Showing papers on "Magnetoresistance published in 2019"

Extremely high conductivity observed in the triple point topological metal MoP

Abstract: Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultra-high electrical conductivity in MoP at low temperature, which has recently been established as a triple point fermion material. We show that the electrical resistivity is 6 nΩ cm at 2 K with a large mean free path of 11 microns. de Haas-van Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures. Although novel topological quasiparticles have recently been evidenced, their electrical transport properties remain elusive. Here, the authors report ultra-low resistivity down to 6 nΩcm at 2 K in MoP with a large mean free path, which hints on the exotic properties of triple point fermions.

Atomically Thin CrCl3: An In-Plane Layered Antiferromagnetic Insulator.

Abstract: The recent discovery of magnetism in atomically thin layers of van der Waals (vdW) crystals has created new opportunities for exploring magnetic phenomena in the two-dimensional (2D) limit. In most 2D magnets studied to date, the c-axis is an easy axis, so that at zero applied field the polarization of each layer is perpendicular to the plane. Here, we demonstrate that atomically thin CrCl3 is a layered antiferromagnetic insulator with an easy-plane normal to the c-axis, that is, the polarization is in the plane of each layer and has no preferred direction within it. Ligand-field photoluminescence at 870 nm is observed down to the monolayer limit, demonstrating its insulating properties. We investigate the in-plane magnetic order using tunneling magnetoresistance in graphene/CrCl3/graphene tunnel junctions, establishing that the interlayer coupling is antiferromagnetic down to the bilayer. From the temperature dependence of the magnetoresistance, we obtain an effective magnetic phase diagram for the bilayer. Our result shows that CrCl3 should be useful for studying the physics of 2D phase transitions and for making new kinds of vdW spintronic devices.

Defect induced, layer-modulated magnetism in ultrathin metallic PtSe 2

Abstract: Defects are ubiquitous in solids and often introduce new properties that are absent in pristine materials. One of the opportunities offered by these crystal imperfections is an extrinsically induced long-range magnetic ordering1, a long-time subject of theoretical investigations1–3. Intrinsic, two-dimensional (2D) magnetic materials4–7 are attracting increasing attention for their unique properties, which include layer-dependent magnetism4 and electric field modulation6. Yet, to induce magnetism into otherwise non-magnetic 2D materials remains a challenge. Here we investigate magneto-transport properties of ultrathin PtSe2 crystals and demonstrate an unexpected magnetism. Our electrical measurements show the existence of either ferromagnetic or antiferromagnetic ground-state orderings that depends on the number of layers in this ultrathin material. The change in the device resistance on the application of a ~25 mT magnetic field is as high as 400 Ω with a magnetoresistance value of 5%. Our first-principles calculations suggest that surface magnetism induced by the presence of Pt vacancies and the Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange couplings across ultrathin films of PtSe2 are responsible for the observed layer-dependent magnetism. Given the existence of such unavoidable growth-related vacancies in 2D materials8,9, these findings can expand the range of 2D ferromagnets into materials that would otherwise be overlooked. Magneto-transport measurements on thin metallic crystals of the transition metal dichalcogenide PtSe2 show signatures of ferro- and antiferromagnetic order depending on the number of layers and first-principles calculations suggest Pt vacancies at the surface as a plausible cause.

Mechanism of Néel Order Switching in Antiferromagnetic Thin Films Revealed by Magnetotransport and Direct Imaging.

Abstract: We probe the current-induced magnetic switching of insulating antiferromagnet-heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely, the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs.

Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction

Abstract: Atomically thin chromium triiodide (CrI3) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of antialigned spin filters, which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of 10 or more). In addition, without switching the state, the TMR can be continuously modulated between 17,000% and 57,000%, due to the combination of spin-dependent tunnel barrier with changing carrier distributions in the graphene contacts. Our work demonstrates new kinds of ma...

