Large Anisotropic Magnetoresistance Research Articles

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Antiferromagnetic spintronics: From metals to functional oxides

Antiferromagnetic spintronics exploits unique properties of antiferromagnetic materials to create new and improved functionalities in future spintronic applications. Here, we briefly review the experimental efforts in our group to unravel spin transport properties in antiferromagnetic materials. Our investigations were initially focused on metallic antiferromagnets, where the first evidence of antiferromagnetic spin-transfer torque was discovered. Because of the lack of metallic antiferromagnets, we then shifted towards antiferromagnetic Mott insulators, where a plethora of transport phenomena was found. For instance, we observed a very large anisotropic magnetoresistance, which can be used to detect the magnetic state of an antiferromagnet. We also observed reversible resistive switching and now provide unequivocal evidence that the resistive switching is associated with structural distortions driven by an electric field. Our findings support the potential of electrically controlled functional oxides for various memory technologies.

Surface anisotropic magnetoresistance in the antiferromagnetic semiconductor CrSb2

We report on large anisotropic magnetoresistance (AMR) in surface states of an antiferromagnetic semiconductor $\mathrm{Cr}{\mathrm{Sb}}_{2}$. The low-temperature measurement of angle-dependent magnetoresistance (ADMR) is performed in high magnetic fields up to 24 T. At 1.4 K, where the surface conduction is dominant, ADMR exhibits clear twofold symmetry consistent with AMR due to magnetic-field-induced change of antiparallel magnetic structure. The AMR magnitude reaches 9.3% despite the small angle dependence of the net magnetic moments, suggesting strong spin-orbit interaction in the surface conduction layer.

Polycrystalline La0.66Gd0.04Ca0.3MnO3 for magnetic-response applications: concurrent anisotropic magnetoresistance and magneto-transport under a low magnetic field

Polycrystalline La0.66Gd0.04Ca0.3MnO3 with a large anisotropic magnetoresistance (−30%) value achieved under a magnetic field of 1 T.

Berry curvature induced antisymmetric in-plane magneto-transport in magnetic Weyl EuB6

In-plane transport properties, including anisotropic magnetoresistance (AMR) and planar Hall effect (PHE), are of great interest in electrical transport and spintronic applications. Unconventional transport behavior emerging from the topological physics has been intensively studied and very much desired. In this study, a large AMR of −18% at a very low magnetic field of 0.2 T is observed in a soft magnetic Weyl semimetal EuB6 based on the characteristics of both high magnetization and large magnetoresistance. Furthermore, the intrinsic antisymmetric AMR and PHE are unambiguously observed and interpreted as the modification in conductivity owing to the Berry curvature in a tilted Weyl system instead of the out-of-plane magnetic field component. Our study provides a strategy for low-magnetic-field applications of large AMR and enriches the transport physics of spintronic devices.

Demonstration of giant anisotropic magnetoresistance, high spin filtering efficiency, and negative differential resistance in nanodevices based on oxygen doped black arsenic phosphorus nanoribbons

We investigate the electrical properties of pure black arsenic phosphorus and oxygen atom doped monolayer black arsenic phosphorus using first principle calculations. The density of states results reveals that oxygen atom doped black arsenic phosphorus is a magnetic semiconductor. The current-voltage characteristic curves of oxygen atom doped monolayer black arsenic phosphorus nanoribbons based devices have been determined using the non-equilibrium Green's function associated with the density function theory. The results demonstrate that the device made of doped black arsenic phosphorus has a substantial current difference between armchair and zigzag black arsenic phosphorus nanoribbons, implying that the device made of nanoribbons has a large anisotropic magnetoresistance. The spin filtering efficiency is nearly 100% at some bias voltage levels, and the transmission spectrums of the associated bias voltage window can explain the significant negative differential resistance. Furthermore, these findings suggest that oxygen atom doped black arsenic-phosphorus nanoribbons are suitable for spintronic devices.

Planar Hall effect and large anisotropic magnetoresistance in a topological superconductor candidate Cu0.05PdTe2

We report the observation of a large anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in a topological superconducting candidate Cu0.05PdTe2. The AMR and PHE data in Cu0.05PdTe2 can be well explained by the semiclassical theory, confirming that the magneto-transport behaviors of the Cu0.05PdTe2 superconductor are related to its topological nature. The AMR ratio in Cu0.05PdTe2 is one order of magnitude larger than those in traditional ferromagnetic metals. The present results suggest that Cu0.05PdTe2 is a promising material in future magnetoresistive devices with low power consumption.

