Showing papers on "Magnetic field published in 2012"

Reviews of Plasma Physics

Abstract: Mechanisms of transverse conductivity and generation of self-consistent electric fields in strongly ionized magnetized plasma V. Rozhansky. 1. Introduction.- 2. Conductivity tensor in partially ionized plasma.- 3. Main mechanisms of perpendicular conductivity in fully ionized plasma.- 4. Acceleration of plasma clouds in an inhomogeneous magnetic field.- 5. Alfven conductivity.- 6. Perpendicular viscosity, radial current, and radial electric field in an infinite cylinder.- 7. Current systems in front of a biased electrode (flush-mounted probe) and spot of emission.- 8. Currents in the vicinity of a biased electrode that is smaller than the ion gyroradius.- 9. Neoclassical perpendicular conductivity in a tokamak.- 10. Transverse conductivity in a reversed field pinch.- 11. Modeling of electric field and currents in the tokamak edge plasma.- 12. Mechanisms of anomalous perpendicular viscosity and viscosity-driven currents.- 13. Transverse conductivity in a stochastic magnetic field.- 14. Electric fields generated in the shielding layer between hot plasma and a solid state.-- Correlations and anomalous transport models O.G. Bakunin. 1. Introduction.- 2. Turbulent diffusion and transport.- 3. Non-local effects and diffusion equations.- 4. The Corrsin conjecture.- 5. Effects of seed diffusivity.- 6. The diffusive tracer equation and averaging.- 7. The quasi-linear approximation.- 8. The diffusive renormalization.- 9. Anomalous transport and convective cells.- 10. Stochastic instability and transport.- 11. Fractal conceptions and turbulence.- 12. Percolation and scalings.- 13. Percolation and turbulent transport scalings.- 14. The temporal hierarchy of scales and correlations.- 15. The stochastic magnetic field and percolation transport.- 16. Percolation in drift flows.- 17. Multiscale flows.- 18. Subdiffusion and traps.- 19. Continuous time random walks.- 20. Fractional differential equations and scalings.- 21. Correlation and phase-space.- 22. Conclusion.

Realizing effective magnetic field for photons by controlling the phase of dynamic modulation

Abstract: By considering a resonator lattice in which the coupling constants between the resonators are harmonically modulated in time and by controlling the spatial distribution of the modulation phases, scientists introduce a scheme that can generate an effective magnetic field for photons, without the use of magneto-optical effects.

Emergent electrodynamics of skyrmions in a chiral magnet

Abstract: When an electron moves in a smoothly varying non-collinear magnetic structure, its spin orientation adapts constantly, thereby inducing forces that act both on the magnetic structure and on the electron. These forces may be described by electric and magnetic fields of an emergent electrodynamics1, 2, 3, 4. The topologically quantized winding number of so-called skyrmions—a type of magnetic whirl discovered recently in chiral magnets5, 6, 7—has been predicted to induce exactly one quantum of emergent magnetic flux per skyrmion. A moving skyrmion is therefore expected to induce an emergent electric field following Faraday’s law of induction, which inherits this topological quantization8. Here we report Hall-effect measurements that establish quantitatively the predicted emergent electrodynamics. We obtain quantitative evidence for the depinning of skyrmions from impurities (at current densities of only 106 A m−2) and their subsequent motion. The combination of exceptionally small current densities and simple transport measurements offers fundamental insights into the connection between the emergent and real electrodynamics of skyrmions in chiral magnets, and might, in the long term, be important for applications.

Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses

Abstract: The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.

