Anatomy of Hidden Dzyaloshinskii-Moriya Interactions and Topological Spin Textures in Centrosymmetric Crystals.

The Dzyaloshinskii-Moriya interaction (DMI) is understood to be forbidden by the symmetry of centrosymmetric systems, thus restricting the candidate types for investigating many correlated physical phenomena. Here, we report the hidden DMI existing in centrosymmetric magnets driven by the local inversion symmetry breaking of specific spin sublattices. The opposite DMI spatially localized on the inverse spin sublattice favors the separated spin spiral with opposite chirality. Furthermore, we elucidate that hidden DMI widely exists in many potential candidates, from the first-principles calculations on the mature crystal database. Interestingly, novel topological spin configurations, such as the anti-chirality-locked merons and antiferromagnetic-ferromagnetic meron chains, are stabilized as a consequence of hidden DMI. Our understanding enables the effective control of DMI by symmetry operations at the atomic level and enlarges the range of currently useful magnets for topological magnetism.

Tunable Spin Textures in a Kagome Antiferromagnetic Semimetal via Symmetry Design.

Kagome antiferromagnetic semimetals such as Mn3Sn have attracted extensive attention for their potential application in antiferromagnetic spintronics. Realizing high manipulation of kagome antiferromagnetic spin states at room temperature can reveal rich emergent phenomena resulting from the quantum interactions between topology, spin, and correlation. Here, we achieved tunable spin textures of Mn3Sn through symmetry design by controlling alternate Mn3Sn and heavy-metal Pt thicknesses. The various topological spin textures were predicted with theoretical simulations, and the skyrmion-induced topological Hall effect, strong spin-dependent scattering, and vertical gradient of spin states were obtained by magnetotransport and magnetic circular dichroism (MCD) spectroscopy measurements in Mn3Sn/Pt heterostructures. Our work provides an effective strategy for the innovative design of topological antiferromagnetic spintronic devices.

Room-Temperature van der Waals Ferromagnet Switching by Spin-Orbit Torques.

The emerging wide varieties of the van der Waals (vdW) magnets with atomically thin and smooth interfaces hold great promise for next-generation spintronic devices. However, due to the lower Curie temperature of the vdW ferromagnets than room temperature, electrically manipulating its magnetization at room temperature has not been realized. In this work, it is demonstrated that the perpendicular magnetization of the vdW ferromagnet Fe3 GaTe2 can be effectively switched at room temperature in the Fe3 GaTe2 /Pt bilayer by spin-orbit torques (SOTs) with a relatively low current density of 1.3 × 107 A cm-2 . Moreover, the high SOT efficiency of ξDL ≈ 0.28 is quantitatively determined by harmonic measurements, which is higher than those in Pt-based heavy metal/conventional ferromagnet devices. The findings of room-temperature vdW ferromagnet switching by SOTs provide a significant basis for the development of vdW-ferromagnet-based spintronic applications.

Large and tunable magnetoresistance in van der Waals ferromagnet/semiconductor junctions.

Magnetic tunnel junctions (MTJs) with conventional bulk ferromagnets separated by a nonmagnetic insulating layer are key building blocks in spintronics for magnetic sensors and memory. A radically different approach of using atomically-thin van der Waals (vdW) materials in MTJs is expected to boost their figure of merit, the tunneling magnetoresistance (TMR), while relaxing the lattice-matching requirements from the epitaxial growth and supporting high-quality integration of dissimilar materials with atomically-sharp interfaces. We report TMR up to 192% at 10 K in all-vdW Fe3GeTe2/GaSe/Fe3GeTe2 MTJs. Remarkably, instead of the usual insulating spacer, this large TMR is realized with a vdW semiconductor GaSe. Integration of semiconductors into the MTJs offers energy-band-tunability, bias dependence, magnetic proximity effects, and spin-dependent optical-selection rules. We demonstrate that not only the magnitude of the TMR is tuned by the semiconductor thickness but also the TMR sign can be reversed by varying the bias voltages, enabling modulation of highly spin-polarized carriers in vdW semiconductors.

Electrically function-switchable magnetic domain-wall memory.

Versatile memory is strongly desired for end users, to protect their information in the information era. In particular, bit-level switchable memory that can be switched from rewritable to read-only function would allow end users to prevent important data being tampered with. However, no such switchable memory has been reported. We demonstrate that the rewritable function can be converted into read-only function by applying a sufficiently large current pulse in a U-shaped domain-wall memory, which comprises an asymmetric Pt/Co/Ru/AlOx heterostructure with strong Dzyaloshinskii-Moriya interaction. Wafer-scale switchable magnetic domain-wall memory arrays on 4-inch Si/SiO2 substrate are demonstrated. Furthermore, we confirm that the information can be stored in rewritable or read-only states at bit level according to the security needs of end users. Our work not only provides a solution for personal confidential data, but also paves the way for developing multifunctional spintronic devices.