Spin–orbit-driven band inversion in bilayer graphene by the van der Waals proximity effect

Abstract: Spin-orbit coupling (SOC) is the key to realizing time-reversal-invariant topological phases of matter1,2. SOC was predicted by Kane and Mele3 to stabilize a quantum spin Hall insulator; however, the weak intrinsic SOC in monolayer graphene4-7 has precluded experimental observation in this material. Here we exploit a layer-selective proximity effect-achieved via a van der Waals contact with a semiconducting transition-metal dichalcogenide8-21-to engineer Kane-Mele SOC in ultra clean bilayer graphene. Using high-resolution capacitance measurements to probe the bulk electronic compressibility, we find that SOC leads to the formation of a distinct, incompressible, gapped phase at charge neutrality. The experimental data agree quantitatively with a simple theoretical model in which the new phase results from SOC-driven band inversion. In contrast to Kane-Mele SOC in monolayer graphene, the inverted phase is not expected to be a time-reversal-invariant topological insulator, despite being separated from conventional band insulators by electric-field-tuned phase transitions where crystal symmetry mandates that the bulk gap must close22. Our electrical transport measurements reveal that the inverted phase has a conductivity of approximately e2/h (where e is the electron charge and h Planck's constant), which is suppressed by exceptionally small in-plane magnetic fields. The high conductivity and anomalous magnetoresistance are consistent with theoretical models that predict helical edge states within the inverted phase that are protected from backscattering by an emergent spin symmetry that remains robust even for large Rashba SOC. Our results pave the way for proximity engineering of strong topological insulators as well as correlated quantum phases in the strong spin-orbit regime in graphene heterostructures.

Giant topological Hall effect in correlated oxide thin films

Abstract: Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca,Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density as a function of magnetic field peaks near the THE maximum. The THE critically depends on carrier concentration and diverges at low doping, near the metal–insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields. A strong Hall effect is observed in a material with spin textures and strong electron correlations. This hints that correlation effects can amplify real-space topological spin transport.

Spin–orbit torque driven by a planar Hall current

Abstract: Spin–orbit torques (SOTs) in multilayers of ferromagnetic (FM) and non-magnetic (NM) metals can manipulate the magnetization of the FM layer efficiently. This is employed, for example, in non-volatile magnetic memories for energy-efficient mobile electronics1,2 and spin torque nano-oscillators3–7 for neuromorphic computing8. Recently, spin torque nano-oscillators also found use in microwave-assisted magnetic recording, which enables ultrahigh-capacity hard disk drives9. Most SOT devices employ spin Hall10,11 and Rashba12 effects, which originate from spin–orbit coupling within the NM layer and at the FM/NM interfaces, respectively. Recently, SOTs generated by the anomalous Hall effect in FM/NM/FM multilayers were predicted13 and experimentally realized14. The control of SOTs through crystal symmetry was demonstrated as well15. Understanding all the types of SOTs that can arise in magnetic multilayers is needed for a formulation of a comprehensive SOT theory and for engineering practical SOT devices. Here we show that a spin-polarized electric current known to give rise to anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) in a FM16 can additionally generate large antidamping SOTs with an unusual angular symmetry in NM1/FM/NM2 multilayers. This effect can be described by a recently proposed magnonic mechanism17. Our measurements reveal that this torque can be large in multilayers in which both spin Hall and Rashba torques are negligible. Furthermore, we demonstrate the operation of a spin torque nano-oscillator driven by this SOT. These findings significantly expand the class of materials that exhibit giant SOTs. A spin-polarized current responsible for the planar Hall effect and anisotropic magnetoresistance is found to generate large antidamping spin–orbit torque in normal metal/ferromagnet/normal metal trilayers.