The Hall effect in metals and alloys

Abstract: 1 The Dynamics of Electrons in Metals: Low-Temperature Effects...- 1.1. Theoretical Conditions.- 1.2. Definition of the Hall Effect.- 1.3. Semiclassical Electronic Motion with Electric and Magnetic Fields.- 1.3.1. Compensation in Metals.- 1.4. Quantum Effects.- 1.4.1. Quantum Oscillations.- 1.4.2. Magnetic Breakdown.- 1.5. Size Effects.- 1.5.1. Classical Size Effects.- 1.5.2. Quantum Size Effects.- 2 The Hall Effect in Limiting Cases.- 2.1. The Hall Effect in the Classical High-Field Limit.- 2.1.1. The Hall Effect with No Open Orbits.- 2.1.2. The Hall Effect with Open Orbits.- 2.1.3. Chambers' Expression for the Conductivity Tensor.- 2.2. The Hall Effect in the Low-Field Limit.- 2.2.1. The Case with Localized Magnetic Moments.- 2.2.2. The Low-Field Spin Component.- 2.3. The Hall Effect in the Two-Band Model.- 3 The Hall Effect in Nearly-Free-Electron Metals.- 3.1. Introduction.- 3.2. The Harrison Construction.- 3.3. The Examples of Al and In.- 3.3.1. Case of the High-Field Limit.- 3.3.2. Case of the Low-Field Limit.- 3.3.3. Case of the Intermediate-Field Region.- 4 The Hall Effect in Group 1B Metals.- 4.1. Introduction.- 4.2. The Hall Effect in the Pure Group 1B Metals.- 4.2.1. The Dependence of R on Magnetic Field.- 4.2.2. The Dependence of R on Temperature.- 4.3. The Hall Effect in Very Dilute Alloys.- 4.4. The Hall Effect in Alloys with B-Metal Solutes.- 5 The Hall Effect in Magnetic Metals.- 5.1. Phenomenological Aspects.- 5.2. Some Conduction-Electron Spin Interactions.- 5.3. Survey of the Theoretical Developments.- 5.3.1. Theories with Itinerant Magnetic Carriers.- 5.3.2. Theories with Localized Magnetic Carriers.- 6Experimental Techniques and the Hall Effect in Unusual Conditions.- 6.1. Introduction.- 6.2. Sources of Error.- 6.3. Methods with Electrical Contact to the Sample.- 6.3.1. dc Methods.- 6.3.2. ac Methods.- 6.4. Methods without Contacts.- 6.5. Measurements under Unusual Conditions.- 6.5.1. Measurements at High Pressures.- 6.5.2. Measurements on Liquid Metals.- 6.5.3. The Hall Effect in Superconductors.- 7 A Guide to the Literature for Metallic Elements and Binary Metallic Alloys.- Appendix A Derivation of Expression (2.59).- Appendix B Units and Conversion Factors.- Notes Added in Proof.- Author Index.

Event-by-event generation of electromagnetic fields in heavy-ion collisions

Abstract: We compute the electromagnetic fields generated in heavy-ion collisions by using the HIJING model. Although after averaging over many events only the magnetic field perpendicular to the reaction plane is sizable, we find very strong electric and magnetic fields both parallel and perpendicular to the reaction plane on the event-by-event basis. We study the time evolution and the spatial distribution of these fields. Especially, the electromagnetic response of the quark-gluon plasma can give non-trivial evolution of the electromagnetic fields. The implications of the strong electromagnetic fields on the hadronic observables are also discussed.

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet.

Abstract: The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

The QCD phase diagram for external magnetic fields

Abstract: The effect of an external (electro)magnetic field on the finite temperature transition of QCD is studied. We generate configurations at various values of the quantized magnetic flux with N f = 2 + 1 flavors of stout smeared staggered quarks, with physical masses. Thermodynamic observables including the chiral condensate and susceptibility, and the strange quark number susceptibility are measured as functions of the field strength. We perform the renormalization of the studied observables and extrapolate the results to the continuum limit using N t = 6, 8 and 10 lattices. We also check for finite volume effects using various lattice volumes. We find from all of our observables that the transition temperature T c significantly decreases with increasing magnetic field. This is in conflict with various model calculations that predict an increasing T c (B). From a finite volume scaling analysis we find that the analytic crossover that is present at B = 0 persists up to our largest magnetic fields eB ≈ 1 GeV2, and that the transition strength increases mildly up to this eB ≈ 1 GeV2.

