Showing papers on "Magnetization published in 2018"

Magnetic hysteresis up to 80 kelvin in a dysprosium metallocene single-molecule magnet

Abstract: Single-molecule magnets (SMMs) containing only one metal center may represent the lower size limit for molecule-based magnetic information storage materials. Their current drawback is that all SMMs require liquid-helium cooling to show magnetic memory effects. We now report a chemical strategy to access the dysprosium metallocene cation [(CpiPr5)Dy(Cp*)]+ (CpiPr5 = penta-iso-propylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl), which displays magnetic hysteresis above liquid-nitrogen temperatures. An effective energy barrier to reversal of the magnetization of Ueff = 1,541 cm–1 is also measured. The magnetic blocking temperature of TB = 80 K for this cation overcomes an essential barrier towards the development of nanomagnet devices that function at practical temperatures.

Controlling magnetism in 2D CrI 3 by electrostatic doping

Abstract: The atomic thickness of two-dimensional materials provides a unique opportunity to control their electrical1 and optical2 properties as well as to drive the electronic phase transitions3 by electrostatic doping. The discovery of two-dimensional magnetic materials4–10 has opened up the prospect of the electrical control of magnetism and the realization of new functional devices11. A recent experiment based on the linear magneto-electric effect has demonstrated control of the magnetic order in bilayer CrI3 by electric fields12. However, this approach is limited to non-centrosymmetric materials11,13–16 magnetically biased near the antiferromagnet–ferromagnet transition. Here, we demonstrate control of the magnetic properties of both monolayer and bilayer CrI3 by electrostatic doping using CrI3–graphene vertical heterostructures. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened/weakened magnetic order with hole/electron doping. Remarkably, in bilayer CrI3, the electron doping above ~2.5 × 1013 cm−2 induces a transition from an antiferromagnetic to a ferromagnetic ground state in the absence of a magnetic field. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages. Electrostatic doping in vertical van der Waals CrI3–graphene heterostructures provides means to control the magnetic properties of monolayer and bilayer CrI3.

Electric-field switching of two-dimensional van der Waals magnets.

Abstract: Controlling magnetism by purely electrical means is a key challenge to better information technology 1 . A variety of material systems, including ferromagnetic (FM) metals2-4, FM semiconductors 5 , multiferroics6-8 and magnetoelectric (ME) materials9,10, have been explored for the electric-field control of magnetism. The recent discovery of two-dimensional (2D) van der Waals magnets11,12 has opened a new door for the electrical control of magnetism at the nanometre scale through a van der Waals heterostructure device platform 13 . Here we demonstrate the control of magnetism in bilayer CrI3, an antiferromagnetic (AFM) semiconductor in its ground state 12 , by the application of small gate voltages in field-effect devices and the detection of magnetization using magnetic circular dichroism (MCD) microscopy. The applied electric field creates an interlayer potential difference, which results in a large linear ME effect, whose sign depends on the interlayer AFM order. We also achieve a complete and reversible electrical switching between the interlayer AFM and FM states in the vicinity of the interlayer spin-flip transition. The effect originates from the electric-field dependence of the interlayer exchange bias.

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit

Abstract: Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSex) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that, in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe2) monolayer, while for thicker films it could originate from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111). Magnetization measurements of monolayer MnSex films on GaSe and SnSe2 epilayers show ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn, which is consistent with the density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe2. Growing MnSex films on GaSe up to a high thickness (∼4...

Controlling magnetism in 2D CrI3 by electrostatic doping

Abstract: The atomic thickness of two-dimensional (2D) materials provides a unique opportunity to control material properties and engineer new functionalities by electrostatic doping. Electrostatic doping has been demonstrated to tune the electrical and optical properties of 2D materials in a wide range, as well as to drive the electronic phase transitions. The recent discovery of atomically thin magnetic insulators has opened up the prospect of electrical control of magnetism and new devices with unprecedented performance. Here we demonstrate control of the magnetic properties of monolayer and bilayer CrI3 by electrostatic doping using a dual-gate field-effect device structure. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened (weakened) magnetic order with hole (electron) doping. Remarkably, in bilayer CrI3 doping drastically changes the interlayer magnetic order, causing a transition from an antiferromagnetic ground state in the pristine form to a ferromagnetic ground state above a critical electron density. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages.

