Manipulation of the Topological Ferromagnetic State in a Weyl Semimetal by Spin-Orbit Torque.

Magnetic Weyl semimetals (MWSMs) exhibit unconventional transport phenomena, such as large anomalous Hall (and Nernst) effects, which are absent in spatial inversion asymmetry WSMs. Compared with its nonmagnetic counterpart, the magnetic state of a MWSM provides an alternative way for the modulation of topology. Spin-orbit torque (SOT), as an effective means of electrically controlling the magnetic states of ferromagnets, may be used to manipulate the topological magnetic states of MWSMs. Here we confirm the MWSM state of high-quality Co2MnGa film by systematically investigating the transport measurements and demonstrating that the magnetization and topology of Co2MnGa can be electrically manipulated. The electrical and magnetic optical measurements further reveal that the current-induced SOT switches the topological magnetic state in a 180-degree manner by applying positive/negative current pulses and in a 90-degree manner by alternately applying two orthogonal current pulses. This work opens up more opportunities for spintronic applications based on topological materials.

Coexistence of Magnon-Induced and Rashba-Induced Unidirectional Magnetoresistance in Antiferromagnets.

Unidirectional magnetoresistance (UMR) has been intensively studied in ferromagnetic systems, which is mainly induced by spin-dependent and spin-flip electron scattering. Yet, UMR in antiferromagnetic (AFM) systems has not been fully understood to date. In this work, we reported UMR in a YFeO3/Pt heterostructure where YFeO3 is a typical AFM insulator. Magnetic-field dependence and temperature dependence of transport measurements indicate that magnon dynamics and interfacial Rashba splitting are two individual origins for AFM UMR, which is consistent with the UMR theory in ferromagnetic systems. We further established a comprehensive theoretical model that incorporates micromagnetic simulation, density functional theory calculation, and the tight-binding model, which explain the observed AFM UMR phenomenon well. Our work sheds light on the intrinsic transport property of the AFM system and may facilitate the development of AFM spintronic devices.

Effective electrical manipulation of a topological antiferromagnet by orbital torques.

The electrical control of the non-trivial topology in Weyl antiferromagnets is of great interest for the development of next-generation spintronic devices. Recent studies suggest that the spin Hall effect can switch the topological antiferromagnetic order. However, the switching efficiency remains relatively low. Here, we demonstrate the effective manipulation of antiferromagnetic order in the Weyl semimetal Mn3Sn using orbital torques originating from either metal Mn or oxide CuOx. Although Mn3Sn can convert orbital current to spin current on its own, we find that inserting a heavy metal layer, such as Pt, of appropriate thickness can effectively reduce the critical switching current density by one order of magnitude. In addition, we show that the memristor-like switching behaviour of Mn3Sn can mimic the potentiation and depression processes of a synapse with high linearity-which may be beneficial for constructing accurate artificial neural networks. Our work paves a way for manipulating the topological antiferromagnetic order and may inspire more high-performance antiferromagnetic functional devices.

Enhancement of Out-of-Plane Spin-Orbit Torque by Interfacial Modification.

Spin-orbit torque (SOT)-induced switching of perpendicular magnetization in the absence of magnetic field is crucial for the application of SOT-based spintronic devices. Recent works have demonstrated that the low-symmetry crystal structure in CuPt/CoPt can give rise to an out-of-plane (OOP) spin torque and lead to deterministic magnetization switching without an external field. However, it is essential to improve OOP effective field for the efficient switching. In this work, the impact of interface oxidation on the generation of OOP effective field in a CuPt/ferromagnet heterostructure is systematically studied. By introducing an oxidized CuPt surface, it is found that the field-free switching performance shows remarkable improvement. OOP effective field measurement indicates that the oxidation treatment can enhance the OOP effective field by more than two times. It is also demonstrated that this oxidation-induced OOP SOT efficiency enhancement is independent of the device shapes, magnetic materials, or magnetization easy axis. This work contributes to improve the performance of SOT devices and provides an effective fabrication guidance for future spintronic devices that utilize OOP SOT.

Efficient Spin-Orbit Torque Switching in a Perpendicularly Magnetized Heusler Alloy MnPtGe Single Layer.

Electrically manipulating magnetic moments by spin-orbit torque (SOT) has great potential applications in magnetic memories and logic devices. Although there have been rich SOT studies on magnetic heterostructures, low interfacial thermal stability and high switching current density still remain an issue. Here, highly textured, polycrystalline Heusler alloy MnxPtyGe (MPG) films with various thicknesses are directly deposited onto thermally oxidized silicon wafers. The perpendicular magnetization of the MPG single layer can be reversibly switched by electrical current pulses with a magnitude as low as 4.1 × 1010Am-2, as evidenced by both the electrical transport and the magnetic optical measurements. The switching is shown to arise from inversion symmetry breaking due to the vertical composition gradient of the films after sample annealing. The SOT effective fields of the samples are analyzed systematically. It is found that the SOT efficiency increases with the film thickness, suggesting a robust bulk-like behavior in the single magnetic layer. Furthermore, a memristive characteristic has been observed due to a multidomain switching property in the single-layer MPG device. Additionally, deterministic field-free switching of magnetization is observed when the electric current flows orthogonal to the direction of the in-plane compositional gradient due to the in-plane symmetry breaking. This work proves that the MPG is a good candidate to be utilized in high-density and efficient magnetoresistive random access memory devices and other spintronic applications.

