Anisotropic Hall Research Articles

Orientation-dependent THz emission in non-collinear antiferromagnetic Mn3Sn and Mn3Sn-based heterostructures

We observe terahertz (THz) emission in Mn3Sn, Mn3Sn/Pt, and Mn3Sn/Co films excited by a femtosecond laser pulse. In the Mn3Sn film and Mn3Sn/Pt heterostructures, the THz emission originates from both magnetic-dipole and superdiffusive transient spin current with different proportions. Our results unambiguously demonstrate that THz emission can be controlled by the spin structure of Mn3Sn. The (0001)-orientated Mn3Sn produces stronger THz emission than the (112¯0)-orientated counterpart because for the latter one, only half of the kagome planes of Mn3Sn are parallel to the field, which can be controlled by the external magnetic field. In the Mn3Sn/Co heterostructure, the Mn3Sn layer serves as a spin-to-charge converter. The (112¯0)-orientated Mn3Sn emits larger THz signals than (0001)-orientated Mn3Sn due to the anisotropic inverse spin Hall effect, determined by the relative relation between spin, charge current, and the kagome plane of Mn3Sn. The spin structure dependent THz radiations in noncollinear antiferromagnetic metal Mn3Sn provide versatility for both spintronics and THz optics.

Magnetic anisotropy and topological Hall effect in the trigonal chromium tellurides Cr5Te8

Nontrivial spin configurations tend to bring about novel transport properties in magnetic materials. We report the magnetic anisotropy and topological Hall effect in trigonal ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ single crystals. Below the magnetic transition temperature, the spins are canted ferromagnetic aligned along $c$ axis with antiferromagnetic components in $ab$ plane. This peculiar magnetic structure gives rise to the anisotropic anomalous Hall effect (AHE) in ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$. When the current flows along the $ab$ plane, ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ exhibits a conventional AHE depicted by skew scattering mechanism. On the other hand, the topological Hall effect appears with current along $c$ axis, which stems from the noncoplanar spin configuration with scalar spin chirality during the spin flop process.

Intrinsic spin currents in ferromagnets

First principles calculations show that electric fields applied to ferromagnets generate spin currents flowing perpendicularly to the electric field. Reduced symmetry in these ferromagnets enables a wide variety of such spin currents. However, the total spin current is approximately the sum of a magnetization-independent spin Hall current and an anisotropic spin anomalous Hall current. Intrinsic spin currents are not subject to dephasing, enabling their spin polarizations to be misaligned with the magnetization, which enables the magnetization-independent spin Hall effect. The spin Hall conductivity and spin anomalous Hall conductivities of transition metal ferromagnets are comparable to those found in heavy metals, opening new avenues for efficient spin current generation in spintronic devices.

Normal-state properties of the quasi-one-dimensional superconductor Ta4Pd3Te16

We examined the physical properties of the quasi-one-dimensional superconductor Ta4Pd3Te16 in the normal state by detailed measurements of susceptibility, in-plane anisotropic resistivity, magnetoresistance, Hall resistivity, and Seebeck coefficient. The large Wilson ratio, as inferred from normal-state susceptibility, indicates strong electron–electron interaction. The Hall and Seebeck coefficients show not only significant temperature-dependent behavior, indicating the multiband effect, but also an obvious anomaly around T1 = 40 K. Analyses of both the Hall resistivity and thermopower using a two-band model indicate that the electrons dominate the electrical transport at low temperatures. Our results imply that it is the quantum fluctuations of the charge order taking place in the temperature range 30–50 K that may result in the abnormal normal-state properties of Ta4Pd3Te16.

Angular evolution of thickness-related unidirectional magnetoresistance in Co/Pt multilayers

We report the magnetoresistance of Co/Pt superlattices having thickness gradients at different orientations relative to an applied current. We measure the magnetoresistance at a fixed field as a function of the out-of-plane field angle, and find a unidirectional magnetoresistance (UMR) in addition to the expected anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Specifically, the UMR signal is the difference in resistance between the (out-of-plane) +z and −z field orientation. The magnitude of UMR is minimized when the thickness gradient is parallel to the applied current and maximized when the gradient is nearly perpendicular to the current. The results imply the possibility of an alternative source of UMR in Co/Pt multilayers in addition to the previously considered anomalous Hall effect.

Unconventional Anisotropic Even-Denominator Fractional Quantum Hall State in a System with Mass Anisotropy.

