics. They share in common that coherent and driven-dissipative quantum dynamics occur on an equal footing, creating genuine non-equilibrium scenarios without immediate counterpart in equilibrium condensed matter physics. This concerns both their non-thermal stationary states and their many-body time evolution. It is a challenge to theory to identify novel instances of universal emergent macroscopic phenomena, which are tied unambiguously and in an observable way to the microscopic drive conditions. In this review, we discuss some recent results in this direction. Moreover, we provide a systematic introduction to the open system Keldysh functional integral approach, which is the proper technical tool to accomplish a merger of quantum optics and many-body physics, and leverages the power of modern quantum field theory to driven open quantum systems.

  6. Keldysh field theory for driven open quantum systems

    NASA Astrophysics Data System (ADS)

    Sieberer, L. M.; Buchhold, M.; Diehl, S.

    2016-09-01

    Recent experimental developments in diverse areas—ranging from cold atomic gases to light-driven semiconductors to microcavity arrays—move systems into the focus which are located on the interface of quantum optics, many-body physics and statistical mechanics. They share in common that coherent and driven-dissipative quantum dynamics occur on an equal footing, creating genuine non-equilibrium scenarios without immediate counterpart in equilibrium condensed matter physics. This concerns both their non-thermal stationary states and their many-body time evolution. It is a challenge to theory to identify novel instances of universal emergent macroscopic phenomena, which are tied unambiguously and in an observable way to the microscopic drive conditions. In this review, we discuss some recent results in this direction. Moreover, we provide a systematic introduction to the open system Keldysh functional integral approach, which is the proper technical tool to accomplish a merger of quantum optics and many-body physics, and leverages the power of modern quantum field theory to driven open quantum systems.

  7. Electric field-driven, magnetically-stabilized ferro-emulsion phase contactor

    DOEpatents

    Scott, T.C.

    1990-07-17

    Methods and systems are disclosed for interfacial surface area contact between a dispersed phase liquid and a continuous phase liquid in counter-current flow for purposes such as solvent extraction. Initial droplets of a dispersed phase liquid material containing ferromagnetic particles functioning as a packing'' are introduced to a counter-current flow of the continuous phase. A high intensity pulsed electric field is applied so as to shatter the initial droplets into a ferromagnetic emulsion comprising many smaller daughter droplets having a greater combined total surface area than that of the initial droplets in contact with the continuous phase material. A magnetic field is applied to control the position of the ferromagnetic emulsion for enhanced coalescence of the daughter droplets into larger reformed droplets. 2 figs.

  8. Electric field-driven, magnetically-stabilized ferro-emulsion phase contactor

    DOEpatents

    Scott, Timothy C.

    1990-01-01

    Methods and systems for interfacial surface area contact between a dispersed phase liquid and a continuous phase liquid in counter-current flow for purposes such as solvent extraction. Initial droplets of a dispersed phase liquid material containing ferromagnetic particles functioning as a "packing" are introduced to a counter-current flow of the continuous phase. A high intensity pulsed electric field is applied so as to shatter the initial droplets into a ferromagnetic emulsion comprising many smaller daughter droplets having a greater combined total surface area than that of the initial droplets in contact with the continuous phase material. A magnetic field is applied to control the position of the ferromagnetic emulsion for enhanced coalescence of the daughter droplets into larger reformed droplets.

  9. Zero-bias-field microwave dynamic magnetic properties in trapezoidal ferromagnetic stripe

    NASA Astrophysics Data System (ADS)

    Bi, Mei; Wang, Xin; Lu, Haipeng; Zhang, Li; Deng, Longjiang; Xie, Jianliang

    2016-06-01

    Dynamic magnetization response of the axially magnetized ferromagnetic stripe with trapezoidal cross section has been studied. The stripe with beveled edges exhibits multiple resonant peaks modes under an in-plane microwave excitation compared with the single resonant of vertical edge surfaces. The complexity of the observed response is attributed to the spatially nonuniform equilibrium spin distribution at the stripe edges. Micromagnetic simulations identify spin waves as spatially localized mode at the modified edges. This one is also described by effective pinning boundary conditions taking into account finite-size effects, which is related to the exchange interaction, surface anisotropy and dipole-dipole interaction. These results provide detailed insights into the nonlinear spin dynamics of microstructures influenced by the edge properties.

  10. Spin-wave modes in ferromagnetic nanodisks, their excitation via alternating currents and fields, and auto-oscillations

    NASA Astrophysics Data System (ADS)

    Mancilla-Almonacid, D.; Arias, R. E.

    2017-06-01

    The excitation of the linear spin wave modes of a soft ferromagnetic free layer of a nanopillar structure through dc-ac currents that traverse the structure is studied, as well as with ac magnetic fields. There is interest in understanding the magnetization dynamics in these structures since they may be used as microwave sources when these nano-oscillators enter into auto-oscillatory regimes. The free layer is a soft ferromagnet, like Permalloy, in the shape of a circular disk, with a very small thickness in the range of the exchange length. Using a description of the magnetization dynamics in terms of a Hamiltonian for weakly interacting waves, we determine the spin wave modes of the structure under two approximations: a very thin film limit, and under a model that includes the effect of the full magnetostatic interaction. We consider direct and parametric excitations of different spin wave modes with ac currents, i.e., with exciting frequency approximately equal to the frequency of the mode or to twice its value, respectively. The Oersted field mainly plays a role in the direct resonant excitation of the modes. Our main conclusion is that for a dc current below the critical value necessary for the development of auto-oscillations, using parametric excitation, a very high value of the ac current is necessary to reach the auto-oscillatory behavior in this geometry. However, if the out-of-plane component of the spin transfer torque is high enough, the ac critical current for auto-oscillations is significantly reduced, leading to a signature for its detection. We comment on parallel pumping and transverse excitation using ac magnetic fields.

  11. CFT driven cosmology and conformal higher spin fields

    NASA Astrophysics Data System (ADS)

    Barvinsky, A. O.

    2016-05-01

    Conformal higher spin (CHS) field theory, which is a solid part of recent advanced checks of AdS/CFT correspondence, finds applications in cosmology. The hidden sector of weakly interacting CHS fields suggests a resolution of the hierarchy problem in the model of initial conditions for inflationary cosmology driven by a conformal field theory. These initial conditions are set by thermal garland-type cosmological instantons in the sub-Planckian energy range for the model of CHS fields with a large positive coefficient β of the Gauss-Bonnet term in their total conformal anomaly and a large number of their polarizations N . The upper bound of this range MP/√{β } is shown to be much lower than the gravitational cutoff MP/√{N } which is defined by the requirement of smallness of the perturbatively nonrenormalizable graviton loop contributions. In this way we justify the approximation scheme in which the nonrenormalizable graviton sector is subject to effective field theory under this cutoff, whereas the renormalizable sector of multiple CHS fields is treated beyond perturbation theory and dynamically generates the bound on the inflation scale of the CFT cosmology MP/√{β }≪MP/√{N }. This confirms recent predictions for the origin of the Starobinsky R2 and Higgs inflation models from the CHS cosmology, which occurs at the energy scale 3 or 4 orders of magnitude below the gravitational cutoff, √{N /β }˜10-3- 10-4 . We also consider cosmological models dominated by fermionic CHS fields with a negative β and anomaly free models of infinite towers of CHS fields with β =0 and briefly discuss the status of unitarity in CHS models.

  12. Transverse complex magnetic susceptibility of single-domain ferromagnetic particles with uniaxial anisotropy subjected to a longitudinal uniform magnetic field

    NASA Astrophysics Data System (ADS)

    Kalmykov, Yu. P.; Coffey, W. T.

    1997-08-01

    The infinite hierarchy of differential-recurrence relations for the equilibrium transverse correlation functions appropriate to magnetic relaxation of single-domain ferromagnetic particles with uniaxial anisotropy subjected to a uniform external magnetic field H0 is derived by averaging Gilbert's equation. Exact expressions in terms of matrix continued fractions for the transverse complex magnetic susceptibility are obtained with the aid of linear-response theory by solving the infinite hierarchy. The principal features of the spectra are emphasized in figures showing the real and imaginary parts of the complex magnetic susceptibility. The accuracy and the range of the applicability of analytical results based on the effective eigenvalue method is established. It is shown that this method provides in general a good approximation to the exact solution with the exception of the range of low-to-intermediate barrier heights of the anisotropy potential where at small H0 there exists essentially a spread of the precession frequencies of the magnetization.

  13. Electric field tunable 60 GHz ferromagnetic resonance response in barium ferrite-barium strontium titanate multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Song, Young-Yeal; Das, Jaydip; Krivosik, Pavol; Mo, Nan; Patton, Carl E.

    2009-05-01

    A magnetic-ferroelectric film heterostructure with a large electric field tuning of the ferromagnetic resonance (FMR) mode was fabricated. Pulse laser deposited 30 nm thick Pt electrodes and 3 μm thick barium strontium titanate films on Nb-doped strontium titanate substrates were capped with an unbonded 200 μm thick single crystal in-plane c-axis barium hexaferrite slab. The structure gives a 60 GHz FMR frequency shift of 16 MHz at a bias of 29 V, for an average response of 0.55 MHz/V. The maximum incremental tuning response at 29 V was 1.3 MHz/V. This is a hundredfold improvement over previous results.

  14. [The influence of ferromagnetic screening from natural electromagnetic fields on the hematological and toxicogenomic indexes in animals].

    PubMed

    Ivanov, S D; Nikitina, V N; Iamshanov, V A; Kovan'ko, E G; Liashko, G G; Monakhov, A S; Koshelevskiĭ, V K

    2010-01-01

    The aim of the study was to reveal during 45 days consequences of ferromagnetic screening (FS) on the hematological and toxicogenomic indexes in rats. An express-method of nucleiod DNA content in blood by help fluorescent indication was used for alive quantitative evaluation of the toxicogenomic effects. The FS for both gender rats during 48 hrs resulted in both decreased magnetic field by 4-10 microT1 and leucopenia by 14 day of experiment. The aneu-/polyploidy index in male leucocytes was increased by 48 hr of FS and lasted out to 12-28 days after the screen removal. The leukocyte apoptosis was enhanced in female in 48 hrs only after the FS start.

  15. Electrically detected ferromagnetic resonance

    SciTech Connect

    Goennenwein, S. T. B.; Schink, S. W.; Brandlmaier, A.; Boger, A.; Opel, M.; Gross, R.; Keizer, R. S.; Klapwijk, T. M.; Gupta, A.; Huebl, H.; Bihler, C.; Brandt, M. S.

    2007-04-16

    We study the magnetoresistance properties of thin ferromagnetic CrO{sub 2} and Fe{sub 3}O{sub 4} films under microwave irradiation. Both the sheet resistance {rho} and the Hall voltage V{sub Hall} characteristically change when a ferromagnetic resonance (FMR) occurs in the film. The electrically detected ferromagnetic resonance (EDFMR) signals closely match the conventional FMR, measured simultaneously, in both resonance fields and line shapes. The sign and the magnitude of the resonant changes {delta}{rho}/{rho} and {delta}V{sub Hall}/V{sub Hall} can be consistently described in terms of a Joule heating effect. Bolometric EDFMR thus is a powerful tool for the investigation of magnetic anisotropy and magnetoresistive phenomena in ferromagnetic micro- or nanostructures.

  16. Ordering in linear multipolar colloids driven by an external field.

    PubMed

    Alarcón-Waess, O

    2006-08-14

    An approach to describe a linear multipolar colloid driven by an external field is developed by considering a colloid which in absence of the field is low structured and its coupling potential is axially symmetric. The equilibrium correlation of one component of the orientation tensor, self and collective, is computed up to linear order in density, which can be measured in an appropriate light scattering experiment. The self-correlation is written in terms of the second and fourth order parameters. All the equilibrium quantities are computed up to two-body level. This is done by assuming that the two-body equilibrium density function is given by the Boltzmann distribution, whereas the one-body density function is computed as solution of the equilibrium N-body Smoluchowski equation in the absence of hydrodynamic interactions. These observables, self and collective, as well as the second and fourth order parameters are able to describe when the colloid would evolve to an orientationally ordered phase. Explicit results for the dipole and quadrupole moments are reported. These results predict a different alignment with the external field for each moment. A relationship is provided between second and fourth order parameters, predicting the critical value of the external field in which the colloid goes into an axially symmetric phase.

  17. Pinning-dependent field-driven domain wall dynamics and thermal scaling in an ultrathin Pt/Co/Pt magnetic film.

    PubMed

    Gorchon, J; Bustingorry, S; Ferré, J; Jeudy, V; Kolton, A B; Giamarchi, T

    2014-07-11

    Magnetic-field-driven domain wall motion in an ultrathin Pt/Co(0.45  nm)/Pt ferromagnetic film with perpendicular anisotropy is studied over a wide temperature range. Three different pinning dependent dynamical regimes are clearly identified: the creep, the thermally assisted flux flow, and the depinning, as well as their corresponding crossovers. The wall elastic energy and microscopic parameters characterizing the pinning are determined. Both the extracted thermal rounding exponent at the depinning transition, ψ=0.15, and the Larkin length crossover exponent, ϕ=0.24, fit well with the numerical predictions.

  18. High-field magnetic behavior and forced-ferromagnetic state in an ErF e11TiH single crystal

    NASA Astrophysics Data System (ADS)

    Kostyuchenko, N. V.; Zvezdin, A. K.; Tereshina, E. A.; Skourski, Y.; Doerr, M.; Drulis, H.; Pelevin, I. A.; Tereshina, I. S.

    2015-09-01

    The crystal-field and exchange parameters are determined for the single-crystalline hydride ErF e11TiH compound by analyzing the experimental magnetization curves obtained in magnetic fields of up to 60 T. By using the calculated parameters we succeeded in modeling theoretical magnetization curves for ErF e11TiH up to 200 S and to study in detail the transition from ferrimagnetic to a ferromagnetic state in the applied magnetic field.

  19. Competition between Superconductor - Ferromagnetic stray magnetic fields in YBa2Cu3O7-x films pierced with Co nano-rods.

    PubMed

    Rouco, V; Córdoba, R; De Teresa, J M; Rodríguez, L A; Navau, C; Del-Valle, N; Via, G; Sánchez, A; Monton, C; Kronast, F; Obradors, X; Puig, T; Palau, A

    2017-07-18

    Superconductivity and ferromagnetism are two antagonistic phenomena that combined can lead to a rich phenomenology of interactions, resulting in novel physical properties and unique functionalities. Here we propose an original hybrid system formed by a high-temperature superconducting film, patterned with antidots, and with ferromagnetic nano-rods grown inside them. This particular structure exhibits the synergic influence of superconductor (SC) - ferromagnetic (FM) stray fields, in both the superconducting behaviour of the film and the three-dimensional (3D) magnetic structure of nano-rods. We show that FM stray fields directly influence the critical current density of the superconducting film. Additional functionalities appear due to the interaction of SC stray fields, associated to supercurrent loops, with the non-trivial 3D remanent magnetic structure of FM nano-rods. This work unravels the importance of addressing quantitatively the effect of stray magnetic fields from both, the superconductor and the ferromagnet in hybrid magnetic nano-devices based on high temperature superconductors.

  20. Magnetic-field-induced quantum criticality in a spin-1 planar ferromagnet with single-ion anisotropy

    NASA Astrophysics Data System (ADS)

    Mercaldo, Maria Teresa; Rabuffo, Ileana; Decesare, Luigi; Caramicod'Auria, Alvaro

    2014-03-01

    The effects of single-ion anisotropy on field-induced quantum criticality in spin-1 planar ferromagnet is explored by means of the two-time Green's function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. Indeed, for values of the uniaxial crystal-field parameter above a positive threshold a re-entrant behavior appears for the critical line, while above this value the conventional magnetic-field-induced quantum critical scenario remains unchanged. M. T. Mercaldo, I. Rabuffo, L. De Cesare, A. Caramico D'Auria, Eur. Phys. J. B 86, 340 (2013)

  1. Laboratory observation of magnetic field growth driven by shear flow

    NASA Astrophysics Data System (ADS)

    Intrator, T. P.; Dorf, L.; Sun, X.; Feng, Y.; Sears, J.; Weber, T.

    2014-04-01

    Two magnetic flux ropes that collide and bounce have been characterized in the laboratory. We find screw pinch profiles that include ion flow vi, magnetic field B, current density J, and plasma pressure. The electron flow ve can be inferred, allowing the evaluation of the Hall J ×B term in a two fluid magnetohydrodynamic Ohm's Law. Flux ropes that are initially cylindrical are mutually attracted and compress each other, which distorts the cylindrical symmetry. Magnetic field is created via the ∇×ve×B induction term in Ohm's Law where in-plane (perpendicular) shear of parallel flow (along the flux rope) is the dominant feature, along with some dissipation and magnetic reconnection. We predict and measure the growth of a quadrupole out-of-plane magnetic field δBz. This is a simple and coherent example of a shear flow driven dynamo. There is some similarity with two dimensional reconnection scenarios, which induce a current sheet and thus out-of-plane flow in the third dimension, despite the customary picture that considers flows only in the reconnection plane. These data illustrate a general and deterministic mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence.

  2. Laboratory observation of magnetic field growth driven by shear flow

    SciTech Connect

    Intrator, T. P. Feng, Y.; Sears, J.; Weber, T.; Dorf, L.; Sun, X.

    2014-04-15

    Two magnetic flux ropes that collide and bounce have been characterized in the laboratory. We find screw pinch profiles that include ion flow v{sub i}, magnetic field B, current density J, and plasma pressure. The electron flow v{sub e} can be inferred, allowing the evaluation of the Hall J×B term in a two fluid magnetohydrodynamic Ohm's Law. Flux ropes that are initially cylindrical are mutually attracted and compress each other, which distorts the cylindrical symmetry. Magnetic field is created via the ∇×v{sub e}×B induction term in Ohm's Law where in-plane (perpendicular) shear of parallel flow (along the flux rope) is the dominant feature, along with some dissipation and magnetic reconnection. We predict and measure the growth of a quadrupole out-of-plane magnetic field δB{sub z}. This is a simple and coherent example of a shear flow driven dynamo. There is some similarity with two dimensional reconnection scenarios, which induce a current sheet and thus out-of-plane flow in the third dimension, despite the customary picture that considers flows only in the reconnection plane. These data illustrate a general and deterministic mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence.

  3. Plasma rotation driven by static nonresonant magnetic fields

    SciTech Connect

    Garofalo, A. M.; Burrell, K. H.; DeBoo, J. C.; DeGrassie, J. S.; Jackson, G. L.; Schaffer, M. J.; Strait, E. J.; Solomon, W. M.; Park, J.-K.; Lanctot, M.; Reimerdes, H.

    2009-05-15

    Recent experiments in high temperature DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 64 (2002)] plasmas reported the first observation of plasma acceleration driven by the application of static nonresonant magnetic fields (NRMFs), with resulting improvement in the global energy confinement time. Although the braking effect of static magnetic field asymmetries is well known, recent theory [A. J. Cole et al., Phys. Rev. Lett. 99, 065001 (2007)] predicts that in some circumstances they lead instead to an increase in rotation frequency toward a 'neoclassical offset' rate in a direction opposed to the plasma current. We report the first experimental confirmation of this surprising result. The measured NRMF torque shows a strong dependence on both plasma density and temperature, above expectations from neoclassical theory. The consistency between theory and experiment improves with modifications to the expression of the NRMF torque accounting for a significant role of the plasma response to the external field and for the beta dependence of the plasma response, although some discrepancy remains. The magnitude and direction of the observed offset rotation associated with the NRMF torque are consistent with neoclassical theory predictions. The offset rotation rate is about 1% of the Alfven frequency or more than double the rotation needed for stable operation at high {beta}{sub N} above the n=1 no-wall kink limit in DIII-D.

  4. Dual-donor (Zn(i) and V(O)) mediated ferromagnetism in copper-doped ZnO micron-scale polycrystalline films: a thermally driven defect modulation process.

    PubMed

    Hu, Liang; Huang, Jun; He, Haiping; Zhu, Liping; Liu, Shijiang; Jin, Yizheng; Sun, Luwei; Ye, Zhizhen

    2013-05-07

    The paper reports robust ferromagnetic Cu-doped ZnO micron-scale polycrystalline films via spin-coating using high-quality doped nanocrystals. A reliable magnetic response is observed in the 900 °C vacuum annealed film without any ferromagnetic contribution from other sources. Post-annealing treatment in terms of atmosphere and temperature can control the proportion of oxygen vacancies (V(O)) and zinc interstitials (Zn(i)) defects and further help to precisely regulate defect-related ferromagnetic behavior. Complex charge transfer processes derived from dual-donor (Zn(i) and V(O)) to Cu acceptor are revealed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra. Based on the above, specific charge transfer (CT)-type Stoner splitting and indirect double-exchange mechanisms are proposed to understand the ferromagnetic origin. The improvable FM performance and annealing-specific modulation further indicate that a thermal driven process can delicately tailor the magnetic property of the transition metal ion-doped ZnO system.

  5. Localized NMR Mediated by Electrical-Field-Induced Domain Wall Oscillation in Quantum-Hall-Ferromagnet Nanowire.

    PubMed

    Miyamoto, S; Miura, T; Watanabe, S; Nagase, K; Hirayama, Y

    2016-03-09

    We present fractional quantum Hall domain walls confined in a gate-defined wire structure. Our experiments utilize spatial oscillation of domain walls driven by radio frequency electric fields to cause nuclear magnetic resonance. The resulting spectra are discussed in terms of both large quadrupole fields created around the wire and hyperfine fields associated with the oscillating domain walls. This provides the experimental fact that the domain walls survive near the confined geometry despite of potential deformation, by which a localized magnetic resonance is allowed in electrical means.

  6. Ising-Type Ferromagnetic Ground State Driven by Anisotropic c-f Hybridization in CeRu2Al2B

    NASA Astrophysics Data System (ADS)

    Matsuno, Haruki; Kotegawa, Hisashi; Matsuoka, Eiichi; Tomiyama, Yo; Sugawara, Hitoshi; Tou, Hideki

    2014-10-01

    The magnetic correlations between Ce 4f electrons and conduction electrons in the new tetragonal compound CeRu2Al2B have been investigated by 27Al nuclear magnetic resonance (NMR). The 27Al NMR spin-lattice relaxation rate 1/T1 exhibits a large magnetic anisotropy for field directions. Within a localized moment picture, the Ce 4f spin-fluctuation rates Γ|| for the c-axis and Γ⊥ for the c-plane are evaluated by taking account of the magnetic anisotropy. The relation of Γ allel ≫ Γ bot , which holds in the entire temperature range, indicates that the c-f hybridization between the ligand conduction electrons and the Ce 4f electrons with the Γ 7(1) crystal electric field ground state is anisotropic. From the temperature dependence of Γ, it is found that the Kondo effect dominates the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction at high temperatures, whereas below 60 K the RKKY interaction overcomes the Kondo effect and causes the magnetic order. These results indicate that the anisotropic c-f hybridization plays a vital role in realization of the Ising-type ferromagnetic magnetic ground state in CeRu2Al2B.

  7. Electric-field control of domain wall nucleation and pinning in a metallic ferromagnet

    NASA Astrophysics Data System (ADS)

    Bernand-Mantel, A.; Herrera-Diez, L.; Ranno, L.; Pizzini, S.; Vogel, J.; Givord, D.; Auffret, S.; Boulle, O.; Miron, I. M.; Gaudin, G.

    2013-03-01

    The electric (E)-field control of magnetic properties opens the prospects of an alternative to magnetic field or electric current activation to control magnetization. Multilayers with perpendicular magnetic anisotropy have proven to be particularly sensitive to the influence of an E-field due to the interfacial origin of their anisotropy. In these systems, E-field effects have been recently applied to assist magnetization switching and control domain wall (DW) velocity. Here we report on two new applications of the E-field in a similar material: controlling domain wall nucleation and stopping DW propagation at the edge of the electrode.

  8. Possibility of ferromagnetic neutron matter

    NASA Astrophysics Data System (ADS)

    Hashimoto, Koji

    2015-04-01

    We study ferromagnetism at high density of neutrons in the QCD hadron phase, by using the simplest chiral effective model incorporating magnetic fields and the chiral anomaly. Under the assumption of spatial homogeneity, we calculate the energy density as a function of neutron density, with a magnetization and a neutral pion condensation in the style of Dautry and Neyman. We find that at a high density the energy of the ferromagnetic order is lower than that of the ordinary neutron matter, and the reduction effect is enhanced by the anomaly. Compared to the inhomogeneous phase with the alternating layer structure, our ferromagnetic phase turns out to be unfavored. However, once an axial vector meson condensation is taken into account in our simplest model, the ferromagnetic energy density is lowered significantly, which still leaves some room for a possible realization of a QCD ferromagnetic phase and ferromagnetic magnetars.

  9. Two-level systems driven by large-amplitude fields

    NASA Astrophysics Data System (ADS)

    Nori, F.; Ashhab, S.; Johansson, J. R.; Zagoskin, A. M.

    2009-03-01

    We analyze the dynamics of a two-level system subject to driving by large-amplitude external fields, focusing on the resonance properties in the case of driving around the region of avoided level crossing. In particular, we consider three main questions that characterize resonance dynamics: (1) the resonance condition, (2) the frequency of the resulting oscillations on resonance, and (3) the width of the resonance. We identify the regions of validity of different approximations. In a large region of the parameter space, we use a geometric picture in order to obtain both a simple understanding of the dynamics and quantitative results. The geometric approach is obtained by dividing the evolution into discrete time steps, with each time step described by either a phase shift on the basis states or a coherent mixing process corresponding to a Landau-Zener crossing. We compare the results of the geometric picture with those of a rotating wave approximation. We also comment briefly on the prospects of employing strong driving as a useful tool to manipulate two-level systems. S. Ashhab, J.R. Johansson, A.M. Zagoskin, F. Nori, Two-level systems driven by large-amplitude fields, Phys. Rev. A 75, 063414 (2007). S. Ashhab et al, unpublished.

  10. Enhancing electric-field control of ferromagnetism through nanoscale engineering of high-Tc MnxGe1−x nanomesh

    PubMed Central

    Nie, Tianxiao; Tang, Jianshi; Kou, Xufeng; Gen, Yin; Lee, Shengwei; Zhu, Xiaodan; He, Qinglin; Chang, Li-Te; Murata, Koichi; Fan, Yabin; Wang, Kang L.

    2016-01-01

    Voltage control of magnetism in ferromagnetic semiconductor has emerged as an appealing solution to significantly reduce the power dissipation and variability beyond current CMOS technology. However, it has been proven to be very challenging to achieve a candidate with high Curie temperature (Tc), controllable ferromagnetism and easy integration with current Si technology. Here we report the effective electric-field control of both ferromagnetism and magnetoresistance in unique MnxGe1−x nanomeshes fabricated by nanosphere lithography, in which a Tc above 400 K is demonstrated as a result of size/quantum confinement. Furthermore, by adjusting Mn doping concentration, extremely giant magnetoresistance is realized from ∼8,000% at 30 K to 75% at 300 K at 4 T, which arises from a geometrically enhanced magnetoresistance effect of the unique mesh structure. Our results may provide a paradigm for fundamentally understanding the high Tc in ferromagnetic semiconductor nanostructure and realizing electric-field control of magnetoresistance for future spintronic applications. PMID:27762320

  11. Electric-Field-Induced Magnetization Reversal in a Ferromagnet-Multiferroic Heterostructure

    DTIC Science & Technology

    2011-11-14

    system. The effect is reversible and mediated by an interfacial magnetic coupling dictated by the multiferroic. Such electric-field control of a...system. The effect is reversible and mediated by an interfacial magnetic coupling dictated by the multiferroic. Such electric-field control of a...by an interfacial magnetic coupling dictated by the multiferroic. Such electric-field control of a magnetoelectric device demonstrates an avenue for

  12. The frequency-domain method of calculation for the pulsed electromagnetic field in a conductive ferromagnetic plate

    NASA Astrophysics Data System (ADS)

    Nosov, G. V.; Kuleshova, E. O.; Lefebvre, S.; Plyusnin, A. A.; Tokmashev, D. M.

    2017-02-01

    The technique for parameters determination of magnetic skin effect on ferromagnetic plate at a specified pulse of magnetic field intensity on the plate surface is proposed. It is based on a frequency-domain method and could be applied for a pulsing transformer, a dynamoelectric pulse generator and a commutating inductor that contains an imbricated core. Due to this technique, such plate parameters as specific heat loss energy, the average power of this energy and the plate temperature raise, the magnetic flux attenuation factor and the plate q-factor could be calculated. These parameters depend on the steel type, the amplitude, the rms value, the duration and the form of the magnetic field intensity impulse on the plate surface. The plate thickness is defined by the value of the flux attenuation factor and the plate q-factor that should be maximal. The reliability of the proposed technique is built on a common frequency-domain usage applicable for pulse transient study under zero boundary conditions of the electric circuit and the conformity of obtained results with the sinusoidal steady-state mode.

  13. Giant electric field control of magnetism and narrow ferromagnetic resonance linewidth in FeCoSiB/Si/SiO2/PMN-PT multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Gao, Y.; Wang, X.; Xie, L.; Hu, Z.; Lin, H.; Zhou, Z.; Nan, T.; Yang, X.; Howe, B. M.; Jones, J. G.; Brown, G. J.; Sun, N. X.

    2016-06-01

    It has been challenging to achieve combined strong magnetoelectric coupling and narrow ferromagnetic resonance (FMR) linewidth in multiferroic heterostructures. Electric field induced large effective field of 175 Oe and narrow FMR linewidth of 40 Oe were observed in FeCoSiB/Si/SiO2/PMN-PT heterostructures with substrate clamping effect minimized through removing the Si substrate. As a comparison, FeCoSiB/PMN-PT heterostructures with FeCoSiB film directly deposited on PMN-PT showed a comparable voltage induced effective magnetic field but a significantly larger FMR linewidth of 283 Oe. These multiferroic heterostructures exhibiting combined giant magnetoelectric coupling and narrow ferromagnetic resonance linewidth offer great opportunities for integrated voltage tunable RF magnetic devices.

  14. Ferromagnetic resonance in a single crystal of iron borate and magnetic field tuning of hybrid oscillations in a composite structure with a dielectric: Experiment and theory

    SciTech Connect

    Popov, M. A.; Zavislyak, I. V.; Chumak, H. L.; Strugatsky, M. B.; Yagupov, S. V.; Srinivasan, G.

    2015-07-07

    The high-frequency properties of a composite resonator comprised single crystal iron borate (FeBO{sub 3}), a canted antiferromagnet with a weak ferromagnetic moment, and a polycrystalline dielectric were investigated at 9–10 GHz. Ferromagnetic resonance in this frequency range was observed in FeBO{sub 3} for bias magnetic fields of ∼250 Oe. In the composite resonator, the magnetic mode in iron borate and dielectric mode are found to hybridize strongly. It is shown that the hybrid mode can be tuned with a static magnetic field. Our studies indicate that coupling between the magnetic mode and the dielectric resonance can be altered from maximum hybridization to a minimum by adjusting the position of resonator inside the waveguide. Magnetic field tuning of the resonance frequency by a maximum of 145 MHz and a change in the transmitted microwave power by as much as 16 dB have been observed for a bias field of 250 Oe. A model is discussed for the magnetic field tuning of the composite resonator and theoretical estimates are in reasonable agreement with the data. The composite resonator with a weak ferromagnet and a dielectric is of interest for application in frequency agile devices with electronically tunable electrodynamic characteristics for the mm and sub-mm wave bands.

  15. Parametric Resonances of a Conductive Pipe Driven by an Alternating Magnetic Field in the Presence of a Static Magnetic Field

    ERIC Educational Resources Information Center

    Donoso, Guillermo; Ladera, Celso L.

    2012-01-01

    The parametric oscillations of an oscillator driven electromagnetically are presented. The oscillator is a conductive pipe hung from a spring, and driven by the oscillating magnetic field of a surrounding coil in the presence of a static magnetic field. It is an interesting case of parametric oscillations since the pipe is neither a magnet nor a…

  16. Parametric Resonances of a Conductive Pipe Driven by an Alternating Magnetic Field in the Presence of a Static Magnetic Field

    ERIC Educational Resources Information Center

    Donoso, Guillermo; Ladera, Celso L.

    2012-01-01

    The parametric oscillations of an oscillator driven electromagnetically are presented. The oscillator is a conductive pipe hung from a spring, and driven by the oscillating magnetic field of a surrounding coil in the presence of a static magnetic field. It is an interesting case of parametric oscillations since the pipe is neither a magnet nor a…

  17. Control of domain wall pinning in ferromagnetic nanowires by magnetic stray fields.

    PubMed

    Ahn, Sung-Min; Moon, Kyoung-Woong; Cho, Cheong-Gu; Choe, Sug-Bong

    2011-02-25

    We have found that the depinning field of domain walls (DWs) in permalloy (Ni(81)Fe(19)) nanowires can be experimentally controlled by interactions between magnetic stray fields and artificial constrictions. A pinning geometry that consists of a notch and a nanobar is considered, where a DW traveling in the nanowire is pinned by the notch with a nanobar vertical to it. We have found that the direction of magnetization of the nanobar affects the shape and local energy minimum of the potential landscape experienced by the DW; therefore, the pinning strength strongly depends on the interaction of the magnetic stray field from the nanobar with the external pinning force of the notch. The mechanism of this pinning behavior is applied for the instant and flexible control of the pinning strength with respect to various DW motions in DW-mediated magnetic memory devices.

  18. Giant Electric Field Control of Magnetism and Narrow Ferromagnetic Resonance Linewidth in FeCoSiB/Si/SiO2/PMN PT Multiferroic Heterostructures (Open Access Author’s Manuscript)

    DTIC Science & Technology

    2016-06-06

    1 Giant electric field control of magnetism and narrow ferromagnetic resonance linewidth in FeCoSiB/Si/SiO2/PMN-PT multiferroic heterostructures...c) 1Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts, 02115, USA 2Materials and Manufacturing...coupling and narrow ferromagnetic resonance (FMR) linewidth in multiferroic heterostructures. Electric field induced large effective field of 175Oe

  19. Cascade of Magnetic-Field-Induced Lifshitz Transitions in the Ferromagnetic Kondo Lattice Material YbNi4 P2

    NASA Astrophysics Data System (ADS)

    Pfau, H.; Daou, R.; Friedemann, S.; Karbassi, S.; Ghannadzadeh, S.; Küchler, R.; Hamann, S.; Steppke, A.; Sun, D.; König, M.; Mackenzie, A. P.; Kliemt, K.; Krellner, C.; Brando, M.