High-Sensitivity and Fast-Speed UV Photodetectors Based on Asymmetric Nanoporous-GaN/Graphene Vertical Junction.

GaN-based photodetectors are strongly desirable in many advanced fields, such as space communication, environmental monitoring, etc. However, the slow photo-response speed in currently reported high-sensitivity GaN-based photodetectors still hinders their applications. Here, we demonstrate a high-sensitivity and fast-speed UV photodetector based on asymmetric Au/nanoporous-GaN/graphene vertical junctions. The nanoporous GaN-based vertical photodetector shows an excellent rectification ratio up to ∼105 at +4 V/-4 V. The photo-responsivity and specific detectivity of the device is up to 1.01 × 104 A/W and 7.84 × 1014 Jones, respectively, more than three orders of magnitude higher than the control planar photodetector. With switching light on and off, the repeatable on/off current ratio of the nanoporous GaN-based vertical photodetector is ∼4.32 × 103, which is about 1.51 × 103 times to that of the control planar device. The measured rise/decay time is 12.2 µs/14.6 µs, which is the fastest value for the high-sensitivity GaN-based photodetectors to date. These results suggest that the asymmetric Au/nanoporous-GaN/graphene structure can improve the sensitivity and the photo-response speed of GaN-based PDs simultaneously.

All-electrical switching of a topological non-collinear antiferromagnet at room temperature.

Non-collinear antiferromagnetic Weyl semimetals, combining the advantages of a zero stray field and ultrafast spin dynamics, as well as a large anomalous Hall effect and the chiral anomaly of Weyl fermions, have attracted extensive interest. However, the all-electrical control of such systems at room temperature, a crucial step toward practical application, has not been reported. Here, using a small writing current density of around 5 × 106 A·cm-2, we realize the all-electrical current-induced deterministic switching of the non-collinear antiferromagnet Mn3Sn, with a strong readout signal at room temperature in the Si/SiO2/Mn3Sn/AlOx structure, and without external magnetic field or injected spin current. Our simulations reveal that the switching originates from the current-induced intrinsic non-collinear spin-orbit torques in Mn3Sn itself. Our findings pave the way for the development of topological antiferromagnetic spintronics.

Topological Spin Textures in a Non-Collinear Antiferromagnet System.

Topologically protected magnetic "whirls" such as skyrmions in antiferromagnetic materials have recently attracted extensive interest due to their nontrivial band topology and potential application in antiferromagnetic spintronics. However, room-temperature skyrmions in natural metallic antiferromagnetic materials with merit of probable convenient electrical manipulation have not been reported. Here, room-temperature skyrmions are realized in a non-collinear antiferromagnet, Mn3 Sn, capped with a Pt overlayer. The evolution of spin textures from coplanar inverted triangular structures to Bloch-type skyrmions is achieved via tuning the magnitude of interfacial Dzyaloshinskii-Moriya interaction. Beyond that, the temperature can induce an unconventional transition from skyrmions to antiferromagnetic meron-like spin textures at ≈220 K in the Mn3 Sn/Pt samples. Combining with the theoretical calculations, it is found that the transition originates from the temperature dependence of antiferromagnetic exchange interaction between kagome sublayers within the Mn3 Sn crystalline unit-cell. These findings open the avenue for the development of topological spin-swirling-based antiferromagnetic spintronics.

Selectively Controlled Ferromagnets by Electric Fields in van der Waals Ferromagnetic Heterojunctions.

Charge transfer plays a key role at the interfaces of heterostructures, which can affect electronic structures and ultimately the physical properties of the materials. However, charge transfer is difficult to manipulate externally once the interface is formed. The recently discovered van der Waals ferromagnets with atomically sharp interfaces provided a perfect platform for the electrical control of interfacial charge transfer. Here, we report magnetoresistance experiments revealing electrically tunable charge transfer in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 all-magnetic van der Waals heterostructures, which can be exploited to selectively modify the switching fields of the top or bottom Fe3GeTe2 electrodes. The directional charge transfer from metallic Fe3GeTe2 to semiconducting Cr2Ge2Te6 is revealed by first-principles calculations, which remarkably modifies the magnetic anisotropy energy of Fe3GeTe2, leading to the dramatically suppressed coercivity. The electrically selective control of magnetism demonstrated in this study could stimulate the development of spintronic devices based on van der Waals magnets.

Progress of GaN-Based Optoelectronic Devices Integrated with Optical Resonances.

