Antiferromagnetic Spintronics Research Articles

Real-time observation of magnetization and magnon dynamics in a two-dimensional topological antiferromagnet MnBi2Te4

Atomically thin van der Waals magnetic materials have not only provided a fertile playground to explore basic physics in the two-dimensional (2D) limit but also created vast opportunities for novel ultrafast functional devices. Here we systematically investigate ultrafast magnetization dynamics and spin wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a function of layer number, temperature, and magnetic field. We find laser-induced (de)magnetization processes can be used to accurately track the distinct magnetic states in different magnetic field regimes, including showing clear odd–even layer number effects. In addition, strongly field-dependent AFM magnon modes with tens of gigahertz frequencies are optically generated and directly observed in the time domain. Remarkably, we find that magnetization and magnon dynamics can be observed in not only the time-resolved magneto-optical Kerr effect but also the time resolved reflectivity, indicating strong correlation between the magnetic state and electronic structure. These measurements present the first comprehensive overview of ultrafast spin dynamics in this novel 2D antiferromagnet, paving the way for potential applications in 2D antiferromagnetic spintronics and magnonics as well as further studies of ultrafast control of both magnetization and topological quantum states.

Visualization of antiferromagnetic domains by nonreciprocal directional dichroism and related optical responses

Nonreciprocal directional dichroism (NDD) is a phenomenon in which the optical absorption is changed by reversing the direction of light propagation or the sign of the magnetic order parameters. While the NDD has mostly been observed in materials with macroscopic magnetization, recent experiments have shown that the NDD can be induced by a specific antiferromagnetic (AFM) spin structure that breaks both space-inversion and time-reversal symmetries. This opens the possibility of visualizing the spatial distribution of AFM domains via the NDD effect. This article reviews the basic features of the NDD, a brief history of the NDD in AFM materials, and recent achievements in visualizing AFM domains via the NDD and related optical responses, and finally provides a perspective on applications of this method for future AFM spintronics research.

Antiferromagnetic second-order topology in two-dimensional NiRuCl6

In recent years, 2D second-order topological insulators (SOTIs) have garnered considerable interest because of their unique properties. However, only the FeSe monolayer with four corner states (two occupied and two unoccupied states) near the Fermi level has been reported to be a candidate for 2D intrinsic antiferromagnetic SOTIs in theory. The limited amount of antiferromagnetic SOTIs has hindered future research, and corner states should be at the Fermi level in order to manifest interesting physics. Herein, we propose NiRuCl6 as a candidate for 2D antiferromagnetic SOTIs with corner states strictly at the Fermi level. Without spin–orbit coupling (SOC), NiRuCl6 is an antiferromagnetic half-metal with a compensating magnetic moment and decoupled spin bands. In the spin-up channel, NiRuCl6 hosts a nontrivial gap of 1.11 eV, where zero-dimensional corner states appear. In the spin-down channels, NiRuCl6 hosts metallically behaved bands, where a spin-polarized quadratic Weyl point emerges. With SOC, two spin bands are coupled, and NiRuCl6 becomes an antiferromagnetic SOTI with three degenerate corner states at the Fermi level inside the SOC-induced gap with a value of 0.11 eV. Remarkably, the corner states in NiRuCl6 are resistant to changes in SOC strength and magnetization orientation. We also reveal that the phononic second-order topology and corner vibrational modes appear in the phonon dispersion curves of NiRuCl6. The presented results improve the general understanding of antiferromagnetic SOTIs and contribute to the prediction of materials with ideal corner states at the Fermi level, thereby advancing the field of topological antiferromagnetic spintronics.

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films.

In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.

Chiral Ruthenium Halide Semiconductor with Strong Antiferromagnetic Coupling

AbstractChiral hybrid organic–inorganic metal halides have attracted wide attention owing to their superior chiroptoelectronic and chirospintronic properties. Here, a new class of chiral lead‐free metal halides, (R/S‐MBA)3Ru2Br9 (MBA = methylbenzylammonium), with unique magnetic properties is reported. (R/S‐MBA)3Ru2Br9 is composed of a surface‐shared octahedron dimer separated by chiral cations and crystallizes into the Sohncke space group of P212121. The crystal structures, chiroptical, magnetic properties, and magnetic circular dichroism of (R/S‐MBA)3Ru2Br9 are systematically investigated. Owing to the strong antiferromagnetic coupling between the Ru atom through bromine bridge in the [Ru2Br9]3− dimer and the weak ferromagnetic coupling between the neighboring [Ru2Br9]3− dimer, (R/S‐MBA)3Ru2Br9 exhibits antiferromagnetic property with Néel temperature at 7 K. Considering the unique magnetic property of ruthenium, together with the rapid progress of antiferromagnetic spintronics and optospintronics recently, this work provides a novel multifunctional lead‐free chiral magnetic candidate toward chiral optoelectronics and antiferromagnetic spintronics.