    2017-09-01

    A ferromagnetic quantum critical point is thought not to exist in two- and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4 (P ,As )2 , possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4 P2 experimentally, but common probes such as angle-resolved photoemission spectroscopy and quantum oscillation measurements are lacking. Here, we study the magnetic-field dependence of transport and thermodynamic properties of YbNi4 P2 . The Kondo effect is continuously suppressed, and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi4 P2 . The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4 f -electron character shaped by the Kondo lattice effect.

  20. Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

    PubMed Central

    Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E.

    2016-01-01

    The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling. PMID:27248368

  1. Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

    SciTech Connect

    Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E.

    2016-06-01

    The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. Lastly, if the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.

  2. Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

    DOE PAGES

    Chesnel, Karine; Safsten, Alex; Rytting, Matthew; ...

    2016-06-01

    The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer.more » The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. Lastly, if the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.« less

  3. Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

    NASA Astrophysics Data System (ADS)

    Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E.

    2016-06-01

    The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.

  4. Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling.

    PubMed

    Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E

    2016-06-01

    The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.

  5. Synchronous precessional motion of multiple domain walls in a ferromagnetic nanowire by perpendicular field pulses.

    PubMed

    Kim, June-Seo; Mawass, Mohamad-Assaad; Bisig, André; Krüger, Benjamin; Reeve, Robert M; Schulz, Tomek; Büttner, Felix; Yoon, Jungbum; You, Chun-Yeol; Weigand, Markus; Stoll, Hermann; Schütz, Gisela; Swagten, Henk J M; Koopmans, Bert; Eisebitt, Stefan; Kläui, Mathias

    2014-03-24

    Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses.

  6. Creep turns linear in narrow ferromagnetic nanostrips.

    PubMed

    Leliaert, Jonathan; Van de Wiele, Ben; Vansteenkiste, Arne; Laurson, Lasse; Durin, Gianfranco; Dupré, Luc; Van Waeyenberge, Bartel

    2016-02-04

    The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.

  7. Creep turns linear in narrow ferromagnetic nanostrips

    NASA Astrophysics Data System (ADS)

    Leliaert, Jonathan; van de Wiele, Ben; Vansteenkiste, Arne; Laurson, Lasse; Durin, Gianfranco; Dupré, Luc; van Waeyenberge, Bartel

    2016-02-01

    The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.

  8. Creep turns linear in narrow ferromagnetic nanostrips

    PubMed Central

    Leliaert, Jonathan; Van de Wiele, Ben; Vansteenkiste, Arne; Laurson, Lasse; Durin, Gianfranco; Dupré, Luc; Van Waeyenberge, Bartel

    2016-01-01

    The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media. PMID:26843125

  9. Application of the Stoner-Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

    NASA Astrophysics Data System (ADS)

    Collocott, S. J.

    2011-08-01

    Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd 60Fe 30Al 10, Nd 60Fe 20Co 10Al 10 and Pr 58Fe 24Al 18 have been measured in applied magnetic fields up to 9 T at temperatures in the range 10-350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner-Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation Ms, anisotropy field Ha, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of Ms, Ha, d and K over the range 20-350 K are found to be qualitatively similar, though there are quantitative differences. In all cases Ms increases with decreasing temperature, both Ha and K increase with decreasing temperature, reaching a peak in the range 75-120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.

  10. Coexistence of superconductivity and ferromagnetism in ferromagnetic metals.

    PubMed

    Karchev, N I; Blagoev, K B; Bedell, K S; Littlewood, P B

    2001-01-29

    We address the question of coexistence of superconductivity and ferromagnetism. Using a field theoretical approach we study a one-fermion effective model of a ferromagnetic superconductor in which the quasiparticles responsible for the ferromagnetism form the Cooper pairs as well. For the first time we solve self-consistently the mean-field equations for the superconducting gap and the spontaneous magnetization. We discuss the physical features which are different in this model and the standard BCS model and consider their experimental consequences.

  11. Spin Mechanics in Ferromagnet/Ferroelectric Hybrid Structures

    NASA Astrophysics Data System (ADS)

    Goennenwein, Sebastian

    2013-03-01

    In most ferromagnets, magnetic and elastic degrees of freedom are coupled - as evident, e.g., from the hum of a transformer. In the ``spin mechanics'' scheme, one intentionally exploits magneto-elastic coupling (inverse magneto-striction) to control the magnetization of ferromagnetic films. On the one hand, I will briefly review spin mechanics in the static limit, taking ferromagnetic nickel thin film/piezoelectric actuator hybrid structures as prototype examples. In these hybrids, the application of an electric field to the actuator results in a uniaxial strain, which is transferred into the Ni film. Due to magneto-elastic coupling, the voltage-controlled strain modifies the magnetic anisotropy and thus induces a magnetization reorientation. This allows for a voltage-controlled, fully reversible magnetization orientation manipulation within a range of approximately 90 degrees at room temperature in these hybrids. On the other hand, I will show that the spin mechanics scheme also is operational at GHz frequencies. In the corresponding experiments, we use surface acoustic waves (SAWs) propagating in Ni/LiNbO3 hybrid devices for the all-elastic excitation and detection of ferromagnetic resonance (FMR). Our SAW magneto-transmission data are consistently described by a modified Landau-Lifshitz-Gilbert approach, in which the magnetization precession is not driven by a conventional, external microwave magnetic field, but rather by a purely virtual, internal tickle field stemming from radio-frequency magneto-elastic interactions. This causes a distinct magnetic field orientation dependence of elastically driven FMR, observed in both simulations and experiment. Last but not least, I will address perspectives for spin mechanics experiments, e.g., the study of magnon-phonon coupling, or acoustic spin pumping in normal metal/ferromagnet hybrid structures.

  12. The behaviors of ferromagnetic nano-particles in and around blood vessels under applied magnetic fields

    NASA Astrophysics Data System (ADS)

    Nacev, A.; Beni, C.; Bruno, O.; Shapiro, B.

    2011-03-01

    In magnetic drug delivery, therapeutic magnetizable particles are typically injected into the blood stream and magnets are then used to concentrate them to disease locations. The behavior of such particles in-vivo is complex and is governed by blood convection, diffusion (in blood and in tissue), extravasation, and the applied magnetic fields. Using physical first-principles and a sophisticated vessel-membrane-tissue (VMT) numerical solver, we comprehensively analyze in detail the behavior of magnetic particles in blood vessels and surrounding tissue. For any blood vessel (of any size, depth, and blood velocity) and tissue properties, particle size and applied magnetic fields, we consider a Krogh tissue cylinder geometry and solve for the resulting spatial distribution of particles. We find that there are three prototypical behaviors (blood velocity dominated, magnetic force dominated, and boundary-layer formation) and that the type of behavior observed is uniquely determined by three non-dimensional numbers (the magnetic-Richardson number, mass Péclet number, and Renkin reduced diffusion coefficient). Plots and equations are provided to easily read out which behavior is found under which circumstances (Figs. 5-8). We compare our results to previously published in-vitro and in-vivo magnetic drug delivery experiments. Not only do we find excellent agreement between our predictions and prior experimental observations, but we are also able to qualitatively and quantitatively explain behavior that was previously not understood.

  13. La Displacement Driven Double-Exchange Like Mediation in Titanium dxy Ferromagnetism at the LaAlO3/SrTiO3

    NASA Astrophysics Data System (ADS)

    Odkhuu, Dorj; Rhim, Sonny H.; Shin, Dongbin; Park, Noejung

    2016-04-01

    The epitaxial atomistic interfaces of two insulating oxides, LaAlO3 (LAO)/SrTiO3 (STO), have attracted great interest owing to rich emergent phenomena such as interface metallicity, thickness dependent insulator-metal transition, superconductivity, ferromagnetism, and even their coexistence. However, the physics origin of ferromagnetic ordering in the n-type LAO/STO interface is in debate. Here, we propose that the polar distortion of La atom can ignite the ferromagnetism at the interface even without oxygen vacancy. The induced hybridization between La dz2 and O px,y states can mediate double-exchange like interaction between Ti dxy electrons. We further suggest that the structural and electrical modification of the outermost surface of LAO or switching the polarization direction of ferroelectric overlayers on LAO/STO can promote such La displacement.

  14. Singlet-Triplet Conversion and the Long-Range Proximity Effect in Superconductor-Ferromagnet Structures with Generic Spin Dependent Fields

    NASA Astrophysics Data System (ADS)

    Bergeret, F. S.; Tokatly, I. V.

    2013-03-01

    The long-range proximity effect in superconductor-ferromagnet (S/F) hybrid nanostructures is observed if singlet Cooper pairs from the superconductor are converted into triplet pairs which can diffuse into the ferromagnet over large distances. It is commonly believed that this happens only in the presence of magnetic inhomogeneities. We show that there are other sources of the long-range triplet component (LRTC) of the condensate and establish general conditions for their occurrence. As a prototypical example, we consider first a system where the exchange field and spin-orbit coupling can be treated as time and space components of an effective SU(2) potential. We derive a SU(2) covariant diffusive equation for the condensate and demonstrate that an effective SU(2) electric field is responsible for the long-range proximity effect. Finally, we extend our analysis to a generic ferromagnet and establish a universal condition for the LRTC. Our results open a new avenue in the search for such correlations in S/F structures and make a hitherto unknown connection between the LRTC and Yang-Mills electrostatics.

  15. Electromagnetically Driven Plasma-Field Dynamics in Modified Ionosphere

    NASA Astrophysics Data System (ADS)

    Kochetov, Andrey; Terina, Galina

    Under sounding of an artificial ionospheric turbulence by short probing radio pulses of ordinary polarization the two types of scattered signals were observed: a "caviton" signal (CS) and a "plasma" signal (PS), which appeared with the heating transmitter switching on and disap-peared after its switching off (G.I. Terina J. Atm. Terr. Phys, 57, 1995, 273, Izv. VUZov, Radiofizika, 39, 1998, 203). The scattered signal of PS type was revealed also after the heating switching off. It was called an "aftereffect plasma signal" (AEPS) (G.I. Terina Izv .VUZov, Radiofizika, 43, 2000, 958). This signal had large time and spatial delays and appeared mostly when corresponding PS had envelope fluctuations. The aftereffect phenomenon was expressed at time on CS by amplitude increasing at once after the heating transmitter turning off. The theoretical model of this phenomenon is proposed in and some peculiarities of the aftereffect phenomena of the scattered signals in modified ionospheric plasma are considered and discussed. For theoretical interpretation of the characteristics of CS and AEPS the numerical solution of nonlinear Shrüdinger equation (NSE) with driven extension were carried out in inhomogeneous plasma layer with linear electron density profile (A.V. Kochetov, V.A. Mironov, G.I. Terina, Adv. Space Reseacrh, 29, 2002, 1369) and for the one with prescribed density depletion (and A.V. Kochetov, G.I. Terina, Adv. Space Reseacrh, 38, 2006, 2490). The simulation results obtained for linear inhomogeneous plasma layer and for plasma one with density depletion al-low us to interpret the aftereffect of CS and PS qualitatively. The field amplitude increase at relaxation stage displayed at calculations allows us to interpret of CS aftereffect. The large time delays of AEPS can be explained as a result of powerful radio waves trapping in the forming at the plasma resonance regions density depletions (E. Mjøhus, J. Geophys. Res. 103, 1998, 14711; B. Eliasson and L. Stenflo, J

  16. Influence of nonuniform surface magnetic fields in wetting transitions in a confined two-dimensional Ising ferromagnet

    NASA Astrophysics Data System (ADS)

    Trobo, Marta L.; Albano, Ezequiel V.

    2013-11-01

    Wetting transitions are studied in the two-dimensional Ising ferromagnet confined between walls where competitive surface fields act. In our finite samples of size L×M, the walls are separated by a distance L, M being the length of the sample. The surface fields are taken to be short-range and nonuniform, i.e., of the form H1,δH1,H1,δH1,..., where the parameter -1≤δ≤1 allows us to control the nonuniformity of the fields. By performing Monte Carlo simulations we found that those competitive surface fields lead to the occurrence of an interface between magnetic domains of different orientation that runs parallel to the walls. In finite samples, such an interface undergoes a localization-delocalization transition, which is the precursor of a true wetting transition that takes place in the thermodynamic limit. By exactly working out the ground state (T=0), we found that besides the standard nonwet and wet phases, a surface antiferromagnetic-like state emerges for δ<-1/3 and large fields (H1>3), H1tr/J=3, δtr=-1/3,T=0, being a triple point where three phases coexist. By means of Monte Carlo simulations it is shown that these features of the phase diagram remain at higher temperatures; e.g., we examined in detail the case T=0.7×Tcb. Furthermore, we also recorded phase diagrams for fixed values of δ, i.e., plots of the critical field at the wetting transition (H1w) versus T showing, on the one hand, that the exact results of Abraham [Abraham, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.44.1165 44, 1165 (1980)] for δ=1 are recovered, and on the other hand, that extrapolations to T→0 are consistent with our exact results. Based on our numerical results we conjectured that the exact result for the phase diagram worked out by Abraham can be extended for the case of nonuniform fields. In fact, by considering a nonuniform surface field of some period λ, with λ≪M, e.g., [H1(x,λ)>0], one can obtain the effective field Heff at a λ coarse-grained level

  17. Influence of nonuniform surface magnetic fields in wetting transitions in a confined two-dimensional Ising ferromagnet.

    PubMed

    Trobo, Marta L; Albano, Ezequiel V

    2013-11-01

    Wetting transitions are studied in the two-dimensional Ising ferromagnet confined between walls where competitive surface fields act. In our finite samples of size L×M, the walls are separated by a distance L, M being the length of the sample. The surface fields are taken to be short-range and nonuniform, i.e., of the form H(1),δH(1),H(1),δH(1),..., where the parameter -1≤δ≤1 allows us to control the nonuniformity of the fields. By performing Monte Carlo simulations we found that those competitive surface fields lead to the occurrence of an interface between magnetic domains of different orientation that runs parallel to the walls. In finite samples, such an interface undergoes a localization-delocalization transition, which is the precursor of a true wetting transition that takes place in the thermodynamic limit. By exactly working out the ground state (T=0), we found that besides the standard nonwet and wet phases, a surface antiferromagnetic-like state emerges for δ<-1/3 and large fields (H(1)>3), H(1)(tr)/J=3, δ(tr)=-1/3,T=0, being a triple point where three phases coexist. By means of Monte Carlo simulations it is shown that these features of the phase diagram remain at higher temperatures; e.g., we examined in detail the case T=0.7×T(cb). Furthermore, we also recorded phase diagrams for fixed values of δ, i.e., plots of the critical field at the wetting transition (H(1w)) versus T showing, on the one hand, that the exact results of Abraham [Abraham, Phys. Rev. Lett. 44, 1165 (1980)] for δ=1 are recovered, and on the other hand, that extrapolations to T→0 are consistent with our exact results. Based on our numerical results we conjectured that the exact result for the phase diagram worked out by Abraham can be extended for the case of nonuniform fields. In fact, by considering a nonuniform surface field of some period λ, with λ0], one can obtain the effective field H(eff) at a λ coarse-grained level given by H(eff)=1/

  18. Dilute ferromagnetic semiconductors: Physics and spintronic structures

    NASA Astrophysics Data System (ADS)

    Dietl, Tomasz; Ohno, Hideo

    2014-01-01

    This review compiles results of experimental and theoretical studies on thin films and quantum structures of semiconductors with randomly distributed Mn ions, which exhibit spintronic functionalities associated with collective ferromagnetic spin ordering. Properties of p-type Mn-containing III-V as well as II-VI, IV-VI, V2-VI3, I-II-V, and elemental group IV semiconductors are described, paying particular attention to the most thoroughly investigated system (Ga,Mn)As that supports the hole-mediated ferromagnetic order up to 190 K for the net concentration of Mn spins below 10%. Multilayer structures showing efficient spin injection and spin-related magnetotransport properties as well as enabling magnetization manipulation by strain, light, electric fields, and spin currents are presented together with their impact on metal spintronics. The challenging interplay between magnetic and electronic properties in topologically trivial and nontrivial systems is described, emphasizing the entangled roles of disorder and correlation at the carrier localization boundary. Finally, the case of dilute magnetic insulators is considered, such as (Ga,Mn)N, where low-temperature spin ordering is driven by short-ranged superexchange that is ferromagnetic for certain charge states of magnetic impurities.

  19. Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

    NASA Astrophysics Data System (ADS)

    Pertsev, N. A.; Kohlstedt, H.

    2010-11-01

    A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spin valves mechanically coupled to a ferroelectric substrate. The switching between low- and high-resistance states results from the strain-driven magnetization reorientations by about 90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weak cubic magnetocrystalline anisotropy. Such reorientations, not requiring external magnetic fields, can be realized experimentally by applying moderate electric field to a thick substrate (bulk or membrane type) made of a relaxor ferroelectric having ultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giant magnetoresistance may be employed as electric-write nonvolatile magnetic memory cells with nondestructive readout.

  20. Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields.

    PubMed

    Wang, C; Cao, J C

    2005-03-01

    We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.

  1. Observation of a very large internal hyperfine field (62.4 T) in the ferromagnetically ordered state of the S = 1 alpha-iron(II) octaethyltetraazaporphyrin.

    PubMed

    Reiff, W M; Frommen, C M; Yee, G T; Sellers, S P

    2000-05-15

    The origins of the extraordinarily large internal hyperfine field (62.4 T) for the three-dimensional (weak) ferromagnetically ordered ground state of alpha-Fe(OETAP) are discussed semiquantitatively in terms of existing physical theory. In particular, the classical Fermi-contact contribution to the internal field is found to be highly enhanced by a very large orbital contribution and a significant dipolar term of the same sign. A rationale for the unexpected ordering of this S = 1 non-Kramers system is also presented.

  2. Magnetically controlled ferromagnetic swimmers

    PubMed Central

    Hamilton, Joshua K.; Petrov, Peter G.; Winlove, C. Peter; Gilbert, Andrew D.; Bryan, Matthew T.; Ogrin, Feodor Y.

    2017-01-01

    Microscopic swimming devices hold promise for radically new applications in lab-on-a-chip and microfluidic technology, diagnostics and drug delivery etc. In this paper, we demonstrate the experimental verification of a new class of autonomous ferromagnetic swimming devices, actuated and controlled solely by an oscillating magnetic field. These devices are based on a pair of interacting ferromagnetic particles of different size and different anisotropic properties joined by an elastic link and actuated by an external time-dependent magnetic field. The net motion is generated through a combination of dipolar interparticle gradient forces, time-dependent torque and hydrodynamic coupling. We investigate the dynamic performance of a prototype (3.6 mm) of the ferromagnetic swimmer in fluids of different viscosity as a function of the external field parameters (frequency and amplitude) and demonstrate stable propulsion over a wide range of Reynolds numbers. We show that the direction of swimming has a dependence on both the frequency and amplitude of the applied external magnetic field, resulting in robust control over the speed and direction of propulsion. This paves the way to fabricating microscale devices for a variety of technological applications requiring reliable actuation and high degree of control. PMID:28276490

  3. Magnetically controlled ferromagnetic swimmers

    NASA Astrophysics Data System (ADS)

    Hamilton, Joshua K.; Petrov, Peter G.; Winlove, C. Peter; Gilbert, Andrew D.; Bryan, Matthew T.; Ogrin, Feodor Y.

    2017-03-01

    Microscopic swimming devices hold promise for radically new applications in lab-on-a-chip and microfluidic technology, diagnostics and drug delivery etc. In this paper, we demonstrate the experimental verification of a new class of autonomous ferromagnetic swimming devices, actuated and controlled solely by an oscillating magnetic field. These devices are based on a pair of interacting ferromagnetic particles of different size and different anisotropic properties joined by an elastic link and actuated by an external time-dependent magnetic field. The net motion is generated through a combination of dipolar interparticle gradient forces, time-dependent torque and hydrodynamic coupling. We investigate the dynamic performance of a prototype (3.6 mm) of the ferromagnetic swimmer in fluids of different viscosity as a function of the external field parameters (frequency and amplitude) and demonstrate stable propulsion over a wide range of Reynolds numbers. We show that the direction of swimming has a dependence on both the frequency and amplitude of the applied external magnetic field, resulting in robust control over the speed and direction of propulsion. This paves the way to fabricating microscale devices for a variety of technological applications requiring reliable actuation and high degree of control.

  4. Relationship between sprite streamer behavior and lightning-driven electric fields

    NASA Astrophysics Data System (ADS)

    Li, Jingbo; Cummer, Steven

    2012-01-01

    The lightning-driven electric fields not only initiate sprite streamers at high altitudes but also control their propagation to lower altitudes until termination. Thus, the relationship between sprite streamer behavior and their causative lightning-driven ambient electric field can reveal the internal microphysics during sprite development. In this work, we combined the measurements of broadband electromagnetic radiation from sprite-producing lightning, high-speed video of sprite optical emissions acquired at 5,000-10,000 frames per second, and numerical simulations to infer the background lightning-driven electric fields during the full extent of downward streamer propagation. For four sprites analyzed, all with positive polarity downward streamers, the observed streamers all terminate at locations where the ambient electric field is approximately 0.05 Ek, independent of the altitude where this field is reached. For two sprites with significant horizontal extent, the points of streamer termination closely follow the spatial contour of the E = 0.05 Ek surface, further confirming the consistency of this termination background field for positive sprite streamers. The positive streamers also begin their significant deceleration where the background field drops below 0.12-0.24 Ek. These measured termination field (Eter) and deceleration field (Edec) are consistent with previous laboratory experiments of positive streamer stopping field and critical field to sustain stable propagation. These results connect sprite streamer behavior with the lightning-driven background electric fields and can be a step to further constrain the existing model of streamer propagation in the mesosphere.

  5. Simulations of microwave electron heating on field-reversed configuration driven by rotating magnetic field

    NASA Astrophysics Data System (ADS)

    Yang, Xiaokang; Petrov, Yuri; Koehn, Alf; Cohen, Sam; Ceccherini, Francesco; Galeotti, Laura; Dettrick, Sean; Binderbauer, Michl

    2016-10-01

    The rotating magnetic field-driven field-reversed configuration (FRC), such as Rotamak or PFRC experiment, was recently proposed as a test bench at Tri Alpha Energy to experimentally pioneer the study of microwave electron heating. In order to provide guidelines to the choice of microwave frequency and antenna position, as well as the desired target plasma profile, extensive simulations have been conducted with use of the GENRAY-C ray-tracing code for a wide range of frequencies from smaller than fundamental electron cyclotron resonant (ECR) frequency up to more than 30 harmonics of ECR. Based on the operational parameters of Rotamak plasma, simulations indicate that microwaves at a frequency around 10 GHz can heat electrons inside the separatrix layer. The physics of heating mechanism is similar for both the Rotamak and the C-2U FRC plasma, meaning that the magnitude of magnetic field goes down along the direction of ray propagation, therefore the rays, after the O-X-B mode conversion, encounter a basin of high harmonic EC resonances and mostly damp the power in the vicinity of the upper-hybrid resonance layer Detailed simulation results and plans for a future test bench will be presented.

  6. Vortex-antivortex nucleation in magnetically nanotextured superconductors: magnetic-field-driven and thermal scenarios.

    PubMed

    Milosević, M V; Peeters, F M

    2005-06-10

    Within the Ginzburg-Landau formalism, we predict two novel mechanisms of vortex-antivortex nucleation in a magnetically nanostructured superconductor. Although counterintuitive, nucleation of vortex-antivortex pairs can be activated in a superconducting (SC) film covered by arrays of submicron ferromagnets (FMs) when exposed to an external homogeneous magnetic field. In another scenario, we predict the thermal induction of vortex-antivortex configurations in SC-FM samples. This phenomenon leads to a new type of Little-Parks oscillations of the FM magnetization-temperature phase boundary of the SC film.

  7. Field-driven dynamics and time-resolved measurement of Dzyaloshinskii-Moriya torque in canted antiferromagnet YFeO3.

    PubMed

    Kim, Tae Heon; Grüenberg, Peter; Han, S H; Cho, B K

    2017-07-03

    Electrical spin switching in an antiferromagnet is one of the key issues for both academic interest and industrial demand in new-type spin devices because an antiferromagnetic system has a negligible stray field due to an alternating sign between sub-lattices, in contrast to a ferromagnetic system. Naturally, questions arise regarding how fast and, simultaneously, how robustly the magnetization can be switched by external stimuli, e.g., magnetic field and spin current. First, the exploitation of ultrafast precessional motion of magnetization in antiferromagnetic oxide has been studied intensively. Regarding robustness, the so-called inertia-driven switching scenario has been generally accepted as the switching mechanism in antiferromagnet system. However, in order to understand the switching dynamics in a canted antiferromagnet, excited by magnetic field, accurate equation of motion and corresponding interpretation are necessary. Here, we re-investigate the inertia-driven switching process, triggered by the strict phase matching between effective driving field, dh/dt, and antiferromagnetic order parameters, l. Such theoretical approaches make it possible to observe the static parameters of an antiferromagnet, hosting Dzyaloshinskii-Moriya (DM) interaction. Indeed, we estimate successfully static parameters, such as DM, exchange, and anisotropy energies, from dynamical behaviour in YFeO3, studied using terahertz time-domain spectroscopy.

  8. Flow-driven cell migration under external electric fields

    PubMed Central

    Li, Yizeng; Mori, Yoichiro; Sun, Sean X.

    2016-01-01

    Electric fields influence many aspects of cell physiology, including various forms of cell migration. Many cells are sensitive to electric fields, and can migrate toward a cathode or an anode, depending on the cell type. In this paper, we examine an actomyosin-independent mode of cell migration under electrical fields. Our theory considers a one-dimensional cell with water and ionic fluxes at the cell boundary. Water fluxes through the membrane are governed by the osmotic pressure difference across the cell membrane. Fluxes of cations and anions across the cell membrane are determined by the properties of the ion channels as well as the external electric field. Results show that without actin polymerization and myosin contraction, electric fields can also drive cell migration, even when the cell is not polarized. The direction of migration with respect to the electric field direction is influenced by the properties of ion channels, and are cell-type dependent. PMID:26765031

  9. Flow-Driven Cell Migration under External Electric Fields

    NASA Astrophysics Data System (ADS)

    Li, Yizeng; Mori, Yoichiro; Sun, Sean X.

    2015-12-01

    Electric fields influence many aspects of cell physiology, including various forms of cell migration. Many cells are sensitive to electric fields, and they can migrate toward a cathode or an anode, depending on the cell type. In this Letter, we examine an actomyosin-independent mode of cell migration under electrical fields. Our theory considers a one-dimensional cell with water and ionic fluxes at the cell boundary. Water fluxes through the membrane are governed by the osmotic pressure difference across the cell membrane. Fluxes of cations and anions across the cell membrane are determined by the properties of the ion channels as well as the external electric field. Results show that without actin polymerization and myosin contraction, electric fields can also drive cell migration, even when the cell is not polarized. The direction of migration with respect to the electric field direction is influenced by the properties of ion channels, and are cell-type dependent.

  10. Device-size dependence of field-free spin-orbit torque induced magnetization switching in antiferromagnet/ferromagnet structures

    NASA Astrophysics Data System (ADS)

    Kurenkov, A.; Zhang, C.; DuttaGupta, S.; Fukami, S.; Ohno, H.

    2017-02-01

    We study spin-orbit torque induced magnetization switching in devices consisting of an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with sizes ranging from 5 μm to 50 nm. As the size decreases, switching behavior changes from analogue-like to stepwise with several intermediate levels. The number of intermediate levels decreases with the decreasing size and finally evolves into a binary mode below a certain threshold. The results are found to be explained by a unique reversal process of this system, where ferromagnetic domains comprising a number of polycrystalline grains reverse individually and among the domains both out-of-plane and in-plane components of exchange bias vary.

  11. Metric-driven RoSy field design and remeshing.

    PubMed

    Lai, Yu-Kun; Jin, Miao; Xie, Xuexiang; He, Ying; Palacios, Jonathan; Zhang, Eugene; Hu, Shi-Min; Gu, Xianfeng

    2010-01-01

    Designing rotational symmetry fields on surfaces is an important task for a wide range of graphics applications. This work introduces a rigorous and practical approach for automatic N-RoSy field design on arbitrary surfaces with user-defined field topologies. The user has full control of the number, positions, and indexes of the singularities (as long as they are compatible with necessary global constraints), the turning numbers of the loops, and is able to edit the field interactively. We formulate N-RoSy field construction as designing a Riemannian metric such that the holonomy along any loop is compatible with the local symmetry of N-RoSy fields. We prove the compatibility condition using discrete parallel transport. The complexity of N-RoSy field design is caused by curvatures. In our work, we propose to simplify the Riemannian metric to make it flat almost everywhere. This approach greatly simplifies the process and improves the flexibility such that it can design N-RoSy fields with single singularity and mixed-RoSy fields. This approach can also be generalized to construct regular remeshing on surfaces. To demonstrate the effectiveness of our approach, we apply our design system to pen-and-ink sketching and geometry remeshing. Furthermore, based on our remeshing results with high global symmetry, we generate Celtic knots on surfaces directly.

  12. A 3D MOF constructed from dysprosium(III) oxalate and capping ligands: ferromagnetic coupling and field-induced two-step magnetic relaxation.

    PubMed

    Liu, Cai-Ming; Zhang, De-Qing; Zhu, Dao-Ben

    2016-04-04

    A novel 3D MOF based on dysprosium(iii) oxalate and 1,10-phenanthroline (phen), {[Dy(C2O4)1.5phen]·0.5H2O}n (1), has been hydrothermally synthesized. The Dy(3+) ion acts as a typical Y-shaped node, linking to each other to generate an interesting 3D topology structure. Complex 1 is the first 3D DyMOF displaying both ferromagnetic coupling and field-induced two-step magnetic relaxation.

  13. One-dimensional spin-1 ferromagnetic Heisenberg model with exchange anisotropy and single-ion anisotropy under external magnetic field

    NASA Astrophysics Data System (ADS)

    Song, Chuang-Chuang; Chen, Yuan; Liu, Ming-Wei

    2010-01-01

    The magnetic properties of the one-dimensional spin-1 ferromagnetic Heisenberg model are investigated by Green's function method. The magnetic properties of the system are treated by the random phase approximation for the exchange interaction term, and the Anderson-Callen approximation for the single-ion anisotropy term. The critical temperature, magnetization, and susceptibility are found to be dependent of the anisotropies. Our results are in agreement with the other theoretical results.

  14. Fundamental properties of field emission-driven direct current microdischarges

    NASA Astrophysics Data System (ADS)

    Rumbach, Paul; Go, David B.

    2012-11-01

    For half a century, it has been known that the onset of field emission in direct current microdischarges with gap sizes less than 10 μm can lead to breakdown at applied voltages far less than predicted by Paschen's law. It is still unclear how field emission affects other fundamental plasma properties at this scale. In this work, a one-dimensional fluid model is used to predict basic scaling laws for fundamental properties including ion density, electric field due to space charge, and current-voltage relations in the pre-breakdown regime. Computational results are compared with approximate analytic solutions. It is shown that field emission provides an abundance of cathode electrons, which in turn create large ion concentrations through ionizing collisions well before Paschen's criterion for breakdown is met. Breakdown due to ion-enhanced field emission occurs when the electric field due to space charge becomes comparable to the applied electric field. Simple scaling analysis of the 1D Poisson equation demonstrates that an ion density of n+ ≈ 0.1VAɛ0/qd2 is necessary to significantly distort the electric field. Defining breakdown in terms of this critical ion density leads analytically to a simple, effective secondary emission coefficient γ' of the same mathematical form initially suggested by Boyle and Kisliuk [Phys. Rev. 97, 255 (1955)].

  15. Plasma-satellite interaction driven magnetic field perturbations

    SciTech Connect

    Saeed-ur-Rehman; Marchand, Richard

    2014-09-15

    We report the first fully kinetic quantitative estimate of magnetic field perturbations caused by the interaction of a spacecraft with space environment. Such perturbations could affect measurements of geophysical magnetic fields made with very sensitive magnetometers on-board satellites. Our approach is illustrated with a calculation of perturbed magnetic fields near the recently launched Swarm satellites. In this case, magnetic field perturbations do not exceed 20 pT, and they are below the sensitivity threshold of the on-board magnetometers. Anticipating future missions in which satellites and instruments would be subject to more intense solar UV radiation, however, it appears that magnetic field perturbations associated with satellite interaction with space environment, might approach or exceed instruments' sensitivity thresholds.

  16. Calculation of magnetic field-induced current densities for humans from EAS countertop activation/deactivation devices that use ferromagnetic cores

    NASA Astrophysics Data System (ADS)

    Li, Qingxiang; Gandhi, Om P.

    2005-01-01

    Compliance testing of electronic article surveillance (EAS) devices requires that induced current densities in central nervous system (CNS) tissues, i.e. brain and the spinal cord, be less than the prescribed safety limits. Even though ferromagnetic cores are mostly used for activation/deactivation of embedded magnetic tags, assumed equivalent air-core coils with guessed increased number of ampere turns have always been used to calculate the magnetic fields for the proximal region to which a customer is exposed. We show that at low frequencies up to several kilohertz, duality of electric and magnetic circuits may be exploited such that the shaped high reluctance core is modelled as though it was a higher conductivity electric circuit of the corresponding shape. The proposed procedure is tested by examples of two magnetic cores typical of countertop activation/deactivation devices. The equivalent exposure magnetic fields obtained from the dual electric fields are shown to be in excellent agreement (within ±5%) with those measured for these ferromagnetic EAS devices. The previously proposed impedance method is then used to calculate the induced current densities for a 1.974 × 1.974 × 2.93 mm resolution anatomic model of a human. For the two considered EAS systems using excitation currents of 5000 A turns at 200 Hz, the maximum 1 cm2 area-averaged induced current densities in the CNS tissues are calculated and found to be less than the ICNIRP safety limits.

  17. From Nagaoka's Ferromagnetism to Flat-Band Ferromagnetism and Beyond --- An Introduction to Ferromagnetism in the Hubbard Model ---

    NASA Astrophysics Data System (ADS)

    Tasaki, H.