Spin-Momentum Locking Induced Anisotropic Magnetoresistance in Monolayer WTe2.

Monolayer WTe2 is predicted to be a quantum spin Hall insulator (QSHI), and its quantized edge transport has recently been demonstrated. However, one of the essential properties of a QSHI, spin-momentum locking of the helical edge states, has yet to be experimentally validated. Here, we measure and observe gate-controlled anisotropic magnetoresistance (AMR) in monolayer WTe2 devices. Electrically tuning the Fermi energy into the band gap, a large in-plane AMR is observed and the minimum of the in-plane AMR occurs when the applied magnetic field is perpendicular to the current direction. In line with the experimental observations, the theoretical predictions based on the band structure of monolayer WTe2 demonstrate that the AMR effect originates from spin-momentum locking in the helical edge states of monolayer WTe2. Our findings reveal that the spin quantization axis of the helical edge states in monolayer WTe2 can be precisely determined from AMR measurements.

The correlation between anisotropic magnetoresistance and phase separation in La0.4Pr0.3Ca0.3MnO3/NGO films

In this paper, the relationship between anisotropic magnetoresistance (AMR) and phase separation in La0.4Pr0.3Ca0.3MnO3 (LPCMO)/NGO films has been investigated. The films were grown on orthorhombic NGO (001) single crystal substrates by the pulsed laser deposition (PLD) method. The as-grown films were ex-situ annealed at 780 °C for 30 min in 400 mTorr oxygen pressures to clarify the effects of annealing on magnetoresistance (MR) and AMR. The X-ray diffraction (XRD) pattern confirmed that the substrate distance from the target is a key parameter for strong preferred crystallographic orientation growth. The temperature-dependent resistivity of the as-grown films revealed the coexistence of ferromagnetic and antiferromagnetic phases and the frozen glassy state at low temperatures. In the film with significant phase coexistence, the considerably large AMR and MR values of about 140% and 100% are achieved under the 5 kOe magnetic field. By tuning the target-substrate distance, different crystalline orientations of LPCMO can be obtained, and as a result, the metal-insulator transition (MIT) temperature, MR, and AMR can be modified.

Silver addition in polycrystalline La0.7Ca0.3MnO3: Large magnetoresistance and anisotropic magnetoresistance for manganite sensors

Perovskite manganite La1−yCayMnO3 has giant and tunable magnetoresistance (MR) and anisotropic magnetoresistance (AMR), still the following two challenges are of great concern: (i) large magnetic field is required for achieving excellent electromagnetic performance; and (ii) metal–insulator (M–I) transition temperature (TMI) significantly different from room temperature; both these factors limit its practical applications. In this study, polycrystalline La0.7Ca0.3MnO3:Agx composites were fabricated by sol–gel and solid–phase addition methods, and large MR and AMR (84.1% and 32%, respectively) were obtained at near room temperature in 1 T magnetic field when x = 0.2. Evidently, it was found that the grain boundary scattering effects were weakened due to Ag−addition, which resulted in an enhancement of MR. The tremendously high AMR value was attributed to the anisotropic spin–orbit coupling (SOC) effect. The electron–scattering and small–polaron hopping models were used to successfully fit the resistivity data, which support the SOC effect under different magnetic fields that enhance the AMR. These results support further development of the perovskite manganese oxide with potential applications in manganite devices.

Temperature Dependence of the Anisotropic Magnetoresistance of the Metallic Antiferromagnet Fe2As

Electrical readout of metallic antiferromagnet (AFM) memories is typically realized by measuring the anisotropic magnetoresistance (AMR), but the mechanisms for enhanced AMR are not yet established. We study AMR of single crystals of AFM ${\mathrm{Fe}}_{2}\mathrm{As}$ from $T=5$ K to above the N\'eel temperature, ${T}_{N}\ensuremath{\approx}353$ K. With an applied magnetic field $B$ rotating in the (001) plane, we observe a peak-to-peak AMR change of 1.3% for $B>1$ T at $T=5$ K, one order of magnitude larger than reported in $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$, a widely studied candidate for AFM spintronics. The AMR varies strongly with temperature, decreasing by a factor of approximately 10 at $T\ensuremath{\approx}200$ K. Our results suggest that large AMR in easy-plane AFMs may require N\'eel temperatures that greatly exceed room temperature.