Implementation of logical operations using antiferromagnetic helical molecule

Abstract For the first time, we explore all possible two-input logical operations (OR, AND, NOT, NOR, NAND, XOR, and XNOR) using a single-stranded antiferromagnetic (AFM) helix system. The AFM helix produces a large amount of spin current in the presence of a transverse electric field. The spin current, considered as the output, can be tuned by the strength and direction of the electric field that are taken as the inputs of the logic gates. All the results are spin-based, which reduces the loss due to heating energy, and since the operations are reprogrammable, one can have all the gates in a single system simply by rearranging the setup. Based on the growth of experimental realization of AFM helical systems we believe that our results can be verified and may lead to a new direction in the branch of AFM spintronics.

Electrochemical manipulation of spin reorientation in FePO[formula omitted] by Li-ion intercalation

Density functional theory plus U and Monte Carlo simulations reveal that the electrochemical lithiation/delithiation process has a dramatic effect on magnetic structure and magnetocrystalline anisotropy (MCA) of an olivine LixFePO4 (0 ≤ x ≤ 1) structure. We demonstrate that MCA undergoes a transition from the magnetic easy axis along the [100] direction for FePO4 (fully delithiated phase) to the [010] direction for the partially and fully lithiated phases of LixFePO4 with x ≥ 0.25. This magnetization reorientation is the result of the interplay between the lithiation/delithiation induced changes in the spin–orbit coupled 3d-electron orbital states near the Fermi level and the high-spin crystal field split of the Fe3＋ and Fe2＋ ionic states. Furthermore, the lithium intercalation substantially reduces the energy deviation of an antiferromagnetic state from a ferromagnetic phase, reflecting the experimental observation of lowering the Néel temperature from 125 K for FePO4 to 52 K for LiFePO4, which are fairly reproduced by the Monte Carlo simulation. These findings open interesting prospects for exploiting electrochemical lithiation/delithiation process to manipulate magnetization direction in antiferromagnet spintronics.

Long-range, non-local switching of spin textures in a frustrated antiferromagnet

Antiferromagnetic spintronics is an emerging area of quantum technologies that leverage the coupling between spin and orbital degrees of freedom in exotic materials. Spin-orbit interactions allow spin or angular momentum to be injected via electrical stimuli to manipulate the spin texture of a material, enabling the storage of information and energy. In general, the physical process is intrinsically local: spin is carried by an electrical current, imparted into the magnetic system, and the spin texture will then rotate in the region of current flow. In this study, we show that spin information can be transported and stored “non-locally" in the material FexNbS2. We propose that collective modes can manipulate the spin texture away from the flowing current, an effect amplified by strong magnetoelastic coupling of the ordered state. This suggests a novel way to store and transport spin information in strongly spin-orbit coupled magnetic systems.

Intrinsic Nonlinear Hall Detection of the Néel Vector for Two-Dimensional Antiferromagnetic Spintronics.

The respective unique merit of antiferromagnets and two-dimensional (2D) materials in spintronic applications inspires us to exploit 2D antiferromagnetic spintronics. However, the detection of the Néel vector in 2D antiferromagnets remains a great challenge because the measured signals usually decrease significantly in the 2D limit. Here we propose that the Néel vector of 2D antiferromagnets can be efficiently detected by the intrinsic nonlinear Hall (INH) effect which exhibits unexpected significant signals. As a specific example, we show that the INH conductivity of the monolayer manganese chalcogenides MnX (X=S, Se, Te) can reach the order of nm·mA/V^{2}, which is orders of magnitude larger than experimental values of paradigmatic antiferromagnetic spintronic materials. The INH effect can be accurately controlled by shifting the chemical potential around the band edge, which is experimentally feasible via electric gating or charge doping. Moreover, we explicitly demonstrate its 2π-periodic dependence on the Néel vector orientation based on an effective k·p model. Our findings enable flexible design schemes and promising material platforms for spintronic memory device applications based on 2D antiferromagnets.

Spin photovoltaic effect in antiferromagnetic materials: Mechanisms, symmetry constraints, and recent progress

Antiferromagnetic (AFM) materials possess unique properties, such as rapid dynamic response, resistance to external magnetic disturbances, and the absence of a stray field. AFM materials are important members in the field of spintronics, and generating the spin current in AFM materials is one of the vital topics for AFM spintronics. The spin photovoltaic effect (SPVE) is the spin counterpart of the bulk photovoltaic effect (BPVE), where the photocurrent is spin-polarized. This effect can generate spin current in a contactless and ultra-fast way. Recently, SPVE has garnered significant interest due to its potential application in AFM spintronics and rich physical content. In this perspective, the mechanism of SPVE, including the relationship between SPVE and BPVE, and symmetry constraints are reviewed. We also provide an overview of recent progress on SPVE in AFM materials. This perspective also offers a viewpoint on this exciting area of research.