    1998-04-01

    It is believed that strong ferromagnetic interactions in some solids are generated by subtle interplay between quantum many-body effects and spin-independent Coulomb interactions between electrons. It is a challenging problem to verify this scenario in the Hubbard model, which is an idealized model for strongly interacting electrons in a solid. Nagaoka's ferromagnetism is a well-known rigorous example of ferromagnetism in the Hubbard model. It deals with the limiting situation in which there is one fewer electron than in the half-filling and the on-site Coulomb interaction is infinitely large. There are relatively new rigorous examples of ferromagnetism in Hubbard models called flat-band ferromagnetism. Flat-band ferromagnetism takes place in carefully prepared models in which the lowest bands (in the single-electron spectra) are ``flat.'' Usually, these two approaches are regarded as two complimentary routes to ferromagnetism in the Hubbard model. In the present paper we describe Nagaoka's ferromagnetism and flat-band ferromagnetism in detail, giving all the necessary background as well as complete (but elementary) mathematical proofs. By studying an intermediate model called the long-range hopping model, we also demonstrate that there is indeed a deep relation between these two seemingly different approaches to ferromagnetism. We further discuss some attempts to go beyond these approaches. We briefly discuss recent rigorous example of ferromagnetism in the Hubbard model which has neither infinitely large parameters nor completely flat bands. We give preliminary discussion regarding possible experimental realizations of the (nearly-)flat-band ferromagnetism. Finally, we focus on some theoretical attempts to understand metallic ferromagnetism. We discuss three artificial one-dimensional models in which the existence of metallic ferromagnetism can be easily proved. We have tried to make the present paper as self-contained as possible, keeping in mind readers who are

  18. Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2

    NASA Astrophysics Data System (ADS)

    von Rohr, Fabian; Krzton-Maziopa, Anna; Pomjakushin, Vladimir; Grundmann, Henrik; Guguchia, Zurab; Schnick, Wolfgang; Schilling, Andreas

    2016-07-01

    We report on the magnetic properties of CsCo2Se2 with ThCr2Si2 structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo2Se2 undergoes a phase transition to an antiferromagnetically ordered state with a Néel temperature of {{T}\\text{N}}≈ 66 K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of \\Theta≈ 51.0 K. We find that the magnetic structure of CsCo2Se2 consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal c-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at T  =  1.5 K is found to be only 0.20(1){μ\\text{Bohr}}  / Co. Already in comparably small magnetic fields of {μ0}H{{}\\text{MM}}(5~K)≈ 0.3 T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo2Se2, with the moments fully ferromagnetically saturated in a magnetic field of {μ0}{{H}\\text{FM}}(5~K)≈ 6.4 T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo2Se2 with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo2Se2, which is chemically and electronically posed closely to the A x Fe2-y Se2 superconductors, as a host of versatile magnetic interactions.

  19. Ellipsoidal Brownian self-driven particles in a magnetic field

    NASA Astrophysics Data System (ADS)

    Fan, Wai-Tong Louis; Pak, On Shun; Sandoval, Mario

    2017-03-01

    We study the two-dimensional Brownian dynamics of an ellipsoidal paramagnetic microswimmer moving at a low Reynolds number and subject to a magnetic field. Its corresponding mean-square displacement, showing the effect of a particles's shape, activity, and magnetic field on the microswimmer's diffusion, is analytically obtained. Comparison between analytical and computational results shows good agreement. In addition, the effect of self-propulsion on the transition time from anisotropic to isotropic diffusion of the ellipse is investigated.

  20. Electric Field Driven Self-Assembly of Colloidal Rods

    NASA Astrophysics Data System (ADS)

    Juarez, Jaime; Chaudhary, Kundan; Chen, Qian; Granick, Steve; Lewis, Jennifer

    2012-02-01

    The ability to assemble anisotropic colloidal building blocks into ordered configurations is of both scientific and technological importance. We are studying how electric field-induced interactions guide the self-assembly of these blocks into well aligned microstructures. Specifically, we present observations of the assembly of colloidal silica rods (L/D ˜ 4) within planar electrode cells as a function of different electric field parameters. Results from video microscopy and image analysis demonstrate that aligned microstructures form due to the competition between equilibrium interactions of induced dipoles and non-equilibrium processes (i.e., electro-osmosis). Under the appropriate electric field conditions (˜ kHZ AC fields), aligned colloidal rod fluids form over large areas on the electrode surface. The superposition of a DC electric field to this aligned colloidal rod fluid initiates their condensation into a vertically oriented crystalline phase. Ongoing work is now focused on exploring how temporal changes to electric fields influence colloidal rod dynamics and, hence, the assembly kinetics of aligned colloidal monolayers.

  1. Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

    SciTech Connect

    Kusa, F.; Echternkamp, K. E.; Herink, G.; Ropers, C.; Ashihara, S.

    2015-07-15

    We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

  2. Precessing Ferromagnetic Needle Magnetometer.

    PubMed

    Jackson Kimball, Derek F; Sushkov, Alexander O; Budker, Dmitry

    2016-05-13

    A ferromagnetic needle is predicted to precess about the magnetic field axis at a Larmor frequency Ω under conditions where its intrinsic spin dominates over its rotational angular momentum, Nℏ≫IΩ (I is the moment of inertia of the needle about the precession axis and N is the number of polarized spins in the needle). In this regime the needle behaves as a gyroscope with spin Nℏ maintained along the easy axis of the needle by the crystalline and shape anisotropy. A precessing ferromagnetic needle is a correlated system of N spins which can be used to measure magnetic fields for long times. In principle, by taking advantage of rapid averaging of quantum uncertainty, the sensitivity of a precessing needle magnetometer can far surpass that of magnetometers based on spin precession of atoms in the gas phase. Under conditions where noise from coupling to the environment is subdominant, the scaling with measurement time t of the quantum- and detection-limited magnetometric sensitivity is t^{-3/2}. The phenomenon of ferromagnetic needle precession may be of particular interest for precision measurements testing fundamental physics.

  3. Enhancement of the thermoelectric figure of merit in a ferromagnet-quantum dot-superconductor device due to intradot spin-flip scattering and ac field

    NASA Astrophysics Data System (ADS)

    Xu, Wei-Ping; Zhang, Yu-Ying; Li, Zhi-Jian; Nie, Yi-Hang

    2017-08-01

    We investigate the thermoelectric properties of a ferromagnet-quantum dot-superconductor hybrid system with the intradot spin-flip scattering and the external microwave field. The results indicate that the increase of figure of merit in the gap is very slight when the spin-flip scattering strength increases, but outside the gap it significantly increases with enhancing spin-flip scattering strength. The presence of microwave field results in photon-assisted Andreev reflection and induces the satellite peaks in conductance spectrum. The appropriate match of spin-flip scattering strength, microwave field strength and frequency can significantly enhances the figure of merit of thermoelectric conversion of the device, which can be used as a scheme improving thermoelectric efficiency using microwave frequency.

  4. Electric-Field-Induced Amplitude Tuning of Ferromagnetic Resonance Peak in Nano-granular Film FeCoB-SiO2/PMN-PT Composites.

    PubMed

    Luo, Mei; Zhou, Peiheng; Liu, Yunfeng; Wang, Xin; Xie, Jianliang

    2016-12-01

    One of the challenges in the design of microwave absorbers lies in tunable amplitude of dynamic permeability. In this work, we demonstrate that electric-field-induced magnetoelastic anisotropy in nano-granular film FeCoB-SiO2/PMN-PT (011) composites can be used to tune the amplitude of ferromagnetic resonance peak at room temperature. The FeCoB magnetic particles are separated from each other by SiO2 insulating matrix and present slightly different in-plane anisotropy fields. As a result, multi-resonances appear in the imaginary permeability (μ″) curve and mixed together to form a broadband absorption peak. The amplitude of the resonance peak could be modulated by external electric field from 118 to 266.

  5. Electric-Field-Induced Amplitude Tuning of Ferromagnetic Resonance Peak in Nano-granular Film FeCoB-SiO2/PMN-PT Composites

    NASA Astrophysics Data System (ADS)

    Luo, Mei; Zhou, Peiheng; Liu, Yunfeng; Wang, Xin; Xie, Jianliang

    2016-11-01

    One of the challenges in the design of microwave absorbers lies in tunable amplitude of dynamic permeability. In this work, we demonstrate that electric-field-induced magnetoelastic anisotropy in nano-granular film FeCoB-SiO2/PMN-PT (011) composites can be used to tune the amplitude of ferromagnetic resonance peak at room temperature. The FeCoB magnetic particles are separated from each other by SiO2 insulating matrix and present slightly different in-plane anisotropy fields. As a result, multi-resonances appear in the imaginary permeability ( μ″) curve and mixed together to form a broadband absorption peak. The amplitude of the resonance peak could be modulated by external electric field from 118 to 266.

  6. Laboratory Observation Of Magnetic Field Growth Driven By Shear Flow

    NASA Astrophysics Data System (ADS)

    Intrator, T.; Sun, X.; Dorf, L.; Sears, J.; Weber, T.; Lapenta, G.

    2012-12-01

    We have measured in the laboratory profiles of magnetic flux ropes, that include ion flow, magnetic field, current density, and plasma pressure. These data allow a complete screw pinch equilibrium with guide magnetic field to be reconstructed, and the electron flows to be inferred. We use this information to evaluate the Hall JxB term in a two fluid magnetohydrodynamic Ohms Law. The difference between ion and electron flows allows us to show experimentally and theoretically that the sheared electron flows can account for the generation of magnetic field. For example we show a measured quadrupole out of plane magnetic field B_z structure that occurs even in the absence of magnetic reconnection. This out of plane quadrupole pattern has historically been used as a signature of magnetic reconnection, especially with small to vanishing guide field. Recent theoretical analyses have pointed out that this presumption need not be true. *Supported by DOE Office of Fusion Energy Sciences under LANS contract DE-AC52-06NA25369, NASA Geospace NNHIOA044I, Basic

  7. A hypothesis-driven approach to optimize field campaigns

    NASA Astrophysics Data System (ADS)

    Nowak, Wolfgang; Rubin, Yoram; de Barros, Felipe P. J.

    2012-06-01

    Most field campaigns aim at helping in specified scientific or practical tasks, such as modeling, prediction, optimization, or management. Often these tasks involve binary decisions or seek answers to yes/no questions under uncertainty, e.g., Is a model adequate? Will contamination exceed a critical level? In this context, the information needs of hydro(geo)logical modeling should be satisfied with efficient and rational field campaigns, e.g., because budgets are limited. We propose a new framework to optimize field campaigns that defines the quest for defensible decisions as the ultimate goal. The key steps are to formulate yes/no questions under uncertainty as Bayesian hypothesis tests, and then use the expected failure probability of hypothesis testing as objective function. Our formalism is unique in that it optimizes field campaigns for maximum confidence in decisions on model choice, binary engineering or management decisions, or questions concerning compliance with environmental performance metrics. It is goal oriented, recognizing that different models, questions, or metrics deserve different treatment. We use a formal Bayesian scheme called PreDIA, which is free of linearization, and can handle arbitrary data types, scientific tasks, and sources of uncertainty (e.g., conceptual, physical, (geo)statistical, measurement errors). This reduces the bias due to possibly subjective assumptions prior to data collection and improves the chances of successful field campaigns even under conditions of model uncertainty. We illustrate our approach on two instructive examples from stochastic hydrogeology with increasing complexity.

  8. Properties of a field emission-driven Townsend discharge

    NASA Astrophysics Data System (ADS)

    Rumbach, Paul; Go, David

    2012-10-01

    For half a century, it has been known that the onset of field emission in direct current (DC) microplasmas with gap sizes less than 10 μm can lead to breakdown at applied voltages far less than predicted by Paschen's law. It is still unclear how field emission affects other fundamental plasma properties at this scale. In this work, a one-dimensional fluid model is used to predict basic scaling laws for fundamental properties such as ion density, electric field due to space charge, and current voltage relations in the pre-breakdown regime. Computational results are compared with approximate analytic solutions. It is shown that ionizing collisions by field-emitted electrons produce significant ion densities well before Paschen's criteria for breakdown is met. When positive space charge densities become sufficiently large, the effect of ion-enhanced field emission leads to breakdown. Defining breakdown mathematically using a solvability condition leads to a full modified Paschen's curve, while defining it physically in terms of a critical ion density leads analytically to an effective secondary emission coefficient, γ', of the form initially suggested by Boyle and Kisliuk.footnotetextBoyle, W.S. and Kisliuk, P., Phys. Rev. 97, 255 (1955).

  9. Near-field optically driven Brownian motors (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Wu, Shao-Hua; Huang, Ningfeng; Jaquay, Eric; Povinelli, Michelle L.

    2016-09-01

    Brownian ratchets are of fundamental interest in fields from statistical physics to molecular motors. The realization of Brownian ratchets in engineered systems opens up the potential to harness thermal energy for directed motion, with applications in transport and sorting of nanoparticles. Implementations based on optical traps provide a high degree of tunability along with precise spatiotemporal control. Near-field optical methods provide particular flexibility and ease of on-chip integration with other microfluidic components. Here, we demonstrate the first all-optical, near-field Brownian ratchet. Our approach uses an asymmetrically patterned photonic crystal and yields an ultra-stable trap stiffness of 253.6 pN/nm-W, 100x greater than conventional optical tweezers. By modulating the laser power, optical ratcheting with transport speed of 1 micron/s can be achieved, allowing a variety of dynamical lab-on-a-chip applications. The resulting transport speed matches well with the theoretical prediction.

  10. Flow field topology of transient mixing driven by buoyancy

    NASA Technical Reports Server (NTRS)

    Duval, Walter M B.

    2004-01-01

    Transient mixing driven by buoyancy occurs through the birth of a symmetric Rayleigh-Taylor morphology (RTM) structure for large length scales. Beyond its critical bifurcation the RTM structure exhibits self-similarity and occurs on smaller and smaller length scales. The dynamics of the RTM structure, its nonlinear growth and internal collision, show that its genesis occurs from an explosive bifurcation which leads to the overlap of resonance regions in phase space. This event shows the coexistence of regular and chaotic regions in phase space which is corroborated with the existence of horseshoe maps. A measure of local chaos given by the topological entropy indicates that as the system evolves there is growth of uncertainty. Breakdown of the dissipative RTM structure occurs during the transition from explosive to catastrophic bifurcation; this event gives rise to annihilation of the separatrices which drives overlap of resonance regions. The global bifurcation of explosive and catastrophic events in phase space for the large length scale of the RTM structure serves as a template for which mixing occurs on smaller and smaller length scales. Copyright 2004 American Institute of Physics.

  11. Flow field topology of transient mixing driven by buoyancy

    NASA Technical Reports Server (NTRS)

    Duval, Walter M B.

    2004-01-01

    Transient mixing driven by buoyancy occurs through the birth of a symmetric Rayleigh-Taylor morphology (RTM) structure for large length scales. Beyond its critical bifurcation the RTM structure exhibits self-similarity and occurs on smaller and smaller length scales. The dynamics of the RTM structure, its nonlinear growth and internal collision, show that its genesis occurs from an explosive bifurcation which leads to the overlap of resonance regions in phase space. This event shows the coexistence of regular and chaotic regions in phase space which is corroborated with the existence of horseshoe maps. A measure of local chaos given by the topological entropy indicates that as the system evolves there is growth of uncertainty. Breakdown of the dissipative RTM structure occurs during the transition from explosive to catastrophic bifurcation; this event gives rise to annihilation of the separatrices which drives overlap of resonance regions. The global bifurcation of explosive and catastrophic events in phase space for the large length scale of the RTM structure serves as a template for which mixing occurs on smaller and smaller length scales. Copyright 2004 American Institute of Physics.

  12. Gravity Driven Universe: Energy from a Unified Field

    NASA Astrophysics Data System (ADS)

    Masters, Roy

    2012-10-01

    One way or another, whether push or pull, we know for sure that gravity is omnidirectional with identical mathematics. With PULL, gravity can be seen as as a property of matter. If so something is wrong. The Moon, lifting the tides twice-daily, should have fallen into orbital decay, with Earth having pulled it down eons ago. It is puzzling that physicists are not troubled by the fact that the Moon not only insists on forever lifting the tides, but, adding insult to injury, keeps moving it about 4 cm further away from Earth each year. Now if instead, we consider gravity as driven by an omnidirectional pressure--a PUSH force, another possibility arises. We can consider that it is mysteriously infusing energy into the Earth-Moon system, sustaining the Moon's orbit with the appearance of raising the tides and actually pushing it away from Earth. Here we can show push and pull, while being identical in their mathematics, have different outcomes. With push, gravity is a property of the universe. If this is true, then gravitation is flowing from an everlasting source, and the Earth/Moon system is one example of many other vacuum energy machines in the universe.

  13. Field-driven oscillation and rotation of a multiskyrmion cluster in a nanodisk

    NASA Astrophysics Data System (ADS)

    Liu, Yan; Yan, Huan; Jia, Min; Du, Haifeng; Du, An; Zang, Jiadong

    2017-04-01

    The field-driven magnetization dynamics of a multiskyrmion cluster in a nanodisk is investigated by micromagnetic simulation and analytical calculation. Under a weak in-plane static magnetic field, the multiskyrmion cluster shows an oscillatory motion around an equilibrium position, which resembles the dynamical behavior of the conventional torsional pendulum. We show that this oscillation is induced by restoring torque acting on the skyrmion generated by the potential energy determined by the angle of the skyrmion orientation. Moreover, the multiskyrmion cluster can be driven to rotate by an in-plane rotating magnetic field. The rotation directions and frequencies are fully determined by the number of the skyrmions.

  14. A model of the radiation-induced bystander effect based on an analogy with ferromagnets. Application to modelling tissue response in a uniform field

    NASA Astrophysics Data System (ADS)

    Vassiliev, O. N.

    2014-12-01

    We propose a model of the radiation-induced bystander effect based on an analogy with magnetic systems. The main benefit of this approach is that it allowed us to apply powerful methods of statistical mechanics. The model exploits the similarity between how spin-spin interactions result in correlations of spin states in ferromagnets, and how signalling from a damaged cell reduces chances of survival of neighbour cells, resulting in correlated cell states. At the root of the model is a classical Hamiltonian, similar to that of an Ising ferromagnet with long-range interactions. The formalism is developed in the framework of the Mean Field Theory. It is applied to modelling tissue response in a uniform radiation field. In this case the results are remarkably simple and at the same time nontrivial. They include cell survival curves, expressions for the tumour control probability and effects of fractionation. The model extends beyond of what is normally considered as bystander effects. It offers an insight into low-dose hypersensitivity and into mechanisms behind threshold doses for deterministic effects.

  15. Spintronics r.f. oscillator driven by magnetic field feedback

    NASA Astrophysics Data System (ADS)

    Tulapurkar, Ashwin; Dixit, Dinesh; Konishi, Katsunori; Tomy, C. V.; Suzuki, Yoshishige

    2013-03-01

    Magnetic tunnel junctions (MTJ) can be used as nano-scale rf oscillators using spin-transfer torque effect. Here we present an alternative novel mechanism of ``magnetic field feedback'' for driving MTJs into precessional states. To realize this effect, MTJ needs to be fabricated on top of a co-planar wave-guide. A dc current is passed through MTJ to produce a fluctuating voltage across it as a combination of thermal fluctuations of free layer and magneto-resistance effect. This voltage is applied across co-planar wave-guide to create a fluctuating magnetic field which acts on the free layer to enhance its fluctuations. If the dc current exceeds a critical value, precessional states of free layer are excited. We have derived expression for the critical current using lineralized LLG equation, modified to include the ``feedback'' magnetic field. We have verified the feedback effect by numerical simulation of stochastic LLG equation including random magnetic field: we find that the damping of the free layer can be increased/decreased by applying -ve/ +ve dc current. Simulations show that by applying dc current more than critical current, large amplitude oscillations with high quality factors are possible.

  16. FLARES IN THE CRAB NEBULA DRIVEN BY UNTWISTING MAGNETIC FIELDS

    SciTech Connect

    Sturrock, Peter; Aschwanden, Markus J. E-mail: aschwanden@lmsal.com

    2012-06-01

    The recent discovery of PeV electrons from the Crab Nebula, produced on rapid timescales of one day or less with a sharply peaked gamma-ray spectrum without hard X-rays, challenges traditional models of diffusive shock acceleration followed by synchrotron radiation. Here, we outline an acceleration model involving a DC electric field parallel to the magnetic field in a twisted toroidal field around the pulsar. Sudden developments of resistivity in localized regions of the twisted field are thought to drive the particle acceleration, up to PeV energies, resulting in flares. This model can reproduce the observed timescales of T Almost-Equal-To 1 day, the peak photon energies of U{sub {Phi},rr} Almost-Equal-To 1 MeV, maximum electron energies of U{sub e,rr} Almost-Equal-To 1 PeV, and luminosities of L Almost-Equal-To 10{sup 36} erg s{sup -1}.

  17. Electric field driven mesoscale phase transition in polarized colloids

    NASA Astrophysics Data System (ADS)

    Khusid, Boris; Elele, Ezinwa; Lei, Qian

    2016-11-01

    A mesoscale phase transition in a polarized suspension was reported by Kumar, Khusid, Acrivos, PRL95, 2005 and Agarwal, Yethiraj, PRL102, 2009. Following the application of a strong AC field, particles aggregated head-to-tail into chains that bridged the interelectrode gap and then formed a cellular pattern, in which large particle-free domains were enclosed by particle-rich thin walls. Cellular structures were not observed in numerous simulations of field induced phase transitions in a polarized suspension. A requirement for matching the particle and fluid densities to avoid particle settling limits terrestrial experiments to negatively polarized particles. We present data on the phase diagram and kinetics of the phase transition in a neutrally buoyant, negatively polarized suspension subjected to a combination of AC and DC. Surprisingly, a weak DC component drastically speeds up the formation of a cellular pattern but does not affect its key characteristic. However, the application of a strong DC field destroys the cellular pattern, but it restores as the DC field strength is reduced. We also discuss the design of experiments to study phase transitions in a suspension of positively polarized, non-buoyancy-matched particles in the International Space Station. Supported by NASA's Physical Science Research Program, NNX13AQ53G.

  18. Development and Fielding of High-Speed Laser Shadowgraphy for Electro-Magnetically Driven Cylindrical Implosions

    DTIC Science & Technology

    2013-06-01

    shockwave in a cylindrical geometry provides fundamental benchmarks used in the modeling of 1-D and 2-D hydrodynamic phenomena from high or solid...DEVELOPMENT AND FIELDING OF HIGH-SPEED LASER SHADOWGRAPHY FOR ELECTRO -MAGNETICALLY DRIVEN CYLINDRICAL IMPLOSIONS J. P. Roberts, G. Rodriguez...an electro -magnetically driven solid density liner implosion in Lucite is described. The laser shadowgraphy system utilizes an advanced high-energy

  19. Phase segregation of superconductivity and ferromagnetism at the LaAlO3/SrTiO3 interface.

    PubMed

    Mohanta, N; Taraphder, A

    2014-01-15

    The highly conductive two-dimensional electron gas formed at the interface between insulating SrTiO3 and LaAlO3 shows low-temperature superconductivity coexisting with inhomogeneous ferromagnetism. The Rashba spin-orbit interaction with the in-plane Zeeman field of the system favors p(x) ± ip(y)-wave superconductivity at finite momentum. Owing to the intrinsic disorder at the interface, the role of spatial inhomogeneity in the superconducting and ferromagnetic states becomes important. We find that, for strong disorder, the system breaks up into mutually excluded regions of superconductivity and ferromagnetism. This inhomogeneity-driven electronic phase separation accounts for the unusual coexistence of superconductivity and ferromagnetism observed at the interface.

  20. Coronal Magnetic Field Measurement Using CME-Driven Shock Observations

    NASA Technical Reports Server (NTRS)

    Gopalswarmy, Nat; Nitta, N.; Yashiro, S.; Makela, P.; Xie, H.; Akiyama, S.

    2012-01-01

    Collisionless shocks form ahead of coronal mass ejections (CMEs) when the CME speed exceeds the Alfven speed of the ambient plasma in the corona and interplanetary medium. The shock stands at a distance from the CME flux rope that depends on the shock Mach number, the geometry of the driver, and the adiabatic index. While the shock ahead of the CME has been observed for a long time in the in situ data, it has been identified recently near the Sun in the coronagraphic and EUV images. Unlike in situ observations, the imaging observations are two dimensional, so one can better discern the CME-shock relationship near the Sun. Gopalswamy and Yashiro demonstrated that the coronal magnetic field can be derived from the shock standoff distance measured in coronagraphic images. The method involves measuring the standoff distance, the radius of curvature of the flux rope, and assuming the value of the adiabatic index and deriving the Alfvenic Mach number. The next step is to derive the Alfvenic Mach number from the measured shock speed and an estimate of the local solar wind speed. The final step involves deriving the magnetic field from the Alfven speed by measuring the local plasma density either from coronagraphic (polarized brightness) images or from the band-splitting of type II radio bursts. In this paper, we derive the combined magnetic field profile from near the Sun to the edge of the LASCO field of view (1.5 to 30 solar radii) and compare it with the current model profiles.

  1. Quantum driven dissipative parametric oscillator in a blackbody radiation field

    SciTech Connect

    Pachón, Leonardo A.; Brumer, Paul

    2014-01-15

    We consider the general open system problem of a charged quantum oscillator confined in a harmonic trap, whose frequency can be arbitrarily modulated in time, that interacts with both an incoherent quantized (blackbody) radiation field and with an arbitrary coherent laser field. We assume that the oscillator is initially in thermodynamic equilibrium with its environment, a non-factorized initial density matrix of the system and the environment, and that at t = 0 the modulation of the frequency, the coupling to the incoherent and the coherent radiation are switched on. The subsequent dynamics, induced by the presence of the blackbody radiation, the laser field, and the frequency modulation, is studied in the framework of the influence functional approach. This approach allows incorporating, in analytic closed formulae, the non-Markovian character of the oscillator-environment interaction at any temperature as well the non-Markovian character of the blackbody radiation and its zero-point fluctuations. Expressions for the time evolution of the covariance matrix elements of the quantum fluctuations and the reduced density-operator are obtained.

  2. Laboratory Observation of Magnetic Field Growth Driven by Shear Flow

    NASA Astrophysics Data System (ADS)

    Intrator, Thomas; Dorf, L.; Sun, X.; Sears, J.; Weber, T.; Feng, Y.

    2013-04-01

    We have measured in the laboratory profiles of magnetic flux ropes, that include ion flow, magnetic field, current density, and plasma pressure. The electron flows v_e can therefore be inferred, and we use this information to evaluate the Hall J × B term in a two fluid magnetohydrodynamic Ohm’s Law. Mutually attracted and compressed flux ropes break the cylindrical symmetry. This simple and coherent example of shear flow supports magnetic field growth corresponding to non vanishing curl × v_e × B. In the absence of magnetic reconnection we measure and predict a quadrupole out of plane magnetic field δBz, even though this has historically been invoked to be the signature of Hall magnetic reconnection. This provides a natural and general mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence. *Supported by DOE Office of Fusion Energy Sciences under LANS contract DE-AC52-06NA25369, NASA Geospace NNHIOA044I, Basic

  3. JET ROTATION DRIVEN BY MAGNETOHYDRODYNAMIC SHOCKS IN HELICAL MAGNETIC FIELDS

    SciTech Connect

    Fendt, Christian

    2011-08-10

    In this paper, we present a detailed numerical investigation of the hypothesis that a rotation of astrophysical jets can be caused by magnetohydrodynamic (MHD) shocks in a helical magnetic field. Shock compression of the helical magnetic field results in a toroidal Lorentz force component that will accelerate the jet material in the toroidal direction. This process transforms magnetic angular momentum (magnetic stress) carried along the jet into kinetic angular momentum (rotation). The mechanism proposed here only works in a helical magnetic field configuration. We demonstrate the feasibility of this mechanism by axisymmetric MHD simulations in 1.5 and 2.5 dimensions using the PLUTO code. In our setup, the jet is injected into the ambient gas with zero kinetic angular momentum (no rotation). We apply different dynamical parameters for jet propagation such as the jet internal Alfven Mach number and fast magnetosonic Mach number, the density contrast of the jet to the ambient medium, and the external sonic Mach number of the jet. The mechanism we suggest should work for a variety of jet applications, e.g., protostellar or extragalactic jets, and internal jet shocks (jet knots) or external shocks between the jet and the ambient gas (entrainment). For typical parameter values for protostellar jets, the numerically derived rotation feature looks consistent with the observations, i.e., rotational velocities of 0.1%-1% of the jet bulk velocity.

  4. Jet Rotation Driven by Magnetohydrodynamic Shocks in Helical Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Fendt, Christian

    2011-08-01

    In this paper, we present a detailed numerical investigation of the hypothesis that a rotation of astrophysical jets can be caused by magnetohydrodynamic (MHD) shocks in a helical magnetic field. Shock compression of the helical magnetic field results in a toroidal Lorentz force component that will accelerate the jet material in the toroidal direction. This process transforms magnetic angular momentum (magnetic stress) carried along the jet into kinetic angular momentum (rotation). The mechanism proposed here only works in a helical magnetic field configuration. We demonstrate the feasibility of this mechanism by axisymmetric MHD simulations in 1.5 and 2.5 dimensions using the PLUTO code. In our setup, the jet is injected into the ambient gas with zero kinetic angular momentum (no rotation). We apply different dynamical parameters for jet propagation such as the jet internal Alfvén Mach number and fast magnetosonic Mach number, the density contrast of the jet to the ambient medium, and the external sonic Mach number of the jet. The mechanism we suggest should work for a variety of jet applications, e.g., protostellar or extragalactic jets, and internal jet shocks (jet knots) or external shocks between the jet and the ambient gas (entrainment). For typical parameter values for protostellar jets, the numerically derived rotation feature looks consistent with the observations, i.e., rotational velocities of 0.1%-1% of the jet bulk velocity.

  5. Rheological properties and orientational distributions of dilute ferromagnetic spherocylinder particle dispersions. Part II. Analysis for the two typical magnetic field directions.

    PubMed

    Aoshima, Masayuki; Satoh, Akira; Chantrell, Roy W; Coverdale, Geoff N

    2002-09-15

    We have investigated the orientational distributions and rheological properties of dilute colloidal dispersions, which consist of ferromagnetic spherocylinder particles. First, the governing equation of the orientational distribution function has been derived for the typical two cases of magnetic field directions: the direction parallel to the shear flow and the direction parallel to the angular velocity vector of the shear flow. The equation has been solved approximately by Galerkin's method. With these numerical solutions we have obtained the results of the orientational distribution and viscosity. The results obtained for the magnetic field in the shear flow direction are summarized as follows. In the case of a weak magnetic field, the particle tends to orient nearly toward the shear flow direction and its opposite direction. As the magnetic field increases, the orientation of the particle is restricted and the viscosity increases significantly. As the influence of the magnetic field becomes dominant, an overshoot in the viscosity curve appears. This is due to the fact that there is a maximum deviation of the averaged particle direction from the magnetic field direction. When the strength of the magnetic field increases significantly, the particle inclines close to the magnetic field direction and the viscosity converges to a constant value. Particles with a larger aspect ratio give rise to a larger increment in the viscosity since such elongated particles induce larger resistance in a flow field. We also have obtained results for the case of the magnetic field in the direction parallel to the angular velocity vector of the shear flow. When the flow field is dominant over both the rotational Brownian motion and the magnetic interaction, the particle rotates in the plane nearly perpendicular to the magnetic field direction. As the magnetic field increases, the particle inclines toward the magnetic direction. For this direction of field, the viscosity is

  6. Steady-state configurations of Dzyaloshinskii domain walls driven by field and current

    NASA Astrophysics Data System (ADS)

    Sánchez-Tejerina, L.; Alejos, O.; Martínez, E.

    2017-02-01

    The dynamics of Dzyaloshinskii domain walls (DDW) in ultrathin ferromagnetic strips with perpendicular magnetic anisotropy, for different values of both perpendicular field and longitudinal current excitation associated to the Spin-Hall effect, has been studied, taking into account different values of the interfacial Dzyaloshinskii-Moriya interaction (DMI). This study has been carried out with the help of the q-Φ one-dimensional model and micromagnetic simulations. We have found that Walker breakdown may be avoided by applying a certain threshold current, even though the inverse effect is also possible. We have also found that, for particular values of field and current, the magnetization within the DDW experiences an abrupt change of orientation, which provokes a change on the contribution of current to the terminal DDW velocity. This effect disappears for sufficiently strong DMI, as it is expected from the model.

  7. Formation and field-driven dynamics of nematic spheroids.

    PubMed

    Fu, Fred; Abukhdeir, Nasser Mohieddin

    2017-07-19

    Unlike the canonical application of liquid crystals (LCs), LC displays, emerging technologies based on LC materials are increasingly leveraging the presence of nanoscale defects. The inherent nanoscale characteristics of LC defects present both significant opportunities as well as barriers for the application of this fascinating class of materials. Simulation-based approaches to the study of the effects of confinement and interface anchoring conditions on LC domains has resulted in significant progress over the past decade, where simulations are now able to access experimentally-relevant length scales while simultaneously capturing nanoscale defect structures. In this work, continuum simulations were performed in order to study the dynamics of micron-scale nematic LC spheroids of varying shape. Nematic spheroids are one of the simplest inherently defect-containing LC structures and are relevant to polymer-dispersed LC-based "smart" window technology. Simulation results include nematic phase formation and external field-switching dynamics of nematic spheroids ranging in shape from oblate to prolate. Results include both qualitative and quantitative insight into the complex coupling of nanoscale defect dynamics and structure transitions to micron-scale reorientation. Dynamic mechanisms are presented and related to structural transitions in LC defects present in the nematic domain. Domain-averaged metrics including order parameters and response times are determined for a range of experimentally-accessible electric field strengths. These results have both fundamental and technological relevance, in that increased understanding of LC dynamics in the presence of defects is a key barrier to continued advancement in the field.