Herschel view of the Taurus B211/3 filament and striations: Evidence of filamentary growth?

Abstract: We present first results from the Herschel Gould Belt survey for the B211/L1495 region in the Taurus molecular cloud. Thanks to their high sensitivity and dynamic range, the Herschel images reveal the structure of the dense, star-forming filament B211 with unprecedented detail, along with the presence of striations perpendicular to the filament and generally oriented along the magnetic field direction as traced by optical polarization vectors. Based on the column density and dust temperature maps derived from the Herschel data, we find that the radial density profile of the B211 filament approaches a power-law behavior {\rho} {\propto} r^(-2.0{\pm}0.4) at large radii and that the temperature profile exhibits a marked drop at small radii. The observed density and temperature profiles of the B211 filament are in good agreement with a theoretical model of a cylindrical filament undergoing gravitational contraction with a polytropic equation of state: P {\propto} {\rho}^{\gamma} and T {\propto} {\rho}^({\gamma}-1), with {\gamma}=0.97{\pm}0.01<1 (i.e. not strictly isothermal). The morphology of the column density map, where some of the perpendicular striations are apparently connected to the B211 filament, further suggests that the material may be accreting along the striations onto the main filament. The typical velocities expected for the infalling material in this picture are ~0.5-1 km/s, which are consistent with the existing kinematical constraints from previous CO observations.

Fixing device, and image forming apparatus

Abstract: Fixing device includes a first cylindrical rotating member having an inner peripheral surface and an outer peripheral surface; a second rotating member which is brought into contact with the first rotating member; magnetic field generation units for generating a magnetic field, the magnetic field generation units being provided at a predetermined distance from the inner or outer peripheral surface of the first rotating member; and a heat generation member that generates heat by a magnetic field, the heat generation member being provided so as to be opposed to the magnetic field generation units across the first rotating member and to be in contact with the first rotating member, having a thickness larger than a skin depth, and containing a magnetic metal material.

Temperature- and magnetic-field-dependent longitudinal spin relaxation in nitrogen-vacancy ensembles in diamond.

Abstract: We present an experimental study of the longitudinal electron-spin relaxation time (T1) of negatively charged nitrogen-vacancy (NV) ensembles in diamond. T1 was studied as a function of temperature from 5 to 475 K and magnetic field from 0 to 630 G for several samples with various NV and nitrogen concentrations. Our studies reveal three processes responsible for T1 relaxation. Above room temperature, a two-phonon Raman process dominates; below room temperature, we observe an Orbach-type process with an activation energy of 73(4) meV, which closely matches the local vibrational modes of the NV center. At yet lower temperatures, sample dependent cross-relaxation processes dominate, resulting in temperature independent values of T1 from milliseconds to minutes. The value of T1 in this limit depends sensitively on the magnetic field and can be tuned by more than 1 order of magnitude.

Spin-wave modes and their intense excitation effects in Skyrmion crystals.

Abstract: We theoretically study spin-wave modes and their intense excitations activated by microwave magnetic fields in the Skyrmion-crystal phase of insulating magnets by numerically analyzing a two-dimensional spin model using the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with frequencies of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GHz}$ are found for in-plane ac magnetic field where distribution of the out-of-plane spin components circulates around each Skyrmion core. Directions of the circulations are opposite between these two modes, and hence the spectra exhibit a salient dependence on the circular polarization of irradiating microwave. A breathing-type mode is also found for an out-of-plane ac magnetic field. By intensively exciting these collective modes, melting of the Skyrmion crystal accompanied by a redshift of the resonant frequency is achieved within nanoseconds.

Identification of kinetic alfven wave turbulence in the solar wind

Abstract: The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfven waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB ∥|/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfven waves with nearly perpendicular wavevectors.