Being direct wide bandgap, III-nitride (III-N) semiconductors have many applications in optoelectronics, including light-emitting diodes, lasers, detectors, photocatalysis, etc. Incorporation of III-N semiconductors with high-efficiency optical resonances including surface plasmons, distributed Bragg reflectors and micro cavities, has attracted considerable interests for upgrading their performance, which can not only reveal the new coupling mechanisms between optical resonances and quasiparticles, but also unveil the shield of novel optoelectronic devices with superior performances. In this review, the content covers the recent progress of GaN-based optoelectronic devices integrated with plasmonics and/or micro resonators, including the LEDs, photodetectors, solar cells, and light photocatalysis. The authors aim to provide an inspiring insight of recent remarkable progress and breakthroughs, as well as a promising prospect for the future highly-integrated, high speed, and efficient GaN-based optoelectronic devices.

Current-assisted magnetization reversal in Fe3GeTe2 van der Waals homojunctions.

Among the numerous two-dimensional van der Waals (vdW) magnetic materials, Fe3GeTe2 (FGT), due to its outstanding properties such as metallicity, high Curie temperature and strong perpendicular magnetic anisotropy, has quickly emerged as a candidate with the most potential for the fabrication of all-vdW spintronic devices. Here, we fabricated a simple vertical homojunction based on two few-layer exfoliated FGT flakes. Under a certain range of external magnetic fields, the magnetization reversal can be achieved by applying a negative or positive pulse current, which can reduce the coercivity through the spin orbit torque of FGT itself in addition to the Joule heat. Moreover, the asymmetrical switching current is caused by the spin transfer torque in the homojunction. As the temperature increases, the magnetization reversal can be observed at a smaller external magnetic field. Our demonstrations of the current-assisted magnetization reversal under a magnetic field in all-vdW structures may provide support for the potential application of vdW magnetism.

A nanopillar-modified high-sensitivity asymmetric graphene-GaN photodetector.

Integration of two-dimensional (2D) materials with three-dimensional (3D) semiconductors leads to intriguing optical and electrical properties that surpass those of the original materials. Here, we report the high performance of a GaN nanopillar-modified graphene/GaN/Ti/Au photodetector (PD). After etching on the surface of a GaN film, GaN nanopillars exhibit multiple functions for improving the detection performance of the PD. Under dark conditions, surface etching reduces the contact area of GaN with the graphene electrode, leading to a reduced dark current for the PD. When illuminated with UV light, the nanopillars enable an enhanced and localized electric field inside GaN, resulting in an ∼20% UV light absorption enhancement and a several-fold increased photocurrent. In addition, the nanopillars are intentionally etched beneath the metal Ti/Au electrode to modify the semiconductor-metal junction. Further investigation shows that the modified GaN/Ti/Au contact triggers a prominent rectifying I-V behaviour. Benefiting from the nanopillar modification, the proposed PD shows a record large detectivity of 1.85 × 1017 Jones, a small dark current of 5.2 nA at +3 V bias, and a nearly three order of magnitude rectification ratio enhancement compared with non-nanopillar PDs. This pioneering work provides a novel nanostructure-modifying method for combining 2D materials and 3D semiconductors to improve the performances of electronic and optoelectronic devices.

Large Tunneling Magnetoresistance in van der Waals Ferromagnet/Semiconductor Heterojunctions.

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe3 GeTe2 as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe3 GeTe2 /InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

Prospect of Spin-Orbitronic Devices and Their Applications.

Science, engineering, and medicine ultimately demand fast information processing with ultra-low power consumption. The recently developed spin-orbit torque (SOT)-induced magnetization switching paradigm has been fueling opportunities for spin-orbitronic devices, i.e., enabling SOT memory and logic devices at sub-nano second and sub-picojoule regimes. Importantly, spin-orbitronic devices are intrinsic of nonvolatility, anti-radiation, unlimited endurance, excellent stability, and CMOS compatibility, toward emerging applications, e.g., processing in-memory, neuromorphic computing, probabilistic computing, and 3D magnetic random access memory. Nevertheless, the cutting-edge SOT-based devices and application remain at a premature stage owing to the lack of scalable methodology on the field-free SOT switching. Moreover, spin-orbitronics poises as an interdisciplinary field to be driven by goals of both fundamental discoveries and application innovations, to open fascinating new paths for basic research and new line of technologies. In this perspective, the specific challenges and opportunities are summarized to exert momentum on both research and eventual applications of spin-orbitronic devices.

Spin-Valve Effect in Fe3GeTe2/MoS2/Fe3GeTe2 van der Waals Heterostructures.