Electrical and thermal transport in antiferromagnet-superconductor junctions

We demonstrate that antiferromagnet-superconductor (AF-S) junctions show qualitatively different transport properties than normal metal-superconductor (N--S) and ferromagnet-superconductor (F-S) junctions. We attribute these transport features to presence of two new scattering processes in AF--S junctions, i.e., specular reflection of holes and retroreflection of electrons. Using the Blonder-Tinkham-Klapwijk formalism, we find that the electrical and thermal conductance depend nontrivially on antiferromagnetic exchange strength. Furthermore, we show that the interplay between the N\'eel vector direction and the interfacial Rashba spin-orbit coupling leads to a large anisotropic magnetoresistance. The unusual transport properties make AF--S interfaces unique among the traditional condensed-matter-system-based superconducting junctions.

Efficient Control of Stochastic Switching via Spin Pumping in Antiferromagnetic Structures

Electrically controlled switching in an antiferromagnet (AFM), utilizing a currentless mechanism, is theoretically examined at finite temperatures. The structure consists of a metallic AFM with biaxial magnetic anisotropy sandwiched between a ferromagnetic spin filter and a semiconductor Schottky junction in a two-terminal pillar configuration. The calculations show that the torque necessary for the desired ${90}^{\ensuremath{\circ}}$ rotation of the N\'eel vector between two easy axes can be provided efficiently by pumping spin-polarized electrons into and out of the AFM through the metallic ferromagnetic layer. Consideration of thermal fluctuations illustrates the stochastic nature of the switching, whose probability distribution can be tailored by the electrical signal pulse as well as by the device dimensions. Detection of the N\'eel-vector state following this rotation may also be achieved straightforwardly via the large anisotropic magnetoresistance of the biaxial antiferromagnetic material. These properties, along with an ultrafast switching speed and a low energy requirement, are expected to be well suited for applications in nonvolatile memory and probabilistic computing.

Experimental observation of large tunneling anisotropic magnetoresistance in a magnetic tunnel junction without heavy metals

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

Pressure‐Induced Superconductivity in Topological Semimetal Candidate TaTe4

AbstractQuasi‐1D transition metal tetrachalcogenide TaTe4 exhibits charge‐density waves (CDW), large anisotropic magnetoresistance, and nontrivial Berry phase at ambient pressure. Pressure‐induced superconductivity is reported in pristine TaTe4 via electrical transport experiments. It is shown that the superconductivity, first observed at Pc ≈ 8.0 GPa, is robust up to 50.6 GPa. No structural transition is detected up to 21.4 GPa, which indicates the tetragonal structure of TaTe4 is stable across Pc. Hall resistivity measurements reveal that both electron and hole densities increase sharply upon compression up to Pc and level off at higher pressures, suggesting that the emergent superconductivity of pressurized TaTe4 can be attributed to the enhancement of the charge carrier density. This finding may provide a new platform to investigate the interplay among superconducting, CDW, and topological orders.

Strong magnetoresistance modulation by Ir insertion in a Ta/Ir/CoFeB trilayer

The spin-orbit torque (SOT) switching of trilayers with two heavy-metal layers on the same side of the ferromagnetic metal layer was studied, realizing tunable spin Hall angle, domain-wall motion, and Dzyaloshinskii-Moriya interaction. However, systematic research on the magnetoresistance in such structures is still lacking. In this work, we investigate the anisotropic magnetoresistance (AMR) and the spin Hall magnetoresistance (SMR) by inserting an ultrathin Ir layer (${t}_{\mathrm{Ir}}\ensuremath{\le}1.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) into Ta/CoFeB. The Ir layer with the thickness larger than 0.4 nm can transform the AMR from positive to negative, which is attributed to the electronic structure of Ir. This process realizes the interfacial modulation of AMR, which is generally considered as a bulk property. The SMR ratio decreases first and then increases with increasing Ir thickness, producing the minimum and maximum at ${t}_{\mathrm{Ir}}=0.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and ${t}_{\mathrm{Ir}}=0.9\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively, which reflects the ultrasmall spin-diffusion length (0.5 nm) and strong spin-memory loss in Ir. Further analyses combined with the SOT switching measurements unravel the existence of the anomalous Hall magnetoresistance, implying the non-negligible spin accumulation due to the anomalous Hall effect of the ferromagnetic metal. The combination of a large negative AMR and comparatively smaller SMR results in a negative planar Hall resistance. Our findings enrich the understanding of the magnetoresistances of heavy-metal/ferromagnetic metal trilayer systems.