A Piezoelectric, Strain-Controlled Antiferromagnetic Memory Insensitive to Magnetic Fields

Abstract: Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields. Different device concepts have been predicted and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves, or room-temperature AFM memory, for which either thermal heating in combination with magnetic fields, or Neel spin-orbit torque is used for the information writing process. On the other hand, piezoelectric materials were employed to control magnetism by electric fields in multiferroic heterostructures, which suppresses Joule heating caused by switching currents and may enable low energy-consuming electronic devices. Here, we combine the two material classes to explore changes of the resistance of the high-Neel-temperature antiferromagnet MnPt induced by piezoelectric strain. We find two non-volatile resistance states at room temperature and zero electric field, which are stable in magnetic fields up to 60 T. Furthermore, the strain-induced resistance switching process is insensitive to magnetic fields. Integration in a tunnel junction can further amplify the electroresistance. The tunneling anisotropic magnetoresistance reaches ~11.2% at room temperature. Overall, we demonstrate a piezoelectric, strain-controlled AFM memory which is fully operational in strong magnetic fields and has potential for low-energy and high-density memory applications.

Antisymmetric magnetoresistance in van der Waals Fe3GeTe2/graphite/Fe3GeTe2 trilayer heterostructures

Abstract: With no requirements for lattice matching, van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next-generation spintronic devices. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in vdW heterostructured Fe3GeTe2 (FGT)/graphite/FGT devices. Unlike conventional giant MR (GMR), which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high-, intermediate-, and low-resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three-resistance behavior was attributed to a spin momentum locking induced spin-polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures.

Absence of Evidence of Electrical Switching of the Antiferromagnetic Néel Vector

Abstract: Much theoretical and experimental attention has been focused on the electrical switching of the antiferromagnetic (AFM) Neel vector via spin-orbit torque. Measurements employing multiterminal patterned structures of Pt/AFM show recurring signals of the supposedly planar Hall effect and magnetoresistance, implying AFM switching. We show in this Letter that similar signals have been observed in structures with and without the AFM layer, and of an even larger magnitude using different metals and substrates. These may not be the conclusive evidence of spin-orbit torque switching of AFM, but the thermal artifacts of patterned metal structure on substrate. Large current densities in the metallic devices, beyond the Ohmic regime, can generate unintended anisotropic thermal gradients and voltages. AFM switching requires unequivocal detection of the AFM Neel vector before and after SOT switching.

Spin-Dependent Transport in van der Waals Magnetic Tunnel Junctions with Fe3GeTe2 Electrodes.

Abstract: van der Waals (vdW) heterostructures, stacking different two-dimensional materials, have opened up unprecedented opportunities to explore new physics and device concepts. Especially interesting are recently discovered two-dimensional magnetic vdW materials, providing new paradigms for spintronic applications. Here, using density functional theory (DFT) calculations, we investigate the spin-dependent electronic transport across vdW magnetic tunnel junctions (MTJs) composed of Fe3GeTe2 ferromagnetic electrodes and a graphene or hexagonal boron nitride (h-BN) spacer layer. For both types of junctions, we find that the junction resistance changes by thousands of percent when the magnetization of the electrodes is switched from parallel to antiparallel. Such a giant tunneling magnetoresistance (TMR) effect is driven by dissimilar electronic structure of the two spin-conducting channels in Fe3GeTe2, resulting in a mismatch between the incoming and outgoing Bloch states in the electrodes and thus suppressed transmission for an antiparallel-aligned MTJ. The vdW bonding between electrodes and a spacer layer makes this result virtually independent of the type of the spacer layer, making the predicted giant TMR effect robust with respect to strain, interface distance, and other parameters, which may vary in the experiment. We hope that our results will further stimulate experimental studies of vdW MTJs and pave the way for their applications in spintronics.

Giant nonvolatile manipulation of magnetoresistance in magnetic tunnel junctions by electric fields via magnetoelectric coupling