  8. Spark-plasma-sintering magnetic field assisted compaction of Co{sub 80}Ni{sub 20} nanowires for anisotropic ferromagnetic bulk materials

    SciTech Connect

    Ouar, Nassima; Schoenstein, Frédéric; Mercone, Silvana; Farhat, Samir; Jouini, Noureddine; Villeroy, Benjamin; Leridon, Brigitte

    2013-10-28

    We developed a two-step process showing the way for sintering anisotropic nanostructured bulk ferromagnetic materials. A new reactor has been optimized allowing the synthesis of several grams per batch of nanopowders via a polyol soft chemistry route. The feasibility of the scale-up has been successfully demonstrated for Co{sub 80}Ni{sub 20} nanowires and a massic yield of ∼97% was obtained. The thus obtained nanowires show an average diameter of ∼6 nm and a length of ∼270 nm. A new bottom-up strategy allowed us to compact the powder into a bulk nanostructured system. We used a spark-plasma-sintering technique under uniaxial compression and low temperature assisted by a permanent magnetic field of 1 T. A macroscopic pellet of partially aligned nanowire arrays has been easily obtained. This showed optimized coercive properties along the direction of the magnetic field applied during compaction (i.e., the nanowires' direction)

  9. Magnetic properties of NiO nano particles: Contributions of the antiferromagnetic and ferromagnetic subsystems in different magnetic field ranges up to 250 kOe

    NASA Astrophysics Data System (ADS)

    Balaev, D. A.; Dubrovskiy, A. A.; Krasikov, A. A.; Popkov, S. I.; Balaev, A. D.; Shaikhutdinov, K. A.; Kirillov, V. L.; Mart'yanov, O. N.

    2017-08-01

    The magnetic properties of antiferromagnetic NiO nanoparticles prepared by thermal decomposition of nickel hydroxocarbonate are investigated. According to the data of magnetization measurements in fields of up to 250 kOe, the magnetic moment linearly grows in strong fields, which is caused by the contribution of the antiferromagnetically ordered nanoparticle core, and the antiferromagnetic susceptibility corresponds to that of bulk polycrystalline NiO. This allowed the antiferromagnetic and ferromagnetic contributions to the total magnetic response of a sample to be quantitatively determined. The latter occurs due to the incomplete spin compensation in an antiferromagnetic nanoparticle caused by defects on its surface. It is demonstrated that to correctly determine the superparamagnetic blocking temperature, it is necessary to take into account the antiferromagnetic susceptibility of the particle core.

  10. Improved understanding of geologic CO{sub 2} storage processes requires risk-driven field experiments

    SciTech Connect

    Oldenburg, C.M.

    2011-06-01

    The need for risk-driven field experiments for CO{sub 2} geologic storage processes to complement ongoing pilot-scale demonstrations is discussed. These risk-driven field experiments would be aimed at understanding the circumstances under which things can go wrong with a CO{sub 2} capture and storage (CCS) project and cause it to fail, as distinguished from accomplishing this end using demonstration and industrial scale sites. Such risk-driven tests would complement risk-assessment efforts that have already been carried out by providing opportunities to validate risk models. In addition to experimenting with high-risk scenarios, these controlled field experiments could help validate monitoring approaches to improve performance assessment and guide development of mitigation strategies.

  11. Resonant parametric excitations driven by lower-hybrid fields

    NASA Astrophysics Data System (ADS)

    Villalon, E.

    1980-11-01

    Three-wave parametric excitation in inhomogeneous plasmas is examined in a two-dimensional geometry relevant to supplementary rf heating of tokamaks. The stabilization of resonant parametric excitation due to a linear mismatch in wavenumbers and to the Landau-damping rates of the decay waves is analyzed, assuming that the magnitude of the pump field is constant in time and in the spatial region where the resonant interaction takes place. Both types of temporally growing modes and spatially amplified instabilities are studied, using a WKB analysis. It is shown that by increasing the strength of the mismatch K prime or the width of the pump L, the growth rate of the fastest growing normal mode will decrease. The amount of spatial amplification is also reduced by the mismatch in wavenumbers and by the damping rates of the excited waves. Because of the finite spatial extent of the pump electric field, the amplification length is smaller than or equal to L, depending on the strength of the mismatch and damping rates.

  12. Kernel-Correlated Levy Field Driven Forward Rate and Application to Derivative Pricing

    SciTech Connect

    Bo Lijun; Wang Yongjin; Yang Xuewei

    2013-08-01

    We propose a term structure of forward rates driven by a kernel-correlated Levy random field under the HJM framework. The kernel-correlated Levy random field is composed of a kernel-correlated Gaussian random field and a centered Poisson random measure. We shall give a criterion to preclude arbitrage under the risk-neutral pricing measure. As applications, an interest rate derivative with general payoff functional is priced under this pricing measure.

  13. Self-assembly of colloidal bands driven by a periodic external field

    SciTech Connect

    Nunes, André S.; Araújo, Nuno A. M. Telo da Gama, Margarida M.

    2016-01-21

    We study the formation of bands of colloidal particles driven by periodic external fields. Using Brownian dynamics, we determine the dependence of the band width on the strength of the particle interactions and on the intensity and periodicity of the field. We also investigate the switching (field-on) dynamics and the relaxation times as a function of the system parameters. The observed scaling relations were analyzed using a simple dynamic density-functional theory of fluids.

  14. Self-assembly of colloidal bands driven by a periodic external field

    NASA Astrophysics Data System (ADS)

    Nunes, André S.; Araújo, Nuno A. M.; Telo da Gama, Margarida M.

    2016-01-01

    We study the formation of bands of colloidal particles driven by periodic external fields. Using Brownian dynamics, we determine the dependence of the band width on the strength of the particle interactions and on the intensity and periodicity of the field. We also investigate the switching (field-on) dynamics and the relaxation times as a function of the system parameters. The observed scaling relations were analyzed using a simple dynamic density-functional theory of fluids.

  15. Nonlinear spin control by terahertz-driven anisotropy fields

    NASA Astrophysics Data System (ADS)

    Baierl, S.; Hohenleutner, M.; Kampfrath, T.; Zvezdin, A. K.; Kimel, A. V.; Huber, R.; Mikhaylovskiy, R. V.

    2016-11-01

    Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light. Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations. Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field. Using the prototypical antiferromagnet thulium orthoferrite (TmFeO3), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe3+ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials.

  16. Microwave-Field Driven Acoustic Modes in Selected DNA Molecules

    NASA Astrophysics Data System (ADS)

    Edwards, Glenn Steven

    The direct coupling of a microwave field to selected DNA molecules is demonstrated using standard dielectrometry. The absorption is resonant with a typical lifetime of 300 picoseconds. Such a long lifetime is unexpected for DNA in aqueous solution at room temperature and has interesting implications for microscopic considerations in future models of solvent damping. Resonant absorption at fundamental and harmonic frequencies for both supercoiled circular and linear DNA agrees with an acoustic mode model. Our associated acoustic velocities for linear DNA are very close to the acoustic velocity of the longitudinal acoustic mode independently observed on DNA fibers using Brillouin Spectroscopy. The difference in acoustic velocities for supercoiled circular and linear DNA is discussed in terms of a conformation dependent model. *This research has been funded by the Office of Naval Research, the Center for Devices and Radiological Health, and the National Science Foundation.

  17. The field-induced laws of thermodynamic properties in the two-dimensional spin-1 ferromagnetic Heisenberg model with the exchange and single-ion anisotropies

    NASA Astrophysics Data System (ADS)

    Pu, Qiurong; Chen, Yuan

    2013-02-01

    Green's function method is applied to investigate the two-dimensional spin-1 ferromagnetic Heisenberg model with the exchange and single-ion anisotropies. In the presence of the magnetic field, the effects of the anisotropies and field on the thermodynamic properties are obtained within the random phase approximation combining with Anderson-Callen approximation. The field-induced laws are found for the thermodynamic properties. Field dependences of heights of the susceptibility maximum and specific heat maximum fit well to power laws. The linear increase at high fields is shown for positions of the susceptibility maximum and specific heat maximum. A power law at low fields occurs for the position of the susceptibility maximum. At the positions of the maxima, the magnetization and internal energy display the power-law increase and linear decrease with the field, respectively. The exponents of the power laws are dependent of the anisotropies, as well as the slopes of the linear laws. Our results do not support the 2/3 power law which was obtained by the Landau theory.

  18. Converging xenon shock waves driven by megagauss magnetic fields

    SciTech Connect

    Shearer, J.W.; Steinberg, D.J.

    1986-07-01

    We attempted to implode a conducting metal linear at high velocity, and our failure to do so led to switching, or rapidly transferring the field from pushing an aluminum conductor to snow-plowing a half-atmosphere of xenon gas. We successfully initiated convergent xenon gas shocks with the use of a magnetohydrodynamic switch and coaxial high-explosive, flux-compression generators. Principal diagnostics used to study the imploding xenon gas were /sup 133/Xe radioactive tracers, continuous x-ray absorption, and neutron output. We compressed the xenon gas about five to sixfold at a velocity of 10 cm/..mu..s at a radius of 4 cm. The snowplow efficiency was good; going from 13- to 4-cm radius, we lost only about 20% of the mass. The temperature of the imploded sheath was determined by mixing deuterium with the xenon and measuring the neutron output. Using reasonable assumptions about the amount, density, and uniformity of the compressed gas, we estimate that we reached temperatures as high as 155 eV. Energy-loss mechanisms that we encountered included wall ablation and Taylor instabilities of the back surface.

  19. Two-level systems driven by large-amplitude fields

    SciTech Connect

    Ashhab, S.; Johansson, J. R.; Zagoskin, A. M.; Nori, Franco

    2007-06-15

    We analyze the dynamics of a two-level system subject to driving by large-amplitude external fields, focusing on the resonance properties in the case of driving around the region of avoided level crossing. In particular, we consider three main questions that characterize resonance dynamics: (1) the resonance condition (2) the frequency of the resulting oscillations on resonance, and (3) the width of the resonance. We identify the regions of validity of different approximations. In a large region of the parameter space, we use a geometric picture in order to obtain both a simple understanding of the dynamics and quantitative results. The geometric approach is obtained by dividing the evolution into discrete time steps, with each time step described by either a phase shift on the basis states or a coherent mixing process corresponding to a Landau-Zener crossing. We compare the results of the geometric picture with those of a rotating wave approximation. We also comment briefly on the prospects of employing strong driving as a useful tool to manipulate two-level systems.

  20. Generation of strong magnetic fields in dense quark matter driven by the electroweak interaction of quarks

    NASA Astrophysics Data System (ADS)

    Dvornikov, Maxim

    2016-12-01

    We study the generation of strong large scale magnetic fields in dense quark matter. The magnetic field growth is owing to the magnetic field instability driven by the electroweak interaction of quarks. We discuss the situation when the chiral symmetry is unbroken in the degenerate quark matter. In this case we predict the amplification of the seed magnetic field 1012G to the strengths (1014 -1015)G. In our analysis we use the typical parameters of the quark matter in the core of a hybrid star or in a quark star. We also discuss the application of the obtained results to describe the magnetic fields generation in magnetars.

  1. 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).

  2. First-order antiferro-ferromagnetic transition in Fe(49)(Rh(0.93)Pd(0.07))(51) under simultaneous application of magnetic field and external pressure.

    PubMed

    Kushwaha, Pallavi; Bag, Pallab; Rawat, R; Chaddah, P

    2012-03-07

    A magnetic field-pressure-temperature (H-P-T) phase diagram for first-order antiferromagnetic (AFM) to ferromagnetic (FM) transitions in Fe(49)(Rh(0.93)Pd(0.07))(51) has been constructed using resistivity measurements under simultaneous application of magnetic field (up to 8 T) and pressure (up to 20 kbar). The temperature dependence of resistivity (ρ-T) shows that the width of the transition and the extent of hysteresis decreases with pressure and increases with magnetic field. By exploiting opposing trends of dT(N)/dP and dT(N)/dH (where T(N) is the first-order transition temperature), the relative effects of temperature, magnetic field and pressure on disorder-broadened first-order transitions has been studied. For this, a set of H and P values are chosen for which T(N)(H(1),P(1)) = T(N)(H(2),P(2)). Measurements for such combinations of H and P show that the temperature dependence of resistivity is similar, i.e. the broadening (in temperature) of transition as well as the extent of hysteresis remains independent of H and P. Isothermal magnetoresistance measurements under various constant pressures show that even though the critical field required for AFM-FM transition depends on applied pressure, the extent of hysteresis as well as transition width (in magnetic field) remains constant with varying pressure.

  3. Electron Inertia Effects in Hall-Driven Magnetic Field Penetration in Electron-Magnetohydrodynamics

    NASA Astrophysics Data System (ADS)

    Richardson, Andrew; Angus, Justin; Swanekamp, Stephen; Schumer, Joseph; Ottinger, Paul

    2015-11-01

    Magnetic field penetration in electron-magnetohydrodynamics (EMHD) can be driven by density gradients through the Hall term. Here we describe the effect of electron inertia on simplified one- and two- dimensional models of a magnetic front. Nonlinear effects due to inertia cause the 1D model to develop peaked solitary waves, while in 2D a shear-driven Kelvin-Helholtz like instability causes the front to break into a series of vortices which propagate into the plasma. The combination of these two effects means that in 2D, Hall driven magnetic field penetration will typically happen in the form of complex vortex-dominated penetration, rather than as a transversely-smooth shock front. This work was supported by the Naval Research Laboratory Base Program.

  4. Optical signatures of electric-field-driven magnetic phase transitions in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Basak, Tista; Shukla, Alok

    2016-06-01

    Experimental challenges in identifying various types of magnetic ordering in graphene quantum dots (QDs) pose a major hurdle in the application of these nanostructures for spintronic devices. Based upon phase diagrams obtained by employing the π -electron Pariser-Parr-Pople (PPP) model Hamiltonian, we demonstrate that the magnetic states undergo phase transition under the influence of an external electric field. Our calculations of the electroabsorption spectra of these QDs indicate that the spectrum in question carries strong signatures of their magnetic state (FM vs AFM), thus suggesting the possibility of an all-optical characterization of their magnetic nature. Further, the gaps for the up and the down spins are the same in the absence of an external electric field, both for the antiferromagnetic (AFM) and the ferromagnetic (FM) states of QDs. But, once the QDs are exposed to a suitably directed external electric field, gaps for different spins split and exhibit distinct variations with respect to the strength of the field. The nature of variation exhibited by the energy gaps corresponding to the up and down spins is different for the AFM and FM configurations of QDs. This selective manipulation of the spin-polarized gap splitting by an electric field in finite graphene nanostructures can open up new frontiers in the design of graphene-based spintronic devices.

  5. Electric-field control of ferromagnetic resonance in monolithic BaFe12O19-Ba0.5Sr0.5TiO3 heterostructures

    NASA Astrophysics Data System (ADS)

    Das, Jaydip; Song, Young-Yeal; Wu, Mingzhong

    2010-08-01

    This paper demonstrates an electric-field tuning of the ferromagnetic resonance (FMR) responses at millimeter wave frequencies for a monolithic magneto-electric heterostructure. The layered stack is comprised of c-axis oriented and low loss barium hexaferrite (BaM) and (111) oriented ferroelectric barium strontium titanate (BSTO) layers along with embedded platinum electrode layers, all fabricated by pulsed laser deposition technique. A tunability of the FMR frequency as large as 3.5 MHz/V has been observed at 60 GHz due to application of bias voltages in the range of several volts. The realization of such a large tunability relies on the quasi-lattice-to-lattice contact between the BaM and BSTO layers as well as the high quality of those layers.

  6. Irreversible magnetic-field dependence of ferromagnetic resonance and inverse spin Hall effect voltage in CoFeB/Pt bilayer

    NASA Astrophysics Data System (ADS)

    Kim, Sang-Il; Seo, Min-Su; Choi, Yeon Suk; Park, Seung-Young

    2017-01-01

    Magnetic field (H) sweeping direction dependences of the mixed voltage Vmix induced by the inverse-spin Hall effect(ISHE) and spin-rectified effect (SRE) in a CoFeB (5 nm)/Pt (10 nm) bilayer structure are investigated using the ferromagnetic resonance in the TE mode cavities and coplanar waveguide methods. Conventionally, the magnitude of ISHE voltage VISH (symmetric) excluding the SRE (antisymmetric component) was unavoidably separated from the fitting curve of Vmix (a sum of a symmetric and an antisymmetric part) for one direction of H-source. By studying the ratio of the two voltage parts with the bi-directional H sweeping, the optimized VISH (no SRE condition) value which also include a well-defined spin Hall angle can be obtained via the linear response relation of ISHE and SRE components.

  7. First-order phase transition from an antiferromagnetic ferroelectric to a cycloidal multiferroic with weak ferromagnetism during the joint action of applied magnetic and electric fields

    SciTech Connect

    Pikin, S. A. Lyubutin, I. S.

    2013-09-15

    The thermodynamics of the phase transition in a perovskite-like multiferroic, in which an antiferromagnetic ferroelectric transforms into a new magnetic state where a spiral spin structure and weak ferromagnetism can coexist in applied magnetic field H, is described. This state forms as a result of a first-order phase transition at a certain temperature (below Neel temperature T{sub N}), where a helicoidal magnetic structure appears due to the Dzyaloshinskii-Moriya effect. In this case, the axes of electric polarization and the helicoid of magnetic moments are mutually perpendicular and lie in the ab plane, which is normal to principal axis c. Additional electric polarization p, which decreases the total polarization of the ferroelectric P, appears in the ab plane. The effect of applied magnetic and electric fields on the properties of a multiferroic with a helicoidal magnetic structure is described. An alternating electric field is shown to cause a field-linear change in magnetic moment m, whose sign is opposite to the sign of the change of electric field E. The detected hysteretic phenomena that determine the temperature ranges of overheating and supercooling of each phase are explained. A comparison with the experimental data is performed.

  8. Electric field tuning of magnetic properties in FeGa films on ferroelastic Pb(Zr,Ti)O3 thin films probed by ferromagnetic resonance

    NASA Astrophysics Data System (ADS)

    Luykx, Arun; Lofland, Samuel; Anbusathaiah, Varatharajan; Nagarajan, Valanoor; Kartawidjaja, Fransiska; Wang, John; Takeuchi, Ichiro

    2009-03-01

    In order to investigate the possibility of fabricating electric field tunable thin film magnetic devices using a multiferroic transduction effect, we have patterned Fe0.7Ga0.3 (FeGa) films sputter-deposited on PbZr0.3Ti0.7O3 (PZT(30/70))/PbZr0.7Ti0.3O3 (PZT(70/30)) tetragonal/rhombohedral bilayers on Pt/Ti/SiO2/Si wafers. Previous piezoforce microscopy studies have shown that the PZT bilayers exhibit presence of ferroelastic domains where the fraction of the local c/a domain ratio can be tuned by an applied electric field. The FeGa top layer was patterned into 20 μm x 20 μm capacitor devices in order to apply electric field to the multilayers, and ferromagnetic resonance (FMR) measurements at 9.2 GHz were performed. Typically, a relatively sharp FMR signal observed before application of the electric field would get substantially broadened after initial application of +4 kV/cm. Angular dependent FMR indicates that magnetic anisotropy in the FeGa is indeed affected by application of electric field.

  9. Large E-field tunability of magnetic anisotropy and ferromagnetic resonance frequency of co-sputtered Fe50Co50-B film

    NASA Astrophysics Data System (ADS)

    Li, Shandong; Xue, Qian; Du, Honglei; Xu, Jie; Li, Qiang; Shi, Zhipeng; Gao, Xiaoyang; Liu, Ming; Nan, Tianxiang; Hu, Zhongqiang; Sun, Nian X.; Shao, Weiquan

    2015-05-01

    Fe27.45Co30.19B42.36 (referred to as FeCoB) films with 100 nm in thickness were co-sputtered on (011)-cut lead zinc niobate-lead titanate (PZN-PT) single crystal substrate under RF powers of 80 W for Fe50Co50 target and 120 W for B target, respectively. The anisotropy field HK of the FeCoB/PZN-PT multiferroic composite is increased by more than 10 times, from 56 to 663 Oe under the E-field from 0 to 7 kV/cm due to the strong magnetoelectric coupling, corresponding to a large tunability of HK of 86.7 Oe cm/kV. At the same time, the self-bias ferromagnetic resonance frequency fFMR is dramatically shifted upwards by an electric field from 2.57 to 9.02 GHz with an increment of 6.45 GHz, corresponding to E-field tunablity of fFMR 921.4 MHz.cm/kV. These features demonstrate that FeCoB/PZN-PT multiferroic laminates prepared under an integrated circuits process are promising in fabrication of E-field tunable monolithic microwave integrated circuits (MMIC) devices and their components.

  10. Temperature limited heater utilizing non-ferromagnetic conductor

    DOEpatents

    Vinegar,; Harold J. , Harris; Kelvin, Christopher [Houston, TX

    2012-07-17

    A heater is described. The heater includes a ferromagnetic conductor and an electrical conductor electrically coupled to the ferromagnetic conductor. The ferromagnetic conductor is positioned relative to the electrical conductor such that an electromagnetic field produced by time-varying current flow in the ferromagnetic conductor confines a majority of the flow of the electrical current to the electrical conductor at temperatures below or near a selected temperature.

  11. Electric-field-driven electron-transfer in mixed-valence molecules

    NASA Astrophysics Data System (ADS)

    Blair, Enrique P.; Corcelli, Steven A.; Lent, Craig S.

    2016-07-01

    Molecular quantum-dot cellular automata is a computing paradigm in which digital information is encoded by the charge configuration of a mixed-valence molecule. General-purpose computing can be achieved by arranging these compounds on a substrate and exploiting intermolecular Coulombic coupling. The operation of such a device relies on nonequilibrium electron transfer (ET), whereby the time-varying electric field of one molecule induces an ET event in a neighboring molecule. The magnitude of the electric fields can be quite large because of close spatial proximity, and the induced ET rate is a measure of the nonequilibrium response of the molecule. We calculate the electric-field-driven ET rate for a model mixed-valence compound. The mixed-valence molecule is regarded as a two-state electronic system coupled to a molecular vibrational mode, which is, in turn, coupled to a thermal environment. Both the electronic and vibrational degrees-of-freedom are treated quantum mechanically, and the dissipative vibrational-bath interaction is modeled with the Lindblad equation. This approach captures both tunneling and nonadiabatic dynamics. Relationships between microscopic molecular properties and the driven ET rate are explored for two time-dependent applied fields: an abruptly switched field and a linearly ramped field. In both cases, the driven ET rate is only weakly temperature dependent. When the model is applied using parameters appropriate to a specific mixed-valence molecule, diferrocenylacetylene, terahertz-range ET transfer rates are predicted.

  12. Electric-field-driven electron-transfer in mixed-valence molecules.

    PubMed

    Blair, Enrique P; Corcelli, Steven A; Lent, Craig S

    2016-07-07

    Molecular quantum-dot cellular automata is a computing paradigm in which digital information is encoded by the charge configuration of a mixed-valence molecule. General-purpose computing can be achieved by arranging these compounds on a substrate and exploiting intermolecular Coulombic coupling. The operation of such a device relies on nonequilibrium electron transfer (ET), whereby the time-varying electric field of one molecule induces an ET event in a neighboring molecule. The magnitude of the electric fields can be quite large because of close spatial proximity, and the induced ET rate is a measure of the nonequilibrium response of the molecule. We calculate the electric-field-driven ET rate for a model mixed-valence compound. The mixed-valence molecule is regarded as a two-state electronic system coupled to a molecular vibrational mode, which is, in turn, coupled to a thermal environment. Both the electronic and vibrational degrees-of-freedom are treated quantum mechanically, and the dissipative vibrational-bath interaction is modeled with the Lindblad equation. This approach captures both tunneling and nonadiabatic dynamics. Relationships between microscopic molecular properties and the driven ET rate are explored for two time-dependent applied fields: an abruptly switched field and a linearly ramped field. In both cases, the driven ET rate is only weakly temperature dependent. When the model is applied using parameters appropriate to a specific mixed-valence molecule, diferrocenylacetylene, terahertz-range ET transfer rates are predicted.

  13. Spin- and valley-dependent electronic band structure and electronic heat capacity of ferromagnetic silicene in the presence of strain, exchange field and Rashba spin-orbit coupling

    NASA Astrophysics Data System (ADS)

    Hoi, Bui Dinh; Yarmohammadi, Mohsen; Kazzaz, Houshang Araghi

    2017-10-01

    We studied how the strain, induced exchange field and extrinsic Rashba spin-orbit coupling (RSOC) enhance the electronic band structure (EBS) and electronic heat capacity (EHC) of ferromagnetic silicene in presence of external electric field (EF) by using the Kane-Mele Hamiltonian, Dirac cone approximation and the Green's function approach. Particular attention is paid to investigate the EHC of spin-up and spin-down bands at Dirac K and K‧ points. We have varied the EF, strain, exchange field and RSOC to tune the energy of inter-band transitions and consequently EHC, leading to very promising features for future applications. Evaluation of EF exhibits three phases: Topological insulator (TI), valley-spin polarized metal (VSPM) and band insulator (BI) at given aforementioned parameters. As a new finding, we have found a quantum anomalous Hall phase in BI regime at strong RSOCs. Interestingly, the effective mass of carriers changes with strain, resulting in EHC behaviors. Here, exchange field has the same behavior with EF. Finally, we have confirmed the reported and expected symmetry results for both Dirac points and spins with the study of valley-dependent EHC.

  14. Driven magnetic reconnection in three dimensions - Energy conversion and field-aligned current generation

    NASA Technical Reports Server (NTRS)

    Sato, T.; Walker, R. J.; Ashour-Abdalla, M.

    1984-01-01

    The energy conversion processes occurring in three-dimensional driven reconnection is analyzed. In particular, the energy conversion processes during localized reconnection in a taillike magnetic configuration are studied. It is found that three-dimensional driven reconnection is a powerful energy converter which transforms magnetic energy into plasma bulk flow and thermal energy. Three-dimensional driven reconnection is an even more powerful energy converter than two-dimensional reconnection, because in the three-dimensional case, plasmas were drawn into the reconnection region from the sides as well as from the top and bottom. Field-aligned currents are generated by three-dimensional driven reconnection. The physical mechanism responsible for these currents which flow from the tail toward the ionosphere on the dawnside of the reconnection region and from the ionosphere toward the tail on the duskside is identified. The field-aligned currents form as the neutral sheet current is diverted through the slow shocks which form on the outer edge of the reconnected field lines (outer edge of the plasma sheet).

  15. Quantum transport in a two-level quantum dot driven by coherent and stochastic fields

    NASA Astrophysics Data System (ADS)

    Ke, Sha-Sha; Miao, Ling-E.; Guo, Zhen; Guo, Yong; Zhang, Huai-Wu; Lü, Hai-Feng

    2016-12-01

    We study theoretically the current and shot noise properties flowing through a two-level quantum dot driven by a strong coherent field and a weak stochastic field. The interaction x(t) between the quantum dot and the stochastic field is assumed to be a Gaussian-Markovian random process with zero mean value and correlation function < x (t) x (t ‧) > = Dκe - κ | t - t ‧ | , where D and κ are the strength and bandwidth of the stochastic field, respectively. It is found that the stochastic field could enhance the resonant effect between the quantum dot and the coherent field, and generate new resonant points. At the resonant points, the state population difference between two levels is suppressed and the current is considerably enhanced. The zero-frequency shot noise of the current varies dramatically between sub- and super-Poissonian characteristics by tuning the stochastic field appropriately.

  16. Quantum Monte Carlo study of the spin-1/2 honeycomb Heisenberg model with mixed antiferromagnetic and ferromagnetic interactions in external magnetic fields

    NASA Astrophysics Data System (ADS)

    Huang, Yi-Zhen; Su, Gang

    2017-05-01

    The continuous imaginary-time quantum Monte Carlo method with the worm update algorithm is applied to explore the ground-state properties of the spin-1/2 Heisenberg model with antiferromagnetic (AF) coupling J >0 and ferromagnetic (F) coupling J'<0 along zigzag and armchair directions, respectively, on honeycomb lattice. It is found that by enhancing the F coupling J' between zigzag AF chains, the system is smoothly crossover from one-dimensional zigzag spin chains to a two-dimensional magnetic ordered state. In absence of an external field, the system is in a stripe-ordered phase. In the presence of uniform and staggered fields, the uniform and staggered out-of-plane magnetizations appear while the stripe order remains in the x y plane, and a second-order quantum phase transition (QPT) at a critical staggered field is observed. The critical exponents of correlation length for QPTs induced by a staggered field for the cases with J >0 , J'<0 and J <0 , J'>0 are obtained to be ν =0.70046 (1 ) and 0.7086 (3 ) , respectively, indicating that both cases belong to O(3) universality. The corresponding dynamic and susceptibility exponent z and γ /ν are fitted to be 1.006572(9), 1.9412(2) and 1.004615(8), 1.96121(9) for the two cases, respectively. The scaling behavior in a staggered field is analyzed, and the ground-state phase diagrams in the plane of coupling ratio and staggered field are presented for two cases. The temperature dependence of susceptibility and specific heat of both systems in external magnetic fields is also discussed. A Kosterlitz-Thouless phase transition is found for the present system in a uniform field.

  17. A Diffusive System Driven by a Battery or by a Smoothly Varying Field

    NASA Astrophysics Data System (ADS)

    Bodineau, T.; Derrida, B.; Lebowitz, J. L.

    2010-08-01

    We consider the steady state of a one dimensional diffusive system, such as the symmetric simple exclusion process (SSEP) on a ring, driven by a battery at the origin or by a smoothly varying field along the ring. The battery appears as the limiting case of a smoothly varying field, when the field becomes a delta function at the origin. We find that in the scaling limit the long range pair correlation functions of the system driven by a battery are very different from the ones known in the steady state of the SSEP maintained out of equilibrium by contact with two reservoirs, even when the steady state density profiles are identical in both models.

  18. Double-electron recombination in high-order-harmonic generation driven by spatially inhomogeneous fields

    NASA Astrophysics Data System (ADS)

    Chacón, Alexis; Ciappina, Marcelo F.; Lewenstein, Maciej

    2016-10-01

    We present theoretical studies of high-order harmonic generation (HHG) driven by plasmonic fields in two-electron atomic systems. Comparing the single- and two-electron active approximation models of the hydrogen negative ion, we provide strong evidence that a nonsequential double-electron recombination mechanism appears to be mainly responsible for the HHG cutoff extension. Our analysis is carried out by means of a reduced one-dimensional numerical integration of the two-electron time-dependent Schrödinger equation, and on investigations of the classical electron trajectories, resulting from the Newton's equation of motion. Additional comparisons between the hydrogen negative ion and the helium atom suggest that the double recombination process depends distinctly on the atomic target. Our research paves the way to the understanding of strong field processes in multielectronic systems driven by spatially inhomogeneous fields.

  19. En route to surface-bound electric field-driven molecular motors.

    PubMed

    Jian, Huahua; Tour, James M

    2003-06-27

    Four caltrop-shaped molecules that might be useful as surface-bound electric field-driven molecular motors have been synthesized. The caltrops are comprised of a pair of electron donor-acceptor arms and a tripod base. The molecular arms are based on a carbazole or oligo(phenylene ethynylene) core with a strong net dipole. The tripod base uses a silicon atom as its core. The legs of the tripod bear sulfur-tipped bonding units, as acetyl-protected benzylic thiols, for bonding to a gold surface. The geometry of the tripod base allows the caltrop to project upward from a metallic surface after self-assembly. Ellipsometric studies show that self-assembled monolayers of the caltrops are formed on Au surfaces with molecular thicknesses consistent with the desired upright-shaft arrangement. As a result, the zwitterionic molecular arms might be controllable when electric fields are applied around the caltrops, thereby constituting field-driven motors.

  20. Data-driven matched field processing for Lamb wave structural health monitoring.

    PubMed

    Harley, Joel B; Moura, José M F

    2014-03-01

    Matched field processing is a model-based framework for localizing targets in complex propagation environments. In underwater acoustics, it has been extensively studied for improving localization performance in multimodal and multipath media. For guided wave structural health monitoring problems, matched field processing has not been widely applied but is an attractive option for damage localization due to equally complex propagation environments. Although effective, matched field processing is often challenging to implement because it requires accurate models of the propagation environment, and the optimization methods used to generate these models are often unreliable and computationally expensive. To address these obstacles, this paper introduces data-driven matched field processing, a framework to build models of multimodal propagation environments directly from measured data, and then use these models for localization. This paper presents the data-driven framework, analyzes its behavior under unmodeled multipath interference, and demonstrates its localization performance by distinguishing two nearby scatterers from experimental measurements of an aluminum plate. Compared with delay-based models that are commonly used in structural health monitoring, the data-driven matched field processing framework is shown to successfully localize two nearby scatterers with significantly smaller localization errors and finer resolutions.

  1. Measurement of pulsed-power-driven magnetic fields via proton deflectometry

    SciTech Connect

    Mariscal, D.; McGuffey, C.; Valenzuela, J.; Beg, F. N.; Wei, M. S.; Chittenden, J. P.; Niasse, N.; Presura, R.; Haque, S.; Wallace, M.; Arias, A.; Covington, A.; Sawada, H.; Wiewior, P.

    2014-12-01

    Measuring magnetic field and current distribution in Z-pinch plasma systems is crucial to the validation of Z-pinch theory. In this letter, the demonstration of proton deflectometry to pulsed-power-driven loads at the mega-amp scale is presented, which is capable of making more detailed field maps in high-density regions of plasmas. In this method, a laser-driven, broad-spectrum, MeV-energy proton beam is directed through a pulsed-power-driven plasma system, and the resulting deflections are measured to examine configuration of magnetic fields and to infer the currents that support them. The technique was first demonstrated on simple short-circuit loads, and the results are in excellent agreement with numerical simulations providing reliable estimates of the field and current configurations. It was then applied to a more complex—radial foil—plasma load. The measurements show unexpected proton deflections that exhibit the complexity of the plasma load and that with further analysis will reveal details about the current and magnetic field topology in this complex configuration.

  2. Asymmetric Ferromagnet-Superconductor-Ferromagnet Switch

    SciTech Connect

    Cadden-Zimansky, P.; Bazaliy, Ya.B.; Litvak, L.M.; Jiang, J.S.; Pearson, J.; Gu, J.Y.; You, Chun-Yeol; Beasley, M.R.; Bader, S.D.

    2011-11-04

    In layered ferromagnet-superconductor-ferromagnet F{sub 1} /S/F{sub 2} structures, the critical temperature T{sub c} of the superconductors depends on the magnetic orientation of the ferromagnetic layers F{sub 1} and F{sub 2} relative to each other. So far, the experimentally observed magnitude of change in T{sub c} for structures utilizing weak ferromagnets has been 2 orders of magnitude smaller than is expected from calculations. We theoretically show that such a discrepancy can result from the asymmetry of F/S boundaries, and we test this possibility by performing experiments on structures where F{sub 1} and F{sub 2} are independently varied. Our experimental results indicate that asymmetric boundaries are not the source of the discrepancy. If boundary asymmetry is causing the suppressed magnitude of T{sub c} changes, it may only be possible to detect in structures with thinner ferromagnetic layers.

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

    PubMed

    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-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.

