Hall Resistance Research Articles

Spin–Orbit Torque-Driven Memristor in L10 FePt Systems with Nanoscale-Thick Layers for Neuromorphic Computing

In this study, a memristor driven by spin–orbit torque (SOT) is realized in the nanoscale thickness L10 FePt systems with high perpendicular magnetization anisotropy (PMA). Due to the domain nucleation and expansion driven by current pulses, multilevel Hall resistance states can be continuously tuned by current density, where the memristive states are retained by the domain wall pinning effects. The properties of multilevel resistance states for samples with different structures are associated with the magnitude of field-like torque, and the larger efficiency of field-like torque enhances multiple resistance characteristics. Furthermore, the stable memristive behavior is obtained in the FePt/MgO/NiFe heterostructure. Finally, a three-layer multilayer perceptron (MLP) neural network is built to perform the MNIST handwritten digit recognition task based on the device’s memristive behaviors, and the accuracy of weight update can reach up to 88.55%. These results pave the way for the application of L10 FePt nanomaterials in neuromorphic computing.

Martensitic Transformation Temperatures and Hall Effect in Ni47-xMn41+xIn12 (x = 0, 1, 2) Alloys.

At present, the question of the relationship between the characteristic martensitic transformation temperatures (MTT) and the electronic parameters of a system has not been fully studied. In the present work, an attempt to establish a similar relationship using the example of the concentration of charge carriers, n, was made. The field dependences of Hall resistivity ρH and magnetization M of the magnetocaloric Ni47-xMn41+xIn12 (x = 0, 1, 2) alloys were measured at T = 4.2 K and in magnetic fields of up to 80 kOe. The MTT were obtained from the temperature dependences of electrical resistivity and magnetization. It was observed that the MTT correlate strongly with both the valence electron concentration e/a and the electronic transport characteristics, which are the coefficient of the normal (NHE) R0 and anomalous (AHE) RS Hall effect and the concentration of charge carriers n.

Investigation of the mechanism of the anomalous Hall effects in Cr2Te3/(BiSb)2(TeSe)3 heterostructure

The interplay between ferromagnetism and the non-trivial topology has unveiled intriguing phases in the transport of charges and spins. For example, it is consistently observed the so-called topological Hall effect (THE) featuring a hump structure in the curve of the Hall resistance (Rxy) vs. a magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a topological insulator (TI). The origin of the hump structure is still controversial between the topological Hall effect model and the multi-component anomalous Hall effect (AHE) model. In this work, we have investigated a heterostructure consisting of BixSb2−xTeySe3−y (BSTS) and Cr2Te3 (CT), which are well-known TI and two-dimensional FM, respectively. By using the so-called “minor-loop measurement”, we have found that the hump structure observed in the CT/BSTS is more likely to originate from two AHE channels. Moreover, by analyzing the scaling behavior of each amplitude of two AHE with the longitudinal resistivities of CT and BSTS, we have found that one AHE is attributed to the extrinsic contribution of CT while the other is due to the intrinsic contribution of BSTS. It implies that the proximity-induced ferromagnetic layer inside BSTS serves as a source of the intrinsic AHE, resulting in the hump structure explained by the two AHE model.Graphical abstract

Hall Effect Anisotropy in the Paramagnetic Phase of Ho0.8Lu0.2B12 Induced by Dynamic Charge Stripes.

A detailed study of charge transport in the paramagnetic phase of the cage-cluster dodecaboride Ho0.8Lu0.2B12 with an instability both of the fcc lattice (cooperative Jahn-Teller effect) and the electronic structure (dynamic charge stripes) was carried out at temperatures 1.9-300 K in magnetic fields up to 80 kOe. Four mono-domain single crystals of Ho0.8Lu0.2B12 samples with different crystal axis orientation were investigated in order to establish the singularities of Hall effect, which develop due to (i) the electronic phase separation (stripes) and (ii) formation of the disordered cage-glass state below T*~60 K. It was demonstrated that a considerable intrinsic anisotropic positive component ρanxy appears at low temperatures in addition to the ordinary negative Hall resistivity contribution in magnetic fields above 40 kOe applied along the [001] and [110] axes. A relation between anomalous components of the resistivity tensor ρanxy~ρanxx1.7 was found for H||[001] below T*~60 K, and a power law ρanxy~ρanxx0.83 for the orientation H||[110] at temperatures T < TS~15 K. It is argued that below characteristic temperature TS~15 K the anomalous odd ρanxy(T) and even ρanxx(T) parts of the resistivity tensor may be interpreted in terms of formation of long chains in the filamentary structure of fluctuating charges (stripes). We assume that these ρanxy(H||[001]) and ρanxy(H||[110]) components represent the intrinsic (Berry phase contribution) and extrinsic (skew scattering) mechanism, respectively. Apart from them, an additional ferromagnetic contribution to both isotropic and anisotropic components in the Hall signal was registered and attributed to the effect of magnetic polarization of 5d states (ferromagnetic nano-domains) in the conduction band of Ho0.8Lu0.2B12.