GRMHD Simulations of Magnetized Advection Dominated Accretion on a Non-Spinning Black Hole: Role of Outflows

Abstract: We present results from two long-duration GRMHD simulations of advection-dominated accretion around a non-spinning black hole. The first simulation was designed to avoid significant accumulation of magnetic flux around the black hole. This simulation was run for a time of 200,000GM/c^3 and achieved inflow equilibrium out to a radius \sim90GM/c^2. Even at this relatively large radius, the mass outflow rate \dot{M}_{out} is found to be only 60% of the net mass inflow rate \dot{M}_{BH} into the black hole. The second simulation was designed to achieve substantial magnetic flux accumulation around the black hole in a magnetically arrested disc. This simulation was run for a shorter time of 100,000GM/c^3. Nevertheless, because the mean radial velocity was several times larger than in the first simulation, it reached inflow equilibrium out to a radius \sim170GM/c^2. Here, \dot{M}_{out} becomes equal to \dot{M}_{BH} at r\sim 160GM/c^2. Since the mass outflow rates in the two simulations do not show robust convergence with time, it is likely that the true outflow rates are lower than our estimates. The effect of black hole spin on mass outflow remains to be explored. Neither simulation shows strong evidence for convection, though a complete analysis including the effect of magnetic fields is left for the future.

Experiments and modeling of iron-particle-filled magnetorheological elastomers

Abstract: Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated rubbers whose mechanical properties are altered by the application of external magnetic fields. Due to their coupled magnetoelastic response, MREs are finding an increasing number of engineering applications. In this work, we present a combined experimental and theoretical study of the macroscopic response of a particular MRE consisting of a rubber matrix phase with spherical carbonyl iron particles. The MRE specimens used in this work are cured in the presence of strong magnetic fields leading to the formation of particle chain structures and thus to an overall transversely isotropic composite. The MRE samples are tested experimentally under uniaxial stresses as well as under simple shear in the absence or in the presence of magnetic fields and for different initial orientations of their particle chains with respect to the mechanical and magnetic loading direction. Using the theoretical framework for finitely strained MREs introduced by Kankanala and Triantafyllidis (2004) , we propose a transversely isotropic energy density function that is able to reproduce the experimentally measured magnetization, magnetostriction and simple shear curves under different prestresses, initial particle chain orientations and magnetic fields. Microscopic mechanisms are also proposed to explain (i) the counterintuitive effect of dilation under zero or compressive applied mechanical loads for the magnetostriction experiments and (ii) the importance of a finite strain constitutive formulation even at small magnetostrictive strains. The model gives an excellent agreement with experiments for relatively moderate magnetic fields but has also been satisfactorily extended to include magnetic fields near saturation.

Experimental Realization of a Magnetic Cloak

Abstract: Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

Spin and valley quantum Hall ferromagnetism in graphene

Abstract: The extra states sometimes observed in graphene’s quantum Hall characteristics have been presumed to be the result of broken SU(4) symmetry. Magnetotransport measurements of high-quality graphene in a tilted magnetic field finally prove this is indeed the case.

Protostellar disk formation and transport of angular momentum during magnetized core collapse

Abstract: Context. Theoretical studies of collapsing clouds have found that even a relatively weak magnetic field may prevent the formation of disks and their fragmentation. However, most previous studies have been limited to cases where the magnetic field and the rotation axis of the cloud are aligned.Aims. We study the transport of angular momentum, and its effects on disk formation, for non-aligned initial configurations and a range of magnetic intensities.Methods. We perform three-dimensional, adaptive mesh, numerical simulations of magnetically supercritical collapsing dense cores using the magneto-hydrodynamic code Ramses. We compute the contributions of all the relevant processes transporting angular momentum, in both the envelope and the region of the disk. We clearly define centrifugally supported disks and thoroughly study their properties.Results. At variance with earlier analyses, we show that the transport of angular momentum acts less efficiently in collapsing cores with non-aligned rotation and magnetic field. Analytically, this result can be understood by taking into account the bending of field lines occurring during the gravitational collapse. For the transport of angular momentum, we conclude that magnetic braking in the mean direction of the magnetic field tends to dominate over both the gravitational and outflow transport of angular momentum. We find that massive disks, containing at least 10% of the initial core mass, can form during the earliest stages of star formation even for mass-to-flux ratios as small as three to five times the critical value. At higher field intensities, the early formation of massive disks is prevented. Conclusions. Given the ubiquity of Class I disks, and because the early formation of massive disks can take place at moderate magnetic intensities, we speculate that for stronger fields, disks will form later, when most of the envelope will have been accreted. In addition, we speculate that some observed early massive disks may actually be outflow cavities, mistaken for disks by projection effects.