The van der Waals (vdW) materials offer an opportunity to build all-two-dimensional (all-2D) spintronic devices with high-quality interfaces regardless of the lattice mismatch. Here, we report on an all-2D vertical spin valve that combines a typical layered semiconductor MoS2 with vdW ferromagnetic metal Fe3GeTe2 (FGT) flakes. The linear current-voltage curves illustrate that Ohmic contacts are formed in FGT/MoS2 interfaces, while the temperature dependence of the junction resistance further demonstrates that the MoS2 interlayer acts as a conducting layer instead of a tunneling layer. In addition, the magnitude of the magnetoresistance (MR) of 3.1% at 10 K is observed, which is around 8 times larger than that of the reported spin valves based on MoS2 sandwiched by conventional ferromagnetic electrodes. The MR decreasing monotonically with increasing temperature follows the Bloch's law. As the bias current decreases exponentially, the MR increases linearly up to a maximum value of 4.1%. Our results reveal the potential opportunities of vdW heterostructures for developing novel spintronic devices.

Magnetic Skyrmions in a Hall Balance with Interfacial Canted Magnetizations.

Magnetic skyrmions are attracting interest as efficient information-storage devices with low energy consumption, and have been experimentally and theoretically investigated in multilayers including ferromagnets, ferrimagnets, and antiferromagnets. The 3D spin texture of skyrmions demonstrated in ferromagnetic multilayers provides a powerful pathway for understanding the stabilization of ferromagnetic skyrmions. However, the manipulation mechanism of skyrmions in antiferromagnets is still lacking. A Hall balance with a ferromagnet/insulating spacer/ferromagnet structure is considered to be a promising candidate to study skyrmions in synthetic antiferromagnets. Here, high-density Néel-type skyrmions are experimentally observed at zero field and room temperature by Lorentz transmission electron microscopy in a Hall balance (core structure [Co/Pt]n /NiO/[Co/Pt]n ) with interfacial canted magnetizations because of interlayer ferromagnetic/antiferromagnetic coupling between top and bottom [Co/Pt]n multilayers, where the Co layers in [Co/Pt]n are always ferromagnetically coupled. Micromagnetic simulations show that the generation and density of skyrmions are strongly dependent on interlayer exchange coupling (IEC) and easy-axis orientation. Direct experimental evidence of skyrmions in synthetic antiferromagnets is provided, suggesting that the proposed approach offers a promising alternative mechanism for room-temperature spintronics.

Boost of single-photon emission by perfect coupling of InAs/GaAs quantum dot and micropillar cavity mode.

We proposed a precise calibration process of Al 0.9Ga0.1As/GaAs DBR micropillar cavity to match the single InAs/GaAs quantum dot (QD) exciton emission and achieve cavity mode resonance and a great enhancement of QD photoluminescence (PL) intensity. Light-matter interaction of single QD in DBR micropillar cavity (Q ∼ 3800) under weak coupling regime was investigated by temperature-tuned PL spectra; a pronounced enhancement (14.6-fold) of QD exciton emission was observed on resonance. The second-order autocorrelation measurement shows g(2)(0)=0.070, and the estimated net count rate before the first objective lens reaches 1.6×107 counts/s under continuous wave excitation, indicating highly pure single-photon emission at high count rates.

Deterministic Magnetization Switching Using Lateral Spin-Orbit Torque.

Current-induced magnetization switching by spin-orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin-orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, deterministic current-induced SOT full magnetization switching by lateral spin-orbit torque in zero external magnetic field is reported. The Pt/Co/Pt magnetic structure is locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, two independent mechanisms giving rise to SOT are identified, i.e., the lateral Pt-Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track.

From two- to multi-state vertical spin valves without spacer layer based on Fe3GeTe2 van der Waals homo-junctions.

Different than covalently bonded magnetic multilayer systems, high-quality interfaces without dangling bonds in van der Waals (vdW) junctions of two-dimensional (2D) layered magnetic materials offer opportunities to realize novel functionalities. Here, we report the fabrication of multi-state vertical spin valves without spacer layers by using vdW homo-junctions in which exfoliated Fe3GeTe2 nanoflakes act as ferromagnetic electrodes and/or interlayers. We demonstrate the typical behavior of two-state and three-state magnetoresistance for devices with two and three Fe3GeTe2 nanoflakes, respectively. Distinct from traditional spin valves with sandwich structures, our novel homo-junction-based spin-valve structure allows the straightforward realization of multi-state magnetic devices. Our work demonstrates the possibility of extend multi-state, non-volatile spin information to 2D magnetic homo-junctions, and it emphasizes the utility of vdW interface as a fundamental building block for spintronic devices.

Atomic origin of spin-valve magnetoresistance at the SrRuO3 grain boundary.

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order.