The In-Plane Anisotropy of the Hall Anomaly in BSCCO Whiskers

No Heading The Hall effect in single crystal BSCCO whiskers has been measured, with the current flowing along the a and b crystallographic directions. The ab-plane anisotropy of the Hall anomaly is found. The magnitude of the anomaly, as well as its anisotropy, are strongly affected by the structural defects.PACS Numbers: 74.72.Hs, 74.80.Dm, 72.15.Gd.     

Planar Hall Effect in Epitaxial Fe Layers on GaAs(001) and GaAs(113)A Substrates

For ferromagnetic layers with in-plane magnetic fields, the longitudinal and transverse resistivities probe the magnetization orientation and its reversal via a spin-orbit-related resistance anisotropy. In the planar Hall effect, we found new contributions in Fe films on GaAs(001) and GaAs(113)A substrates, which cannot be understood within the conventionally used model of the resistance anisotropy. To understand its origin, we adopted a method to determine the orientation of the magnetization from magnetoresistance data. As a result, we were able to identify a symmetric fourfold and a twofold saturated asymmetric in-plane Hall effect for the Fe/GaAs(001) and Fe/GaAs(113)A systems, respectively. Since these new contributions almost perfectly compensate the planar Hall effect based on the resistance anisotropy, we argue about an intrinsic origin of the planar Hall effect in terms of scattering at spin textures within the Fe films.     

Anisotropy of electron-phonon Umklapp scattering and the hall coefficient: Model results for an alkali metal

The large anisotropy in the scattering time and hence the anisotropy in the Hall coefficient at low temperatures is shown to be a consequence of the anisotropic nature of Umkalpp electron-phonon scattering.     

Stress Induced Enhancement of Magnetization Reversal Process of DyFeCo Films With Perpendicular Magnetization

A method to control magnetization reversal in magnetic films with perpendicular magnetic anisotropy was demonstrated in this study. Alloys of rare earth-transition metals (RE-TM) with their extremely large anomalous Hall coefficient and relatively large magnetostriction constants were suitable to observe the stress induced anisotropy using anomalous Hall effect. Stress applied to the RE-TM thin films significantly reduced the perpendicular coercivity and nucleation field of the film in comparison with that of the film in stress-free condition. Dy_x(Fe₉₀Co₁₀)_1-x thin films revealed large Hall voltage and remarkable change in the coercivity under the mechanical tensile stress     

Anisotropic electrical properties of a eutectic InSb + MnSb composite

The temperature dependences of the Hall coefficient, electrical conductivity, and thermoelectric power for a eutectic InSb + MnSb composite have been studied in the temperature range from 80 to 700 K. Electron-microscopic results confirm that the system is in a two-phase state and consists of an InSb matrix and needle-like MnSb metallic inclusions. The inclusions are surrounded by interfacial zones ~0.3 μm in width. The observed anisotropy in the transport properties of the material is attributed to a short-circuiting effect of the metallic inclusions. Interpretation in terms of effective medium theory with allowance for the interfacial zones suggests that they make a significant contribution to the electrical conductivity anisotropy.     

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

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.     

Influence of grain size on plastic anisotropy in low-carbon steels

Abstract

The plastic anisotropy of textured polycrystalline materials has often been described by the r-value – the ratio of the width and thickness strains. The measured –-values of low-carbon steels for larger grain sizes are found to agree reasonably well with values calculated by the authors on the basis of the Taylor theory. In steels with small grain sizes the absolute level of r-value is not predicted particularly well, but the variation with testing direction (planar anisotropy) is predicted well. Calculations showed that in all cases the instantaneous anisotropy parameter ρ as a function of strain, when extrapolated to zero strain, agreed with the value predicted from the measured texture. With small grain sizes a sudden drop in ρ was observed with increasing strain, this being complete within 2–3% of strain. The influence of grain size on the plastic anisotropy can be understood if it is assumed that the grain boundary resistance parameter K in the Hall–Petch relation depends on the contraction ratio – in a different way from the Taylor factor M(q). Measurements of the Hall–Petch constant in plane-strain compression support this view.