Strong Coulomb drag and broken symmetry in double-layer graphene

Abstract: C oulomb drag is a frictional coupling between electric currents flowing in spatially separated conducting layers. It is caused by interlayer electron‐electron interactions. Previously, only the regime of weak (d l) to intermediate (d l) coupling could be studied experimentally, where d is the interlayer separation and l is the characteristic distance between charge carriers. Here we use graphene‐boron-nitride heterostructures withd down to 1 nm to probe Coulomb drag in the limitd l such that the two Dirac liquids effectively nest within the same plane, but can still be tuned and measured independently. The strongly interacting regime reveals many unexpected features. In particular, although drag vanishes because of electron‐hole symmetry when either layer is neutral, we often find drag strongest when both layers are neutral. Under this circumstance, drag is positive in zero magnetic field but changes its sign and rapidly grows in strength with field. The drag remains strong at room temperature. The broken electron‐hole symmetry is attributed to mutual polarization of closely spaced interacting layers.

Foreign object detection in wireless energy transfer systems

Abstract: The disclosure features apparatus, methods, and systems for wireless power transfer that include a power source featuring at least one resonator, a power receiver featuring at least one resonator, a first detector featuring one or more loops of conductive material and configured to generate an electrical signal based on a magnetic field between the power source and the power receiver, a second detector featuring conductive material, and control electronics coupled to the first and second detectors, where during operation, the control electronics are configured to measure the electrical signal of the first detector and compare the measured electrical signal of the first detector to baseline electrical information for the first detector to determine information about whether debris is positioned between the power source and the power receiver.

Manipulation and control of molecular beams.

Abstract: A study was conducted to demonstrate the manipulation of molecular beams with electric and magnetic fields. Seeded pulsed supersonic expansions were employed to conduct the investigations. The conservative forces exerted further downstream by the electric and magnetic fields enabled the researchers to manipulate and control the shape and the position of the distribution in the six-dimensional phase-space. All of the original experimental geometries were devised to create strong magnetic or electric field gradients to efficiently deflect particles from the beam axis. It was demonstrated that a detailed understanding of the influence of the external field on the energy level structure of the molecules was required for the manipulation of molecules with electric or magnetic fields.

Magnetic-field-dependent photodynamics of single NV defects in diamond: an application to qualitative all-optical magnetic imaging

Abstract: Magnetometry and magnetic imaging with nitrogen–vacancy (NV) defects in diamond rely on the optical detection of electron spin resonance (ESR). However, this technique is inherently limited to magnetic fields that are weak enough to avoid electron spin mixing. Here, we focus on the high off-axis magnetic field regime where spin mixing alters the NV defect spin dynamics. We first study, in a quantitative manner, the dependence of the NV defect optical properties on the magnetic field vector B. Magnetic-field-dependent time-resolved photoluminescence (PL) measurements are compared to a seven-level model of the NV defect that accounts for field-induced spin mixing. The model reproduces decreases in (i) ESR contrast, (ii) PL intensity and (iii) excited level lifetime with an increasing off-axis magnetic field. We next demonstrate that these effects can be used to perform all-optical imaging of the magnetic field component |B⊥| orthogonal on the NV defect axis. Using a scanning NV defect microscope, we map the stray field of a magnetic hard disc through both PL and fluorescence lifetime imaging. This all-optical method for high magnetic field imaging at the nanoscale might be of interest in the field of nanomagnetism, where samples producing fields in excess of several tens of milliteslas are typically found.