The fractional quantum Hall state (FQHS) observed at a half-filled Landau level in an interacting two-dimensional electron system (2DES) is among the most exotic states of matter as its quasiparticles are expected to be Majorana excitations with non-Abelian statistics. We demonstrate here the unexpected presence of such a state in a novel 2DES with a strong band-mass anisotropy. The FQHS we observe has unusual characteristics. While its Hall resistance is well quantized at low temperatures, it exhibits highly anisotropic in-plane transport resembling compressible stripe or nematic charge-density-wave phases. More striking, the anisotropy sets in suddenly below a critical temperature, suggesting a finite-temperature phase transition. Our observations highlight how anisotropy modifies the many-body phases of a 2DES, and should further fuel the discussion surrounding the enigmatic even-denominator FQHS.

Guidelines for the evaluation of magnetotransport parameters from measurements on thin strip-shaped samples of bulk metallic ferromagnets with finite residual resistivity

In the present paper, the formulae are summarized for the evaluation of magnetoresistance data from the measured field dependence of the resistivity obtained on thin strip-shaped samples of ferromagnetic metals and alloys with a finite residual resistivity for various relative orientations of the magnetic field and the measuring current. This sample shape simplifies the problem in that the specimen shape can be approximated with a general ellipsoid for which the demagnetizing fields can be estimated with sufficient accuracy from tabulated data. First, the various experimental configurations are defined for the magnetotransport measurements. Then, the formulae for describing the field-dependence of the resistivity in metallic ferromagnets with finite residual resistivity are discussed, by treating separately the case of low and high temperatures, low and high being defined with respect to the Curie temperature of the ferromagnet under study. The extraction of some magnetotransport parameters (anisotropic magnetoresistance and Hall effect) often requires an extrapolation of the measured data to zero magnetic field or zero magnetic induction and this will also be discussed. Finally, a summary of the relations between the experimentally measured magnetotransport parameters and those derived from theoretical calculations will be given.

Giant anisotropic magnetoresistance and planar Hall effect in Sr0.06Bi2Se3

We report the observation of giant negative anisotropic magnetoresistance and planar Hall effect in superconducting topological insulator Sr0.06Bi2Se3. It is found that giant anisotropic magnetoresistance and planar Hall effect with non-zero transverse voltage are developed by tilting the in-plane magnetic field away from the direction of the electrical current. Quantitative analyses of the measured data suggest that the observed anisotropic magnetoresistance and planar Hall effect originate from the chiral anomaly behavior of the material. The large anisotropic magnetoresistance and planar Hall effect demonstrate that this material has potential to be utilized in magnetoresistive devices with low power consumption.

Geometry of flux attachment in anisotropic fractional quantum Hall states

Fractional quantum Hall (FQH) states are known to possess an internal metric degree of freedom that allows them to minimize their energy when contrasting geometries are present in the problem (e.g., electron band mass and dielectric tensor). We investigate the internal metric of several incompressible FQH states by probing its response to band mass anisotropy using infinite DMRG simulations on a cylinder geometry. We test and apply a method to extract the internal metric of a FQH state from its guiding center structure factor. We find that the response to band mass anisotropy is approximately the same for states in the same Jain sequence, but changes substantially between different sequences. We provide a theoretical explanation of the observed behavior of primary states at filling $\nu = 1/m$ in terms of a minimal microscopic model of flux attachment.

Free and forced Barkhausen noises in magnetic thin film based cross-junctions

Barkhausen noise, driven by thermal fluctuations in stationary magnetic field, and Barkhausen jumps, driven by sweeping magnetic field, are demonstrated to be effects of different orders of magnitude. The critical magnetic field for domain walls depinning, followed by avalanched and irreversible magnetization jumps, is determined. Magnetoresistive response of NiFe/M/NiFe (M = Au, Ta, Ag) trilayers to stationary and sweeping magnetic field is studied by means of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements. Thermal fluctuations result in local and reversible changes of magnetization of the layers in thin film magnetic junctions, while the sweeping magnetic field results in reversible and irreversible avalanched domain motion, dependently on the ratio between the values of sweeping magnetic field and domain wall depinning field. The correlation between AMR and PHE responses to Barkhausen jumps is studied. The value of this correlation is found to be dependent on the α angle between the directions of magnetic field and current path.

Publisher’s Note: Investigating Anisotropic Quantum Hall States with Bimetric Geometry [Phys. Rev. Lett. 119 , 146602 (2017)

This corrects the article DOI: 10.1103/PhysRevLett.119.146602.