MST/84     

Magnetic Anisotropies and (Ga,Mn)As‐based Spintronic Devices

In this Review, we discuss the rich anisotropic properties of the ferromagnetic semiconductor (Ga,Mn)As, and their implications in transport studies. We review the various sources and types of anisotropy seen in the material, discuss its magnetization reversal process, and demonstrate how basic transport properties, such as resistivity and Hall measurements, can be used as very sensitive tools to investigate the magnetization properties of the material. We also discuss how the magnetic anisotropy, coupled with large spin–orbit coupling, leads to an anisotropy in the transport density of states, which in turn leads to fundamentally novel behavior such as tunneling anisotropic magnetoresistance (TAMR).     

Memory Behavior of the Planar Hall Effect in Ferromagnetic (Ga, Mn)As/GaAs Superlattices

The planar Hall effect (PHE) has been studied in short period (Ga,?Mn)As/GaAs superlattices displaying a ferromagnetic interlayer coupling between the magnetic layers. Complex dependence of the PHE on applied magnetic field is explained by taking into account the magnetocrystalline anisotropy of the (Ga,?Mn)As layers, which results from biaxial compressive strain in the layers epitaxially grown on GaAs. Two-state behavior of the planar Hall resistance at zero magnetic field provides its usefulness for applications in nonvolatile memory devices. In addition, using an appropriate sequence of applied magnetic fields four different states of the planar Hall resistance, suitable for quaternary memory devices, can be realized owing to the formation of a stable multidomain structure in the Hall bar.     

Anisotropic magnetoresistance and planar hall effect in GaAs structure with Mn-delta-doped layer

Anisotropic magnetoresistance and the planar Hall effect are discovered and studied in a GaAs structure containing a single layer delta-doped with manganese to its total content equivalent to 0.18 monolayer. The Mn-delta-doped structures were obtained by combining the methods of metalorganic chemical vapor deposition (MOCVD) in a hydride system and laser ablation of solid targets in a common technological cycle. The character of the dependence of the Hall resistance on the magnetic field is indicative of the presence of a planar cubic magnetic anisotropy.     

Electrical properties of magnesium-intercalated InSe

Indium monoselenide single crystals have been intercalated with magnesium from the vapor phase, and their electron concentration, electron Hall mobility, anisotropic electrical conductivity, and thermoelectric power have been measured as functions of temperature. The effects of vacuum annealing and Mg intercalation on the electrical properties of InSe are analyzed. Magnesium intercalation has the strongest effect on the conductivity anisotropy in the crystals.     

Use of the asymmetric planar hall resistance of an Fe film for possible multi-value memory device applications

Systematic planar Hall measurements have been performed on a ferromagnetic Fe film grown on a standard (001) GaAs substrate at room temperature. The angular dependence of the planar Hall effect revealed the presence of both four-fold (cubic) and two-fold (uniaxial) anisotropies in the 7 nm thick Fe film. The dominance of the four-fold symmetric anisotropy, however, provided four magnetic easy axes near the (100) direction, which results in a two step switching phenomenon in the magnetization reversal process. An interesting asymmetric hysteresis loop was observed in the planar Hall resistance (PHR) when the turning point of the field scan is set at the value in the region of the second transition. The intermediate resistance states appearing in the asymmetric PHR loop were understood in terms of mutli-domain structures formed during the second switching of magnetization. Such multi-domain structure of the Fe film showing robust time stability provided additional Hall resistance states, which can be used for multi-valued memory device applications.     

Growth,characterization and electrical anisotropy in GaTe—a natural semiconducting superlattice

GaTe is a III–VI semiconductor which has layered structure with large anisotropy in electrical properties. Growth of single crystals by the Bridgman technique permitted the measurement of thermoelectric power in orthogonal directions from which the anisotropy of hole effective masses were determined for the first time. From resistivity and Hall effect measurements the carrier activation energies and scattering mechanisms between 10–300°K were found. Study of the temperature dependence of conductivity revealed a variety of conduction mechanisms including weak localization below 20°K, hopping conduction between 20–50 K and band conduction in and across the layer planes atT>70 K. Weak localization was confirmed through observation of negative magnetoresistance. TheI–V characteristics showed quantized behaviour due to tunneling across potential barriers, which may be due to stacking faults between layer planes as observed by TEM studies.     