Energy spectrum and quantum Hall effect in twisted bilayer graphene

Abstract: We investigate the electronic structure and the quantum Hall effect in twisted bilayer graphenes with various rotation angles in the presence of magnetic field. Using a low-energy approximation, which incorporates the rigorous interlayer interaction, we computed the energy spectrum and the quantized Hall conductivity in a wide range of magnetic field from the semiclassical regime to the fractal spectrum regime. In weak magnetic fields, the low-energy conduction band is quantized into electronlike and holelike Landau levels at energies below and above the van Hove singularity, respectively, and the Hall conductivity sharply drops from positive to negative when the Fermi energy goes through the transition point. In increasing magnetic field, the spectrum gradually evolves into a fractal band structure called Hofstadter's butterfly, where the Hall conductivity exhibits a nonmonotonic behavior as a function of Fermi energy. The typical electron density and magnetic field amplitude characterizing the spectrum monotonically decrease as the rotation angle is reduced, indicating that the rich electronic structure may be observed in a moderate condition.

Magnetic stripes and skyrmions with helicity reversals

Abstract: It was recently realized that topological spin textures do not merely have mathematical beauty but can also give rise to unique functionalities of magnetic materials. An example is the skyrmion—a nano-sized bundle of noncoplanar spins—that by virtue of its nontrivial topology acts as a flux of magnetic field on spin-polarized electrons. Lorentz transmission electron microscopy recently emerged as a powerful tool for direct visualization of skyrmions in noncentrosymmetric helimagnets. Topologically, skyrmions are equivalent to magnetic bubbles (cylindrical domains) in ferromagnetic thin films, which were extensively explored in the 1970s for data storage applications. In this study we use Lorentz microscopy to image magnetic domain patterns in the prototypical magnetic oxide–M-type hexaferrite with a hint of scandium. Surprisingly, we find that the magnetic bubbles and stripes in the hexaferrite have a much more complex structure than the skyrmions and spirals in helimagnets, which we associate with the new degree of freedom—helicity (or vector spin chirality) describing the direction of spin rotation across the domain walls. We observe numerous random reversals of helicity in the stripe domain state. Random helicity of cylindrical domain walls coexists with the positional order of magnetic bubbles in a triangular lattice. Most unexpectedly, we observe regular helicity reversals inside skyrmions with an unusual multiple-ring structure.

Hall Viscosity and Electromagnetic Response

Abstract: We show that, for Galilean invariant quantum Hall states, the Hall viscosity appears in the electromagnetic response at finite wave numbers $q$. In particular, the leading $q$ dependence of the Hall conductivity at small $q$ receives a contribution from the Hall viscosity. The coefficient of the ${q}^{2}$ term in the Hall conductivity is universal in the limit of strong magnetic field.

Room temperature giant and linear magnetoresistance in topological insulator Bi2Te3 nanosheets

Abstract: Topological insulators, a new class of condensed matter having bulk insulating states and gapless metallic surface states, have demonstrated fascinating quantum effects. However, the potential practical applications of the topological insulators are still under exploration worldwide. We demonstrate that nanosheets of a ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ topological insulator several quintuple layers thick display giant and linear magnetoresistance. The giant and linear magnetoresistance achieved is as high as over 600% at room temperature, with a trend towards further increase at higher temperatures, as well as being weakly temperature-dependent and linear with the field, without any sign of saturation at measured fields up to 13 T. Furthermore, we observed a magnetic field induced gap below 10 K. The observation of giant and linear magnetoresistance paves the way for 3D topological insulators to be useful for practical applications in magnetoelectronic sensors such as disk reading heads, mechatronics, and other multifunctional electromagnetic applications.

Exchange biasing of magnetoelectric composites

Abstract: Magnetoelectric composite materials are of interest for sensitive magnetic-field sensors. The realization of a magnetoelectric composite that does not require an applied external magnetic field, but instead relies on internal bias via exchange coupling, promises sensitive sensors even for small magnetic fields.

Electric-field control of magnetic domain wall motion and local magnetization reversal

Abstract: Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to electric-field driven spintronics.