Anisotropic anomalous Hall effect in triangular itinerant ferromagnet Fe3GeTe2

Magnetic frustrated materials are of great interest for their novel spin-dependent transport properties. We report an anisotropic anomalous Hall effect in the triangular itinerant ferromagnet ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. When the current flows along the $ab$ plane, ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ exhibits the conventional anomalous Hall effect below the Curie temperature ${T}_{c}$, which can be depicted by Karplus--Luttinger theory. On the other hand, the topological Hall effect shows up below ${T}_{c}$ with current along the $c$ axis. The enhancement of Hall resistivity can be attributed to the chiral effect during the spin-flop process.

Investigating Anisotropic Quantum Hall States with Bimetric Geometry.

We construct a low energy effective theory of anisotropic fractional quantum Hall (FQH) states. We develop a formalism similar to that used in the bimetric approach to massive gravity, and apply it to describe Abelian anisotropic FQH states in the presence of external electromagnetic and geometric backgrounds. We derive a relationship between the shift, the Hall viscosity, and a new quantized coupling to anisotropy, which we term anisospin. We verify this relationship by numerically computing the Hall viscosity for a variety of anisotropic quantum Hall states using the density matrix renormalization group. Finally, we apply these techniques to the problem of nematic order and clarify certain disagreements that exist in the literature about the meaning of the coefficient of the Berry phase term in the nematic effective action.

Semitransparent anisotropic and spin Hall magnetoresistance sensor enabled by spin-orbit torque biasing

We demonstrate an ultrathin and semitransparent anisotropic and spin Hall magnetoresistance sensor based on NiFe/Pt heterostructures. The use of a spin-orbit torque effective field for transverse biasing allows us to reduce the total thickness of the sensors down to 3–4 nm, thereby leading to the semitransparency. Despite the extremely simple design, the spin-orbit torque effective field biased NiFe/Pt sensor exhibits levels of linearity and sensitivity comparable to those of sensors using more complex linearization schemes. In a proof-of-concept design using a full Wheatstone bridge comprising four sensing elements, we obtained a sensitivity up to 202.9 mΩ Oe−1, a linearity error below 5%, and a detection limit down to 20 nT. The transmittance of the sensor is over 50% in the visible range.

Six-fold crystalline anisotropic magnetoresistance in the (111) LaAlO3/SrTiO3 oxide interface

We measured the magnetoresistance of the 2D electron liquid formed at the (111) LaAlO$_3$/SrTiO$_3$ interface. The hexagonal symmetry of the interface is manifested in a six-fold crystalline component appearing in the anisotropic magnetoresistance (AMR) and planar Hall data, which agree well with symmetry analysis we performed. The six-fold component increases with carrier concentration, reaching 15% of the total AMR signal. Our results suggest the coupling between higher itinerant electronic bands and the crystal as the origin of this effect and demonstrate that the (111) oxide interface is a unique hexagonal system with tunable magnetocrystalline effects.

Generalized Pseudopotentials for the Anisotropic Fractional Quantum Hall Effect.

We generalize the notion of Haldane pseudopotentials to anisotropic fractional quantum Hall (FQH) systems that are physically realized, e.g., in tilted magnetic field experiments or anisotropic band structures. This formalism allows us to expand any translation-invariant interaction over a complete basis, and directly reveals the intrinsic metric of incompressible FQH fluids. We show that purely anisotropic pseudopotentials give rise to new types of bound states for small particle clusters in the infinite plane, and can be used as a diagnostic of FQH nematic order. We also demonstrate that generalized pseudopotentials quantify the anisotropic contribution to the effective interaction potential, which can be particularly large in models of fractional Chern insulators.

Magnetic-proximity-induced magnetoresistance on topological insulators

We theoretically study the magnetoresistance (MR) of two-dimensional massless Dirac electrons as found on the surface of three-dimensional topological insulators (3D TIs) that is capped by a ferromagnetic insulator (FI). We calculate charge and spin transport by Kubo and Boltzmann theories, taking into account the ladder-vertex correction and the in-scattering due to normal and magnetic disorder. The induced exchange splitting is found to generate an electric conductivity that depends on the magnetization orientation, but its form is very different from both the anisotropic and spin Hall MR. The in-plane MR vanishes identically for non-magnetic disorder, while out-of-plane magnetizations cause a large MR ratio. On the other hand, we do find an in-plane MR and planar Hall effect in the presence of magnetic disorder aligned with the FI magnetization. Our results may help understand recent transport measurements on TI|FI systems.