Current-induced self-switching of perpendicular magnetization in CoPt single layer.

All-electric switching of perpendicular magnetization is a prerequisite for the integration of fast, high-density, and low-power magnetic memories and magnetic logic devices into electric circuits. To date, the field-free spin-orbit torque (SOT) switching of perpendicular magnetization has been observed in SOT bilayer and trilayer systems through various asymmetric designs, which mainly aim to break the mirror symmetry. Here, we report that the perpendicular magnetization of CoxPt100-x single layers within a special composition range (20 < x < 56) can be deterministically switched by electrical current in the absence of external magnetic field. Specifically, the Co30Pt70 shows the largest out-of-plane effective field efficiency and best switching performance. We demonstrate that this unique property arises from the cooperation of two structural mechanisms: the low crystal symmetry property at the Co platelet/Pt interfaces and the composition gradient along the thickness direction. Compared with that in bilayers or trilayers, the field-free switching in CoxPt100-x single layer greatly simplifies the SOT structure and avoids additional asymmetric designs.

Enhanced Tunneling Magnetoresistance Effect via Ferroelectric Control of Interface Electronic/Magnetic Reconstructions.

Magnetic tunnel junctions (MTJs) with tunable tunneling magnetoresistances (TMR) have already been proven to have great potential for spintronics. Especially, when ferroelectric materials are used as insulating barriers, more novel functions of MTJs can be realized due to interface magnetoelectric coupling. Here, we demonstrate a very large ferroelectric modulation of TMR (as high as 570% in low-resistance state) in the ferroelectric/magnetic La0.5Sr0.5MnO3/BaTiO3 (LSMO/BTO) junctions and find robust interfacial electronic and magnetic reconstructions via ferroelectric polarization switching. Through electrical, magnetic, and optical measurements combined with X-ray absorption and magnetic circular dichroism, we reveal that the interfacial electronic and magnetic (ferromagnetic/antiferromagnetic phase transition) reconstructions originate from strong electromagnetic coupling between BTO and LSMO at the interface and are driven by the modulation of hole/electron doping at the interface of LSMO/BTO through ferroelectric polarization switching. As a result, the ferroelectrically controlled interface barrier height and width and spin filter effect enable a giant electrical modulation of TMR. Our results shed new light on the intrinsic mechanisms governing magnetoelectric coupling and offering a new route to enhance magnetoelectric coupling for spin control in spintronic devices.

Spin-Orbit Torque Switching of a High-Quality Perpendicularly Magnetized Ferrimagnetic Heusler Mn3Ge Film.

Current-induced spin-orbit torque (SOT) switching of magnetization has attracted great interest due to its potential application in magnetic memory devices, which offer low-energy consumption and high-speed writing. However, most of the SOT studies on perpendicularly magnetized anisotropy (PMA) magnets have been limited to heterostructures with interfacial PMA and poor thermal stability. Here, we experimentally demonstrate a SOT magnetization switching for a ferrimagnetic D022-Mn3Ge film with high bulk PMA and robust thermal stability factor under a critical current density of 6.6 × 1011 A m-2 through the spin Hall effect of an adjacent capping Pt and a buffer Cr layer. A large effective damping-like SOT efficiency of 2.37 mT/1010 A m-2 is determined using harmonic measurements in the structure. The effect of the double-spin source layers and the negative-exchange interaction of the ferrimagnet may explain the large SOT efficiency and the manifested magnetization switching of Mn3Ge. Our findings demonstrate that D022-Mn3Ge is a promising candidate for application in high-density SOT magnetic random-access memory devices.

2D magnetic MoS2-Fe3O4 hybrid nanostructures for ultrasensitive exosome detection in GMR sensor.

Exosome released from cells plays an important role in intercellular communication and show great clinical potential in early cancer screening and prognosis. Herein, an ultrasensitive Giant Magnetoresistance (GMR) biosensor was developed for exosome detection, which was based on 2D MoS2-Fe3O4 nanostructures (MOFE) as magnetic responsive probes for signal amplification. The MOFE can be readily synthesized with simple phase transfer method. Compared to pure Fe3O4 magnetic nanoparticles (MNPs), the layered MoS2 function as a template for recruiting high loading density of MNPs as magnetic probes. After modified by aptamer, we discover that the 2D magnetic MOFE hybrid nanostructures enable both multidentate targeting and multi-magnetic particle-based signal amplification, increasing the magnetic sensor performance, especially in sensitivity and output signal. Moreover, the 2D magnetic nanocomposites afford high selectivity and excellent reproducibility with detection limit of 100 exosomes in GMR sensor. Results demonstrate that the magnetic strategy based on 2D structures introduced here provide a new dimension for exosome detection, which show great potential of engineering 2D magnetic nanobioprobes for GMR based liquid biopsy application.

Bipolar resistance switching in transparent ITO/LaAlO3/SrTiO3 memristors.

We report reversible bipolar resistance switching behaviors in transparent indium-tin oxide (ITO)/LaAlO3/SrTiO3 memristors at room temperature. The memristors exhibit high optical transparency, long retention, and excellent antifatigue characteristics. The high performances are promising for employing ITO/LaAlO3/SrTiO3 memristors in nonvolatile transparent memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the migration of positively charged oxygen vacancies from the SrTiO3 substrate to the LaAlO3 film, resulting in Poole-Frenkel emission for the low resistance state and thermionic emission for the high resistance state.