Direct visualization of current-induced spin accumulation in topological insulators.

Abstract: Charge-to-spin conversion in various materials is the key for the fundamental understanding of spin-orbitronics and efficient magnetization manipulation. Here we report the direct spatial imaging of current-induced spin accumulation at the channel edges of Bi2Se3 and BiSbTeSe2 topological insulators as well as Pt by a scanning photovoltage microscope at room temperature. The spin polarization is along the out-of-plane direction with opposite signs for the two channel edges. The accumulated spin direction reverses sign upon changing the current direction and the detected spin signal shows a linear dependence on the magnitude of currents, indicating that our observed phenomena are current-induced effects. The spin Hall angle of Bi2Se3, BiSbTeSe2, and Pt is determined to be 0.0085, 0.0616, and 0.0085, respectively. Our results open up the possibility of optically detecting the current-induced spin accumulations, and thus point towards a better understanding of the interaction between spins and circularly polarized light.

Large intrinsic anomalous Hall effect in half-metallic ferromagnet Co 3 Sn 2 S 2 with magnetic Weyl fermions.

Abstract: The origin of anomalous Hall effect (AHE) in magnetic materials is one of the most intriguing aspects in condensed matter physics and has been a controversial topic for a long time. Recent studies indicate that the intrinsic AHE is closely related to the Berry curvature of occupied electronic states. In a magnetic Weyl semimetal with broken time-reversal symmetry, there are significant contributions to Berry curvature around Weyl nodes, possibly leading to a large intrinsic AHE. Here, we report the quite large AHE in the half-metallic ferromagnet Co3Sn2S2 single crystal. By systematically mapping out the electronic structure of Co3Sn2S2 both theoretically and experimentally, we demonstrate that the intrinsic AHE from the Weyl fermions near the Fermi energy is dominating. The intrinsic anomalous Hall conductivity depends linearly on the magnetization and can be reproduced by theoretical simulation, in which the Weyl nodes monotonically move with the constrained magnetic moment on Co atom.

Electric-field control of magnetism in a few-layered van der Waals ferromagnetic semiconductor.

Abstract: Manipulating a quantum state via electrostatic gating has been of great importance for many model systems in nanoelectronics. Until now, however, controlling the electron spins or, more specifically, the magnetism of a system by electric-field tuning has proven challenging1–4. Recently, atomically thin magnetic semiconductors have attracted significant attention due to their emerging new physical phenomena5–13. However, many issues are yet to be resolved to convincingly demonstrate gate-controllable magnetism in these two-dimensional materials. Here, we show that, via electrostatic gating, a strong field effect can be observed in devices based on few-layered ferromagnetic semiconducting Cr2Ge2Te6. At different gate doping, micro-area Kerr measurements in the studied devices demonstrate bipolar tunable magnetization loops below the Curie temperature, which is tentatively attributed to the moment rebalance in the spin-polarized band structure. Our findings of electric-field-controlled magnetism in van der Waals magnets show possibilities for potential applications in new-generation magnetic memory storage, sensors and spintronics. Few-layer semiconducting Cr2Ge2Te6 shows bipolar gate-tuned magnetism below its ferromagnetic Curie temperature.

Electric-field control of magnetism in few-layered van der Waals magnet

Abstract: Manipulating quantum state via electrostatic gating has been intriguing for many model systems in nanoelectronics. When it comes to the question of controlling the electron spins, more specifically, the magnetism of a system, tuning with electric field has been proven to be elusive. Recently, magnetic layered semiconductors have attracted much attention due to their emerging new physical phenomena. However, challenges still remain in the demonstration of a gate controllable magnetism based on them. Here, we show that, via ionic gating, strong field effect can be observed in few-layered semiconducting Cr$_{2}$Ge$_{2}$Te$_{6}$ devices. At different gate doping, micro-area Kerr measurements in the studied devices demonstrate tunable magnetization loops below the Curie temperature, which is tentatively attributed to the moment re-balance in the spin-polarized band structure. Our findings of electric-field controlled magnetism in van der Waals magnets pave the way for potential applications in new generation magnetic memory storage, sensors, and spintronics.

Room-temperature high spin-orbit torque due to quantum confinement in sputtered BixSe(1-x) films.