  4. 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

  5. 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

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

    SciTech Connect

    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-vacancy 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.

  7. Magnetic field-driven induction of ZENK in the trigeminal system of pigeons (Columba livia)

    PubMed Central

    Lefeldt, Nele; Heyers, Dominik; Schneider, Nils-Lasse; Engels, Svenja; Elbers, Dana; Mouritsen, Henrik

    2014-01-01

    Magnetoreception remains one of the few unsolved mysteries in sensory biology. The upper beak, which is innervated by the ophthalmic branch of the trigeminal nerve (V1), has been suggested to contain magnetic sensors based on ferromagnetic structures. Recently, its existence in pigeons has been seriously challenged by studies suggesting that the previously described iron-accumulations are macrophages, not magnetosensitive nerve endings. This raised the fundamental question of whether V1 is involved in magnetoreception in pigeons at all. We exposed pigeons to either a constantly changing magnetic field (CMF), to a zero magnetic field providing no magnetic information, or to CMF conditions after V1 was cut bilaterally. Using immediate early genes as a marker of neuronal responsiveness, we report that the trigeminal brainstem nuclei of pigeons, which receive V1 input, are activated under CMF conditions and that this neuronal activation disappears if the magnetic stimuli are removed or if V1 is cut. Our data suggest that the trigeminal system in pigeons is involved in processing magnetic field information and that V1 transmits this information from currently unknown, V1-associated magnetosensors to the brain. PMID:25232052

  8. Magnetic field-driven induction of ZENK in the trigeminal system of pigeons (Columba livia).

    PubMed

    Lefeldt, Nele; Heyers, Dominik; Schneider, Nils-Lasse; Engels, Svenja; Elbers, Dana; Mouritsen, Henrik

    2014-11-06

    Magnetoreception remains one of the few unsolved mysteries in sensory biology. The upper beak, which is innervated by the ophthalmic branch of the trigeminal nerve (V1), has been suggested to contain magnetic sensors based on ferromagnetic structures. Recently, its existence in pigeons has been seriously challenged by studies suggesting that the previously described iron-accumulations are macrophages, not magnetosensitive nerve endings. This raised the fundamental question of whether V1 is involved in magnetoreception in pigeons at all. We exposed pigeons to either a constantly changing magnetic field (CMF), to a zero magnetic field providing no magnetic information, or to CMF conditions after V1 was cut bilaterally. Using immediate early genes as a marker of neuronal responsiveness, we report that the trigeminal brainstem nuclei of pigeons, which receive V1 input, are activated under CMF conditions and that this neuronal activation disappears if the magnetic stimuli are removed or if V1 is cut. Our data suggest that the trigeminal system in pigeons is involved in processing magnetic field information and that V1 transmits this information from currently unknown, V1-associated magnetosensors to the brain.

  9. 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.

  10. Controlling magnetic domain wall positions with an external magnetic field and a low spin-polarized current in chamfered L-shaped ferromagnetic thin ribbons

    NASA Astrophysics Data System (ADS)

    Honda, Syuta; Yamamoto, Daiki; Ohsawa, Tomokatsu; Gushi, Toshiki; Ito, Keita; Suemasu, Takashi

    2016-09-01

    Current-induced magnetic domain wall (DW) motion in ferromagnetic ribbons is utilized in spintronic devices. The direction of the motion changes in response to the sign of the spin-polarizability of the current through the ribbon. The DW motion is expected to measure the sign. In this study, we investigate the magnetic structures of chamfered L-shaped nano-ribbons using micro-magnetic simulations, and show that the position at which the DW is produced can be controlled by applying an external magnetic field with a low spin-polarized current (SPC). In particular, we use the material parameters of Fe4N and permalloy to simulate the magnetic structure of the ribbon. The DW can be produced at either of two locations in a chamfered corner of the ribbon, and disappears upon applying an external magnetic field. From this point, after the field is removed, a new DW is produced at either of two locations, and its position can be controlled by adjusting the low SPC.

  11. Achieving High Performance in AC-Field Driven Organic Light Sources

    NASA Astrophysics Data System (ADS)

    Xu, Junwei; Carroll, David L.; Smith, Gregory M.; Dun, Chaochao; Cui, Yue

    2016-04-01

    Charge balance in organic light emitting structures is essential to simultaneously achieving high brightness and high efficiency. In DC-driven organic light emitting devices (OLEDs), this is relatively straight forward. However, in the newly emerging, capacitive, field-activated AC-driven organic devices, charge balance can be a challenge. In this work we introduce the concept of gating the compensation charge in AC-driven organic devices and demonstrate that this can result in exceptional increases in device performance. To do this we replace the insulator layer in a typical field-activated organic light emitting device with a nanostructured, wide band gap semiconductor layer. This layer acts as a gate between the emitter layer and the voltage contact. Time resolved device characterization shows that, at high-frequencies (over 40 kHz), the semiconductor layer allows for charge accumulation in the forward bias, light generating part of the AC cycle and charge compensation in the negative, quiescent part of the AC cycle. Such gated AC organic devices can achieve a non-output coupled luminance of 25,900 cd/m2 with power efficiencies that exceed both the insulator-based AC devices and OLEDs using the same emitters. This work clearly demonstrates that by realizing balanced management of charge, AC-driven organic light emitting devices may well be able to rival today’s OLEDs in performance.

  12. Achieving High Performance in AC-Field Driven Organic Light Sources

    PubMed Central

    Xu, Junwei; Carroll, David L.; Smith, Gregory M.; Dun, Chaochao; Cui, Yue

    2016-01-01

    Charge balance in organic light emitting structures is essential to simultaneously achieving high brightness and high efficiency. In DC-driven organic light emitting devices (OLEDs), this is relatively straight forward. However, in the newly emerging, capacitive, field-activated AC-driven organic devices, charge balance can be a challenge. In this work we introduce the concept of gating the compensation charge in AC-driven organic devices and demonstrate that this can result in exceptional increases in device performance. To do this we replace the insulator layer in a typical field-activated organic light emitting device with a nanostructured, wide band gap semiconductor layer. This layer acts as a gate between the emitter layer and the voltage contact. Time resolved device characterization shows that, at high-frequencies (over 40 kHz), the semiconductor layer allows for charge accumulation in the forward bias, light generating part of the AC cycle and charge compensation in the negative, quiescent part of the AC cycle. Such gated AC organic devices can achieve a non-output coupled luminance of 25,900 cd/m2 with power efficiencies that exceed both the insulator-based AC devices and OLEDs using the same emitters. This work clearly demonstrates that by realizing balanced management of charge, AC-driven organic light emitting devices may well be able to rival today’s OLEDs in performance. PMID:27063414

  13. Energetic-Particle-Driven Instabilities and Their Effect on Fast Ions in a Reversed Field Pinch

    NASA Astrophysics Data System (ADS)

    Lin, L.; Ding, W. X.; Brower, D. L.; Koliner, J. J.; Eilerman, S.; Reusch, J.; Anderson, J. K.; Almagri, A. F.; Chapman, B. E.; Nornberg, M. D.; Sarff, J. S.; Waksman, J.; Liu, D.

    2012-10-01

    During 1 MW tangential neutral-beam injection (NBI) into the MST reversed field pinch, multiple, bursty instabilities (n=5, 4 and -1) are detected by various fluctuation diagnostics. The spatial structure of associated density fluctuations peaks near the core where fast ions reside. Significant bicoherence among them is measured, indicating nonlinear three-wave coupling. These instabilities are also observed by a laser-based Faraday-rotation diagnostic, containing critical information on the internal magnetic field fluctuations. A tangential-view high-energy neutral particle analyzer (NPA) is used to study the fast-ion population. The measured NPA signal decreases by 15% following NBI-driven instabilities, indicating fluctuation-induced fast-ion transport. The NBI also reduces the amplitude of the innermost-resonant tearing mode by up to 65%. This mode-suppression is lessened following the NBI-driven bursts, consistent with fast ion loss/redistribution weakening the suppression effect.

  14. Modeling of ferromagnetic semiconductor devices for spintronics

    NASA Astrophysics Data System (ADS)

    Lebedeva, N.; Kuivalainen, P.

    2003-06-01

    We develop physical models for magnetic semiconductor devices, where a part of the device structure consists of a ferromagnetic semiconductor layer. First we calculate the effect of the exchange interaction between the charge carrier spins and the spins of the localized magnetic electrons on the electronic states, recombination processes, and charge transport in ferromagnetic semiconductors such as (Ga,Mn)As. Taking into account, e.g., the splitting of the conduction and valence bands due to the exchange interaction, we model the electrical characteristics of the basic magnetic semiconductor devices such as Schottky diodes consisting of a nonmagnetic metal/ferromagnetic semiconductor interface, pn diodes consisting of a ferromagnetic/nonmagnetic junction and bipolar transistors having a ferromagnetic emitter. The models predict that at temperatures close to the Curie temperature TC the electrical properties of the magnetic semiconductor devices become strongly dependent on the average spin polarization of the magnetic atoms. A feature in the models is that many device parameters such as diffusion lengths or potential barriers become spin dependent in magnetic semiconductor devices. In a ferromagnetic Schottky diode the sensitivity of the device current I to the external magnetic field may be as large as (∂I/∂B)I-1≈1/T at temperatures close to TC. In a ferromagnetic pn diode both the ideal and recombination currents become magnetic field dependent. In a ferromagnetic bipolar transistor the current gain shows the same sensitivity to the spin polarization as the dc current in the ferromagnetic pn diodes. According to our model calculations optimal structures showing the largest magnetization dependence of the electrical characteristics in III-V ferromagnetic semiconductor devices would be those where the magnetic side of the junction is of n type.

  15. Ferromagnetism in ruthenate perovskites

    NASA Astrophysics Data System (ADS)

    Dang, Hung T.; Mravlje, Jernej; Millis, Andrew J.

    2014-03-01

    In apparent contrast to the usual rule that stronger correlations favor magnetism and other forms of order, while weaker correlations lead to Fermi liquid metals, it has been experimentally established that CaRuO3, a more correlated material, is a paramagnetic metal with a Fermi liquid ground state while SrRuO3, which is less strongly correlated, is ferromagnetic below a Curie temperature of 160K. We present density functional plus dynamical mean field theory calculations which resolve this conundrum. We show that in these materials ferromagnetism occurs naturally for cubic perovskite systems at moderate correlations but is suppressed both by proximity to the Mott insulating phase and by increasing the amplitude of a GdFeO3 distortion. These factors are strongly related to the differences between Ca and Sr ruthenates and are used as the keys to solve the problem. Placement of the ruthenate materials on the metal-insulator phase diagram and comparison to previous works on the Ruddlesden-Popper materials are also discussed. Supported by the Basic Energy Sciences Program of the US Department of Energy under grant DOE ER046169 and the Columbia-Ecole Polytechnique Alliance program.

  16. Electric-field-driven dynamics of magnetic domain walls in magnetic nanowires patterned on ferroelectric domains

    NASA Astrophysics Data System (ADS)

    Van de Wiele, Ben; Leliaert, Jonathan; Franke, Kévin J. A.; van Dijken, Sebastiaan

    2016-03-01

    Strong coupling of magnetic domain walls onto straight ferroelastic boundaries of a ferroelectric layer enables full and reversible electric-field control of magnetic domain wall motion. In this paper, the dynamics of this new driving mechanism is analyzed using micromagnetic simulations. We show that transverse domain walls with a near-180° spin structure are stabilized in magnetic nanowires and that electric fields can move these walls with high velocities. Above a critical velocity, which depends on material parameters, nanowire geometry and the direction of domain wall motion, the magnetic domain walls depin abruptly from the ferroelastic boundaries. Depinning evolves either smoothly or via the emission and annihilation of a vortex or antivortex core (Walker breakdown). In both cases, the magnetic domain wall slows down after depinning in an oscillatory fashion and eventually comes to a halt. The simulations provide design rules for hybrid ferromagnetic-ferroelectric domain-wall-based devices and indicate that material disorder and structural imperfections only influence Walker-breakdown-like depinning at high domain wall velocities.

  17. Fault tolerant filtering and fault detection for quantum systems driven by fields in single photon states

    SciTech Connect

    Gao, Qing Dong, Daoyi Petersen, Ian R.; Rabitz, Herschel

    2016-06-15

    The purpose of this paper is to solve the fault tolerant filtering and fault detection problem for a class of open quantum systems driven by a continuous-mode bosonic input field in single photon states when the systems are subject to stochastic faults. Optimal estimates of both the system observables and the fault process are simultaneously calculated and characterized by a set of coupled recursive quantum stochastic differential equations.

  18. Exact asymptotics of the current in boundary-driven dissipative quantum chains in large external fields

    NASA Astrophysics Data System (ADS)

    Lenarčič, Zala; Prosen, Tomaž

    2015-03-01

    A boundary-driven quantum master equation for a general inhomogeneous (nonintegrable) anisotropic Heisenberg spin-1 /2 chain, or an equivalent nearest neighbor interacting spinless fermion chain, is considered in the presence of a strong external field f . We present an exact closed form expression for large f asymptotics of the current in the presence of a pure incoherent source and sink dissipation at the boundaries. In application, we demonstrate an arbitrary large current rectification in the presence of the interaction.

  19. Statics and field-driven dynamics of transverse domain walls in biaxial nanowires under uniform transverse magnetic fields

    NASA Astrophysics Data System (ADS)

    Lu, Jie

    2016-06-01

    In this work, we report analytical results on transverse domain wall (TDW) statics and field-driven dynamics in quasi-one-dimensional biaxial nanowires under arbitrary uniform transverse magnetic fields (TMFs) based on the Landau-Lifshitz-Gilbert equation. Without axial driving fields, the static TDW should be symmetric about its center while twisted in its azimuthal angle distribution. By decoupling polar and azimuthal degrees of freedom, an approximate solution is provided which reproduces these features to a great extent. When an axial driving field is applied, the dynamical behavior of a TDW is viewed as the response of its static profile to external excitations. By means of the asymptotic expansion method, the TDW velocity in the traveling-wave mode is obtained, which provides the extent and boundary of the "velocity-enhancement" effect of TMFs on TDWs in biaxial nanowires. Finally, numerical simulations are performed and strongly support our analytics.

  20. Lightning-driven electric fields measured in the lower ionosphere: Implications for transient luminous events

    NASA Astrophysics Data System (ADS)

    Thomas, Jeremy N.; Barnum, Benjamin H.; Lay, Erin; Holzworth, Robert H.; Cho, Mengu; Kelley, Michael C.

    2008-12-01

    Transient luminous events above thunderstorms such as sprites, halos, and elves require large electric fields in the lower ionosphere. Yet very few in situ measurements in this region have been successfully accomplished, since it is typically too low in altitude for rockets and satellites and too high for balloons. In this article, we present some rare examples of lightning-driven electric field changes obtained at 75-130 km altitude during a sounding rocket flight from Wallops Island, Virginia, in 1995. We summarize these electric field changes and present a few detailed case studies. Our measurements are compared directly to a 2D numerical model of lightning-driven electromagnetic fields in the middle and upper atmosphere. We find that the in situ electric field changes are smaller than predicted by the model, and the amplitudes of these fields are insufficient for elve production when extrapolated to a 100 kA peak current stroke. This disagreement could be due to lightning-induced ionospheric conductivity enhancement, or it might be evidence of flaws in the electromagnetic pulse mechanism for elves.

  1. Electric-field-driven Phenomena for Manipulating Particles in Micro-Devices

    NASA Technical Reports Server (NTRS)

    Khusid, Boris; Acrivos, Andreas

    2004-01-01

    Compared to other available methods, ac dielectrophoresis is particularly well-suited for the manipulation of minute particles in micro- and nano-fluidics. The essential advantage of this technique is that an ac field at a sufficiently high frequency suppresses unwanted electric effects in a liquid. To date very little has been achieved towards understanding the micro-scale field-and shear driven behavior of a suspension in that, the concepts currently favored for the design and operation of dielectrophoretic micro-devices adopt the approach used for macro-scale electric filters. This strategy considers the trend of the field-induced particle motions by computing the spatial distribution of the field strength over a channel as if it were filled only with a liquid and then evaluating the direction of the dielectrophoretic force, exerted on a single particle placed in the liquid. However, the exposure of suspended particles to a field generates not only the dielectrophoretic force acting on each of these particles, but also the dipolar interactions of the particles due to their polarization. Furthermore, the field-driven motion of the particles is accompanied by their hydrodynamic interactions. We present the results of our experimental and theoretical studies which indicate that, under certain conditions, these long-range electrical and hydrodynamic interparticle interactions drastically affect the suspension behavior in a micro-channel due to its small dimensions.

  2. Instability of magnetic fields in electroweak plasma driven by neutrino asymmetries

    SciTech Connect

    Dvornikov, Maxim; Semikoz, Victor B. E-mail: semikoz@yandex.ru

    2014-05-01

    The magnetohydrodynamics (MHD) is modified to incorporate the parity violation in the Standard Model leading to a new instability of magnetic fields in the electroweak plasma in the presence of nonzero neutrino asymmetries. The main ingredient for such a modified MHD is the antisymmetric part of the photon polarization tensor in plasma, where the parity violating neutrino interaction with charged leptons is present. We calculate this contribution to the polarization tensor connected with the Chern-Simons term in effective Lagrangian of the electromagnetic field. The general expression for such a contribution which depends on the temperature and the chemical potential of plasma as well as on the photon's momentum is derived. The instability of a magnetic field driven by the electron neutrino asymmetry for the ν-burst during the first second of a supernova explosion can amplify a seed magnetic field of a protostar, and, perhaps, can explain the generation of strongest magnetic fields in magnetars. The growth of a cosmological magnetic field driven by the neutrino asymmetry density Δn{sub ν} = n{sub ν}−n{sub ν-bar}≠0 is provided by a lower bound on |ξ{sub ν{sub e}}| = |μ{sub ν{sub e}}|/T which is consistent with the well-known Big Bang nucleosynthesis (upper) bound on neutrino asymmetries in a hot universe plasma.

  3. Absence of a magnetic field driven metal-insulator transition in WTe{sub 2}.

    SciTech Connect

    Wang, Y. L.; Thoutam, L. R.; Xiao, Z. L.; Hu, J.; Das, S.; Mao, Z. Q.; Wei, J.; Divan, R.; Luican-Mayer, A.; Crabtree, G. W.; Kwok, W. K.

    2015-11-03

    A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative ‘turn-on’ temperature behavior: when the applied magnetic field H is above certain value, the resistivity versus temperature ρ(T) curve shows a minimum at a field dependent temperature T*, which was seemingly interpreted as a magnetic field driven metal-insulator transition. Here, we demonstrate that ρ(T) curves with ubiquitous turn-on behavior in the newly discovered XMR material WTe2 can be scaled as MR ~ (H/ρ0)m with m ≈ 2 and ρ0 being the resistivity at zero-field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature T* ~ (H-Hc)ν with ν ≈ 1/2, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity ρ* ≈ 2ρ0 at the onset of the XMR behavior, which fits the data remarkably well. These results evidently exclude the possible existence of a magnetic field driven metal-insulator transition in WTe2. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials.

  4. Suppressing of slow magnetic relaxation in tetracoordinate Co(II) field-induced single-molecule magnet in hybrid material with ferromagnetic barium ferrite

    PubMed Central

    Nemec, Ivan; Herchel, Radovan; Trávníček, Zdeněk

    2015-01-01

    The novel field-induced single-molecule magnet based on a tetracoordinate mononuclear heteroleptic Co(II) complex involving two heterocyclic benzimidazole (bzi) and two thiocyanido ligands, [Co(bzi)2(NSC)2], (CoL4), was prepared and thoroughly characterized. The analysis of AC susceptibility data resulted in the spin reversal energy barrier U = 14.7 cm−1, which is in good agreement with theoretical prediction, Utheor. = 20.2 cm−1, based on axial zero-field splitting parameter D = −10.1 cm−1 fitted from DC magnetic data. Furthermore, mutual interactions between CoL4 and ferromagnetic barium ferrite BaFe12O19 (BaFeO) in hybrid materials resulted in suppressing of slow relaxation of magnetization in CoL4 for 1:2, 1:1 and 2:1 mass ratios of CoL4 and BaFeO despite the lack of strong magnetic interactions between two magnetic phases. PMID:26039085

  5. AN UPGRADE OF MAGNET-FIELD-DRIVEN TIMING SYSTEMS AT THE AGS.

    SciTech Connect

    TIAN, Y.; OERTER, B.

    2005-10-10

    An upgrade of the main magnet-field-driven timing systems at Brookhaven National Laboratory's Alternating Gradient Synchrotron (AGS) and Booster accelerators will be described in this paper. A novel approach using content addressable memory (CAM) is applied to overcome a weakness in the previous systems, which required a reproducible dwell field for proper operation. Upgraded from a multibus-based system to a VME-based system, the new timing system also proves easier to maintain and to diagnose. Details of the system architecture, as well as its application in other timing systems will be discussed.

  6. Observation of Plasma Rotation Driven by Static Nonaxisymmetric Magnetic Fields in a Tokamak

    SciTech Connect

    Garofalo, A. M.; Burrell, K. H.; DeBoo, J. C.; DeGrassie, J. S.; Jackson, G. L.; Schaffer, M. J.; Strait, E. J.; Lanctot, M.; Reimerdes, H.; Solomon, W. M.

    2008-11-07

    We present the first evidence for the existence of a neoclassical toroidal rotation driven in a direction counter to the plasma current by nonaxisymmetric, nonresonant magnetic fields. At high beta and with large injected neutral beam momentum, the nonresonant field torque slows down the plasma toward the neoclassical 'offset' rotation rate. With small injected neutral beam momentum, the toroidal rotation is accelerated toward the offset rotation, with resulting improvement in the global energy confinement time. The observed magnitude, direction, and radial profile of the offset rotation are consistent with neoclassical theory predictions [A. J. Cole et al., Phys. Rev. Lett. 99, 065001 (2007)].

  7. Fabrication 3D buried channel optical waveguide modulators on field-driven ion exchange process

    NASA Astrophysics Data System (ADS)

    Zhou, Zigang; Chen, Wenqiang; Zhu, Li; Li, Jing; Luo, Xiaoying

    2010-10-01

    A high electric field technique was developed to fabricate buried optical waveguide modulator on K9 optical glass. The 80V voltage was applied on the glass to accelerate the field-driven ion exchange process by expeditiously replacing host sodium ions in the glass with silver ions. As a result, the optical loss for optical waveguide modulator was measured using the edge coupling technique with a 0.6328μm He-Ne laser. Loss of 0.20 dB/cm was obtained for channel waveguides of 25μm in depth, relatively low for waveguides of such depth at red wavelength.

  8. Longitudinal detection of ferromagnetic resonance using x-ray transmission measurements.

    PubMed

    Boero, G; Rusponi, S; Kavich, J; Rizzini, A Lodi; Piamonteze, C; Nolting, F; Tieg, C; Thiele, J-U; Gambardella, P

    2009-12-01

    We describe a setup for the x-ray detection of ferromagnetic resonance in the longitudinal geometry using element-specific transmission measurements. Thin magnetic film samples are placed in a static magnetic field collinear with the propagation direction of a polarized soft x-ray beam and driven to ferromagnetic resonance by a continuous wave microwave magnetic field perpendicular to it. The transmitted photon flux is measured both as a function of the x-ray photon energy and as a function of the applied static magnetic field. We report experiments performed on a 15 nm film of doped Permalloy (Ni(73)Fe(18)Gd(7)Co(2)) at the L(3)/L(2)-edges of Fe, Co, and Ni. The achieved ferromagnetic resonance sensitivity is about 0.1 monolayers/square root(Hz). The obtained results are interpreted in the framework of a conductivity tensor based formalism. The factors limiting the sensitivity as well as different approaches for the x-ray detection of ferromagnetic resonance are discussed.

  9. Longitudinal detection of ferromagnetic resonance using x-ray transmission measurements

    SciTech Connect

    Boero, G.; Rusponi, S.; Kavich, J.; Rizzini, A. Lodi; Piamonteze, C.; Nolting, F.; Tieg, C.; Thiele, J.-U.; Gambardella, P.

    2009-12-15

    We describe a setup for the x-ray detection of ferromagnetic resonance in the longitudinal geometry using element-specific transmission measurements. Thin magnetic film samples are placed in a static magnetic field collinear with the propagation direction of a polarized soft x-ray beam and driven to ferromagnetic resonance by a continuous wave microwave magnetic field perpendicular to it. The transmitted photon flux is measured both as a function of the x-ray photon energy and as a function of the applied static magnetic field. We report experiments performed on a 15 nm film of doped Permalloy (Ni{sub 73}Fe{sub 18}Gd{sub 7}Co{sub 2}) at the L{sub 3}/L{sub 2}-edges of Fe, Co, and Ni. The achieved ferromagnetic resonance sensitivity is about 0.1 monolayers/{radical}(Hz). The obtained results are interpreted in the framework of a conductivity tensor based formalism. The factors limiting the sensitivity as well as different approaches for the x-ray detection of ferromagnetic resonance are discussed.

  10. Optical properties of a quantum well driven by a THz electric field

    NASA Astrophysics Data System (ADS)

    Maslov, Alexey V.

    2001-07-01

    A systematic study of linear optical properties of a quantum well driven by a periodic electric field with period in the THz frequency range is performed. The THz field is oriented in the growth direction of the quantum well. We present a general approach to characterize the optical properties of a modulated medium and discuss the use of short optical pulses (shorter than the modulation period) to obtain the optical properties in the frequency domain. Mixing of the quantum well subbands (both in the conduction and valence band) for strong THz fields is treated in terms of the states dressed by the THz field. Relations between the dressed states and the optical properties of the quantum well are given. In particular, our approach allowed us to find simple relations for the efficiency of the energy conversion of the incident light into the sidebands and generalize the rate of the exciton radiative decay for the case of THz-dressed exciton. We also predict the effect of mutual transparency of several coherent laser beams which are resonant with different Fourier components of the dressed exciton state. Finally, results of realistic calculations of the absorption spectra of THz-field driven quantum wells using multisubband semiconductor Bloch equations in the linear regime are presented.

  11. Receptive field properties of rod-driven horizontal cells in the skate retina

    PubMed Central

    1992-01-01

    The large receptive fields of retinal horizontal cells result primarily from extensive intercellular coupling via gap (electrical) junctions; thus, the extent of the receptive field provides an index of the degree to which the cells are electrically coupled. For rod-driven horizontal cells in the dark-adapted skate retina, a space constant of 1.18 +/- 0.15 mm (SD) was obtained from measurements with a moving slit stimulus, and a comparable value (1.43 +/- 0.55 mm) was obtained with variation in spot diameter. These values, and the extensive spread of a fluorescent dye (Lucifer Yellow) from the site of injection to neighboring cells, indicate that the horizontal cells of the all-rod retina of skate are well coupled electrically. Neither the receptive field properties nor the gap-junctional features of skate horizontal cells were influenced by the adaptive state of the retina: (a) the receptive field organization was unaffected by light adaptation, (b) similar dye coupling was seen in both dark- and light-adapted retinae, and (c) no significant differences were found in the gap-junctional particle densities measured in dark- and light-adapted retinas, i.e., 3,184 +/- 286/microns 2 (n = 8) and 3,073 +/- 494/microns 2 (n = 11), respectively. Moreover, the receptive fields of skate horizontal cells were not altered by either dopamine, glycine, GABA, or the GABAA receptor antagonists bicuculline and picrotoxin. We conclude that the rod-driven horizontal cells of the skate retina are tightly coupled to one another, and that the coupling is not affected by photic and pharmacological conditions that are known to modulate intercellular coupling between cone-driven horizontal cells in other species. PMID:1359000

  12. Dynamical response of vibrating ferromagnets

    NASA Astrophysics Data System (ADS)

    Gaganidze, E.; Esquinazi, P.; Ziese, M.

    2000-02-01

    The resonance frequency of vibrating ferromagnetic reeds in a homogeneous magnetic field can be substantially modified by intrinsic and extrinsic field-related contributions. Searching for the physical reasons of the field-induced resonance frequency change and to study the influence of the spin glass state on it, we have measured the low-temperature magnetoelastic behavior and the dynamical response of vibrating amorphous and polycrystalline ferromagnetic ribbons. We show that the magnetoelastic properties depend strongly on the direction of the applied magnetic field. The influence of the re-entrant spin glass transition on these properties is discussed. We present clear experimental evidence that for applied fields perpendicular to the main area of the samples the behavior of ferromagnetic reeds is rather independent of the material composition and magnetic state, exhibiting a large decrease of the resonance frequency. This effect can be very well explained with a model based on the dynamical response of the reed and the magnetomechanical pole effect within a domain rotation model and is not related to magnetoelasticity.

  13. Kilotesla Magnetic Field due to a Capacitor-Coil Target Driven by High Power Laser

    PubMed Central

    Fujioka, Shinsuke; Zhang, Zhe; Ishihara, Kazuhiro; Shigemori, Keisuke; Hironaka, Youichiro; Johzaki, Tomoyuki; Sunahara, Atsushi; Yamamoto, Naoji; Nakashima, Hideki; Watanabe, Tsuguhiro; Shiraga, Hiroyuki; Nishimura, Hiroaki; Azechi, Hiroshi

    2013-01-01

    Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 μm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 μm, and 5 × 1016 W/cm2). PMID:23378905

  14. Double criticality in the magnetic field driven transition of a high-TC superconductor

    NASA Astrophysics Data System (ADS)

    Leridon, Brigitte; Vanacken, Johan; Moshchalkov, V. V.; Vignolle, Baptiste; Porwal, Rajni; Budhani, R. C.

    2015-03-01

    The magnetic-field driven transition of a set of high critical temperature La2 - x Srx CuO4 superconducting thin films has been investigated using high pulsed magnetic fields. For the underdoped samples, the existence of two distinct critical regions in the superconductor/insulator transition has been evidenced for the first time. The first quantum critical region is observed at intermediate magnetic fields (~= 19 T)and temperatures and gives way at lower temperature to a quantum critical point at about twice critical magnetic field and resistance per square. The critical exponents inferred from scaling behaviour are markedly different for the two regions. We attribute this behaviour to the existence of a clean/dirty crossover due to the presence of electronic inhomogeneities. This work has been supported by a SESAME grant from Region Ile-de-France. Part of the experiments at KULeuven have been founded by EuroMagNET II under the EU Contract Number 228043.

  15. Thermally generated magnetic fields in laser-driven compressions and explosions

    NASA Technical Reports Server (NTRS)

    Tidman, D. A.

    1975-01-01

    The evolution of thermally generated magnetic fields in a plasma undergoing a nearly spherically symmetric adiabatic compression or expansion is calculated. The analysis is applied to obtain approximate results for the development of magnetic fields in laser-driven compression and explosion of a pellet of nuclear fuel. Localized sources, such as those occurring at composition boundaries in structured pellets or at shock fronts, give stronger fields than those deriving from smoothly distributed asymmetries. Although these fields may approach 10 million G in the late stages of compression, this is not expected to present difficulties for the compression process. Assuming ignition of a nuclear explosion occurs, the sources become much stronger, and values of approximately 10 billion G are obtained at tamper boundaries assuming a 20% departure from spherical symmetry during the explosion.

  16. Phase-sensitive x-ray absorption driven by strong infrared fields

    SciTech Connect

    Guimaraes, F.F.; Felicissimo, V.C.; Kimberg, V.; Gel'mukhanov, F.; Aagren, H.; Cesar, A.

    2005-04-01

    In this paper it is demonstrated that electron vibrational absorption of molecules driven by strong IR field provides rich physical interpretations of dynamical processes on a short time scale. The phase of an infrared field influences strongly the trajectory of the nuclear wave packet and the probing spectrum. It is shown that the probe spectrum keeps memory of the infrared phase even after that the pump field left the system. The phase effect takes maximum value when the duration of the probe pulse is of the order of the infrared field period, and can be enhanced by a proper control of the duration and intensity of the pump pulse. The phase effect is different for oriented and disordered molecules and depends strongly on the intensity of pump radiation. It can be an effective tool to study charge transfer processes like proton transfer in hydrogen bonded networks.

  17. Angular and Linear Momentum of Excited Ferromagnets

    NASA Astrophysics Data System (ADS)

    Yan, Peng; Kamra, Akashdeep; Cao, Yunshan; Bauer, Gerrit

    2014-03-01

    The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are not conserved separately anymore. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to spin transfer torques and identify the emergence of a non-adiabatic effective field acting on domain walls in ferromagnetic insulators

  18. Improved leakage current and ferromagnetic properties in magnetic field annealed BiFeO{sub 3}-based ceramics

    SciTech Connect

    Yin, L.H.; Zhao, B.C.; Fang, J.; Zhang, R.R.; Tang, X.W.; Song, W.H.; Dai, J.M.; Sun, Y.P.

    2012-10-15

    Single-phase Bi{sub 0.85}La{sub 0.15}FeO{sub 3} ceramics were synthesized under various magnetic fields (H{sub a}=0 T, 3 T, 5 T). Substantially reduced leakage current and hence modified ferroelectric (FE) properties were obtained with magnetic field annealing (MA). The largest magnetization and lowest leakage current with large FE polarization (P{sub r}{approx}33 {mu}C/cm{sup 2}) were found in the sample annealed with H{sub a}=3 T. Great changes were also observed in the Raman spectra. All the observed features originate mainly from the different FE domain wall structures induced by MA. These results demonstrate that MA is an effective way to tune the multiferroic and magnetoelectric properties in BiFeO{sub 3}-based materials. - Graphical abstract: Bright field TEM micrograph of the representative domain structures in the samples (a) BLF0, (b) BLF3 and (c) BLF5. Highlights: Black-Right-Pointing-Pointer Bi{sub 0.85}La{sub 0.15}FeO{sub 3} ceramics were synthesized under various magnetic fields. Black-Right-Pointing-Pointer Substantially reduced leakage current with improved ferroelectricity were obtained. Black-Right-Pointing-Pointer Enhanced magnetization with moderate annealing magnetic field.

  19. Study of shockwave method for diagnosing the radiation fields of laser-driven gold hohlraums

    NASA Astrophysics Data System (ADS)

    Li, Yongsheng; Lan, Ke; Huo, Wenyi; Lai, Dongxian; Gao, Yaoming; Pei, Wenbing

    2013-11-01

    Besides the routinely used broad-band x-ray spectrometer (Dante or SXS), ablative shock-wave method is often used to diagnose the radiation fields of laser-driven Hohlraums. The x-ray ablation process of Aluminum and Titanium is studied numerically with a 1-D radiation hydrodynamic code RDMG [F. Tinggui et al., Chin. J. Comput. Phys. 16, 199 (1999)], based on which a new scaling relation of the equivalent radiation temperature with the ablative shock velocity in Aluminum plates is proposed, and a novel method is developed for determining simultaneously the radiation temperature and the M-band (2-4 keV) fraction in laser-driven gold Hohlraums.