Abstract: Electrically switchable magnetization is considered a milestone in the development of ultralow power spintronic devices, and it has been a long sought-after goal for electric-field control of magnetoresistance in magnetic tunnel junctions with ultralow power consumption. Here, through integrating spintronics and multiferroics, we investigate MgO-based magnetic tunnel junctions on ferroelectric substrate with a high tunnel magnetoresistance ratio of 235%. A giant, reversible and nonvolatile electric-field manipulation of magnetoresistance to about 55% is realized at room temperature without the assistance of a magnetic field. Through strain-mediated magnetoelectric coupling, the electric field modifies the magnetic anisotropy of the free layer leading to its magnetization rotation so that the relative magnetization configuration of the magnetic tunnel junction can be efficiently modulated. Our findings offer significant fundamental insight into information storage using electric writing and magnetic reading and represent a crucial step towards low-power spintronic devices. Electric field controlled magnetism provides an energy efficient way for the operations in the spintronic devices. Here the authors show strain induced, reversible, nonvolatile electric field control of magnetization and magnetoresistance in a magnetic tunnel junction on a ferroelectric substrate at room temperature and zero magnetic field.

A piezoelectric, strain-controlled antiferromagnetic memory insensitive to magnetic fields.

Abstract: Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields1–3. Different device concepts have been predicted4,5 and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves6, or room-temperature AFM memory, for which either thermal heating in combination with magnetic fields7 or Neel spin–orbit torque8 is used for the information writing process. On the other hand, piezoelectric materials were employed to control magnetism by electric fields in multiferroic heterostructures9–12, which suppresses Joule heating caused by switching currents and may enable low-energy-consuming electronic devices. Here, we combine the two material classes to explore changes in the resistance of the high-Neel-temperature antiferromagnet MnPt induced by piezoelectric strain. We find two non-volatile resistance states at room temperature and zero electric field that are stable in magnetic fields up to 60 T. Furthermore, the strain-induced resistance switching process is insensitive to magnetic fields. Integration in a tunnel junction can further amplify the electroresistance. The tunnelling anisotropic magnetoresistance reaches ~11.2% at room temperature. Overall, we demonstrate a piezoelectric, strain-controlled AFM memory that is fully operational in strong magnetic fields and has the potential for low-energy and high-density memory applications. An antiferromagnetic memory with piezoelectric strain control can be operated in high magnetic fields and combines a small device footprint with low switching power.

Singular angular magnetoresistance in a magnetic nodal semimetal

Abstract: Transport coefficients of correlated electron systems are often useful for mapping hidden phases with distinct symmetries. Here we report a transport signature of spontaneous symmetry breaking in the magnetic Weyl semimetal cerium-aluminum-germanium (CeAlGe) system in the form of singular angular magnetoresistance (SAMR). This angular response exceeding 1000% per radian is confined along the high-symmetry axes with a full width at half maximum reaching less than 1° and is tunable via isoelectronic partial substitution of silicon for germanium. The SAMR phenomena is explained theoretically as a consequence of controllable high-resistance domain walls, arising from the breaking of magnetic point group symmetry strongly coupled to a nearly nodal electronic structure. This study indicates ingredients for engineering magnetic materials with high angular sensitivity by lattice and site symmetries.

Isolated zero field sub-10 nm skyrmions in ultrathin Co films

Abstract: Due to their exceptional topological and dynamical properties magnetic skyrmions—localized stable spin structures—show great promise for spintronic applications. To become technologically competitive, isolated skyrmions with diameters below 10 nm stable at zero magnetic field and at room temperature are desired. Despite finding skyrmions in a wide spectrum of materials, the quest for a material with these envisioned properties is ongoing. Here we report zero field isolated skyrmions at T = 4 K with diameters below 5 nm observed in the virgin ferromagnetic state coexisting with 1 nm thin domain walls in Rh/Co atomic bilayers on Ir(111). These spin structures are investigated by spin-polarized scanning tunneling microscopy and can also be detected using non-spin-polarized tips via the noncollinear magnetoresistance. We demonstrate that sub-10 nm skyrmions are stabilized in these ferromagnetic Co films at zero field due to strong frustration of exchange interaction, together with Dzyaloshinskii–Moriya interaction and large magnetocrystalline anisotropy. Isolated skyrmions with diameters below 10 nm stabilized at zero magnetic field are of great technological relevance to the future spintronic applications. Here the authors report stabilization of zero field isolated skyrmions with diameters smaller than 5 nm in Rh/Co atomic bilayers on the Ir(111) surface.