Abstract: The spin–orbit torque (SOT) that arises from materials with large spin–orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1–x) thin films in BixSe(1–x)/Co20Fe60B20 heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θS) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the BixSe(1–x) was observed at room temperature with a low critical magnetization switching current density of 4.3 × 105 A cm–2. Quantum transport simulations using a realistic sp3 tight-binding model suggests that the high SOT in sputtered BixSe(1–x) is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θS, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1–x) films provide an avenue for the use of BixSe(1–x) as a spin density generator in SOT-based memory and logic devices. Sputtered BixSe(1–x) thin films can generate very large current-induced spin–orbit torque, capable to switch both in-plane and out-of-plane magnetized CoFeB-based structures deposited on top, at room temperature.

Exploring the Magnetoelectric Coupling at the Composite Interfaces of FE/FM/FE Heterostructures.

Abstract: Multiferroic materials have attracted considerable attention as possible candidates for a wide variety of future microelectronic and memory devices, although robust magnetoelectric (ME) coupling between electric and magnetic orders at room temperature still remains difficult to achieve. In order to obtain robust ME coupling at room temperature, we studied the Pb(Fe0.5Nb0.5)O3/Ni0.65Zn0.35Fe2O4/Pb(Fe0.5Nb0.5)O3 (PFN/NZFO/PFN) trilayer structure as a representative FE/FM/FE system. We report the ferroelectric, magnetic and ME properties of PFN/NZFO/PFN trilayer nanoscale heterostructure having dimensions 70/20/70 nm, at room temperature. The presence of only (00l) reflection of PFN and NZFO in the X-ray diffraction (XRD) patterns and electron diffraction patterns in Transmission Electron Microscopy (TEM) confirm the epitaxial growth of multilayer heterostructure. The distribution of the ferroelectric loop area in a wide area has been studied, suggesting that spatial variability of ferroelectric switching behavior is low, and film growth is of high quality. The ferroelectric and magnetic phase transitions of these heterostructures have been found at ~575 K and ~650 K, respectively which are well above room temperature. These nanostructures exhibit low loss tangent, large saturation polarization (Ps ~ 38 µC/cm2) and magnetization (Ms ~ 48 emu/cm3) with strong ME coupling at room temperature revealing them as potential candidates for nanoscale multifunctional and spintronics device applications.

Antiferromagnetic opto-spintronics

Abstract: Control and detection of spin order in ferromagnetic materials is the main principle enabling magnetic information to be stored and read in current technologies. Antiferromagnetic materials, on the other hand, are far less utilized, despite having some appealing features. For instance, the absence of net magnetization and stray fields eliminates crosstalk between neighbouring devices, and the absence of a primary macroscopic magnetization makes spin manipulation in antiferromagnets inherently faster than in ferromagnets. However, control of spins in antiferromagnets requires exceedingly high magnetic fields, and antiferromagnetic order cannot be detected with conventional magnetometry. Here we provide an overview and illustrative examples of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets. We also discuss possible research directions that are anticipated to be among the main topics defining the future of this rapidly developing field. An overview of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets, and possible future research directions.

A conductive topological insulator with large spin Hall effect for ultralow power spin-orbit torque switching.

Abstract: Spin–orbit torque switching using the spin Hall effect in heavy metals and topological insulators has a great potential for ultralow power magnetoresistive random-access memory. To be competitive with conventional spin-transfer torque switching, a pure spin current source with a large spin Hall angle (θSH > 1) and high electrical conductivity (σ > 105 Ω−1 m−1) is required. Here we demonstrate such a pure spin current source: conductive topological insulator BiSb thin films with σ ≈ 2.5 × 105 Ω−1 m−1, θSH ≈ 52 and spin Hall conductivity σSH ≈ 1.3 × 107 $$\frac{\hbar }{{2e}}$$ Ω−1 m−1 at room temperature. We show that BiSb thin films can generate a very large spin–orbit field of 2.3 kOe MA–1 cm2 and a critical switching current density as low as 1.5 MA cm–2 in Bi0.9Sb0.1/MnGa bilayers, which underlines the potential of BiSb for industrial applications. A large spin–orbit torque, generated in a conductive topological insulator (TI) Bi0.9Sb0.1 is further employed to effectively switch the magnetization of MnGa in a BiSb/MnGa bilayer Hall-bar device at room temperature.