Experimental Investigation and DFT Study of Tin-Oxide for Its Application as Light Absorber Layer in Optoelectronic Devices

Experimental investigation of SnO film has been performed to analyse the effect of oxygen ratio variation on its optical and electrical properties. The oxygen composition in SnO film has been varied and correspondingly its extinction coefficient and band gap have been obtained. Also, density functional theory (DFT) study of SnO film has been carried out in order to obtain its optical properties such as extinction coefficient and corresponding bandgap. The experimental and theoretical trends related to its optical properties are in good agreement with each other. The SnO film may be used as a prospective light absorber layer in various opto-electronic sensor devices, solar cell in particular, and its optoelectronic properties can be tuned with change in oxygen mole fraction ratio of SnO films which are detailed out in this paper. Further, some important electrical parameters of such as Hall mobility, carrier concentration and resistivity of SnO films for its different Sn and O ratios have been obtained. The investigation of tunable optical and electrical properties of SnO thin film will pave the way for a wide range of opto-electronic devices.

Anisotropy and delayed relaxation of the anomalous Hall effect in GdFeCo/Ir/GdFeCo synthetic ferrimagnets

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation. Keywords: spin Hall effect, spin-orbit torque, magnetic relaxation, domain wall dynamics.

Measurement of the Resistor Load Effect Using the Combined Quantum Hall Resistance

Measurement of the Resistor Load Effect Using the Combined Quantum Hall Resistance

Анизотропия и медленная релаксация аномального эффекта Холла в синтетических ферримагнетиках GdFeCo/Ir/GdFeCo

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation.

Performance and Stability Assessment of Graphene-Based Quantum Hall Devices for Resistance Metrology

For more than three decades, GaAs/AlGaAs quantum Hall (QH) devices are being used as standards for the realization of the dc resistance unit ohm. Recently, the outstanding performance of graphene-based quantum Hall resistance (QHR) devices signifies an immense potential for a transition from GaAs to graphene-based devices in the field of resistance and also impedance metrology, with equally good accuracy. The recent developments in epitaxial graphene (EG) quality and fabrication of EG-based doped QHR devices and their improved performance tested in several national metrology institutes (NMIs) all over the world have demonstrated a more user-friendly dissemination of the unit ohm since they can be operated at relatively low magnetic fields and higher temperatures in comparison to existing GaAs-based devices. In this work, we have extensively tested the temporal stability of the magneto-transport properties of our graphene QH devices fabricated in the PTB and the resistance quantization performance by means of repeated high-accuracy cryogenic current comparator (CCC) measurements with several devices. Together with interlaboratory comparisons between NMI’s, the performance and stability assessment demonstrate the readiness of the technology for calibration purposes in resistance metrology.

Anomalous Hall effect in Pt/La<sub>0.67</sub>Sr<sub>0.33</sub>MnO<sub>3</sub> heterojunctions