Unconventional superconductivity in heavy fermion Ute2

Abstract: We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected ...

Magnetoresistance from Fermi surface topology

Abstract: Extremely large nonsaturating magnetoresistance has recently been reported for a large number of both topologically trivial and nontrivial materials. Different mechanisms have been proposed to explain the observed magnetotransport properties, yet without arriving to definitive conclusions or portraying a global picture. In this work, we investigate the transverse magnetoresistance of materials by combining the Fermi surfaces calculated from first principles with the Boltzmann transport theory approach relying on the semiclassical model and the relaxation time approximation. We first consider a series of simple model Fermi surfaces to provide a didactic introduction into the charge-carrier compensation and open-orbit mechanisms leading to nonsaturating magnetoresistance. We then address in detail magnetotransport in three representative materials: (i) copper, a prototypical nearly free-electron metal characterized by the open Fermi surface that results in an intricate angular magnetoresistance, (ii) bismuth, a topologically trivial semimetal in which very large magnetoresistance is known to result from charge-carrier compensation, and (iii) tungsten diphosphide ${\mathrm{WP}}_{2}$, a recently discovered type-II Weyl semimetal that holds the record of magnetoresistance in compounds. In all three cases our calculations show excellent agreement with both the field dependence of magnetoresistance and its anisotropy measured at low temperatures. Furthermore, the calculations allow for a full interpretation of the observed features in terms of the Fermi surface topology. Our study thus establishes guidelines to clarifying the physical mechanisms underlying the magnetotransport properties in a broad range of materials. These results will help addressing a number of outstanding questions, such as the role of the topological phase in the pronounced large nonsaturating magnetoresistance observed in topological materials.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction.

Abstract: Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe2/MoS2 heterojunction, where the VSe2 monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS2, SOT is promising for the magnetization switching of VSe2 free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe2/MoS2 provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Isolated zero field sub-10 nm skyrmions in ultrathin Co films

Abstract: Due to their exceptional topological and dynamical properties magnetic skyrmions - localized stable spin structures on the nanometre scale - show great promise for future spintronic applications. To become technologically competitive, isolated skyrmions with diameters below 10 nm that are stable at zero magnetic field and room temperature are desired. Despite finding skyrmions in a wide spectrum of materials, the quest for a material with these envisioned properties is still ongoing. Here we report zero field isolated skyrmions with diameters smaller than 5 nm coexisting with 1 nm thin domain walls in Rh/Co atomic bilayers on the Ir(111) surface. These spin structures are characterized by spin-polarized scanning tunnelling microscopy and can also be detected using non-spin-polarized tips due to a large non-collinear magnetoresistance. We demonstrate that sub-10 nm skyrmions are stabilised in these ferromagnetic Co films at zero field due to strong frustration of exchange interaction, together with Dzyaloshinskii-Moriya interaction and a large magnetocrystalline anisotropy.

Anisotropies and magnetic phase transitions in insulating antiferromagnets determined by a Spin-Hall magnetoresistance probe

Abstract: Antiferromagnets possess a number of intriguing and promising properties for electronic devices, which include a vanishing net magnetic moment and thus insensitivity to large magnetic fields and characteristic terahertz frequency dynamics. However, probing the antiferromagnetic ordering is challenging without synchrotron-based facilities. Here, we determine the material parameters of the insulating iron oxide hematite, α-Fe2O3, using the surface sensitive spin-Hall magnetoresistance (SMR). Combined with a simple analytical model, we extract the antiferromagnetic anisotropies and the bulk Dzyaloshinskii-Moriya field over a wide range of temperatures and magnetic fields. Across the Morin phase transition, we show that the electrical response is dominated by the antiferromagnetic Neel vector rather than by the emergent weak magnetic moment. Our results highlight that the surface sensitivity of SMR enables access to the magnetic anisotropies of antiferromagnetic crystals, and also of thin films, where other methods to determine anisotropies such as bulk-sensitive magnetic susceptibility measurements do not provide sufficient sensitivity. Antiferromagnets are expected to be a key part of next generation electronic devices however their magnetic interactions prove difficult to access. Here, the authors demonstrate that the surface sensitive spin-Hall magnetoresistance, along with a simple analytical model, can successfully probe the internal anisotropies of the model antiferromagnet hematite (α-Fe2O3).