Electric-field switching of two-dimensional van der Waals magnets

Abstract: Controlling magnetism by purely electrical means is a key challenge to better information technology1. A variety of material systems, including ferromagnetic (FM) metals2,3,4, FM semiconductors5, multiferroics6,7,8 and magnetoelectric (ME) materials9,10, have been explored for the electric-field control of magnetism. The recent discovery of two-dimensional (2D) van der Waals magnets11,12 has opened a new door for the electrical control of magnetism at the nanometre scale through a van der Waals heterostructure device platform13. Here we demonstrate the control of magnetism in bilayer CrI3, an antiferromagnetic (AFM) semiconductor in its ground state12, by the application of small gate voltages in field-effect devices and the detection of magnetization using magnetic circular dichroism (MCD) microscopy. The applied electric field creates an interlayer potential difference, which results in a large linear ME effect, whose sign depends on the interlayer AFM order. We also achieve a complete and reversible electrical switching between the interlayer AFM and FM states in the vicinity of the interlayer spin-flip transition. The effect originates from the electric-field dependence of the interlayer exchange bias.

Spin currents and spin-orbit torques in ferromagnetic trilayers

Abstract: Magnetic torques generated through spin–orbit coupling1–8 promise energy-efficient spintronic devices. For applications, it is important that these torques switch films with perpendicular magnetizations without an external magnetic field9–14. One suggested approach 15 to enable such switching uses magnetic trilayers in which the torque on the top magnetic layer can be manipulated by changing the magnetization of the bottom layer. Spin currents generated in the bottom magnetic layer or its interfaces transit the spacer layer and exert a torque on the top magnetization. Here we demonstrate field-free switching in such structures and show that its dependence on the bottom-layer magnetization is not consistent with the anticipated bulk effects 15 . We describe a mechanism for spin-current generation16,17 at the interface between the bottom layer and the spacer layer, which gives torques that are consistent with the measured magnetization dependence. This other-layer-generated spin–orbit torque is relevant to energy-efficient control of spintronic devices. Spin–orbit torques are reported in ferromagnetic trilayers that lead to the switching of perpendicular magnetizations without an external magnetic field.

Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

Abstract: In conducting ferromagnets, an anomalous Nernst effect—the generation of an electric voltage perpendicular to both the magnetization and an applied temperature gradient—can be driven by the nontrivial geometric structure, or Berry curvature, of the wavefunction of the electrons1,2 Here, we report the observation of a giant anomalous Nernst effect at room temperature in the full-Heusler ferromagnet Co2MnGa, an order of magnitude larger than the previous maximum value reported for a magnetic conductor3,4 Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions5–7 is responsible for the observed –Tlog(T) behaviour, with T denoting the temperature, and the enhanced value of the transverse thermoelectric conductivity The temperature dependence of the thermoelectric response in experiments and numerical calculations can be understood in terms of a quantum critical-scaling function predicted by the low-energy effective theory over more than a decade of temperatures Moreover, the observation of an unsaturated positive longitudinal magnetoconductance, or chiral anomaly8–10, also provides evidence for the existence of Weyl fermions11,12 in Co2MnGa A magnetic field and temperature gradient produce a large electric potential in a ferromagnet, indicating the possible presence of Weyl points The specific structure of Weyl points gives the electrons quantum-critical properties

Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures.

Abstract: The "magnetoelectric effect" arises from the coupling between magnetic and electric properties in materials. The Z_{2} invariant of topological insulators (TIs) leads to a quantized version of this phenomenon, known as the topological magnetoelectric (TME) effect. This effect can be realized in a new topological phase called an "axion insulator" whose surface states are all gapped but the interior still obeys time reversal symmetry. We demonstrate such a phase using electrical transport measurements in a quantum anomalous Hall (QAH) sandwich heterostructure, in which two compositionally different magnetic TI layers are separated by an undoped TI layer. Magnetic force microscopy images of the same sample reveal sequential magnetization reversals of the top and bottom layers at different coercive fields, a consequence of the weak interlayer exchange coupling due to the spacer. When the magnetization is antiparallel, both the Hall resistance and Hall conductance show zero plateaus, accompanied by a large longitudinal resistance and vanishing longitudinal conductance, indicating the realization of an axion insulator state. Our findings thus show evidence for a phase of matter distinct from the established QAH state and provide a promising platform for the realization of the TME effect.