Optical spin Hall effects in plasmonic chains

Observation of optical spin Hall effects (OSHEs) manifested by a spin-dependent momentum redirection is presented. The effect occurring solely as a result of the curvature of the coupled localized plasmonic chain is regarded as the locally isotropic OSHE, while the locally anisotropic OSHE arises from the interaction between the optical spin and the local anisotropy of the plasmonic mode rotating along the chain. A wavefront phase dislocation was observed in a circular curvature, in which the dislocation strength was enhanced by the locally anisotropic effect.     

A Note on the Galvanomagnetic and Thermoelectric Coefficients of Tetragonal Crystalline Materials

The independent Hall, magnetoresistive, and thermoelectric coefficients for a tetragonal crystal have been tabulated and geometric configurations for their experimental measurement have been determined. These coefficients have been calculated on assumptions of several simple ellipsoidal models, in the range of nondegenerate statistics. Implications of experimentally observed isotropy or anisotropy of transport properties on the structure of the energy surfaces are noted.     

Anisotropic Quantum Hall Liquids at Intermediate Magnetic Fields

The observation of significant magnetoresistance anisotropy of a two-dimensional electronic system in very clean GaAs/AlGaAs heterostructure samples in presence of moderately large perpendicular magnetic fields is a striking example of novel anisotropic behavior in the quantum Hall regime. Anisotropy appears to be the strongest at quantum Hall even-denominator filled states for filling factors ν>4 where several Landau levels are occupied. A possible explanation of these findings is due to the existence of charge density waves that are known to cause interesting phase transitions at high Landau levels. An alternative explanation of this phenomenon is to argue that the strongly correlated electronic system has stabilized in an orientationally ordered anisotropic quantum Hall liquid state. Quantum Monte Carlo calculations with a translationally invariant wave function in which rotation symmetry is broken indicate that this might be the case.     

Electronic transport in two stacked graphene monolayers

We report on interlayer and lateral electronic transport measurements in two stacked graphene monolayers which have separate electrical contacts. The current-voltage characteristic across the two layers shows linear Ohmic behavior at zero magnetic field. At high magnetic fields, sequences of quantum Hall plateaus of the overlap region with filling factors 4, 8, and 12 are observed which can be explained by equilibration of the edge channel potentials of the individual graphene layers. An anomaly is observed at total filling factors ±2 in the overlap region. The I-V characteristic for interlayer transport turns nonlinear, and the Hall signal vanishes, indicating a magnetic field induced electrical decoupling of the two graphene layers.     

The anisotropy field Hkof thin ferromagnetic films, measured by magnetoresistive methods

Several methods are described based on the magnetoresistance and the planar Hall effect by which the anisotropy field strength Hkmay be determined. The Hkvalues deduced from the different methods are discussed especially with respect to their dependence upon the magnetization ripple and on distinctH_{c}/H_{k}ratios. Although some of the reviewed methods yield Hkvalues not influenced by the ripple hysteresis, only the planar Hall effect method and the modified Kobelev method enable rapid measurements or continuous recording of Hk. These methods are, therefore, found to be especially suitable for measurements of the Hkrelaxation during annealing experiments.     

Hall Coefficient of Equilibrium Supercurrents Flowing inside Superconductors

We study augmented quasiclassical equations of superconductivity with the Lorentz force, which is missing from the standard Ginzburg–Landau and Eilenberger equations. It is shown that the magnetic Lorentz force on equilibrium supercurrents induces a finite charge distribution and the resulting electric field to balance the Lorentz force. An analytic expression is obtained for the corresponding Hall coefficient of clean type-II superconductors with simultaneously incorporating the Fermi-surface and gap anisotropies. It has the same sign and magnitude at zero temperature as the normal state for an arbitrary pairing, having no temperature dependence specifically for s-wave pairing. The gap anisotropy may bring about a considerable temperature dependence in the Hall coefficient and can lead to its sign change as a function of temperature, as exemplified for a model d-wave pairing with a two-dimensional Fermi surface. The sign change may be observed in some high-T _c superconductors.     

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr_1+xTe₂ with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr_1.53Te₂ with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.