Anisotropic electronic states in the fractional quantum Hall regime

Recent experiments indicate the presence of new anisotropic fractional quantum Hall states at regimes not anticipated before. These experiments raise many fundamental questions regarding the inner nature of the electronic system that leads to such anisotropic states. Interplay between electron mass anisotropy and electron-electron correlation effects in a magnetic field can create a rich variety of possibilities. Several anisotropic electronic states ranging from anisotropic quantum Hall liquids to anisotropic Wigner solids may stabilize due to such effects. The electron mass anisotropy in a two-dimensional electron gas effectively leads to an anisotropic Coulomb interaction potential between electrons. An anisotropic interaction potential may strongly influence the stability of various quantum phases that are close in energy since the overall stability of an electronic system is very sensitive to local order. As a result there is a possibility that various anisotropic electronic phases may emerge even in the lowest Landau level in regimes where one would not expect them. In this work we study the state with filling factor 1/6 in the lowest Landau level, a state which is very close to the critical filling factor where the liquid-solid transition takes place. We investigate whether an anisotropic Coulomb interaction potential is able to stabilize an anisotropic electronic liquid state at this filling factor. We describe such an anisotropic state by means of a liquid crystalline wave function with broken rotational symmetry which can be adiabatically connected to the actual wave function for the corresponding isotropic phase. We perform quantum Monte Carlo simulations in a disk geometry to study the properties of the anisotropic electronic liquid state under consideration. The findings indicate stability of liquid crystalline order in presence of an anisotropic Coulomb interaction potential. The results are consistent with the existence of an anisotropic electronic liquid state in the lowest Landau level.

Reduced models accounting for parallel magnetic perturbations: gyrofluid and finite Larmor radius–Landau fluid approaches

Reduced models are derived for a strongly magnetized collisionless plasma at scales which are large relative to the electron thermal gyroradius and in two asymptotic regimes. One corresponds to cold ions and the other to far sub-ion scales. By including the electron pressure dynamics, these models improve the Hall reduced magnetohydrodynamics (MHD) and the kinetic Alfvén wave model of Boldyrev et al. (2013 Astrophys. J., vol. 777, 2013, p. 41), respectively. We show that the two models can be obtained either within the gyrofluid formalism of Brizard (Phys. Fluids, vol. 4, 1992, pp. 1213–1228) or as suitable weakly nonlinear limits of the finite Larmor radius (FLR)–Landau fluid model of Sulem and Passot (J. Plasma Phys., vol 81, 2015, 325810103) which extends anisotropic Hall MHD by retaining low-frequency kinetic effects. It is noticeable that, at the far sub-ion scales, the simplifications originating from the gyroaveraging operators in the gyrofluid formalism and leading to subdominant ion velocity and temperature fluctuations, correspond, at the level of the FLR–Landau fluid, to cancellation between hydrodynamic contributions and ion finite Larmor radius corrections. Energy conservation properties of the models are discussed and an explicit example of a closure relation leading to a model with a Hamiltonian structure is provided.

Giant Anomalous Hall Effect in the Chiral Antiferromagnet Mn3Ge

The external field control of antiferromagnetism is a significant subject both for basic science and technological applications. As a useful macroscopic response to detect magnetic states, the anomalous Hall effect (AHE) is known for ferromagnets, but it has never been observed in antiferromagnets until the recent discovery in Mn3Sn. Here we report another example of the AHE in a related antiferromagnet, namely, in the hexagonal chiral antiferromagnet Mn3Ge. Our single-crystal study reveals that Mn3Ge exhibits a giant anomalous Hall conductivity $|\sigma_{xz}|$ ~ 60 per ohm per cm at room temperature and approximately 380 per ohm per cm at 5 K in zero field, reaching nearly half of the value expected for the quantum Hall effect per atomic layer with Chern number of unity. Our detailed analyses on the anisotropic Hall conductivity indicate that in comparison with the in-plane-field components $|\sigma_{xz}|$ and $|\sigma_{zy}|$, which are very large and nearly comparable in size, we find $|\sigma_{yx}|$ obtained in the field along the c axis is found to be much smaller. The anomalous Hall effect shows a sign reversal with the rotation of a small magnetic field less than 0.1 T. The soft response of the AHE to magnetic field should be useful for applications, for example, to develop switching and memory devices based on antiferromagnets.