Giant Anisotropy of Gilbert Damping in Epitaxial CoFe Films.

Abstract: Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co_{50}Fe_{50} thin films with a maximum-minimum damping ratio of 400%, determined by broadband spin-torque ferromagnetic resonance as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborated from the magnitude of the anisotropic magnetoresistance in Co_{50}Fe_{50}.

Large anomalous Hall effect in ferromagnetic Weyl semimetal candidate PrAlGe

Abstract: We present the structure and anisotropic magnetic and electrical transport properties on the flux-grown PrAlGe single crystal, a recently proposed magnetic Weyl semimetal candidate. From the powder and single-crystal x-ray diffraction analysis, the grown crystal is revealed to crystallize in tetragonal LaPtSi-type structure with space group of I41md. The PrAlGe exhibits strong Ising-type magnetic anisotropy with ferromagnetic moments ∼2.32μB/Pr along easy c-axis below transition temperature Tc ∼ 15 K. Accordingly, anomalous Hall effect (AHE) is observed for field (H) along c axis in contrast to a axis, and large anomalous Hall conductivity value reaches up to ∼680 Ω−1 cm−1 close to the theoretical expected value based on intrinsic Berry-curvature mechanism. Moreover, the linear scaling behaviors between the anomalous Hall resistivity and longitudinal resistivity also support the intrinsic Karplus-Luttinger mechanism as a dominant role on the observed AHE rather than extrinsic scattering mechanism.

Nonlinear Planar Hall Effect.

Abstract: An intriguing property of a three-dimensional (3D) topological insulator (TI) is the existence of surface states with spin-momentum locking, which offers a new frontier of exploration in spintronics. Here, we report the observation of a new type of Hall effect in a 3D TI Bi_{2}Se_{3} film. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall effect and the anisotropic magnetoresistance. This novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry breaking, which also exists in a wide class of noncentrosymmetric materials with a large span of magnitude. It provides a new way to characterize and utilize the nonlinear spin-to-charge conversion in a variety of topological quantum materials.

Spin Filtering in CrI3 Tunnel Junctions.

Abstract: The recently discovered magnetism of two-dimensional (2D) van der Waals crystals has attracted a lot of attention. Among these materials is CrI3, a magnetic semiconductor, exhibiting transitions between ferromagnetic and antiferromagnetic orderings under the influence of an applied magnetic field. Here, using first-principles methods based on density functional theory, we explore spin-dependent transport in tunnel junctions formed of face-centered cubic Cu(111) electrodes and a CrI3 tunnel barrier. We find about 100% spin polarization of the tunneling current for a ferromagnetically ordered four-monolayer CrI3 and a tunneling magnetoresistance of about 3000% associated with a change of magnetic ordering in CrI3. This behavior is understood in terms of the spin and wave-vector-dependent evanescent states in CrI3, which control the tunneling conductance. We find a sizable charge transfer from Cu to CrI3, which adds new features to the mechanism of spin filtering in CrI3-based tunnel junctions. Our results elucidate the mechanisms of spin filtering in CrI3 tunnel junctions and provide important insights for the design of magnetoresistive devices based on 2D magnetic crystals.

Active Magneto-Optical Control of Near-Field Radiative Heat Transfer between Graphene Sheets

Abstract: Graphene provides an ideal platform for active modulation of near-field radiative heat transfer. While much research here focuses on tuning the chemical potential by doping or gating, this work proposes using a magnetic field to control near-field heat flux between graphene sheets. The authors predict giant thermal magnetoresistance and negative thermal magnetoresistance of the near-field radiative heat flux for different Fermi energies, and Shubnikov-de-Haas-like oscillations in the spectral heat flux. In a static magnetic field, coupling of excited magnetoplasmon-polariton modes of graphene boosts radiative heat transfer beyond the blackbody limit by several orders of magnitude.