  20. Seeding magnetic fields for laser-driven flux compression in high-energy-density plasmas.

    PubMed

    Gotchev, O V; Knauer, J P; Chang, P Y; Jang, N W; Shoup, M J; Meyerhofer, D D; Betti, R

    2009-04-01

    A compact, self-contained magnetic-seed-field generator (5 to 16 T) is the enabling technology for a novel laser-driven flux-compression scheme in laser-driven targets. A magnetized target is directly irradiated by a kilojoule or megajoule laser to compress the preseeded magnetic field to thousands of teslas. A fast (300 ns), 80 kA current pulse delivered by a portable pulsed-power system is discharged into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 Omega) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity-a way of reaching higher gains than is possible with conventional ICF.

  1. Seeding Magnetic Fields for Laser-Driven Flux Compression in High-Energy-Density Plasmas

    SciTech Connect

    Gotchev, O.V.; Knauer, J.P.; Chang, P.Y.; Jang, N.W.; Shoup III, M.J.; Meyerhofer, D.D.; Betti, R.

    2010-03-23

    A compact, self-contained magnetic-seed-field generator (5 to 16 T) is the enabling technology for a novel laser-driven flux-compression scheme in laser-driven targets. A magnetized target is directly irradiated by a kilojoule or megajoule laser to compress the preseeded magnetic field to thousands of teslas. A fast (300 ns), 80 kA current pulse delivered by a portable pulsed-power system is discharged into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 Omega) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity—a way of reaching higher gains than is possible with conventional ICF.

  2. Seeding magnetic fields for laser-driven flux compression in high-energy-density plasmas

    SciTech Connect

    Gotchev, O. V.; Knauer, J. P.; Shoup, M. J. III; Chang, P. Y.; Jang, N. W.; Meyerhofer, D. D.; Betti, R.

    2009-04-15

    A compact, self-contained magnetic-seed-field generator (5 to 16 T) is the enabling technology for a novel laser-driven flux-compression scheme in laser-driven targets. A magnetized target is directly irradiated by a kilojoule or megajoule laser to compress the preseeded magnetic field to thousands of teslas. A fast (300 ns), 80 kA current pulse delivered by a portable pulsed-power system is discharged into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 {Omega}) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity--a way of reaching higher gains than is possible with conventional ICF.

  3. Magnetization-driven random-field Ising model at T=0

    NASA Astrophysics Data System (ADS)

    Illa, Xavier; Rosinberg, Martin-Luc; Shukla, Prabodh; Vives, Eduard

    2006-12-01

    We study the hysteretic evolution of the random field Ising model at T=0 when the magnetization M is controlled externally and the magnetic field H becomes the output variable. The dynamics is a simple modification of the single-spin-flip dynamics used in the H -driven situation and consists in flipping successively the spins with the largest local field. This allows one to perform a detailed comparison between the microscopic trajectories followed by the system with the two protocols. Simulations are performed on random graphs with connectivity z=4 (Bethe lattice) and on the three-dimensional cubic lattice. The same internal energy U(M) is found with the two protocols when there is no macroscopic avalanche and it does not depend on whether the microscopic states are stable or not. On the Bethe lattice, the energy inside the macroscopic avalanche also coincides with the one that is computed analytically with the H -driven algorithm along the unstable branch of the hysteresis loop. The output field, defined here as ΔU/ΔM , exhibits very large fluctuations with the magnetization and is not self-averaging. The relation to the experimental situation is discussed.

  4. Field-induced quantum critical point in planar Heisenberg ferromagnets with long-range interactions: Two-time Green's function framework

    NASA Astrophysics Data System (ADS)

    Campana, L. S.; de Cesare, L.; Esposito, U.; Mercaldo, M. T.; Rabuffo, I.

    2010-07-01

    The two-time Green’s function method is used to study the critical properties and crossover phenomena near the field-induced quantum critical point (QCP) of a d -dimensional spin- S planar Heisenberg ferromagnet with long-range interactions decaying as r-α (with α>d ) with the distance r between spins. We adopt the Callen scheme for spin S and the Tyablikov decoupling procedure which is expected to provide suitable results at low temperatures. Different quantum critical regimes are found in the (α,d) plane and the global structure of the phase diagram is determined showing the typical V-shaped region close to the QCP. Depending on the values of α , we find that also for dimensionalities d⩽2 a finite-temperature critical line, ending in the QCP, exists with asymptotic behaviors and crossovers which can be employed as a useful guide for experimental studies. Moreover, these crossovers are shown to be suitably described in terms of (α,d) -dependent scaling functions and effective critical exponents.

  5. Measurement of high frequency conductivity of oxide-doped anti-ferromagnetic thin film with a near-field scanning microwave microscope

    NASA Astrophysics Data System (ADS)

    Wu, Z.; Souza, A. D.; Peng, B.; Sun, W. Q.; Xu, S. Y.; Ong, C. K.

    2014-04-01

    In this manuscript, we describe how the map of high frequency conductivity distribution of an oxide-doped anti-ferromagnetic 200 nm thin film can be obtained from the quality factor (Q) measured by a near-field scanning microwave microscope (NSMM). Finite element analysis (FEA) is employed to simulate the NSMM tip-sample interaction and obtain a curve related between the simulated quality factor (Q) and conductivity. The curve is calibrated by a standard Cu thin film with thickness of 200 nm, together with NSMM measured Q of Ag, Au, Fe, Cr and Ti thin films. The experimental conductivity obtained by the NSMM for IrMn thin films with various doped concentrations of Al2O3 is found consistent with conventional voltammetry measurement in the same tendency. That conductivity decreases as the content of doped Al2O3 increases. The results and images obtained demonstrate that NSMM can be employed in thin film analysis for characterization of local electrical properties of materials in a non-destructive manner and for obtaining a map of conductivity distribution on the same film.

  6. Tunable zero-field ferromagnetic resonance frequency from S to X band in oblique deposited CoFeB thin films

    PubMed Central

    Li, Chengyi; Chai, Guozhi; Yang, Chengcheng; Wang, Wenfeng; Xue, Desheng

    2015-01-01

    Tunable zero-field ferromagnetic resonance frequency in wide range is very useful for the application of microwave devices. We performed an investigation of the static and high frequency magnetic properties for oblique sputtered CoFeB thin films. The static magnetic results revealed that oblique sputtered CoFeB thin films possess well defined in-plane uniaxial magnetic anisotropy, which increases monotonically from 50.1 to 608.8 Oe with the increasing of deposition angle from 10° to 70°. Continuous modification of the resonance frequency of CoFeB thin films in a range of 2.83–9.71 GHz (covers three microwave bands including S, C and X bands) has been achieved. This behavior can be explained as the result of the microstructure due to the self-shadowing effect mainly. These CoFeB thin films with tunable magnetic properties may be good candidates for usage in microwave devices. PMID:26593035

  7. Magnetic resonance in ferromagnetic films, multilayers and nanoparticle composites

    NASA Astrophysics Data System (ADS)

    Noginova, Natalia; Bates, Brittany; Greene, Nicole

    2014-03-01

    Incorporation of magnetic materials into metamaterial systems provides an opportunity to tune microwave permeability with external magnetic field. We studied magnetically dependent microwave properties of polymer composites with iron oxide nanoparticles, ferromagnetic films and ferromagnetic/dielectric multilayers. We show that the permeability of such systems can be magnetically tuned from positive to negative values in the range of ferromagnetic resonance, strongly affecting wave propagation. Strong changes in mu-metal permeability in low field range provides an additional possibility of tuning.

  8. Frequency-based nanoparticle sensing over large field ranges using the ferromagnetic resonances of a magnetic nanodisc

    NASA Astrophysics Data System (ADS)

    Albert, Maximilian; Beg, Marijan; Chernyshenko, Dmitri; Bisotti, Marc-Antonio; Carey, Rebecca L.; Fangohr, Hans; Metaxas, Peter J.

    2016-11-01

    Using finite element micromagnetic simulations, we study how resonant magnetisation dynamics in thin magnetic discs with perpendicular anisotropy are influenced by magnetostatic coupling to a magnetic nanoparticle. We identify resonant modes within the disc using direct magnetic eigenmode calculations and study how their frequencies and spatial profiles are changed by the nanoparticle’s stray magnetic field. We demonstrate that particles can generate shifts in the resonant frequency of the disc’s fundamental mode which exceed resonance linewidths in recently studied spin torque oscillator devices. Importantly, it is shown that the simulated shifts can be maintained over large field ranges (here up to 1 T). This is because the resonant dynamics (the basis of nanoparticle detection here) respond directly to the nanoparticle stray field, i.e. detection does not rely on nanoparticle-induced changes to the magnetic ground state of the disc. A consequence of this is that in the case of small disc-particle separations, sensitivities to the particle are highly mode- and particle-position-dependent, with frequency shifts being maximised when the intense stray field localised directly beneath the particle can act on a large proportion of the disc’s spins that are undergoing high amplitude precession.

  9. Analysis and measurement of the 3D magnetic field in a rotating magnetic field driven FRC

    NASA Astrophysics Data System (ADS)

    Velas, K. M.; Milroy, R. D.

    2012-10-01

    A translatable three-axis probe was installed on TCSU shortly before its shutdown. The probe has 90 windings that simultaneously measure Br, Bθ, and Bz at 30 radial positions. Positioning the probe at multiple axial positions and taking multiple repeatable shots allows for a full r-z map of the magnetic field. Probe measurements are used to calculate the end-shorting torque and the rotating magnetic field (RMF) torque. The torque applied to the plasma is the RMF torque reduced by the shorting torque. An estimate of the plasma resistivity is made based on the steady state balance between the applied torque and the resistive torque. The steady state data from applying a 10 kHz low pass filter used in conjunction with data at the RMF frequency yields a map of the full 3D rotating field structure. Data from even- and odd-parity experiments will be presented. The NIMROD code has been adapted to simulate the TCSU experiment using boundary conditions adjusted to match both even- and odd-parity experimental conditions. A comparison of the n=0 components of the calculated fields to the 3-axis probe measurements shows agreement in the magnetic field structure of the FRC as well as in the jet region.

  10. Superconducting magnetoresistance in ferromagnet/superconductor/ferromagnet trilayers.

    PubMed

    Stamopoulos, D; Aristomenopoulou, E

    2015-08-26

    Magnetoresistance is a multifaceted effect reflecting the diverse transport mechanisms exhibited by different kinds of plain materials and hybrid nanostructures; among other, giant, colossal, and extraordinary magnetoresistance versions exist, with the notation indicative of the intensity. Here we report on the superconducting magnetoresistance observed in ferromagnet/superconductor/ferromagnet trilayers, namely Co/Nb/Co trilayers, subjected to a parallel external magnetic field equal to the coercive field. By manipulating the transverse stray dipolar fields that originate from the out-of-plane magnetic domains of the outer layers that develop at coercivity, we can suppress the supercurrent of the interlayer. We experimentally demonstrate a scaling of the magnetoresistance magnitude that we reproduce with a closed-form phenomenological formula that incorporates relevant macroscopic parameters and microscopic length scales of the superconducting and ferromagnetic structural units. The generic approach introduced here can be used to design novel cryogenic devices that completely switch the supercurrent 'on' and 'off', thus exhibiting the ultimate magnetoresistance magnitude 100% on a regular basis.

  11. Superconducting magnetoresistance in ferromagnet/superconductor/ferromagnet trilayers

    PubMed Central

    Stamopoulos, D.; Aristomenopoulou, E.

    2015-01-01

    Magnetoresistance is a multifaceted effect reflecting the diverse transport mechanisms exhibited by different kinds of plain materials and hybrid nanostructures; among other, giant, colossal, and extraordinary magnetoresistance versions exist, with the notation indicative of the intensity. Here we report on the superconducting magnetoresistance observed in ferromagnet/superconductor/ferromagnet trilayers, namely Co/Nb/Co trilayers, subjected to a parallel external magnetic field equal to the coercive field. By manipulating the transverse stray dipolar fields that originate from the out-of-plane magnetic domains of the outer layers that develop at coercivity, we can suppress the supercurrent of the interlayer. We experimentally demonstrate a scaling of the magnetoresistance magnitude that we reproduce with a closed-form phenomenological formula that incorporates relevant macroscopic parameters and microscopic length scales of the superconducting and ferromagnetic structural units. The generic approach introduced here can be used to design novel cryogenic devices that completely switch the supercurrent ‘on’ and ‘off’, thus exhibiting the ultimate magnetoresistance magnitude 100% on a regular basis. PMID:26306543

  12. Propelling and spinning of microsheets in nematic liquid crystals driven by ac electric field

    NASA Astrophysics Data System (ADS)

    Rasna, M. V.; Ramudu, U. V.; Chandrasekar, R.; Dhara, Surajit

    2017-01-01

    Dynamics of microparticles in isotropic liquids by transducing the energy of an applied electric field have been studied for decades. Recently, such studies in anisotropic media like liquid crystals have opened up new perspectives in colloid science. Here, we report studies on ac-electric-field-driven dynamics of microsheets in nematic liquid crystals. In planar aligned liquid crystals, with negative dielectric anisotropy, the microsheets are propelled parallel to the director. A steady spinning of the microsheets is observed in homeotropic cells with positive dielectric anisotropy liquid crystals. The velocity of propelling and the angular frequency of spinning depends on the amplitude and the frequency of the applied electric field. The electrokinetic studies of anisotropic microparticles are important as they are potential for applications in microfluidics and in areas where the controlled transport or rotation is required.

  13. A simple model for estimating a magnetic field in laser-driven coils

    SciTech Connect

    Fiksel, Gennady; Fox, William; Gao, Lan; Ji, Hantao

    2016-09-26

    Magnetic field generation by laser-driven coils is a promising way of magnetizing plasma in laboratory high-energy-density plasma experiments. A typical configuration consists of two electrodes—one electrode is irradiated with a high-intensity laser beam and another electrode collects charged particles from the expanding plasma. The two electrodes are separated by a narrow gap forming a capacitor-like configuration and are connected with a conducting wire-coil. The charge-separation in the expanding plasma builds up a potential difference between the electrodes that drives the electrical current in the coil. A magnetic field of tens to hundreds of Teslas generated inside the coil has been reported. This paper presents a simple model that estimates the magnetic field using simple assumptions. Lastly, the results are compared with the published experimental data.

  14. Attosecond control of spin polarization in electron-ion recollision driven by intense tailored fields

    NASA Astrophysics Data System (ADS)

    Ayuso, David; Jiménez-Galán, Alvaro; Morales, Felipe; Ivanov, Misha; Smirnova, Olga

    2017-07-01

    Tunnel ionization of noble gas atoms driven by a strong circularly polarized laser field in combination with a counter-rotating second harmonic generates spin-polarized electrons correlated to the spin-polarized ionic core. Crucially, such two-color field can bring the spin-polarized electrons back to the parent ion, enabling the scattering of the spin-polarized electron on the spin-polarized parent ion. Here we show how one can control the degree of spin polarization as a function of electron energy and recollision time by tuning the laser parameters, such as the relative intensities of the counter-rotating fields. The attosecond precision of the control over the degree of spin polarization opens the door for attosecond control and spectroscopy of spin-resolved dynamics.

  15. A simple model for estimating a magnetic field in laser-driven coils

    SciTech Connect

    Fiksel, Gennady; Fox, William; Gao, Lan; Ji, Hantao

    2016-09-26

    Magnetic field generation by laser-driven coils is a promising way of magnetizing plasma in laboratory high-energy-density plasma experiments. A typical configuration consists of two electrodes—one electrode is irradiated with a high-intensity laser beam and another electrode collects charged particles from the expanding plasma. The two electrodes are separated by a narrow gap forming a capacitor-like configuration and are connected with a conducting wire-coil. The charge-separation in the expanding plasma builds up a potential difference between the electrodes that drives the electrical current in the coil. A magnetic field of tens to hundreds of Teslas generated inside the coil has been reported. This paper presents a simple model that estimates the magnetic field using simple assumptions. Lastly, the results are compared with the published experimental data.

  16. A simple model for estimating a magnetic field in laser-driven coils

    NASA Astrophysics Data System (ADS)

    Fiksel, Gennady; Fox, William; Gao, Lan; Ji, Hantao

    2016-09-01

    Magnetic field generation by laser-driven coils is a promising way of magnetizing plasma in laboratory high-energy-density plasma experiments. A typical configuration consists of two electrodes—one electrode is irradiated with a high-intensity laser beam and another electrode collects charged particles from the expanding plasma. The two electrodes are separated by a narrow gap forming a capacitor-like configuration and are connected with a conducting wire-coil. The charge-separation in the expanding plasma builds up a potential difference between the electrodes that drives the electrical current in the coil. A magnetic field of tens to hundreds of Teslas generated inside the coil has been reported. This paper presents a simple model that estimates the magnetic field using simple assumptions. The results are compared with the published experimental data.

  17. A simple model for estimating a magnetic field in laser-driven coils

    DOE PAGES

    Fiksel, Gennady; Fox, William; Gao, Lan; ...

    2016-09-26

    Magnetic field generation by laser-driven coils is a promising way of magnetizing plasma in laboratory high-energy-density plasma experiments. A typical configuration consists of two electrodes—one electrode is irradiated with a high-intensity laser beam and another electrode collects charged particles from the expanding plasma. The two electrodes are separated by a narrow gap forming a capacitor-like configuration and are connected with a conducting wire-coil. The charge-separation in the expanding plasma builds up a potential difference between the electrodes that drives the electrical current in the coil. A magnetic field of tens to hundreds of Teslas generated inside the coil has beenmore » reported. This paper presents a simple model that estimates the magnetic field using simple assumptions. Lastly, the results are compared with the published experimental data.« less

  18. Influence of frequency of the excitation magnetic field and material's electric conductivity on domain wall dynamics in ferromagnetic materials

    NASA Astrophysics Data System (ADS)

    Chávez-González, A. F.; Pérez-Benítez, J. A.; Espina-Hernández, J. H.; Grössinger, R.; Hallen, J. M.

    2016-03-01

    The present work analyzes the influence of electric conductivity on the Magnetic Barkhausen Noise (MBN) signal using a microscopic model which includes the influence of eddy currents. This model is also implemented to explain the dependence of MBN on the frequency of the applied magnetic field. The results presented in this work allow analyzing the influence of eddy currents on MBN signals for different values of the material's electric conductivity and for different frequencies of applied magnetic field. Additionally, the outcomes of this research can be used as a reference to differentiate the influence of eddy currents from that of second phase particles in the MBN signal, which has been reported in previous works.

  19. Improved leakage current and ferromagnetic properties in magnetic field annealed BiFeO3-based ceramics

    NASA Astrophysics Data System (ADS)

    Yin, L. H.; Zhao, B. C.; Fang, J.; Zhang, R. R.; Tang, X. W.; Song, W. H.; Dai, J. M.; Sun, Y. P.

    2012-10-01

    Single-phase Bi0.85La0.15FeO3 ceramics were synthesized under various magnetic fields (Ha=0 T, 3 T, 5 T). Substantially reduced leakage current and hence modified ferroelectric (FE) properties were obtained with magnetic field annealing (MA). The largest magnetization and lowest leakage current with large FE polarization (Pr˜33 μC/cm2) were found in the sample annealed with Ha=3 T. Great changes were also observed in the Raman spectra. All the observed features originate mainly from the different FE domain wall structures induced by MA. These results demonstrate that MA is an effective way to tune the multiferroic and magnetoelectric properties in BiFeO3-based materials.

  20. Ferromagnetic Resonance detection using stroboscopic magneto optical Kerr effect

    NASA Astrophysics Data System (ADS)

    Yoon, Seungha; Moriyama, Takahiro; McMichael, Robert

    2015-03-01

    Ferromagnetic resonance (FMR) is a powerful method for measuring the magnetic properties of ferromagnets. A number of related optical techniques have become popular, including time-resolved magneto-optical Kerr effect (TR-MOKE) microscopy and Brillouin light scattering (BLS). In this presentation we describe a new, stroboscopic method of measuring FMR based on the magneto-optical Kerr effect (MOKE). We use a polarized telecommunications fiber laser (wavelength = 1550 nm) and a fiber modulator driven at a frequency of interest (1 GHz to 10 GHz) to create pulsed, linearly polarized light incident on a CoFeB thin film sample. Precession in the sample is driven via a coplanar waveguide in the sample holder while the reflected light is split by a polarizing beam splitter and detected by a balanced detector. As the magnetic field is swept, oscillations in the Kerr angle and in the light intensity mix to produce a DC resonance signal. The spectra are Lorentzian, with a superposition of symmetric and anti-symmetric shapes that depends on the phase of the optical and microwave signals. In the presentation, we will also discuss phase sensitive measurements with this technique as well as the advantages over other FMR techniques.

  1. Nonlinear magnetization dynamics of the classical ferromagnet with two single-ion anisotropies in an external magnetic field

    NASA Astrophysics Data System (ADS)

    Liu, Wu-Ming; Zhang, Wu-Shou; Pu, Fu-Cho; Zhou, Xin

    1999-11-01

    By using a stereographic projection of the unit sphere of magnetization vector onto a complex plane for the equations of motion, the effect of an external magnetic field for integrability of the system is discussed. The properties of the Jost solutions and the scattering data are then investigated through introducing transformations other than the Riemann surface in order to avoid double-valued functions of the usual spectral parameter. The exact multisoliton solutions are investigated by means of the Binet-Cauchy formula. The results showed that under the action of an external magnetic field nonlinear magnetization depends essentially on two parameters: its center moves with a constant velocity, while its shape changes with another constant velocity; its amplitude and width vary periodically with time, while its shape is also dependent on time and is unsymmetric with respect to its center. The orientation of the nonlinear magnetization in the plane orthogonal to the anisotropy axis changes with an external magnetic field. The total magnetic momentum and the integral of the motion coincident with its z component depend on time. The mean number of spins derivated from the ground state in a localized magnetic excitations is dependent on time. The asymptotic behavior of multisoliton solutions, the total displacement of center, and the phase shift of the jth peak are also analyzed.

  2. Micromagnetic modeling of the shielding properties of nanoscale ferromagnetic layers

    NASA Astrophysics Data System (ADS)

    Iskandarova, I. M.; Knizhnik, A. A.; Popkov, A. F.; Potapkin, B. V.; Stainer, Q.; Lombard, L.; Mackay, K.

    2016-09-01

    Ferromagnetic shields are widely used to concentrate magnetic fields in a target region of space. Such shields are also used in spintronic nanodevices such as magnetic random access memory and magnetic logic devices. However, the shielding properties of nanostructured shields can differ considerably from those of macroscopic samples. In this work, we investigate the shielding properties of nanostructured NiFe layers around a current line using a finite element micromagnetic model. We find that thin ferromagnetic layers demonstrate saturation of magnetization under an external magnetic field, which reduces the shielding efficiency. Moreover, we show that the shielding properties of nanoscale ferromagnetic layers strongly depend on the uniformity of the layer thickness. Magnetic anisotropy in ultrathin ferromagnetic layers can also influence their shielding efficiency. In addition, we show that domain walls in nanoscale ferromagnetic shields can induce large increases and decreases in the generated magnetic field. Therefore, ferromagnetic shields for spintronic nanodevices require careful design and precise fabrication.

  3. Microwave metamaterials with ferromagnetic microwires

    NASA Astrophysics Data System (ADS)

    Panina, L. V.; Ipatov, M.; Zhukova, V.; Zhukov, A.; Gonzalez, J.

    2011-06-01

    This paper discusses a new type of wire media based on amorphous ferromagnetic microwires. The combination of two effects, namely, a strong dispersion of the effective permittivity in metallic wire composites (resonance or plasmonic type) and giant magnetoimpedance effect in wires, will result in unusual property that an effective dielectric response may strongly depend on the wire magnetization which can be changed with external stimuli: magnetic field, mechanical stress and temperature.

  4. Measurements accounting for the impediment of ion spin-up in rotating magnetic field driven field reversed configurations

    SciTech Connect

    Deards, C. L.; Hoffman, A. L.; Steinhauer, L. C.

    2011-11-15

    Improved vacuum hygiene, wall conditioning, and reduced recycling in the rotating magnetic field (RMF) driven translation, confinement, and sustainment-upgrade (TCSU) field reversed configuration experiment have made possible a more accurate assessment of the forces affecting ion spin-up. This issue is critical in plasmas sustained by RMFs, such as TCSU since ion spin-up can substantially reduce or cancel the RMF current drive effect. Several diagnostics are brought to bear, including a 3-axis translatable magnetic probe allowing the first experimental measurement of the end shorting effect. These results show that the ion rotation is determined by a balance between electron-ion friction, the end shorting effect, and ion drag against neutrals.

  5. 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.

  6. Micromagnetic simulations of spin-wave normal modes and the spin-transfer-torque driven magnetization dynamics of a ferromagnetic cross

    SciTech Connect

    Pramanik, Tanmoy Roy, Urmimala; Register, Leonard F.; Banerjee, Sanjay K.; Tsoi, Maxim

    2014-05-07

    We studied spin-transfer-torque (STT) switching of a cross-shaped magnetic tunnel junction in a recent report [Roy et al., J. Appl. Phys. 113, 223904 (2013)]. In that structure, the free layer is designed to have four stable energy states using the shape anisotropy of a cross. STT switching showed different regions with increasing current density. Here, we employ the micromagnetic spectral mapping technique in an attempt to understand how the asymmetry of cross dimensions and spin polarization direction of the injected current affect the magnetization dynamics. We compute spatially averaged frequency-domain spectrum of the time-domain magnetization dynamics in the presence of the current-induced STT term. At low currents, the asymmetry of polarization direction and that of the arms are observed to cause a splitting of the excited frequency modes. Higher harmonics are also observed, presumably due to spin-wave wells caused by the regions of spatially non-uniform effective magnetic field. The results could be used towards designing a multi-bit-per-cell STT-based random access memory with an improved storage density.

  7. 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 La0.7Sr0.3MnO3 and antiferromagnetic LaFeO3 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 La0.7Sr0.3MnO3 single-layer micromagnets, but the coercivity withinmore » this materials system can be tuned over a wide range by careful balance of material properties.« less

  8. Conductance spectra of asymmetric ferromagnet/ferromagnet/ferromagnet junctions

    NASA Astrophysics Data System (ADS)

    Pasanai, K.

    2017-01-01

    A theory of tunneling spectroscopy of ferromagnet/ferromagnet/ferromagnet junctions was studied. We applied a delta-functional approximation for the interface scattering properties under a one-dimensional system of a free electron approach. The reflection and transmission probabilities were calculated in the ballistic regime, and the conductance spectra were then calculated using the Landauer formulation. The magnetization directions were set to be either parallel (P) or anti-parallel (AP) alignments, for comparison. We found that the conductance spectra was suppressed when increasing the interfacial scattering at the interfaces. Moreover, the electron could exhibit direct transmission when the thickness was rather thin. Thus, there was no oscillation in this case. However, in the case of a thick layer the conductance spectra oscillated, and this oscillation was most prominent when the middle layer thickness increased. In the case of direct transmission, the conductance spectra of P and AP systems were definitely suppressed with increased exchange energy of the middle ferromagnet. This also refers to an increase in the magnetoresistance of the junction. In the case of oscillatory behavior, the positions of the resonance peaks were changed as the exchange energy was changed.

  9. Mean-field approximation for a limit order driven market model

    NASA Astrophysics Data System (ADS)

    Slanina, František

    2001-11-01

    A mean-field variant of the model of limit order driven market introduced recently by Maslov is formulated and solved. The agents do not have any strategies and the memory of the system is kept within the order book. We show that the evolution of the order book is governed by a matrix multiplicative process. The resulting stationary distribution of step-to-step price changes is calculated. It exhibits a power-law tail with exponent 2. We obtain also the price autocorrelation function, which agrees qualitatively with the experimentally observed negative autocorrelation for short times.

  10. Pressure driven tearing and interchange modes in the reversed field pinch

    SciTech Connect

    Paccagnella, R.

    2013-01-15

    In this work, the magneto-hydro-dynamic stability of pressure driven modes in the reversed field pinch has been analyzed. It is shown that at low and intermediate {beta}'s, i.e., typically for values below 20-25%, the tearing parity is dominant, while only at very high {beta}, well above the achieved experimental values, at least part of the modes are converted to ideal interchange instabilities. Before their transition to ideal instabilities, according to their Lundquist number scaling, they can be classified as resistive-g modes.

  11. Quantum hall ferromagnets

    NASA Astrophysics Data System (ADS)

    Kumar, Akshay

    We study several quantum phases that are related to the quantum Hall effect. Our initial focus is on a pair of quantum Hall ferromagnets where the quantum Hall ordering occurs simultaneously with a spontaneous breaking of an internal symmetry associated with a semiconductor valley index. In our first example ---AlAs heterostructures--- we study domain wall structure, role of random-field disorder and dipole moment physics. Then in the second example ---Si(111)--- we show that symmetry breaking near several integer filling fractions involves a combination of selection by thermal fluctuations known as "order by disorder" and a selection by the energetics of Skyrme lattices induced by moving away from the commensurate fillings, a mechanism we term "order by doping". We also study ground state of such systems near filling factor one in the absence of valley Zeeman energy. We show that even though the lowest energy charged excitations are charge one skyrmions, the lowest energy skyrmion lattice has charge > 1 per unit cell. We then broaden our discussion to include lattice systems having multiple Chern number bands. We find analogs of quantum Hall ferromagnets in the menagerie of fractional Chern insulator phases. Unlike in the AlAs system, here the domain walls come naturally with gapped electronic excitations. We close with a result involving only topology: we show that ABC stacked multilayer graphene placed on boron nitride substrate has flat bands with non-zero local Berry curvature but zero Chern number. This allows access to an interaction dominated system with a non-trivial quantum distance metric but without the extra complication of a non-zero Chern number.

  12. The Behaviors of Ferro-Magnetic Nano-Particles In and Around Blood Vessels under Applied Magnetic Fields

    PubMed Central

    Nacev, A.; Beni, C.; Bruno, O.; Shapiro, B.

    2010-01-01

    In magnetic drug delivery, therapeutic magnetizable particles are typically injected into the blood stream and magnets are then used to concentrate them to disease locations. The behavior of such particles in-vivo is complex and is governed by blood convection, diffusion (in blood and in tissue), extravasation, and the applied magnetic fields. Using physical first-principles and a sophisticated vessel-membrane-tissue (VMT) numerical solver, we comprehensively analyze in detail the behavior of magnetic particles in blood vessels and surrounding tissue. For any blood vessel (of any size, depth, and blood velocity) and tissue properties, particle size and applied magnetic fields, we consider a Krogh tissue cylinder geometry and solve for the resulting spatial distribution of particles. We find that there are three prototypical behaviors (blood velocity dominated, magnetic force dominated, and boundary-layer formation) and that the type of behavior observed is uniquely determined by three non-dimensional numbers (the magnetic-Richardson number, mass Péclet number, and Renkin reduced diffusion coefficient). Plots and equations are provided to easily read out which behavior is found under which circumstances (Figures 5, 6, 7, and 8). We compare our results to previously published in-vitro and in-vivo magnetic drug delivery experiments. Not only do we find excellent agreement between our predictions and prior experimental observations, but we are also able to qualitatively and quantitatively explain behavior that was previously not understood. PMID:21278859

  13. High-order-harmonic generation from Rydberg atoms driven by plasmon-enhanced laser fields

    NASA Astrophysics Data System (ADS)

    Tikman, Y.; Yavuz, I.; Ciappina, M. F.; Chacón, A.; Altun, Z.; Lewenstein, M.

    2016-02-01

    We theoretically investigate high-order-harmonic generation (HHG) in Rydberg atoms driven by spatially inhomogeneous laser fields, induced, for instance, by plasmonic enhancement. It is well known that the laser intensity should exceed a certain threshold in order to stimulate HHG when noble gas atoms in their ground state are used as an active medium. One way to enhance the coherent light coming from a conventional laser oscillator is to take advantage of the amplification obtained by the so-called surface plasmon polaritons, created when a low-intensity laser field is focused onto a metallic nanostructure. The main limitation of this scheme is the low damage threshold of the materials employed in the nanostructure engineering. In this work we propose the use of Rydberg atoms, driven by spatially inhomogeneous, plasmon-enhanced laser fields, for HHG. We exhaustively discuss the behavior and efficiency of these systems in the generation of coherent harmonic emission. Toward this aim we numerically solve the time-dependent Schrödinger equation for an atom, with an electron initially in a highly excited n th Rydberg state, located in the vicinity of a metallic nanostructure. In this zone the electric field changes spatially on scales relevant for the dynamics of the laser-ionized electron. We first use a one-dimensional model to investigate systematically the phenomena. We then employ a more realistic situation, in which the interaction of a plasmon-enhanced laser field with a three-dimensional hydrogen atom is modeled. We discuss the scaling of the relevant input parameters with the principal quantum number n of the Rydberg state in question and demonstrate that harmonic emission can be achieved from Rydberg atoms well below the damage threshold, thus without deterioration of the geometry and properties of the metallic nanostructure.

  14. Magnetization reversal in ferromagnetic spirals via domain wall motion

    NASA Astrophysics Data System (ADS)

    Schumm, Ryan D.; Kunz, Andrew

    2016-11-01

    Domain wall dynamics have been investigated in a variety of ferromagnetic nanostructures for potential applications in logic, sensing, and recording. We present a combination of analytic and simulated results describing the reliable field driven motion of a domain wall through the arms of a ferromagnetic spiral nanowire. The spiral geometry is capable of taking advantage of the benefits of both straight and circular wires. Measurements of the in-plane components of the spirals' magnetization can be used to determine the angular location of the domain wall, impacting the magnetoresistive applications dependent on the domain wall location. The spirals' magnetization components are found to depend on the spiral parameters: the initial radius and spacing between spiral arms, along with the domain wall location. The magnetization is independent of the parameters of the rotating field used to move the domain wall, and therefore the model is valid for current induced domain wall motion as well. The speed of the domain wall is found to depend on the frequency of the rotating driving field, and the domain wall speeds can be reliably varied over several orders of magnitude. We further demonstrate a technique capable of injecting multiple domain walls and show the reliable and unidirectional motion of domain walls through the arms of the spiral.