Tunneling Spin Valves Based on Fe3GeTe2/hBN/Fe3GeTe2 van der Waals Heterostructures.

Abstract: Thin van der Waals (vdW) layered magnetic materials hold the possibility of realizing vdW heterostructures with new functionalities. Here, we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers and a very sharp magnetization switching at magnetic field values that depends slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2 heterostructures, we observe textbook behavior of the tunneling resistance, which is minimum (maximum) when the magnetization in the two electrodes is parallel (antiparallel) to each other. The magnetoresistance is 160% at low temperature, from which we determine the spin polarization of Fe3GeTe2 to be 0.66, corresponding to 83% and 17% o...

Field-free switching of a perpendicular magnetic tunnel junction through the interplay of spin–orbit and spin-transfer torques

Abstract: Magnetization switching in magnetic tunnel junctions using spin-transfer torque and spin–orbit torque is key to the development of future spintronic memories. However, both switching mechanisms suffer from intrinsic limitations. In particular, the switching current in spin-transfer torque devices needs to be lowered, whereas an external magnetic field is required for spin–orbit torque devices to achieve deterministic switching in perpendicular magnetic tunnel junctions. Here, we experimentally demonstrate field-free switching of three-terminal perpendicular-anisotropy magnetic tunnel junction devices through the interaction between spin–orbit and spin-transfer torques. We show that the threshold current density of spin–orbit torque switching can be reduced by increasing the spin-transfer torque current density, and thus an optimal point for low-power perpendicular magnetic tunnel junction switching can be found by tuning the two current densities. Furthermore, and due to this interplay, low-power switching in two-terminal perpendicular magnetic tunnel junctions without an external magnetic field is also achieved. The interplay between spin–orbit and spin-transfer torques can be used to develop a low-power route to magnetization switching of perpendicular magnetic tunnel junctions without an external magnetic field.

Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates

Abstract: It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance.

Abstract: Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. However, the major challenge of current-induced switching in the nanopillars with both a large tunnel magnetoresistance ratio and a low junction resistance is still to be met. Here, we report spin transfer torque switching in nano-scale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 Ω µm2, which consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient resonant tunnelling transmission induced by the atom-thick W layers could contribute to the larger magnetoresistance ratio than conventional structures with Ta layers, in addition to the robustness of W layers against high-temperature diffusion during annealing. The critical switching current density could be lower than 3.0 MA cm−2 for devices with a 45-nm radius. Perpendicular magnetic tunnel junctions with large tunnel magnetoresistance and low junction resistance are promising for the magnetic random access memories. Here the authors achieve the spin-transfer-torque switching in perpendicular magnetic tunnel junctions with 249% tunnel magnetoresistance and low resistance-area product.

Single Ion Magnets from 3d to 5f: Developments and Strategies.

Abstract: Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc2 Tb]- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Hybrid chiral domain walls and skyrmions in magnetic multilayers

Abstract: Noncollinear spin textures in ferromagnetic ultrathin films are currently the subject of renewed interest since the discovery of the interfacial Dzyaloshinskii-Moriya interaction (DMI). This antisymmetric exchange interaction selects a given chirality for the spin textures and allows stabilizing configurations with nontrivial topology including chiral domain walls (DWs) and magnetic skyrmions. Moreover, it has many crucial consequences on the dynamical properties of these topological structures. In recent years, the study of noncollinear spin textures has been extended from single ultrathin layers to magnetic multilayers with broken inversion symmetry. This extension of the structures in the vertical dimension allows room temperature stability and very efficient current-induced motion for both Neel DWs and skyrmions. We show how, in these multilayered systems, the interlayer interactions can actually lead to hybrid chiral magnetization arrangements. The described thickness-dependent reorientation of DWs is experimentally confirmed by studying demagnetized multilayers through circular dichroism in x-ray resonant magnetic scattering. We also demonstrate a simple yet reliable method for determining the magnitude of the DMI from static domain measurements even in the presence of these hybrid chiral structures by taking into account the actual profile of the DWs. The existence of these novel hybrid chiral textures has far-reaching implications on how to stabilize and manipulate DWs, as well as skymionic structures in magnetic multilayers.