Field-free Magnetization Switching by Utilizing the Spin Hall Effect and Interlayer Exchange Coupling of Iridium.

Abstract: Magnetization switching by spin-orbit torque (SOT) via spin Hall effect represents as a competitive alternative to that by spin-transfer torque (STT) used for magnetoresistive random access memory (MRAM), as it doesn’t require high-density current to go through the tunnel junction. For perpendicular MRAM, however, SOT driven switching of the free layer requires an external in-plane field, which poses limitation for viability in practical applications. Here we demonstrate field-free magnetization switching of a perpendicular magnet by utilizing an Iridium (Ir) layer. The Ir layer not only provides SOTs via spin Hall effect, but also induce interlayer exchange coupling with an in-plane magnetic layer that eliminates the need for the external field. Such dual functions of the Ir layer allows future build-up of magnetoresistive stacks for memory and logic applications. Experimental observations show that the SOT driven field-free magnetization reversal is characterized as domain nucleation and expansion. Micromagnetic modeling is carried out to provide in-depth understanding of the perpendicular magnetization reversal process in the presence of an in-plane exchange coupling field.

Giant anisotropic magnetoresistance and nonvolatile memory in canted antiferromagnet Sr 2 IrO 4

Abstract: Antiferromagnets have been generating intense interest in the spintronics community, owing to their intrinsic appealing properties like zero stray field and ultrafast spin dynamics. While the control of antiferromagnetic (AFM) orders has been realized by various means, applicably appreciated functionalities on the readout side of AFM-based devices are urgently desired. Here, we report the remarkably enhanced anisotropic magnetoresistance (AMR) as giant as ~160% in a simple resistor structure made of AFM Sr2IrO4 without auxiliary reference layer. The underlying mechanism for the giant AMR is an indispensable combination of atomic scale giant-MR-like effect and magnetocrystalline anisotropy energy, which was not accessed earlier. Furthermore, we demonstrate the bistable nonvolatile memory states that can be switched in-situ without the inconvenient heat-assisted procedure, and robustly preserved even at zero magnetic field, due to the modified interlayer coupling by 1% Ga-doping in Sr2IrO4. These findings represent a straightforward step toward the AFM spintronic devices. The determination of the antiferromagnetic (AFM) orders by the anisotropic magnetoresistance (AMR) is one of the key components to the AFM-spintronics. Here the authors report enhanced AMR ~160% in Sr2IrO4 due to the combination of atomic scale giant-MR-like effect and magnetocrystalline anisotropy energy.

Nonmagnetic single-molecule spin-filter based on quantum interference.

Abstract: Key spin transport phenomena, including magnetoresistance and spin transfer torque, cannot be activated without spin-polarized currents, in which one electron spin is dominant. At the nanoscale, the relevant length-scale for modern spintronics, spin current generation is rather limited due to unwanted contributions from poorly spin-polarized frontier states in ferromagnetic electrodes, or too short length-scales for efficient spin splitting by spin-orbit interaction and magnetic fields. Here, we show that spin-polarized currents can be generated in silver-vanadocene-silver single molecule junctions without magnetic components or magnetic fields. In some cases, the measured spin currents approach the limit of ideal ballistic spin transport. Comparison between conductance and shot-noise measurements to detailed calculations reveals a mechanism based on spin-dependent quantum interference that yields very efficient spin filtering. Our findings pave the way for nanoscale spintronics based on quantum interference, with the advantages of low sensitivity to decoherence effects and the freedom to use non-magnetic materials. Quantum interference can be used to control electronic transport with high sensitivity at the nanoscale. Pal et al. show that without the need for magnetic materials, quantum interference can also filter spin transport approaching the limit of ideal spin-polarized ballistic transport in molecular junctions.