  15. Evaluation of standoff distance method to determine the coronal magnetic field using CME-driven shocks

    NASA Astrophysics Data System (ADS)

    Suresh, K.; Shanmugaraju, A.; Syed Ibrahim, M.

    2016-11-01

    We have analyzed the propagation characteristics of four limb coronal mass ejections (CMEs) with their shocks. These CMEs were observed in 18 frames up to 18 solar radii using LASCO white light images. Gopalswamy and Yashiro (Astrophys. J. 736:L17, 2011) introduced the standoff distance method (SOD) to find the magnetic field in the corona using CME-driven shock. In this paper, we have used this technique to determine the magnetic field strength and to study the propagation/shock formation condition of these CMEs at 18 different locations. Since the thickness of shock sheath (standoff distance or SOD) is not constant around CME, we estimate the shock parameters and their variation in large and small SOD regions of the shock. The Mach number ranges from 1.7 to 2.8 and Alfvén speed varies from 197 to 857 km s^{-1}. Finally, we estimate the magnetic field variation in the corona. The magnetic field strength ranges from 4.9 to 26.2 mG from 8.3 to 17.5 solar radii. The estimated magnetic field strength in this study is consistent with the literature value (7.6 to 45.8 mG from Gopalswamy and Yashiro (Astrophys. J. 736:L17, 2011), and 6 to 105 mG from Kim et al. (Astrophys. J. 746:118, 2012)) and it smoothly follows the general coronal magnetic field profile.

  16. Flow Driven by an Archimedean Helical Permanent Magnetic Field. Part I: Flow Patterns and Their Transitions

    NASA Astrophysics Data System (ADS)

    Wang, Bo; Wang, Xiaodong; Etay, Jacqueline; Na, Xianzhao; Zhang, Xinde; Fautrelle, Yves

    2016-04-01

    In this study, an Archimedean helical permanent magnetic field was constructed and its driving effects on liquid metal were examined. A magnetic stirrer was constructed using a series of arc-like magnets. The helical distribution of its magnetic field, which was confirmed via Gauss probe measurements and numerical simulations, can be considered a combination of rotating and traveling magnetic fields. The characteristics of the flow patterns, particularly the transitions between the meridian secondary flow (two vortices) and the global axial flow (one vortex), driven by this magnetic field were quantitatively measured using ultrasonic Doppler velocimetry. The transient and modulated flow behaviors will be presented in a companion article. The D/ H dimension ratio was used to characterize the transitions of these two flow patterns. The results demonstrated that the flow patterns depend on not only the intrinsic structure of the magnetic field, e.g., the helix lead angle, but also the performance parameters, e.g., the dimensional ratio of the liquid bulk. The notable opposing roles of these two flow patterns in the improvement of macrosegregations when imposing such magnetic fields near the solidifying front were qualitatively addressed.

  17. Transport driven plasma flows in the scrape-off layer of ADITYA Tokamak in different orientations of magnetic field

    SciTech Connect

    Sangwan, Deepak; Jha, Ratneshwar; Brotankova, Jana; Gopalkrishna, M. V.

    2014-06-15

    Parallel plasma flows in the scrape-off layer of ADITYA tokamak are measured in two orientations of total magnetic field. In each orientation, experiments are carried out by reversing the direction of the toroidal magnetic field and the plasma current. The transport-driven component is determined by averaging flow Mach numbers, measured in two directions of the toroidal magnetic field and the plasma current for the same orientation. It is observed that there is a significant transport-driven component in the measured flow and the component depends on the field orientation.

  18. Conserved momenta of a ferromagnetic soliton

    NASA Astrophysics Data System (ADS)

    Tchernyshyov, Oleg

    2015-12-01

    Linear and angular momenta of a soliton in a ferromagnet are commonly derived through the application of Noether's theorem. We show that these quantities exhibit unphysical behavior: they depend on the choice of a gauge potential in the spin Lagrangian and can be made arbitrary. To resolve this problem, we exploit a similarity between the dynamics of a ferromagnetic soliton and that of a charged particle in a magnetic field. For the latter, canonical momentum is also gauge-dependent and thus unphysical; the physical momentum is the generator of magnetic translations, a symmetry combining physical translations with gauge transformations. We use this analogy to unambiguously define conserved momenta for ferromagnetic solitons. General considerations are illustrated on simple models of a domain wall in a ferromagnetic chain and of a vortex in a thin film.

  19. Conserved momenta of a ferromagnetic soliton

    SciTech Connect

    Tchernyshyov, Oleg

    2015-12-15

    Linear and angular momenta of a soliton in a ferromagnet are commonly derived through the application of Noether’s theorem. We show that these quantities exhibit unphysical behavior: they depend on the choice of a gauge potential in the spin Lagrangian and can be made arbitrary. To resolve this problem, we exploit a similarity between the dynamics of a ferromagnetic soliton and that of a charged particle in a magnetic field. For the latter, canonical momentum is also gauge-dependent and thus unphysical; the physical momentum is the generator of magnetic translations, a symmetry combining physical translations with gauge transformations. We use this analogy to unambiguously define conserved momenta for ferromagnetic solitons. General considerations are illustrated on simple models of a domain wall in a ferromagnetic chain and of a vortex in a thin film.

  20. Passive Superconducting Flux Conservers for Rotating-Magnetic-Field-Driven Field-Reversed Configurations

    SciTech Connect

    Oz, E.; Myers, C. E.; Edwards, M. R.; Berlinger, B.; Brooks, A.; Cohen, S. A.

    2011-01-05

    The Princeton Field-Reversed Configuration (PFRC) experiment employs an odd-parity rotating magnetic field (RMFo) current drive and plasma heating system to form and sustain high-Β plasmas. For radial confinement, an array of coaxial, internal, passive, flux-conserving (FC) rings applies magnetic pressure to the plasma while still allowing radio-frequency RMFo from external coils to reach the plasma. The 3 ms pulse duration of the present experiment is limited by the skin time (τfc) of its room-temperature copper FC rings. To explore plasma phenomena with longer characteristic times, the pulse duration of the next-generation PFRC-2 device will exceed 100 ms, necessitating FC rings with (τfc > 300 ms. In this paper we review the physics of internal, discrete, passive FCs and describe the evolution of the PFRC's FC array. We then detail new experiments that have produced higher performance FC rings that contain embedded high-temperature superconducting (HTS) tapes. Several HTS tape winding configurations have been studied and a wide range of extended skin times, from 0.4 s to over 103 s, has been achieved. The new FC rings must carry up to 3 kA of current to balance the expected PFRC-2 plasma pressure, so the dependence of the HTS-FC critical current on the winding configuration and temperature was also studied. From these experiments, the key HTS-FC design considerations have been identified and HTS-FC rings with the desired performance characteristics have been produced.

  1. Ferromagnetic (Ga,Mn)As nanostructures for spintronic applications

    SciTech Connect

    Wosinski, Tadeusz; Andrearczyk, Tomasz; Figielski, Tadeusz; Makosa, Andrzej; Wrobel, Jerzy; Sadowski, Janusz

    2013-12-04

    Magneto-resistive, cross-like nanostructures have been designed and fabricated by electron-beam lithography patterning and chemical etching from thin epitaxial layers of the ferromagnetic semiconductor (Ga,Mn)As. The nanostructures, composed of two perpendicular nanostripes crossing in the middle of their length, represent four-terminal devices, in which an electric current can be driven through any of the two nanostripes. In these devices, a novel magneto-resistive memory effect, related to a rearrangement of magnetic domain walls in the central part of the device, has been demonstrated. It consists in that the zero-field resistance of a nanostripe depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells.

  2. Phase-resolved ferromagnetic resonance using heterodyne detection method

    PubMed Central

    Yoon, Seungha; Liu, Jason; McMichael, Robert D.

    2016-01-01

    This paper describes a phase-resolved ferromagnetic resonance (FMR) measurement using a heterodyne method. Spin precession is driven by microwave fields and detected by 1550 nm laser light that is modulated at a frequency slightly shifted with respected to the FMR driving frequency. The evolving phase difference between the spin precession and the modulated light produces a slowly oscillating Kerr rotation signal with a phase equal to the precession phase plus a phase due to the path length difference between the excitation microwave signal and the optical signal. We estimate the accuracy of the precession phase measurement to be 0.1 rad. This heterodyne FMR detection method eliminates the need for field modulation and allows a stronger detection signal at higher intermediate frequency where the 1/f noise floor is reduced. PMID:27453957

  3. Current-driven domain wall motion enhanced by the microwave field

    SciTech Connect

    Wang, Xi-guang; Guo, Guang-hua Nie, Yao-zhuang; Wang, Dao-wei; Li, Zhi-xiong; Tang, Wei; Zeng, Zhong-ming

    2014-07-14

    The magnetic domain wall (DW) motion driven by a spin-polarized current opens a new concept for memory and logic devices. However, the critical current density required to overcome the intrinsic and/or extrinsic pinning of DW remains too large for practical applications. Here, we show, by using micromagnetic simulations and analytical approaches, that the application of a microwave field offers an effective solution to this problem. When a transverse microwave field is applied, the adiabatic spin-transfer torque (STT) alone can sustain a steady-state DW motion without the sign of Walker breakdown, meaning that the intrinsic pinning disappears. The extrinsic pinning can also be effectively reduced. Moreover, the DW velocity is increased greatly for the microwave-assisted DW motion. This provides a new way to manipulate the DW motion at low current densities.

  4. Nonlinear resonance and dynamical chaos in a diatomic molecule driven by a resonant ir field

    SciTech Connect

    Berman, G.P.; Bulgakov, E.N.; Holm, D.D. ||||

    1995-10-01

    We consider the transition from regular motion to dynamical chaos in a classical model of a diatomic molecule which is driven by a circularly polarized resonant ir field. Under the conditions of a nearly two-dimensional case, the Hamiltonian reduces to that for the nonintegrable motion of a charged particle in an electromagnetic wave [A. J. Lichtenberg and M. A. Lieberman, {ital Regular} {ital and} {ital Stochastic} {ital Motion} (Springer-Verlag, City, 1983)]. In the general case, the transition to chaos is connected with the overlapping of vibrational-rotational nonlinear resonances and appears even at rather low radiation field intensity, {ital S}{approx_gt}1 GW/cm{sup 2}. We also discuss the possibility of experimentally observing this transition.

  5. Neutron production using a pyroelectric driven target coupled with a gated field ionization source

    SciTech Connect

    Ellsworth, J. L.; Tang, V.; Falabella, S.; Naranjo, B.; Putterman, S.

    2013-04-19

    A palm sized, portable neutron source would be useful for widespread implementation of detection systems for shielded, special nuclear material. We present progress towards the development of the components for an ultracompact neutron generator using a pulsed, meso-scale field ionization source, a deuterated (or tritiated) titanium target driven by a negative high voltage lithium tantalate crystal. Neutron production from integrated tests using an ion source with a single, biased tungsten tip and a 3 Multiplication-Sign 1 cm, vacuum insulated crystal with a plastic deuterated target are presented. Component testing of the ion source with a single tip produces up to 3 nA of current. Dielectric insulation of the lithium tantalate crystals appears to reduce flashover, which should improve the robustness. The field emission losses from a 3 cm diameter crystal with a plastic target and 6 cm diameter crystal with a metal target are compared.

  6. Electric-field-driven magnetization reversal in square-shaped nanomagnet-based multiferroic heterostructure

    SciTech Connect

    Peng, Ren-Ci; Nan, Ce-Wen E-mail: cwnan@tsinghua.edu.cn; Wang, J. J. E-mail: cwnan@tsinghua.edu.cn; Chen, Long-Qing; Hu, Jia-Mian

    2015-04-06

    Based on phase field modeling and thermodynamic analysis, purely electric-field-driven magnetization reversal was shown to be possible in a multiferroic heterostructure of a square-shaped amorphous Co{sub 40}Fe{sub 40}B{sub 20} nanomagnet on top of a ferroelectric layer through electrostrain. The reversal is made possible by engineering the mutual interactions among the built-in uniaxial magnetic anisotropy, the geometry-dependent magnetic configuration anisotropy, and the magnetoelastic anisotropy. Particularly, the incorporation of the built-in uniaxial anisotropy made it possible to reverse magnetization with one single unipolar electrostrain pulse, which is simpler than previous designs involving the use of bipolar electrostrains and may alleviate ferroelectric fatigue. Critical conditions for triggering the magnetization reversal are identified.

  7. Resonant field amplification with feedback-stabilized regime in current driven resistive wall mode

    SciTech Connect

    Liu Yueqiang; In, Y.; Okabayashi, M.

    2010-07-15

    The stability and resonant field response of current driven resistive wall modes are numerically studied for DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] low pressure plasmas. The resonant field response of the feedback-stabilized resistive wall mode is investigated both analytically and numerically, and compared with the response from intrinsically stable or marginally stable modes. The modeling qualitatively reproduces the experimental results. Furthermore, based on some recent results and on the indirect numerical evidence in this work, it is suggested that the mode stability behavior observed in DIII-D experiments is due to the kink-peeling mode stabilization by the separatrix geometry. The phase inversion radius of the computed plasma displacement does not generally coincide with the radial locations of rational surfaces, also supporting experimental observations.

  8. Field observations of wave-driven circulation over spur and groove formations on a coral reef

    NASA Astrophysics Data System (ADS)

    Rogers, Justin S.; Monismith, Stephen G.; Dunbar, Robert B.; Koweek, David

    2015-01-01

    and groove (SAG) formations are found on the forereefs of many coral reefs worldwide. Modeling results have shown that SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells, but these have never been observed in the field. We present results from two separate field studies of SAG formations on Palmyra Atoll which show their effect on waves to be small, but reveal a persistent order 1 cm/s depth-averaged Lagrangian offshore flow over the spur and onshore flow over the grooves. This circulation was stronger for larger, directly incident waves and low alongshore flow conditions, consistent with predictions from modeling. Favorable forcing conditions must be maintained on the order of 1 h to accelerate and develop the SAG circulation cells. The primary cross and alongshore depth-averaged momentum balances were between the pressure gradient, radiation stress gradient, and nonlinear convective terms, and the bottom drag was similar to values found on other reefs. The vertical structure of these circulation cells was previously unknown and the results show a complex horizontal offshore Lagrangian flow over the spurs near the surface driven by alongshore variability in radiation stress gradients. Vertical flow was downward over the spur and upward over the groove, likely driven by alongshore differences in bottom stress and not by vortex forcing.

  9. Electric-Field-Driven Dual Vacancies Evolution in Ultrathin Nanosheets Realizing Reversible Semiconductor to Half-Metal Transition.

    PubMed

    Lyu, Mengjie; Liu, Youwen; Zhi, Yuduo; Xiao, Chong; Gu, Bingchuan; Hua, Xuemin; Fan, Shaojuan; Lin, Yue; Bai, Wei; Tong, Wei; Zou, Youming; Pan, Bicai; Ye, Bangjiao; Xie, Yi

    2015-12-02

    Fabricating a flexible room-temperature ferromagnetic resistive-switching random access memory (RRAM) device is of fundamental importance to integrate nonvolatile memory and spintronics both in theory and practice for modern information technology and has the potential to bring about revolutionary new foldable information-storage devices. Here, we show that a relatively low operating voltage (+1.4 V/-1.5 V, the corresponding electric field is around 20,000 V/cm) drives the dual vacancies evolution in ultrathin SnO2 nanosheets at room temperature, which causes the reversible transition between semiconductor and half-metal, accompanyied by an abrupt conductivity change up to 10(3) times, exhibiting room-temperature ferromagnetism in two resistance states. Positron annihilation spectroscopy and electron spin resonance results show that the Sn/O dual vacancies in the ultrathin SnO2 nanosheets evolve to isolated Sn vacancy under electric field, accounting for the switching behavior of SnO2 ultrathin nanosheets; on the other hand, the different defect types correspond to different conduction natures, realizing the transition between semiconductor and half-metal. Our result represents a crucial step to create new a information-storage device realizing the reversible transition between semiconductor and half-metal with flexibility and room-temperature ferromagnetism at low energy consumption. The as-obtained half-metal in the low-resistance state broadens the application of the device in spintronics and the semiconductor to half-metal transition on the basis of defects evolution and also opens up a new avenue for exploring random access memory mechanisms and finding new half-metals for spintronics.

  10. Simulation Study of Magnetic Fields generated by the Electromagnetic Filamentation Instability driven by Pair Loading

    NASA Technical Reports Server (NTRS)

    Nishikawa, K.-I.; Ramirez-Ruiz, E.; Hededal, C.; Hardee, P.; Mizuno, Y.; Fishman, G. J.

    2007-01-01

    Using a 3-D relativistic particle-in-cell (RPIC) code, we have investigated particle acceleration associated with a relativistic electron-positron (cold) jet propagating into ambient electron-positron and electron-ion plasmas without initial magnetic fields in order to investigate the nonlinear stage of the Weibel instability. We have also performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which are assumed to be created by the photon annihilation. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions and ambient plasma. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-ion ambient plasma are larger than those in the electron-positron ambient plasma. We have shown that plasma instabilities driven by these streaming electron-positron pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. These fields maintain a strong saturated level on timescales much longer than the electron skin depth at least for the duration of the simulations. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between electron-positron pairs and ions, and may help explain the origin of large upstream fields in GRB shock.

  11. Field-driven dynamics of microcapillaries filled with nematic liquid crystal

    NASA Astrophysics Data System (ADS)

    Fu, Fred; Khayyatzadeh, Pouya; Abukhdeir, Nasser M.

    Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of study for their unique electro-optical properties and the feasibility of manufacturing them on a large scale, resulting in applications such as switchable windows. LC domains within PDLCs are typically spheroidal, as opposed to rectangular in LCD technology, and thus exhibit substantially different behaviour in the presence of an external field. In this work, continuum simulations were performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. A simplified elliptic cylinder (microcapillary) geometry is used and the effects of varying aspect ratio, surface anchoring, and external field strength were studied using the Landau-de Gennes model. The observed nematic formation and reorientation dynamics were found to be governed by the presence and motion of defects within the domain. Aspect ratio was found to strongly influence domain texture by providing regions of high curvature to which defects are attracted. Simulations also predict the presence of a geometry-controlled transition from nematic order enhanced by an external field (low aspect ratio) to nematic order frustrated by an external field (high aspect ratio). This work was made possible by the Natural Sciences and Engineering Research Council of Canada and Compute Ontario.

  12. Multiphoton transitions in a spin system driven by strong bichromatic field

    SciTech Connect

    Saiko, A. P. Fedoruk, G. G.; Markevich, S. A.

    2007-11-15

    EPR transient nutation spectroscopy is used to measure the effective field (Rabi frequency) for multiphoton transitions in a two-level spin system bichromatically driven by a transverse microwave (MW) field and a longitudinal radio-frequency (RF) field. The behavior of the effective field amplitude is examined in the case of a relatively strong MW field, when the derivation of the effective Hamiltonian cannot be reduced to first-order perturbation theory in {omega}{sub 1}/{omega}{sub rf}({omega}{sub 1} is the microwave Rabi frequency, {omega}{sub rf} is the RF frequency). Experimental results are consistently interpreted by taking into account the contributions of second and third order in {omega}{sub 1}/{omega}{sub rf} evaluated by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously broadened EPR line, the third-order correction modifies the nutation frequency, while the second-order correction gives rise to a change in the nutation amplitude due to a Bloch-Siegert shift.

  13. Convectively Driven Sinks and Magnetic Fields in the Quiet-Sun

    NASA Astrophysics Data System (ADS)

    Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot Rubio, Luis R.; Martínez Pillet, Valentín; Solanki, Sami K.; Schmidt, Wolfgang

    2017-03-01

    We study the relation between mesogranular flows, convectively driven sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We obtain the horizontal velocity flow fields of two quiet-Sun regions (31.2 × 31.2 Mm2) via local correlation tracking. Mesogranular lanes and the central position of sinks are identified using Lagrange tracers. We find 6.7× {10}-2 sinks per Mm2 in the two observed regions. The sinks are located at the mesogranular vertices and turn out to be associated with (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts. The spatial distribution of magnetic fields in the quiet-Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal components of the magnetic field stronger than 500 G are located in the close neighborhood of sinks. In contrast, the small-scale magnetic loops detected by Martínez González et al. in the same two observed areas do not show any preferential distribution at mesogranular scales. The study of individual examples reveals that sinks can play an important role in the evolution of quiet-Sun magnetic features.

  14. Electric-field-driven domain wall dynamics in perpendicularly magnetized multilayers

    NASA Astrophysics Data System (ADS)

    López González, Diego; Shirahata, Yasuhiro; Van de Wiele, Ben; Franke, Kévin J. A.; Casiraghi, Arianna; Taniyama, Tomoyasu; van Dijken, Sebastiaan

    2017-03-01

    We report on reversible electric-field-driven magnetic domain wall motion in a Cu/Ni multilayer on a ferroelectric BaTiO3 substrate. In our heterostructure, strain-coupling to ferroelastic domains with in-plane and perpendicular polarization in the BaTiO3 substrate causes the formation of domains with perpendicular and in-plane magnetic anisotropy, respectively, in the Cu/Ni multilayer. Walls that separate magnetic domains are elastically pinned onto ferroelectric domain walls. Using magneto-optical Kerr effect microscopy, we demonstrate that out-of-plane electric field pulses across the BaTiO3 substrate move the magnetic and ferroelectric domain walls in unison. Our experiments indicate an exponential increase of domain wall velocity with electric field strength and opposite domain wall motion for positive and negative field pulses. The application of a magnetic field does not affect the velocity of magnetic domain walls, but independently tailors their internal spin structure, causing a change in domain wall dynamics at high velocities.

  15. Experimental Study on Flow Field behind Backward-Facing Step Using Detonation-Driven Shock Tunnel

    NASA Astrophysics Data System (ADS)

    Kim, Tae-Hwan; Obara, Tetsuro; Ohyagi, Shigeharu; Yoshikawa, Masato

    As a research to develop a SCRAM-jet engine is actively conducted, a necessity to produce a high-enthalpy flow in a laboratory is increasing. In order to develop the SCRAM-jet engine, stabilized combustion in a supersonic flow-field should be attained, in which a duration time of flow is extremely short. Therefore, a mixing process of breathed air and fuel, which is injected into supersonic flow-fields is one of the most important problem. Since, the flow inside SCRAM-jet engine has high-enthalpy, an experimental facility is required to produce such high-enthalpy flow-field. In this study, a detonation-driven shock tunnel was built and was used to produce high-enthalpy flow. At first, a performance of this facility was investigated in order to obtain a Tayloring condition. Furthermore, SCRAM-jet combustor model equipped backward-facing step was installed at test section and flow-fields were visualized using color-schlieren technique. The fuel was injected perpendicular to the flow of Mach number three behind step. The height of backward-facing step and injection pressure were changed to investigate effects of the step on a mixing characteristic between air and fuel. The schlieren photograph and pressure histories show that the fuel was ignited behind step and the height of step is important factor to ignite a fuel in a supersonic flow-field.

  16. Mesh-driven vector field clustering and visualization: an image-based approach.

    PubMed

    Peng, Zhenmin; Grundy, Edward; Laramee, Robert S; Chen, Guoning; Croft, Nick

    2012-02-01

    Vector field visualization techniques have evolved very rapidly over the last two decades, however, visualizing vector fields on complex boundary surfaces from computational flow dynamics (CFD) still remains a challenging task. In part, this is due to the large, unstructured, adaptive resolution characteristics of the meshes used in the modeling and simulation process. Out of the wide variety of existing flow field visualization techniques, vector field clustering algorithms offer the advantage of capturing a detailed picture of important areas of the domain while presenting a simplified view of areas of less importance. This paper presents a novel, robust, automatic vector field clustering algorithm that produces intuitive and insightful images of vector fields on large, unstructured, adaptive resolution boundary meshes from CFD. Our bottom-up, hierarchical approach is the first to combine the properties of the underlying vector field and mesh into a unified error-driven representation. The motivation behind the approach is the fact that CFD engineers may increase the resolution of model meshes according to importance. The algorithm has several advantages. Clusters are generated automatically, no surface parameterization is required, and large meshes are processed efficiently. The most suggestive and important information contained in the meshes and vector fields is preserved while less important areas are simplified in the visualization. Users can interactively control the level of detail by adjusting a range of clustering distance measure parameters. We describe two data structures to accelerate the clustering process. We also introduce novel visualizations of clusters inspired by statistical methods. We apply our method to a series of synthetic and complex, real-world CFD meshes to demonstrate the clustering algorithm results.

  17. Quantum transport in ferromagnetic graphene: Role of Berry curvature

    SciTech Connect

    Chowdhury, Debashree; Basu, Banasri

    2014-12-10

    The magnetic effects in ferromagnetic graphene basically depend on the principle of exchange interaction when ferromagntism is induced by depositing an insulator layer on graphene. Here we deal with the consequences of non-uniformity in the exchange coupling strength of the ferromagnetic graphene. We discuss how the in- homogeneity in the coordinate and momentum of the exchange vector field can provide interesting results in the conductivity analysis of the ferromagnetic graphene. Our analysis is based on the Kubo formalism of quantum transport.

  18. Ferromagnetic resonance in FeCoNi electroplated wires

    NASA Astrophysics Data System (ADS)

    García-Miquel, H.; Bhagat, S. M.; Lofland, S. E.; Kurlyandskaya, G. V.; Svalov, A. V.

    2003-08-01

    We have investigated the microwave properties (ferromagnetic resonance and ferromagnetic antiresonance) of FeCoNi magnetic tubes created by electroplating on CuBe wire. Important parameters such as the g factor, magnetization, anisotropy field, and damping parameter were obtained from the measurements. One sample, prepared by a method which entails rf-sputtering deposition of an additional FeNi layer, shows a clear ferromagnetic antiresonance.

  19. Giant electric field modulation of double exchange ferromagnetism at room temperature in the perovskite manganite/titanate p-n junction.

    PubMed

    Tanaka, Hidekazu; Zhang, Jun; Kawai, Tomoji

    2002-01-14

    We report on the electrical modulation of double exchange ferromagnetism at room temperature in hole-doped manganites of a metal oxide p-n junction. In this (La0.9Ba0.1)MnO(3)/Nb doped SrTiO3 p-n junction, the temperature dependence of the junction resistance shows a metal-insulator transition whose temperature, corresponding to that of ferromagnetic transition, is hugely modulated from 290 to 340 K by a bias voltage increasing from +1.0 to +1.8 V. The magnetoresistance can also be modulated electrically.

  20. Magnetic pinning in superconductor-ferromagnet multilayers

    SciTech Connect

    Bulaevskii, L. N.; Chudnovsky, E. M.; Maley, M. P.

    2000-05-01

    We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 10{sup 6}-10{sup 7} A/cm{sup 2} at high temperatures (but not very close to T{sub c}) at least in magnetic fields below 0.1 T. (c) 2000 American Institute of Physics.

  1. Surface ferromagnetism in close-packed structures

    NASA Astrophysics Data System (ADS)

    Sanchez, J. M.; Morán-López, J. L.

    The temperature-magnetic field equilibrium phase diagram for the (111) surface of an fcc spin- {1}/{2} Ising ferromagnet is calculated using the tetrahedron aproximation of the cluster variation method. Among the new features found in the model is a triple point corresponding to a ferromagnetic first-order phase transition at zero field. Some characteristics of the model, such as the increase in the surface transition temperature with the magnetic field, may be relevant to recent observations in Gd(0001) by Weller and Alvarado.

  2. Terahertz-driven magnetism dynamics in the orthoferrite DyFeO3

    NASA Astrophysics Data System (ADS)

    Reid, A. H. M.; Rasing, Th.; Pisarev, R. V.; Dürr, H. A.; Hoffmann, M. C.

    2015-02-01

    Terahertz (THz) driven magnetization dynamics are explored in the orthoferrite DyFeO3. A high-field, single cycle THz pulse is used to excite magnon modes in the crystal together with other resonances. Both quasi-ferromagnetic and quasi-antiferromagnetic magnon modes are excited and appear in time-resolved measurements of the Faraday rotation. Other modes are also observed in the measurements of the time-resolved linear birefringence. Analysis of the excitation process reveals that despite larger than expected electro-optical susceptibility, it is mainly the THz magnetic field that couples to the quasi-ferromagnetic and quasi-antiferromagnetic magnon branches.

  3. Electrically-induced ferromagnetism at room temperature in (Ti,Co)O2: carrier-mediated ferromagnetism

    NASA Astrophysics Data System (ADS)

    Fukumura, Tomoteru

    2013-03-01

    Oxide-diluted magnetic semiconductors (DMS) is expected to have high Curie temperature via carrier-mediated ferromagnetism through heavy electron mass and large electron carrier density. We have studied various oxide-DMS such as (Zn,Mn)O, and discovered room temperature ferromagnetism in (Ti,Co)O2. The origin of ferromagnetism has been discussed for a decade. Previously, the control of ferromagnetism was demonstrated through carrier control by chemical doping. But it was difficult to exclude the defect-mediated ferromagnetism, since the electron donor was the oxygen vacancy. In order to evidence the carrier-mediated ferromagnetism, the electric field control of ferromagnetism is useful. The control of ferromagnetism at room temperature is also important for implementation of spintronic devices. By gating with electric double layer transistor, the ferromagnetism was induced at room temperature, representing electron carrier-mediated ferromagnetism. Chemical doping study in (Ti,Co)O2 for wider range of carrier density exhibited clearer paramagnetic insulator to ferromagnetic metal transition with increasing carrier density. At a medium carrier density, a ferromagnetic insulator phase appeared possibly related with a phase separation between ferromagnetic and paramagnetic phases. Also, a superparamagnetic phase appeared for excessively reduced sample. Taking all these results into account, previously proposed extrinsic mechanisms such as oxygen vacancy-mediated mechanism, metal segregation, and superparamagnetism are not correct picture of the ferromagnetism. This study was in collaboration with Y. Yamada, K. Ueno, M. Kawasaki, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, A. Fujimori, and T. Mizokawa. This research was in part supported by JSPS through NEXT Program initiated by CSTP.

  4. Flocking ferromagnetic colloids

    PubMed Central

    Kaiser, Andreas; Snezhko, Alexey; Aranson, Igor S.

    2017-01-01

    Assemblages of microscopic colloidal particles exhibit fascinating collective motion when energized by electric or magnetic fields. The behaviors range from coherent vortical motion to phase separation and dynamic self-assembly. Although colloidal systems are relatively simple, understanding their collective response, especially under out-of-equilibrium conditions, remains elusive. We report on the emergence of flocking and global rotation in the system of rolling ferromagnetic microparticles energized by a vertical alternating magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms, leading to the emergence of large-scale collective motion: spontaneous symmetry breaking of the clockwise/counterclockwise particle rotation, collisional alignment of particle velocities, and random particle reorientations due to shape imperfections. We have also shown that hydrodynamic interactions between the particles do not have a qualitative effect on the collective dynamics. Our findings shed light on the onset of spatial and temporal coherence in a large class of active systems, both synthetic (colloids, swarms of robots, and biopolymers) and living (suspensions of bacteria, cell colonies, and bird flocks). PMID:28246633

  5. Flocking ferromagnetic colloids

    DOE PAGES

    Kaiser, Andreas; Snezhko, Alexey; Aranson, Igor S.

    2017-02-15

    Assemblages of microscopic colloidal particles exhibit fascinating collective motion when energized by electric or magnetic fields. The behaviors range from coherent vortical motion to phase separation and dynamic self-assembly. While colloidal systems are relatively simple, understanding their collective response, especially in out of equilibrium conditions, remains elusive. Here, we report on the emergence of flocking and global rotation in the system of rolling ferromagnetic microparticles energized by a vertical alternating magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms leading to the emergence of largescale collective motion: spontaneous symmetry breaking of the clock /more » counterclockwise particle rotation, collisional alignment of particle velocities, and random particle re-orientations due to shape imperfections. We have also shown that hydrodynamic interactions between the particles do not have a qualitative effect on the collective dynamics. Lastly, our findings shed light on the onset of spatial and temporal coherence in a large class of active systems, both synthetic (colloids, swarms of robots, biopolymers) and living (suspensions of bacteria, cell colonies, bird flocks).« less

  6. Electronic transport in the ferromagnetic pyrochlore L u2V2O7 : Role of magnetization

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaohang; Zhou, Haidong; Yu, Liuqi; Gardner, H. Jeffery; von Molnár, Stephan; Wiebe, Christopher; Xiong, Peng

    2015-05-01

    This paper reports on a study of the resistivity and Hall effect of the ferromagnetic pyrochlore L u2V2O7 . The temperature dependence of the resistivity of single crystalline L u2V2O7 exhibits overall activation behavior with a metalliclike exception at intermediate temperatures near the Curie temperature (TC). This temperature dependence bears a surprising resemblance to that of doped semiconductors. The ferromagnetic oxide shows a negative magnetoresistance (MR) which scales quadratically with the reduced magnetization at temperatures above TC; however, the scaling factor is significantly smaller than the value expected for a ferromagnetic system in the pure spin scattering regime, which suggests that other scattering processes may be at work. Concomitant with the negative MR, a distinct switch in the Hall resistivity slope is observed at temperatures near TC. Our analysis suggests that the nonlinear Hall effect is associated with a change in the effective carrier density at a constant critical magnetization induced by an external magnetic field. We argue that within a picture that incorporates high temperature activation transport with a magnetization-driven charge percolation transition, the observed complex electronic transport in the ferromagnetic pyrochlore can be quantitatively described.

  7. 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.

  8. Dynamics of a reconnection-driven runaway ion tail in a reversed field pinch plasma

    NASA Astrophysics Data System (ADS)

    Anderson, J. K.; Kim, J.; Bonofiglo, P. J.; Capecchi, W.; Eilerman, S.; Nornberg, M. D.; Sarff, J. S.; Sears, S. H.