Many emergent and novel phenomena occur in nonmagnetic/ferromagnet heterostructures. In particular, Pt/ferromagnet heterostructures where the Pt has strong spin-orbit coupling and thus can convert spin current into charge current, has attracted a great attention recently. The anomalous Hall effect (AHE) has been found in many Pt/ferromagnet heterostructures. However, the underlying physics remains elusive, so it is necessary to find more heterostructures in order to provide more experimental data. In this work, we investigate anomalous Hall resistances (AHRs) in Pt thin films sputtered on epitaxial La<sub>0.67</sub>Sr<sub>0.33</sub>MnO<sub>3</sub> (LSMO) ferromagnetic films. High-quality Pt/LSMO heterojunctions are fabricated by pulsed laser deposition and RF-magnetron sputtering. The physical properties of LSMO films are characterized by the measurements of magnetic and transport properties. The AHR mainly contributed by Pt in the Pt/LSMO heterojunction increases sharply with temperature decreasing and changes its sign below 40 K. Furthermore, the AHR decreases sharply with the increase of Pt thickness. Those facts suggest that the ferromagnetism of Pt originates from interface due to magnetic proximity effect. Interestingly, this heterojunction can exhibit possible signal of topological Hall effect under low applied magnetic field. The above results provide an experimental basis for further understanding the interactions between electron spin and charge transport in nonmagnetic/ferromagnetic heterostructures.

Anomalous metallic state regulated by magnetic field at LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface

<sec>Since the discovery of two-dimensional electron gas with high mobility at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface, many physical properties such as two-dimensional superconductivity, magnetism and spin-orbit coupling have been widely studied. The origin of the transition from quantum superconductor to metal at zero temperature in two-dimensional superconductor is still an open problem, which has been discussed intensely. According to the conventional theory, when the temperature is close to zero, the superconductor-insulator transition can be observed by applying a magnetic field or magnetic field effect of disorder, and the ground state should be superconducting or insulating.</sec><sec>However, when Jaeger et al. (Jaeger H M, Haviland D B, Orr B G, Goldman A M <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevB.40.182">1989 <i>Phys. Rev. B</i> <b>40</b> 182</ext-link>) studied the relationship between superconductivity evolution and thickness and temperature in a superconducting granular film, they found that there exists an intermediate metal state that can destroy the direct transition between superconducting and insulating. The intermediate metal state is characterized by the existence of saturation resistance at superconducting transition temperature, and the corresponding ground state is called anomalous metallic state. In addition to the saturation of resistance at low temperature, the characteristics of an anomalous metallic state also include the linear current-voltage (<i>I-V</i>) characteristics in the low current range, the giant positive magnetoresistance (MR), the vanishing of Hall resistance (<i>R</i><sub><i>xy</i></sub>), and the tuning capability adjusted by changing a variety of parameters including degree of disorder, gate voltage and magnetic field.</sec><sec>In this work, we systematically investigate the electrical transport properties of LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface in a perpendicular magnetic field at low temperature. The <i>R-T</i> curves and the <i>I-V</i> characteristics in zero magnetic field show that LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface is in a superconducting state. However, after a small magnetic field is applied, the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface has the characteristics of resistance saturation at low temperature, linear <i>I-V</i> characteristics, giant positive MR, abnormal Hall response, indicating the clear characteristics of an anomalous metallic state. The sample undergoes a transition from quantum superconductor to metal at temperatures approaching to zero.</sec><sec>In addition, we observe that the anomalous metallic state in an unusually large region under the action of magnetic field, and our main observations are summarized in the <i>H-T</i> phase diagram. By analyzing the relationship between the resistance of the anomalous metallic regime and the magnetic field, and the vanish of Hall resistance, we infer that the anomalous metallic state observed in LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface can be explained by Bose metal model. According to our findings, the magnetic field regulated LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface appears as a special platform to study the details of anomalous metallic state in a controllable way.</sec>

Large negative magnetoresistance beyond chiral anomaly in topological insulator candidate CeCuAs<sub>2</sub> with spin-glass-like behavior

<p>Large negative magnetoresistance (NMR), an important property for spintronics, requires experimental realization owing to the lack of suitable structural motifs. Herein, a remarkable NMR of up to -15% under 9 T at 2 K is demonstrated in a 112-type topological insulator candidate CeCuAs<sub>2</sub> single crystal containing an As square net. Due to the presence of Dirac points coming from both the As square net and Ce�CCu�CAs layer in the paramagnetic state of CeCuAs<sub>2</sub>, the possibility of chiral anomaly is examined and eliminated by investigating magnetoresistance (MR) with different magnetic field configurations and angle-dependant MR, which show no specific restriction on the configurations under the applied magnetic fields. Upon investigation of the anisotropic magnetism, a spin-glass-like behavior with <i>T</i><sub><i>f</i></sub> ~ 4.5 K is observed in CeCuAs<sub>2</sub>, indicating that the large NMR could be attributed to the spin-dependent scattering induced by the possible spin-glass state. Hall resistivity exhibits a multiband feature and hole-dominated transport properties, corresponding well with the calculated band structure. This study not only offers a new building block for large NMR but also serves as a guide for the investigating the interplay among transport properties, topology, and magnetism, and it is expected to broaden the research on spintronics.</p>