Robust spin torque switching of noncollinear antiferromagnet Mn3Sn

Electrical switching of topological antiferromagnetic states in Mn3Sn thin films has been a subject of active investigation. However, dependences of switching behaviors on the film thickness, external field, and crystal orientation remain to be fully explored, which motivate the present study. In this work, (112̄0)-orientated Mn3Sn thin films are fabricated on sapphire substrates, in which a large anomalous Hall effect over a wide temperature range (270–400 K) can be identified. The current-induced spin–orbit torques (SOTs) are utilized to electrically manipulate the topological antiferromagnetic states in Mn3Sn/Pt bilayers. The robust SOT switching can be realized in Mn3Sn films with thicknesses up to 100 nm and with in-plane fields up to 1200 mT. Furthermore, SOT switching behaviors that are independent of the choice of crystal orientations are clearly revealed. Our results could be useful for implementing Mn3Sn films for efficient and stable antiferromagnetic spintronics.

Antiferromagnetic spintronics: From metals to functional oxides

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

Perspective on antiferromagnetic iridates for spintronics

Antiferromagnetic (AFM) iridates are recently discovered to be a fertile playground for exploring emergent phenomena relevant to the intriguing interplay among multiple degrees of freedom, such as strong spin–orbit coupling, electron correlation, and the crystal field effect. These phenomena offer interesting routes for probing as well as controlling AFM order in iridate, which is essential in AFM spintronics. In this perspective, we will briefly review recent studies on AFM iridates that host large potential for advancing the reading (anisotropic magnetoresistance effect, etc.) and writing (magnetic field control of AFM order, etc.) functionalities of AFM spintronics. We will also discuss promising directions for expanding the research of AFM iridate based spintronics from the perspectives of material growth, manipulation protocol, and characterization technique.

Degeneracy Removal of Spin Bands in Collinear Antiferromagnets with Non-Interconvertible Spin-Structure Motif Pair.

Energy bands in antiferromagnets are supposed to be spin degenerate in the absence of spin-orbit coupling (SOC). Recent studies [Physical Review B 102, 01 4422 (2020)] identified formal symmetry conditions for antiferromagnetic crystals in which this degeneracy can be lifted - spin splitting - even in the vanishing SOC (i.e., non-relativistic) limit. Materials having such symmetries could enable spin-split antiferromagnetic spintronics without the burden of using heavy atom compounds. However, the symmetry conditions that involve spin and magnetic symmetry are not always effective as practical material selection filters. Furthermore, these symmetry conditions do not readily disclose trends in the magnitude and momentum dependence of the spin splitting energy. Here, we show that the formal symmetry conditions enabling spin-split antiferromagnets can be interpreted in terms of local motif pairs, such as octahedra or tetrahedra, each carrying opposite magnetic moments. Collinear antiferromagnets with such spin-structure motif pair whose components interconvert by neither translation nor spatial inversion will show spin splitting. Such a real-space motif-based approach enables an easy way to identify and design materials (illustrated in real example materials) having spin splitting without the need for spin-orbit coupling and offers insights into the momentum dependence and magnitude of the spin splitting. This article is protected by copyright. All rights reserved.

NiSi: New Venue for Antiferromagnetic Spintronics.

Envisaging antiferromagnetic spintronics pivots on two key criteria of high transition temperature and tuning of underlying magnetic order using straightforward application of magnetic field or electric current. Here, weshow that NiSi metal can provide suitable new platform in this quest. First, ourstudy unveils high temperature antiferromagnetism in single crystal NiSi with TN ⩾ 700 K. Antiferromagnetic order in NiSi is accompanied by the non-centrosymmetric magnetic character with small ferromagnetic component in a-c plane. Second, wefind that NiSi manifests distinct magnetic and electronic hysteresis responses to field applications due to the disparity in two moment directions. While magnetic hysteresis is characterized by one-step switching between ferromagnetic states of uncompensated moment, electronic behavior is ascribed to metamagnetic switching phenomena between non-collinear spin configurations. Importantly, the switching behaviors persist to high temperature. The properties underscore the importance of NiSi in the pursuit of antiferromagneticspintronics. This article is protected by copyright. All rights reserved.

Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure.