Observation of the nonlinear Hall effect under time reversal symmetric conditions.

Abstract: The electrical Hall effect is the production of a transverse voltage under an out-of-plane magnetic field. Historically, studies of the Hall effect have led to major breakthroughs including the discoveries of Berry curvature and the topological Chern invariants. In magnets, the internal magnetization allows Hall conductivity in the absence of external magnetic field. This anomalous Hall effect (AHE) has become an important tool to study quantum magnets. In nonmagnetic materials without external magnetic fields, the electrical Hall effect is rarely explored because of the constraint by time-reversal symmetry. However, strictly speaking, only the Hall effect in the linear response regime, i.e., the Hall voltage linearly proportional to the external electric field, identically vanishes due to time-reversal symmetry. The Hall effect in the nonlinear response regime, on the other hand, may not be subject to such symmetry constraints. Here, we report the observation of the nonlinear Hall effect (NLHE) in the electrical transport of the nonmagnetic 2D quantum material, bilayer WTe2. Specifically, flowing an electrical current in bilayer WTe2 leads to a nonlinear Hall voltage in the absence of magnetic field. The NLHE exhibits unusual properties sharply distinct from the AHE in metals: The NLHE shows a quadratic I-V characteristic; It strongly dominates the nonlinear longitudinal response, leading to a Hall angle of about 90 degree. We further show that the NLHE directly measures the "dipole moment" of the Berry curvature, which arises from layer-polarized Dirac fermions in bilayer WTe2. Our results demonstrate a new Hall effect and provide a powerful methodology to detect Berry curvature in a wide range of nonmagnetic quantum materials in an energy-resolved way.

A coronene-based semiconducting two-dimensional metal-organic framework with ferromagnetic behavior

Abstract: Metal–organic frameworks (MOFs) have so far been highlighted for their potential roles in catalysis, gas storage and separation. However, the realization of high electrical conductivity (>10−3 S cm−1) and magnetic ordering in MOFs will afford them new functions for spintronics, which remains relatively unexplored. Here, we demonstrate the synthesis of a two-dimensional MOF by solvothermal methods using perthiolated coronene as a ligand and planar iron-bis(dithiolene) as linkages enabling a full π-d conjugation. This 2D MOF exhibits a high electrical conductivity of ~10 S cm−1 at 300 K, which decreases upon cooling, suggesting a typical semiconductor nature. Magnetization and 57Fe Mossbauer experiments reveal the evolution of ferromagnetism within nanoscale magnetic clusters below 20 K, thus evidencing exchange interactions between the intermediate spin S = 3/2 iron(III) centers via the delocalized π electrons. Our results illustrate that conjugated 2D MOFs have potential as ferromagnetic semiconductors for application in spintronics.

Spin Direction-Controlled Electronic Band Structure in Two-Dimensional Ferromagnetic CrI3.

Abstract: Manipulating physical properties using the spin degree of freedom constitutes a major part of modern condensed matter physics and is a key aspect for spintronics devices. Using the newly discovered two-dimensional van der Waals ferromagnetic CrI3 as a prototype material, we theoretically demonstrated a giant magneto band-structure (GMB) effect whereby a change of magnetization direction significantly modifies the electronic band structure. Our density functional theory calculations and model analysis reveal that rotating the magnetic moment of CrI3 from out-of-plane to in-plane causes a direct-to-indirect bandgap transition, inducing a magnetic field controlled photoluminescence. Moreover, our results show a significant change of Fermi surface with different magnetization directions, giving rise to giant anisotropic magnetoresistance. Additionally, the spin reorientation is found to modify the topological states. Given that a variety of properties are determined by band structures, our predicted GMB effec...

Highly Efficient Spin-Current Generation by the Spin Hall Effect in Au 1 − x Pt x

Abstract: Current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnet systems are promising for efficiently manipulating magnetization in nanoscale spintronics, but the energy efficiency of SOT operations remains limited by a combination of material parameters. The authors report very efficient generation of spin current via the spin Hall effect in Au-Pt alloy, which combines a giant internal spin Hall ratio with a relatively low resistivity. This work establishes Au${}_{0.25}$Pt${}_{0.75}$ as a milestone spin-current generator, more energy-efficient than other heavy metals or topological insulators, to benefit the development of fast, efficient SOT-driven magnetic memory and other devices.