Deterministic field-free switching of perpendicular magnetization by spin–orbit torques originating from in-plane magnetized Co2MnAl

Spin–orbit torques (SOTs) induced magnetization switching in a perpendicularly magnetized CoFeB film deposited on a Ti/in-plane magnetized Co2MnAl stack was investigated. Deterministic switching of the CoFeB magnetization was demonstrated by applying a current pulse to the stack in a direction parallel or antiparallel to the Co2MnAl magnetization. We found that the hysteresis loops of the anomalous Hall resistance for CoFeB under the constant current is shifted in the out-of-plane magnetic field axis direction depending on the directions of both the applied current and the magnetization of Co2MnAl. The shift amount exhibits an almost linear increase as the current magnitude increases. These results are consistent with the effects caused by SOTs originating from spin currents having an in-plane polarization orthogonal to the Co2MnAl magnetization.

Probing antiferromagnetism in exfoliated Fe3GeTe2 using magneto-transport measurements.

Among the material class of van der Waals magnets, Fe3GeTe2 (FGT) has emerged as one of the most studied owing to features such as its relatively high Curie temperature, metallic nature, and large spin polarization. Though most studies only investigate its explicitly ferromagnetic properties, FGT is also predicted to have an antiferromagnetic phase in the out-of-plane direction emerging at temperatures below 150 K, leading to a blend of ferromagnetic and antiferromagnetic ordering. Here, we explore the emergence of this phase and its effects in FGT/h-BN heterostructures using magneto-transport measurements. The devices' anomalous Hall and magnetoresistance responses exhibit a complex trend with temperature that is consistent with multiple magnetic phases. In addition to the usual out-of-plane sensing, we also rotate the applied field to the in-plane direction and observe behavior resembling the planar topological Hall effect. Intriguingly, this response follows a similar temperature trend to the out-of-plane response. We also use the out-of-plane anomalous Hall response to show that, at sufficiently low temperatures, both positive and negative field-cooling results in an increased saturation Hall resistance. Such a field-cooling divergence is consistent with antiferromagnetic ordering resulting in a spin-glass like state in the sample. In addition to providing insight into one of the most exciting candidate materials for 2D magnetic devices, our work demonstrates the power of magneto-transport measurements to probe complex behavior in vdW magnets where common magnetometry techniques used on bulk samples may not be viable.

An anomalous Hall effect in edge-bonded monolayer graphene.

An anomalous Hall effect (AHE) is usually presumed to be absent in pristine graphene due to its diamagnetism. In this work, we report that a gate-tunable Hall resistance Rxy can be obtained in edge-bonded monolayer graphene without an external magnetic field. In a perpendicular magnetic field, Rxy consists of a sum of two terms: one from the ordinary Hall effect and the other from the AHE (RAHE). Plateaus of Rxy ∼ 0.94h/3e2 and RAHE ∼ 0.88h/3e2 have been observed while the longitudinal resistance Rxx decreases at a temperature of 2 K, which are indications of the quantum version of the AHE. At a temperature of 300 K, Rxx shows a positive, giant magnetoresistance of ∼177% and RAHE still has a value of ∼400 Ω. These observations indicate the existence of a long-range ferromagnetic order in pristine graphene, which may lead to new applications in pure carbon-based spintronics.

Phase-change control of anomalous Hall effect in ferromagnetic MnBi thin films

We have experimentally demonstrated a phase change control of Hall effects in ferromagnetic MnBi thin films at room temperature. Two distinct ferromagnetic phases, i.e., a low-temperature phase and a quenched high-temperature phase, were obtained by slow-cooling or quenching the sputtered MnBi films. The measurement of Hall effects showed that the anomalous Hall resistivity has different coercivity and remanence between the two phases. By optimizing the Mn/Bi ratio, it is even possible to repeatedly switch the sign of Hall resistivity, which could be utilized as a concept of phase-change memory based on ferromagnetic transport.