Quantum geometry in condensed-matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in two-dimensional electron gases and the anomalous Hall effect (AHE) in ferromagnets, the quantum metric has rarely been explored. Here, we report a nonlinear Hall effect induced by the quantum metric dipole by interfacing even-layered MnBi2Te4 with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the antiferromagnetic (AFM) spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.

Spin-Chirality-Driven Quantum Anomalous and Quantum Topological Hall Effects in Chiral Magnets.

The quantum anomalous Hall effect (QAHE) is a highly researched topic in condensed matter physics due to its ability to enable dissipationless transport. Previous studies have mainly focused on the ferromagnetic QAHE, which arises from the combination of collinear ferromagnetism and two-dimensional (2D) Z2 topological insulator phases. In our study, we demonstrate the emergence of the spin-chirality-driven QAHE and the quantum topological Hall effect (QTHE) by sandwiching a 2D Z2 topological insulator between two chiral kagome antiferromagnetic single-layers synthesized experimentally. The QAHE is surprisingly realized with fully compensated noncollinear antiferromagnetism in contrast to conventional collinear ferromagnetism. The Chern number can be regulated periodically with the interplay between vector- and scalar-spin chiralities, and the QAHE emerges even without spin-orbit coupling, indicating the rare QTHE. Our findings open a new avenue for realizing antiferromagnetic quantum spintronics based on the unconventional mechanisms from chiral spin textures.

Terahertz Spin Current Pulses in Antiferromagnetic Oxide: The Role of Vacancy‐Induced Ferromagnetism

Antiferromagnetic oxides have attracted increasing attention for their outstanding peculiarities in spintronics. Crystal lattice defects that are present in antiferromagnetic oxides can influence their physical properties, such as vacancy‐induced ferromagnetism. Meanwhile, the generation and manipulation of ultrafast spin currents of antiferromagnetic insulators are highly desired. Although the generation and detection of terahertz spin current pulses in antiferromagnetic oxides have been realized, the effect of vacancy‐induced ferromagnetism on spin current in antiferromagnetic oxides is not yet known. Herein, the role of vacancy‐induced ferromagnetism on the terahertz spin current in antiferromagnetic nickel oxide thin films is reported. Structural and magnetic characterizations reveal that nickel vacancies effectively break the strong antiferromagnetic exchange coupling, giving rise to the coexistence of antiferromagnetism and ferromagnetism in NiO thin films. Notably, the enhancement of terahertz radiation associated with the photo‐induced ultrafast spin current of NiO thin film with the strongest ferromagnetism is the most significant. Besides, the nonlinear susceptibility tensor parameters related to the antiferromagnetic property of NiO thin films also change distinctly. The findings indicate that the defects of antiferromagnetic materials play a decisive role in the application of antiferromagnetic spintronics in the terahertz field.

Néel Spin Currents in Antiferromagnets.

Ferromagnets are known to support spin-polarized currents that control various spin-dependent transport phenomena useful for spintronics. On the contrary, fully compensated antiferromagnets are expected to support only globally spin-neutral currents. Here, we demonstrate that these globally spin-neutral currents can represent the Néel spin currents, i.e., staggered spin currents flowing through different magnetic sublattices. The Néel spin currents emerge in antiferromagnets with strong intrasublattice coupling (hopping) and drive the spin-dependent transport phenomena such as tunneling magnetoresistance (TMR) and spin-transfer torque (STT) in antiferromagnetic tunnel junctions (AFMTJs). Using RuO_{2} and Fe_{4}GeTe_{2} as representative antiferromagnets, we predict that the Néel spin currents with a strong staggered spin polarization produce a sizable fieldlike STT capable of the deterministic switching of the Néel vector in the associated AFMTJs. Our work uncovers the previously unexplored potential of fully compensated antiferromagnets and paves a new route to realize the efficient writing and reading of information for antiferromagnetic spintronics.

Electrically controllable spin polarization in collinear antiferromagnetic junctions

Antiferromagnetic spintronics is a rapidly growing subfield of spintronics in condensed-matter physics and information technology. Electrical current in collinear antiferromagnetic materials is typically spin unpolarized, limiting the realization of antiferromagnetic spintronics effects. Here we study the transport in the collinear antiferromagnetic junctions by applying a transverse electric field E y to the antiferromagnets (AFs). The band structures of the collinear AFs may become spin-polarized when the combined time reversal and lattice translation symmetry is broken by E y . The separation between spin-up and spin-down bands is controlled by E y . Full spin polarization originating from spin-polarized states near the band gap’s edges is observed at high exchange energy. In particular, as E y increases, the region capable of generating high spin polarization broadens due to the increased separation between spin-up and spin-down bands. The amplitude and sign of spin polarization can be controlled by E y . These characteristics indicate that collinear AF materials are ideal for future spintronics applications.