    2016-05-01

    While reconnection-driven ion heating is common in laboratory and astrophysical plasmas, the underlying mechanisms for converting magnetic to kinetic energy remain not fully understood. Reversed field pinch discharges are often characterized by rapid ion heating during impulsive reconnection, generating an ion distribution with an enhanced bulk temperature, mainly perpendicular to magnetic field. In the Madison Symmetric Torus, a subset of discharges with the strongest reconnection events develop a very anisotropic, high energy tail parallel to magnetic field in addition to bulk perpendicular heating, which produces a fusion neutron flux orders of magnitude higher than that expected from a Maxwellian distribution. Here, we demonstrate that two factors in addition to a perpendicular bulk heating mechanism must be considered to explain this distribution. First, ion runaway can occur in the strong parallel-to-B electric field induced by a rapid equilibrium change triggered by reconnection-based relaxation; this effect is particularly strong on perpendicularly heated ions which experience a reduced frictional drag relative to bulk ions. Second, the confinement of ions varies dramatically as a function of velocity. Whereas thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), sufficiently energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of toroidal plasma current are nearly classically confined, while counter-propagating ions experience an intermediate confinement, greater than that of thermal ions but significantly less than classical expectations. The details of ion confinement tend to reinforce the asymmetric drive of the parallel electric field, resulting in a very asymmetric, anisotropic distribution.

  9. Dynamics of a reconnection-driven runaway ion tail in a reversed field pinch plasma

    SciTech Connect

    Anderson, J. K. Kim, J.; Bonofiglo, P. J.; Capecchi, W.; Eilerman, S.; Nornberg, M. D.; Sarff, J. S.; Sears, S. H.

    2016-05-15

    While reconnection-driven ion heating is common in laboratory and astrophysical plasmas, the underlying mechanisms for converting magnetic to kinetic energy remain not fully understood. Reversed field pinch discharges are often characterized by rapid ion heating during impulsive reconnection, generating an ion distribution with an enhanced bulk temperature, mainly perpendicular to magnetic field. In the Madison Symmetric Torus, a subset of discharges with the strongest reconnection events develop a very anisotropic, high energy tail parallel to magnetic field in addition to bulk perpendicular heating, which produces a fusion neutron flux orders of magnitude higher than that expected from a Maxwellian distribution. Here, we demonstrate that two factors in addition to a perpendicular bulk heating mechanism must be considered to explain this distribution. First, ion runaway can occur in the strong parallel-to-B electric field induced by a rapid equilibrium change triggered by reconnection-based relaxation; this effect is particularly strong on perpendicularly heated ions which experience a reduced frictional drag relative to bulk ions. Second, the confinement of ions varies dramatically as a function of velocity. Whereas thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), sufficiently energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of toroidal plasma current are nearly classically confined, while counter-propagating ions experience an intermediate confinement, greater than that of thermal ions but significantly less than classical expectations. The details of ion confinement tend to reinforce the asymmetric drive of the parallel electric field, resulting in a very asymmetric, anisotropic distribution.

  10. Molecular dynamics study of nanoconfined water flow driven by rotating electric fields under realistic experimental conditions.

    PubMed

    De Luca, Sergio; Todd, B D; Hansen, J S; Daivis, Peter J

    2014-03-25

    In our recent work, J. Chem. Phys. 2013, 138, 154712, we demonstrated the feasibility of unidirectional pumping of water, exploiting translational-rotational momentum coupling using nonequilibrium molecular dynamics simulations. Flow can be sustained when the fluid is driven out of equilibrium by an external spatially uniform rotating electric field and confined between two planar surfaces exposing different degrees of hydrophobicity. The permanent dipole moment of water follows the rotating field, thus inducing the molecules to spin, and the torque exerted by the field is continuously injected into the fluid, enabling a steady conversion of spin angular momentum into linear momentum. The translational-rotational coupling is a sensitive function of the rotating electric field parameters. In this work, we have found that there exists a small energy dissipation region attainable when the frequency of the rotating electric field matches the inverse of the dielectric relaxation time of water and when its amplitude lies in a range just before dielectric saturation effects take place. In this region, that is, when the frequency lies in a small window of the microwave region around ∼20 GHz and amplitude ∼0.03 V Å(-1), the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ∼2 ms(-1) with only moderate fluid heating. In this work, we also confine water to a realistic nanochannel made of graphene giving a hydrophobic surface on one side and β-cristobalite giving a hydrophilic surface on the other, reproducing slip-and-stick velocity boundary conditions, respectively. This enables us to demonstrate that in a realistic environment, the coupling can be effectively exploited to achieve noncontact pumping of water at the nanoscale. A quantitative comparison between nonequilibrium molecular dynamics and analytical solutions of the extended Navier-Stokes equations, including an external rotating electric field has been performed

  11. Electromagnetic field redistribution induced selective plasmon driven surface catalysis in metal nanowire-film systems

    PubMed Central

    Pan, Liang; Huang, Yingzhou; Yang, Yanna; Xiong, Wen; Chen, Guo; Su, Xun; Wei, Hua; Wang, Shuxia; Wen, Weijia

    2015-01-01

    For the novel interpretation of Raman spectrum from molecule at metal surface, the plasmon driven surface catalysis (PDSC) reactions have become an interesting topic in the research field of surface enhanced Raman scattering (SERS). In this work, the selective PDSC reactions of p,p’-dimercaptoazobenzene (DMAB) produced from para-aminothiophenol (PATP) or 4-nitrobenzenethiol (4NBT) were demonstrated in the Ag nanowires dimer-Au film systems. The different SERS spectra collected at individual part and adjacent part of the same nanowire-film system pointed out the importance of the electromagnetic field redistribution induced by image charge on film in this selective surface catalysis, which was confirmed by the simulated electromagnetic simulated electro- magnetic field distributions. Our result indicated this electromagnetic field redistribution induced selective surface catalysis was largely affected by the polarization and wavelength of incident light but slightly by the difference in diameters between two nanowires. Our work provides a further understanding of PDSC reaction in metal nanostructure and could be a deep support for the researches on surface catalysis and surface analysis. PMID:26601698

  12. Exciton Absorption in Semiconductor Quantum Wells Driven by a Strong Intersubband Pump Field

    NASA Technical Reports Server (NTRS)

    Liu, Ansheng; Ning, Cun-Zheng

    1999-01-01

    Optical interband excitonic absorption of semiconductor quantum wells (QW's) driven by a coherent pump field is investigated based on semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW's. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron-hole pairs in the QW structure is low, the pump field induces an Autler-Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron-hole pairs is increased, the split contrast (the ratio between the maximum and minimum values) is decreased because the exciton effect is suppressed at higher densities due to the many-body screening.

  13. Numerical modeling of laser-driven experiments of colliding jets: Turbulent amplification of seed magnetic fields

    NASA Astrophysics Data System (ADS)

    Tzeferacos, Petros; Fatenejad, Milad; Flocke, Norbert; Graziani, Carlo; Gregori, Gianluca; Lamb, Donald; Lee, Dongwook; Meinecke, Jena; Scopatz, Anthony; Weide, Klaus

    2014-10-01

    In this study we present high-resolution numerical simulations of laboratory experiments that study the turbulent amplification of magnetic fields generated by laser-driven colliding jets. The radiative magneto-hydrodynamic (MHD) simulations discussed here were performed with the FLASH code and have assisted in the analysis of the experimental results obtained from the Vulcan laser facility. In these experiments, a pair of thin Carbon foils is placed in an Argon-filled chamber and is illuminated to create counter-propagating jets. The jets carry magnetic fields generated by the Biermann battery mechanism and collide to form a highly turbulent region. The interaction is probed using a wealth of diagnostics, including induction coils that are capable of providing the field strength and directionality at a specific point in space. The latter have revealed a significant increase in the field's strength due to turbulent amplification. Our FLASH simulations have allowed us to reproduce the experimental findings and to disentangle the complex processes and dynamics involved in the colliding flows. This work was supported in part at the University of Chicago by DOE NNSA ASC.

  14. Ultrafast probing of magnetic field growth inside a laser-driven solenoid.

    PubMed

    Goyon, C; Pollock, B B; Turnbull, D P; Hazi, A; Divol, L; Farmer, W A; Haberberger, D; Javedani, J; Johnson, A J; Kemp, A; Levy, M C; Grant Logan, B; Mariscal, D A; Landen, O L; Patankar, S; Ross, J S; Rubenchik, A M; Swadling, G F; Williams, G J; Fujioka, S; Law, K F F; Moody, J D

    2017-03-01

    We report on the detection of the time-dependent B-field amplitude and topology in a laser-driven solenoid. The B-field inferred from both proton deflectometry and Faraday rotation ramps up linearly in time reaching 210 ± 35 T at the end of a 0.75-ns laser drive with 1 TW at 351 nm. A lumped-element circuit model agrees well with the linear rise and suggests that the blow-off plasma screens the field between the plates leading to an increased plate capacitance that converts the laser-generated hot-electron current into a voltage source that drives current through the solenoid. ALE3D modeling shows that target disassembly and current diffusion may limit the B-field increase for longer laser drive. Scaling of these experimental results to a National Ignition Facility (NIF) hohlraum target size (∼0.2cm^{3}) indicates that it is possible to achieve several tens of Tesla.

  15. Ultrafast probing of magnetic field growth inside a laser-driven solenoid

    NASA Astrophysics Data System (ADS)

    Goyon, C.; Pollock, B. B.; Turnbull, D. P.; Hazi, A.; Divol, L.; Farmer, W. A.; Haberberger, D.; Javedani, J.; Johnson, A. J.; Kemp, A.; Levy, M. C.; Grant Logan, B.; Mariscal, D. A.; Landen, O. L.; Patankar, S.; Ross, J. S.; Rubenchik, A. M.; Swadling, G. F.; Williams, G. J.; Fujioka, S.; Law, K. F. F.; Moody, J. D.

    2017-03-01

    We report on the detection of the time-dependent B-field amplitude and topology in a laser-driven solenoid. The B-field inferred from both proton deflectometry and Faraday rotation ramps up linearly in time reaching 210 ± 35 T at the end of a 0.75-ns laser drive with 1 TW at 351 nm. A lumped-element circuit model agrees well with the linear rise and suggests that the blow-off plasma screens the field between the plates leading to an increased plate capacitance that converts the laser-generated hot-electron current into a voltage source that drives current through the solenoid. ALE3D modeling shows that target disassembly and current diffusion may limit the B-field increase for longer laser drive. Scaling of these experimental results to a National Ignition Facility (NIF) hohlraum target size (˜0.2 cm3 ) indicates that it is possible to achieve several tens of Tesla.

  16. Electromagnetic field redistribution induced selective plasmon driven surface catalysis in metal nanowire-film systems

    NASA Astrophysics Data System (ADS)

    Pan, Liang; Huang, Yingzhou; Yang, Yanna; Xiong, Wen; Chen, Guo; Su, Xun; Wei, Hua; Wang, Shuxia; Wen, Weijia

    2015-11-01

    For the novel interpretation of Raman spectrum from molecule at metal surface, the plasmon driven surface catalysis (PDSC) reactions have become an interesting topic in the research field of surface enhanced Raman scattering (SERS). In this work, the selective PDSC reactions of p,p’-dimercaptoazobenzene (DMAB) produced from para-aminothiophenol (PATP) or 4-nitrobenzenethiol (4NBT) were demonstrated in the Ag nanowires dimer-Au film systems. The different SERS spectra collected at individual part and adjacent part of the same nanowire-film system pointed out the importance of the electromagnetic field redistribution induced by image charge on film in this selective surface catalysis, which was confirmed by the simulated electromagnetic simulated electro- magnetic field distributions. Our result indicated this electromagnetic field redistribution induced selective surface catalysis was largely affected by the polarization and wavelength of incident light but slightly by the difference in diameters between two nanowires. Our work provides a further understanding of PDSC reaction in metal nanostructure and could be a deep support for the researches on surface catalysis and surface analysis.

  17. Velocity field control of a class of electrically-driven manipulators

    NASA Astrophysics Data System (ADS)

    Moreno-Valenzuela, Javier; Campa, Ricardo; Santibáñez, Víctor

    2014-03-01

    This article addresses the control of robotic manipulators under the assumption that the desired motion in the operational space is encoded through a velocity field. In other words, a vectorial function assigns a velocity vector to each point in the robot workspace. Thus, the control objective is to design a control input such that the actual operational space velocity of the robot end-effector asymptotically tracks the desired velocity from the velocity field. This control formulation is known in the literature as velocity field control. A new velocity field controller together with a rigorous stability analysis is introduced in this article. The controller is developed for a class of electrically-driven manipulators. In this class of manipulators, the passivity property from the servo-amplifier voltage input to the joint velocity is not satisfied. However, global exponential stability of the state space origin of the closed-loop system is proven. Furthermore, the closed-loop system is proven to be and output strictly passive map from an auxiliary input to a filtered error signal. To confirm the theoretical conclusions, a detailed experimental study in a two degrees-of-freedom direct-drive manipulator is provided. Particularly, experiments consist of comparing the performance of a simple PI controller and a high-gain PI controller with respect to the new control scheme.

  18. Field-Driven Mott Gap Collapse and Resistive Switch in Correlated Insulators

    NASA Astrophysics Data System (ADS)

    Mazza, G.; Amaricci, A.; Capone, M.; Fabrizio, M.

    2016-10-01

    Mott insulators are "unsuccessful metals" in which Coulomb repulsion prevents charge conduction despite a metal-like concentration of conduction electrons. The possibility to unlock the frozen carriers with an electric field offers tantalizing prospects of realizing new Mott-based microelectronic devices. Here we unveil how such unlocking happens in a simple model that shows the coexistence of a stable Mott insulator and a metastable metal. Considering a slab subject to a linear potential drop, we find, by means of the dynamical mean-field theory, that the electric breakdown of the Mott insulator occurs via a first-order insulator-to-metal transition characterized by an abrupt gap collapse in sharp contrast to the standard Zener breakdown. The switch on of conduction is due to the field-driven stabilization of the metastable metallic phase. Outside the region of insulator-metal coexistence, the electric breakdown occurs through a more conventional quantum tunneling across the Hubbard bands tilted by the field. Our findings rationalize recent experimental observations and may offer a guideline for future technological research.

  19. Statistical properties of Barkhausen noise in amorphous ferromagnetic films.

    PubMed

    Bohn, F; Corrêa, M A; Carara, M; Papanikolaou, S; Durin, G; Sommer, R L

    2014-09-01

    We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between 100 and 1000 nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents τ,α,1/σνz, and ϑ. We also focus on the average shape of the avalanches, which gives further indications on the domain-wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents τ=1.28±0.02,α=1.52±0.3, and 1/σνz=ϑ=1.83±0.03, values compatible with that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain-wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.

  20. Statistical properties of Barkhausen noise in amorphous ferromagnetic films

    NASA Astrophysics Data System (ADS)

    Bohn, F.; Corrêa, M. A.; Carara, M.; Papanikolaou, S.; Durin, G.; Sommer, R. L.

    2014-09-01

    We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between 100 and 1000 nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents τ,α,1/σνz, and ϑ. We also focus on the average shape of the avalanches, which gives further indications on the domain-wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents τ =1.28±0.02,α =1.52±0.3, and 1/σνz=ϑ=1.83±0.03, values compatible with that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain-wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.

  1. Joint optimization of fluence field modulation and regularization in task-driven computed tomography

    NASA Astrophysics Data System (ADS)

    Gang, G. J.; Siewerdsen, J. H.; Stayman, J. W.

    2017-03-01

    Purpose: This work presents a task-driven joint optimization of fluence field modulation (FFM) and regularization in quadratic penalized-likelihood (PL) reconstruction. Conventional FFM strategies proposed for filtered-backprojection (FBP) are evaluated in the context of PL reconstruction for comparison. Methods: We present a task-driven framework that leverages prior knowledge of the patient anatomy and imaging task to identify FFM and regularization. We adopted a maxi-min objective that ensures a minimum level of detectability index (d') across sample locations in the image volume. The FFM designs were parameterized by 2D Gaussian basis functions to reduce dimensionality of the optimization and basis function coefficients were estimated using the covariance matrix adaptation evolutionary strategy (CMA-ES) algorithm. The FFM was jointly optimized with both space-invariant and spatially-varying regularization strength (β) - the former via an exhaustive search through discrete values and the latter using an alternating optimization where β was exhaustively optimized locally and interpolated to form a spatially-varying map. Results: The optimal FFM inverts as β increases, demonstrating the importance of a joint optimization. For the task and object investigated, the optimal FFM assigns more fluence through less attenuating views, counter to conventional FFM schemes proposed for FBP. The maxi-min objective homogenizes detectability throughout the image and achieves a higher minimum detectability than conventional FFM strategies. Conclusions: The task-driven FFM designs found in this work are counter to conventional patterns for FBP and yield better performance in terms of the maxi-min objective, suggesting opportunities for improved image quality and/or dose reduction when model-based reconstructions are applied in conjunction with FFM.

  2. 2D profile of poloidal magnetic field diagnosed by a laser-driven ion-beam trace probe (LITP)

    SciTech Connect

    Yang, Xiaoyi; Xiao, Chijie Chen, Yihang; Xu, Tianchao; Lin, Chen; Wang, Long; Xu, Min; Yu, Yi

    2016-11-15

    Based on large energy spread of laser-driven ion beam (LIB), a new method, the Laser-driven Ion-beam Trace Probe (LITP), was suggested recently to diagnose the poloidal magnetic field (B{sub p}) and radial electric field (E{sub r}) in toroidal devices. Based on another property of LIB, a wide angular distribution, here we suggested that LITP could be extended to get 2D B{sub p} profile or 1D profile of both poloidal and radial magnetic fields at the same time. In this paper, we show the basic principle, some preliminary simulation results, and experimental preparation to test the basic principle of LITP.

  3. Dynamical band flipping in fermionic lattice systems: an ac-field-driven change of the interaction from repulsive to attractive.

    PubMed

    Tsuji, Naoto; Oka, Takashi; Werner, Philipp; Aoki, Hideo

    2011-06-10

    We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

  4. Dynamical Band Flipping in Fermionic Lattice Systems: An ac-Field-Driven Change of the Interaction from Repulsive to Attractive

    SciTech Connect

    Tsuji, Naoto; Oka, Takashi; Aoki, Hideo; Werner, Philipp

    2011-06-10

    We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

  5. Spin-orbit ferromagnetic resonance

    NASA Astrophysics Data System (ADS)

    Ferguson, Andrew

    2013-03-01

    In conventional magnetic resonance techniques the magnitude and direction of the oscillatory magnetic field are (at least approximately) known. This oscillatory field is used to probe the properties of a spin ensemble. Here, I will describe experiments that do the inverse. I will discuss how we use a magnetic resonance technique to map out the current-induced effective magnetic fields in the ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P). These current-induced fields have their origin in the spin-orbit interaction. Effective magnetic fields are observed with symmetries which resemble the Dresselhaus and Rashba spin-orbit interactions and which depend on the diagonal and off-diagonal strain respectively. Ferromagnetic semiconductor materials of different strains, annealing conditions and concentrations are studied and the results compared with theoretical calculations. Our original study measured the rectification voltage coming from the product of the oscillatory magnetoresistance, during magnetisation precession, and the alternating current. More recently we have developed an impedance matching technique which enables us to extract microwave voltages from these high resistance (10 k Ω) samples. In this way we measure the microwave voltage coming from the product of the oscillating magneto-resistance and a direct current. The direct current is observed to affect the magnetisation precession, indicating that anti-damping as well as field-like torques can originate from the spin-orbit interaction.

  6. Ferromagnetism in neutron matter and its implication for the neutron star equation of state

    NASA Astrophysics Data System (ADS)

    Diener, J. P. W.; Scholtz, F. G.

    2011-09-01

    We investigate the possible contribution of the ferromagnetic phase of neutron matter in the neutron star interior to the star's magnetic field. We introduce a relativistic, self-consistent calculation of the ferromagnetic phase in neutron matter within the context of the relativistic mean-field approximation. The presence of the ferromagnetic phase stiffens the star's equation of state which implies a larger neutron star radius compared to the non-ferromagnetic case.

  7. Ferromagnetism in neutron matter and its implication for the neutron star equation of state

    SciTech Connect

    Diener, J. P. W.; Scholtz, F. G.

    2011-09-21

    We investigate the possible contribution of the ferromagnetic phase of neutron matter in the neutron star interior to the star's magnetic field. We introduce a relativistic, self-consistent calculation of the ferromagnetic phase in neutron matter within the context of the relativistic mean-field approximation. The presence of the ferromagnetic phase stiffens the star's equation of state which implies a larger neutron star radius compared to the non-ferromagnetic case.

  8. Photoassociation dynamics driven by second- and third-order phase-modulated laser fields

    NASA Astrophysics Data System (ADS)

    Wang, Meng; Chen, Mao-Du; Hu, Xue-Jin; Li, Jing-Lun; Cong, Shu-Lin

    2016-05-01

    We investigate theoretically the photoassociation dynamics of ultracold 85Rb atoms driven by second- and third-order phase-modulated laser fields. The interplay between the second-order and third-order terms of the phase-modulated pulse has an obvious influence on photoassociation dynamics. The different combinations of the second-order and third-order phase coefficients lead to different pulse shapes. Most of the molecular population in the excited electronic state driven only by the third-order phase pulses can be distributed in a single vibrational level. The second-order term of the phase-modulated pulse can change the instantaneous frequency, and therefore the final population is distributed on several resonant vibrational levels, instead of concentrating on a single level. Although the second- and third-order phase-modulated pulse covers more resonant vibrational levels, the total population on the resonant vibrational levels is much smaller than that controlled only by the third-order phase pulse. In particular, the third-order term of the phase-modulated pulse can weaken the ‘multiple interaction’ to some degree.

  9. Quasistationary magnetic field generation with a laser-driven capacitor-coil assembly

    NASA Astrophysics Data System (ADS)

    Tikhonchuk, V. T.; Bailly-Grandvaux, M.; Santos, J. J.; Poyé, A.

    2017-08-01

    Recent experiments are showing possibilities to generate strong magnetic fields on the excess of 500 T with high-energy nanosecond laser pulses in a compact setup of a capacitor connected to a single turn coil. Hot electrons ejected from the capacitor plate (cathode) are collected at the other plate (anode), thus providing the source of a current in the coil. However, the physical processes leading to generation of currents exceeding hundreds of kiloamperes in such a laser-driven diode are not sufficiently understood. Here we present a critical analysis of previous results and propose a self-consistent model for the high current generation in a laser-driven capacitor-coil assembly. It accounts for three major effects controlling the diode current: the space charge neutralization, the plasma magnetization between the capacitor plates, and the Ohmic heating of the external circuit—the coil-shaped connecting wire. The model provides the conditions necessary for transporting strongly super-Alfvenic currents through the diode on the time scale of a few nanoseconds. The model validity is confirmed by a comparison with the available experimental data.

  10. Towards ferromagnetic quantum criticality in FeGa3 -xGex :71Ga NQR as a zero-field microscopic probe

    NASA Astrophysics Data System (ADS)

    Majumder, M.; Wagner-Reetz, M.; Cardoso-Gil, R.; Gille, P.; Steglich, F.; Grin, Y.; Baenitz, M.

    2016-02-01

    71Ga NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, 71(1 /T1T ) exhibits a pronounced T-4 /3 power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x =0.2 sample (TC≈6 K), 71(1 /T1T ) could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with 1 /T1T ˜χ . In contrast to this, the low-doping regime nicely displays local moment behavior where 1 /T1T ˜χ2 is valid. For T →0 , the Sommerfeld ratio γ =(C /T ) is enhanced (70 mJ /mole K2 for x =0.1 ) , which indicates the formation of heavy 3 d electrons.

  11. Driven chemical kinetics: Optimalization of catalytic action of membrane proteins by rectangular alternating electric field

    NASA Astrophysics Data System (ADS)

    Fuliński, Andrzej

    1992-03-01

    The chemical kinetics driven by external force in the form of a train of alternating rectangular impulses is discussed. The model of the conformational transition of a membrane protein exposed to an ac electric field, proposed by R. D. Astumian and B. Robertson [J. Chem. Phys. 91, 4891 (1989)], is reconsidered. On the example of this model we show that the use of the driving field in the form of rectangular impulses has two distinct advantages over the usual sinusoidal driving. The first one is that the use of a rectangular driving field makes it possible to obtain the exact solution of the basic kinetic equation of the system. This in turn enables one to write down the simple and very good approximate solution for any form of the driving field, better than the harmonic expansion used by Astumian and Robertson. A more important advantage is the greater flexibility of the rectangular driving, which makes possible the better optimalization of the process of interest. Astumian and Robertson demonstrated that the movement of charge within the catalytic cycle provides a mechanism for the enzyme to absorb energy from an ac electric field and to use that energy to enhance the catalyzed process. In this paper we show that the use of the driving ac field in the form of alternating rectangular impulses of variable duration and amplitude (instead of the usual sinusoidal modulation) leads to further optimalization of the process. The efficiency of the energy transduction, for example, can be increased from about 25% for sinusoidal driving to about 37% for suitably chosen alternating rectangular pulses.

  12. Proximity effect and its enhancement by ferromagnetism in high-temperature superconductor-ferromagnet structures.

    PubMed

    Volkov, A F; Efetov, K B

    2009-02-20

    We consider a bilayer consisting of a d-wave layered superconductor and diffusive ferromagnet with a domain wall (DW). The c axis in the superconductor and DW in the ferromagnet are assumed to be perpendicular to the interface. We demonstrate that in such a heterostructure the inhomogeneous exchange field enhances the proximity effect. It is shown that, whereas in the absence of the exchange field the d-wave condensate decays in the normal metal on the mean free path l, the superconductivity penetrates the ferromagnet along the DW over much larger distances. This happens because the presence of the DW results in a generation of an odd-frequency triplet s-wave component of the condensate. The phenomenon discovered here may help to explain a recent experiment on high-temperature superconductor-ferromagnet bilayers.

  13. Ferromagnetic viscoelastic liquid crystalline materials

    NASA Astrophysics Data System (ADS)

    Schlesier, Cristina; Shibaev, Petr; McDonald, Scott

    2012-02-01

    Novel ferromagnetic liquid crystalline materials were designed by mixing ferromagnetic nanoparticles with glass forming oligomers and low molar mass liquid crystals. The matrix in which nanoparticles are embedded is highly viscous that reduces aggregation of nanoparticles and stabilizes the whole composition. Mechanical and optical properties of the composite material are studied in the broad range of nanoparticle concentrations. The mechanical properties of the viscoelastic composite material resemble those of chemically crosslinked elastomers (elasticity and reversibility of deformations). The optical properties of ferromagnetic cholesteric materials are discussed in detail. It is shown that application of magnetic field leads to the shift of the selective reflection band of the cholesteric material and dramatically change its color. Theoretical model is suggested to account for the observed effects; physical properties of the novel materials and liquid crystalline elastomers are compared and discussed. [1] P.V. Shibaev, C. Schlesier, R. Uhrlass, S. Woodward, E. Hanelt, Liquid Crystals, 37, 1601 (2010) [2] P.V. Shibaev, R. Uhrlass, S. Woodward, C. Schlesier, Md R. Ali, E. Hanelt, Liquid Crystals, 37, 587 (2010)

  14. Are the outflows in FU Orionis systems driven by the stellar magnetic field?

    NASA Astrophysics Data System (ADS)

    Königl, Arieh; Romanova, Marina M.; Lovelace, Richard V. E.

    2011-09-01

    FU Orionis (FUOR) outbursts are major optical brightening episodes in low-mass protostars that evidently correspond to rapid mass accretion events in the innermost region of a protostellar disc. The outbursts are accompanied by strong outflows, with the inferred mass outflow rates reaching ˜10 per cent of the mass inflow rates. Shu et al. proposed that the outflows represent accreted disc material that is driven centrifugally from the spun-up surface layers of the protostar by the stellar magnetic field. This model was critiqued by Calvet et al., who argued that it cannot reproduce the photospheric absorption-line shifts observed in the prototype object FU Ori. Calvet et al. proposed that the wind is launched, instead, from the surface of the disc on scales of a few stellar radii by a non-stellar magnetic field. In this paper we present results from numerical simulations of disc accretion on to a slowly rotating star with an aligned magnetic dipole moment that gives rise to a kilogauss-strength surface field. We demonstrate that, for parameters appropriate to FU Ori, such a system can develop a strong, collimated disc outflow of the type previously identified by Romanova et al. in simulations of protostars with low and moderate accretion rates. At the high accretion rate that characterizes the FUOR outburst phase, the radius rm at which the disc is truncated by the stellar magnetic field moves much closer to the stellar surface, but the basic properties of the outflow, which is launched from the vicinity of rm along opened-up stellar magnetic field lines, remain the same. These properties are distinct from those of the X-celerator (or the closely related X-wind) mechanism proposed by Shu et al. - in particular, the outflow is driven from the start by the magnetic pressure gradient force, not centrifugally, and it is more strongly collimated. We show that the simulated outflow can in principle account for the main observed characteristics of FUOR winds, including

  15. Current-driven magnetohydrodynamic thermal instabilities in sheared fields. [of solar corona

    NASA Technical Reports Server (NTRS)

    Bodo, G.; Ferrari, A.; Massaglia, S.; Rosner, R.

    1987-01-01

    Approximate analytic solutions are sought for the dispersion relation for the MHD stability of magnetized medium in current-driven filamentation modes such as those observed in the solar atmosphere. The magnetic field is assumed to have a self-consistent sheared equilibrium structure. The analysis is carried out in the small wavenumber regime, where shear length is similar to the mode wavelength. Instability is found to depend on the ratio between the thermal and magnetic diffusivities, i.e., the Prandtl number, which identifies the unstable transverse wavenumbers. The instability conditions are expressed in an algebraic equation amenable to numerical solution. Results are provided from use of the model to determine the maximum growth rate and typical scale lengths of instabilities in a precoronal atmosphere and the lower transition region.

  16. Centrifugal Force Based Magnetic Micro-Pump Driven by Rotating Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Kim, S. H.; Hashi, S.; Ishiyama, K.

    2011-01-01

    This paper presents a centrifugal force based magnetic micro-pump for the pumping of blood. Most blood pumps are driven by an electrical motor with wired control. To develop a wireless and battery-free blood pump, the proposed pump is controlled by external rotating magnetic fields with a synchronized impeller. Synchronization occurs because the rotor is divided into multi-stage impeller parts and NdFeB permanent magnet. Finally, liquid is discharged by the centrifugal force of multi-stage impeller. The proposed pump length is 30 mm long and19 mm in diameter which much smaller than currently pumps; however, its pumping ability satisfies the requirement for a blood pump. The maximum pressure is 120 mmHg and the maximum flow rate is 5000ml/min at 100 Hz. The advantage of the proposed pump is that the general mechanical problems of a normal blood pump are eliminated by the proposed driving mechanism.

  17. A compact broadband ion beam focusing device based on laser-driven megagauss thermoelectric magnetic fields

    SciTech Connect

    Albertazzi, B.; D'Humières, E.; Lancia, L.; Antici, P.; Dervieux, V.; Nakatsutsumi, M.; Romagnani, L.; Fuchs, J.; Böcker, J.; Swantusch, M.; Willi, O.; Bonlie, J.; Cauble, B.; Shepherd, R.; Breil, J.; Feugeas, J. L.; Nicolaï, P.; Tikhonchuk, V. T.; Chen, S. N.; Sentoku, Y.; and others

    2015-04-15

    Ultra-intense lasers can nowadays routinely accelerate kiloampere ion beams. These unique sources of particle beams could impact many societal (e.g., proton-therapy or fuel recycling) and fundamental (e.g., neutron probing) domains. However, this requires overcoming the beam angular divergence at the source. This has been attempted, either with large-scale conventional setups or with compact plasma techniques that however have the restriction of short (<1 mm) focusing distances or a chromatic behavior. Here, we show that exploiting laser-triggered, long-lasting (>50 ps), thermoelectric multi-megagauss surface magnetic (B)-fields, compact capturing, and focusing of a diverging laser-driven multi-MeV ion beam can be achieved over a wide range of ion energies in the limit of a 5° acceptance angle.

  18. A compact broadband ion beam focusing device based on laser-driven megagauss thermoelectric magnetic fields.

    PubMed

    Albertazzi, B; d'Humières, E; Lancia, L; Dervieux, V; Antici, P; Böcker, J; Bonlie, J; Breil, J; Cauble, B; Chen, S N; Feugeas, J L; Nakatsutsumi, M; Nicolaï, P; Romagnani, L; Shepherd, R; Sentoku, Y; Swantusch, M; Tikhonchuk, V T; Borghesi, M; Willi, O; Pépin, H; Fuchs, J

    2015-04-01

    Ultra-intense lasers can nowadays routinely accelerate kiloampere ion beams. These unique sources of particle beams could impact many societal (e.g., proton-therapy or fuel recycling) and fundamental (e.g., neutron probing) domains. However, this requires overcoming the beam angular divergence at the source. This has been attempted, either with large-scale conventional setups or with compact plasma techniques that however have the restriction of short (<1 mm) focusing distances or a chromatic behavior. Here, we show that exploiting laser-triggered, long-lasting (>50 ps), thermoelectric multi-megagauss surface magnetic (B)-fields, compact capturing, and focusing of a diverging laser-driven multi-MeV ion beam can be achieved over a wide range of ion energies in the limit of a 5° acceptance angle.

  19. Dynamical features and electric field strengths of double layers driven by currents. [in auroras

    NASA Technical Reports Server (NTRS)

    Singh, N.; Thiemann, H.; Schunk, R. W.

    1985-01-01

    In recent years, a number of papers have been concerned with 'ion-acoustic' double layers. In the present investigation, results from numerical simulations are presented to show that the shapes and forms of current-driven double layers evolve dynamically with the fluctuations in the current through the plasma. It is shown that double layers with a potential dip can form even without the excitation of ion-acoustic modes. Double layers in two-and one-half-dimensional simulations are discussed, taking into account the simulation technique, the spatial and temporal features of plasma, and the dynamical behavior of the parallel potential distribution. Attention is also given to double layers in one-dimensional simulations, and electrical field strengths predicted by two-and one-half-dimensional simulations.

  20. Quantum system driven by incoherent a.c fields: Multi-crossing Landau Zener dynamics

    NASA Astrophysics Data System (ADS)

    Jipdi, M. N.; Fai, L. C.; Tchoffo, M.

    2016-10-01

    The paper investigates the multi-crossing dynamics of a Landau-Zener (LZ) system driven by two sinusoidal a.c fields applying the Dynamic Matrix approach (DMA). The system is shown to follow one-crossing and multi-crossing dynamics for low and high frequency regime respectively. It is shown that in low frequency regime, the resonance phenomenon occurs and leads to the decoupling of basis states; the effective gap vanishes and then the complete blockage of the system. For high frequency, the system achieves multi-crossing dynamics with two fictitious crossings; the system models a Landau-Zener-Stückelberg (LZS) interferometer with critical parameters that tailor probabilities. The system is then shown to depend only on the phase that permits the easiest control with possible application in implementing logic gates.