Spin–orbit proximity effect in Bi/Co multilayer: The role of interface scattering

The spin–orbit proximity effect is the raise of spin–orbit coupling at a layer near to the interface with a strong spin–orbit material. It has been seen in several system such as graphene and ferromagnetic layers. The control of the spin–orbit coupling can be a pathway to discover novel and exotic phases in superconductor and semimetallic systems. Here, we study the magnetoelectrical transport, i.e., magnetoresistance and anomalous Hall effect, in cobalt/bismuth multilayers looking for traces of spin–orbit proximity effect and evaluate the origin of such effect. Our results point for an increase of spontaneous magnetic anisotropy of resistivity and anomalous Hall resistivity at very low thicknesses of cobalt. The analysis of the anomalous Hall resistivity indicate that the bismuth layers change the scattering mechanism of Hall effect to the extrinsic skew-scattering type, indicating that the spin–orbit proximity effect could be related to the elastic scattering of cobalt free carriers by bismuth sites at the interface.

Antisite disorder and Berry curvature driven anomalous Hall effect in the spin gapless semiconducting Mn2CoAl Heusler compound

Spin gapless semiconductors exhibit a finite band gap for one spin channel and a closed gap for another spin channel, and they have emerged as a new state of magnetic materials with a great potential for spintronic applications. The first experimental evidence for spin gapless semiconducting behavior was observed in an inverse Heusler compound ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Here, we report a detailed investigation of the crystal structure and anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ using experimental and theoretical studies. The analysis of the high-resolution synchrotron x-ray diffraction data shows antisite disorder between Mn and Al atoms within the inverse Heusler structure. The temperature-dependent resistivity shows semiconducting behavior and follows Mooij's criteria for disordered metal. The scaling behavior of the anomalous Hall resistivity suggests that the anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ is primarily governed by an intrinsic mechanism due to the Berry curvature in momentum space. The experimental intrinsic anomalous Hall conductivity (AHC) is found to be $\ensuremath{\sim}35$ S/cm, which is considerably larger than the theoretically predicted value for ordered ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Our first-principles calculations conclude that the antisite disorder between Mn and Al atoms enhances the Berry curvature and hence the value of intrinsic AHC, which is in very good agreement with the experiment.

Modulating saturation magnetization and topological Hall resistivity of flexible ferrimagnetic Mn4N films by bending strains

The ferrimagnetic rare-earth-free Mn4N films are considered as a good candidate in spintronics due to its low saturation magnetization (MS) and high Néel temperature. Here, Mn4N films are directly deposited on flexible mica to investigate strain-related magnetic and electronic transport properties. The MS variation of 11.0 nm Mn4N film reaches 453% at tensile strain of radius of curvature (ROC) = 2 mm. Bending strains cannot affect anomalous Hall resistivity and magnetoresistance. However, the topological Hall resistivity of 147.0 nm Mn4N film increases by 58% at tensile strain of ROC = 5 mm due to frustrated exchange interactions. The flexible Mn4N films have great potential applications in flexible magnetic sensor and strain gauge due to strain modulated MS, resistance, and stable magnetoresistance.

Thermoelectric properties and low-temperature transport anomalies in the p -type full-Heusler compounds Fe2−xCrxVAl

Elemental substitutions are successfully used for the optimization of thermoelectric properties of a specific material; it requires, however, a deep understanding of its impact. For this purpose, based on the full-Heusler material ${\mathrm{Fe}}_{2}\mathrm{VAl}$, various compounds (${\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{VAl}$) were synthesized by substituting Cr for Fe in a wide range from $x$ = 0.005 up to $x$ = 0.4. X-ray diffraction analysis revealed full solubility of Cr for all concentrations. Bulk thermoelectric properties, such as electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall resistivity were measured from 2 K up to 780 K, and all results were discussed in the context of the outcome of density functional calculations. Transport anomalies, resembling Kondo scattering, were observed for all samples below 30 K. Finally, an increased effective number of valence electrons of 7 for Cr was phenomenologically determined, which revealed good agreement with other $p$-type doping studies of ${\mathrm{Fe}}_{2}\mathrm{VAl}$.