Tunneling anisotropic magnetoresistance of Pb and Bi adatoms and dimers on Mn/W(110): A first-principles study

We show that Pb and Bi adatoms and dimers have a large tunneling anisotropic magnetoresistance (TAMR) of up to 60% when adsorbed on a magnetic transition-metal surface due to strong spin-orbit coupling and the hybridization of 6p orbitals with 3d states of the magnetic layer. Using density functional theory, we have explored the TAMR effect of Pb and Bi adatoms and dimers adsorbed on a Mn monolayer on W(110). This surface exhibits a noncollinear cycloidal spin spiral ground state with an angle of 173$^\circ$ between neighboring spins which allows to rotate the spin quantization axis of an adatom or dimer quasi-continuously and is ideally suited to explore the angular dependence of TAMR using scanning tunneling microscopy (STM). We find that the induced magnetic moments of Pb and Bi adatoms and dimers are small, however, the spin-polarization of the local density of states (LDOS) is still very large. The TAMR obtained from the anisotropy of the vacuum LDOS is up to 50-60 % for adatoms. For dimers the TAMR depends sensitively on the dimer orientation with respect to the crystallographic directions of the surface due to the formation of bonds between the adatoms with the Mn surface atoms and the symmetry of the spin-orbit coupling induced mixing. Dimers oriented along the spin spiral direction of the Mn monolayer display the largest TAMR of 60 % which is due to hybrid 6p-3d states of the dimers and the Mn layer

Tunable giant magnetoresistance in a single-molecule junction

Controlling electronic transport through a single-molecule junction is crucial for molecular electronics or spintronics. In magnetic molecular devices, the spin degree-of-freedom can be used to this end since the magnetic properties of the magnetic ion centers fundamentally impact the transport through the molecules. Here we demonstrate that the electron pathway in a single-molecule device can be selected between two molecular orbitals by varying a magnetic field, giving rise to a tunable anisotropic magnetoresistance up to 93%. The unique tunability of the electron pathways is due to the magnetic reorientation of the transition metal center, resulting in a re-hybridization of molecular orbitals. We obtain the tunneling electron pathways by Kondo effect, which manifests either as a peak or a dip line shape. The energy changes of these spin-reorientations are remarkably low and less than one millielectronvolt. The large tunable anisotropic magnetoresistance could be used to control electronic transport in molecular spintronics.

Anisotropic magnetoresistance in multiband systems: Two-dimensional electron gases and polar metals at oxide interfaces

Low density two-dimensional electron gases (2DEGs) with spin-orbit coupling are highly sensitive to an in-plane magnetic field, which impacts their Fermi surfaces and transport properties. Such 2DEGs, formed at transition metal oxide surfaces or interfaces, can also undergo surface phase transitions leading to polar metals which exhibit electronic nematicity. Motivated by experiments on such systems, we theoretically study magnetotransport in $t_{2g}$ orbital systems, using Hamiltonians which include atomic spin-orbit coupling (SOC) and broken inversion symmetry, for both square symmetry (001) and hexagonal symmetry (111) 2DEGs. Using a numerical solution to the full multiband matrix-Boltzmann equation, together with insights gleaned from the impurity scattering overlap matrix, we explore the anisotropic magnetoresistance (AMR) in the presence of impurities which favor small momentum scattering. We find that transport in the (001) 2DEG is dominated by a single pair of bands, weakly coupled by impurity scattering, one of which has a larger Fermi velocity while the other provides an efficient current-relaxation mechanism. This leads to strong angle-dependent current damping and a large AMR with many angular harmonics. In contrast, AMR in the (111) 2DEG typically features a single $\cos(2\vartheta)$ harmonic, with the angle-averaged magnetoresistance being highly tunable by a symmetry-allowed trigonal distortion. We also explore how the (111) 2DEG Fermi sufaces are impacted by electronic nematicity via a surface phase transition into a 2D polar metal for which we discuss a Landau theory, and show that this leads to distinct symmetry components and higher angular harmonics in the AMR. Our results are in qualitative agreement with experiments from various groups for 2DEGs at the SrTiO$_3$ surface or the LaAlO$_3$-SrTiO$_3$ interface.