Terahertz electrical writing speed in an antiferromagnetic memory

Extreme terahertz magnon multiplication induced by resonant magnetic pulse pairs

Nonlinear interactions of spin-waves and their quanta, magnons, have emerged as prominent candidates for interference-based technology, ranging from quantum transduction to antiferromagnetic spintronics. Yet magnon multiplication in the terahertz (THz) spectral region represents a major challenge. Intense, resonant magnetic fields from THz pulse-pairs with controllable phases and amplitudes enable high order THz magnon multiplication, distinct from non-resonant nonlinearities such as the high harmonic generation by below-band gap electric fields. Here, we demonstrate exceptionally high-order THz nonlinear magnonics. It manifests as 7th-order spin-wave-mixing and 6th harmonic magnon generation in an antiferromagnetic orthoferrite. We use THz two-dimensional coherent spectroscopy to achieve high-sensitivity detection of nonlinear magnon interactions up to six-magnon quanta in strongly-driven many-magnon correlated states. The high-order magnon multiplication, supported by classical and quantum spin simulations, elucidates the significance of four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking. Moreover, our results shed light on the potential quantum fluctuation properties inherent in nonlinear magnons.

Antiferromagnetic Chromium-Doped Tin Clusters

The trend toward further miniaturization of micronano antiferromagnetic (AFM) spintronic devices has led to a strong demand for low-dimensional materials. The assembly of AFM clusters to produce such materials is a potential pathway that promotes studies on such clusters. In this work, we report on the discovery of the AFM Cr2Snx (x = 3-20) clusters with a stepwise growth at the density functional theory (DFT) level. In comparison, the two Cr atoms tend to stay together and be buried by Sn atoms, forming endohedral structures with one Cr atom encapsulated at size 9 and finally forming a full-encapsulated structure at size 17. Each successive cluster size is composed of its predecessor with an extra Sn atom adsorbed onto the face, giving evidence of stepwise growth. All these Cr2Snx (x = 3-20) clusters are antiferromagnets, except for the triplet-state ferrimagnetic Cr2Sn11, and all their singly negatively and positively charged ions are ferromagnets. The found stable Cr2Sn17 cluster can dimerize, yielding dimers and trimers without noticeably distorting the geometrical structure and magnetic properties of each of its constituent cluster monomers, making it possible as a building block for AFM materials.

Emerging Antiferromagnets for Spintronics

Antiferromagnets constitute promising contender materials for next-generation spintronic devices with superior stability, scalability, and dynamics. Nevertheless, the perception of well-established ferromagnetic spintronics underpinned by spontaneous magnetization seemed to indicate the inadequacy of antiferromagnets for spintronics-their compensated magnetization has been perceived to result in uncontrollable antiferromagnetic order and subtle magnetoelectronic responses. However, remarkable advancements have been achieved in antiferromagnetic spintronics in recent years, with consecutive unanticipated discoveries substantiating the feasibility of antiferromagnet-centered spintronic devices. It is emphasized that, distinct from ferromagnets, the richness in complex antiferromagnetic crystal structures is the unique and essential virtue of antiferromagnets that can open up their endless possibilities of novel phenomena and functionality for spintronics. In this Perspective, the recent progress in antiferromagnetic spintronics is reviewed, with a particular focus on that based on several kinds of antiferromagnets with special antiferromagnetic crystal structures. The latest developments in efficiently manipulating antiferromagnetic order, exploring novel antiferromagnetic physical responses, and demonstrating prototype antiferromagnetic spintronic devices are discussed. An outlook on future research directions is also provided. It is hoped that this Perspective can serve as guidance for readers who are interested in this field and encourage unprecedented studies on antiferromagnetic spintronic materials, phenomena, and devices.

Electrically Controlled All-Antiferromagnetic Tunnel Junctions on Silicon with Large Room-Temperature Magnetoresistance

Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon-based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures are not grown using a silicon-compatible deposition process, and controlling their AFM order required external magnetic fields. Here it is shown three-terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn3 , sputter-deposited on silicon. The devices simultaneously exhibit electrical switching using electric currents, and electrical readout by a large room-temperature TMR effect. First-principles calculations explain the TMR in terms of the momentum-resolved spin-dependent tunneling conduction in tunnel junctions with noncollinear AFM electrodes.

Ptychographic Nanoscale Imaging of the Magnetoelectric Coupling in Freestanding BiFeO3

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80 nm thin freestanding film of the room-temperature multiferroic BiFeO3 (BFO). The antiferromagnetic spin cycloid of period 64 nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L3 edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71° between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with soft X-rays.

Spin-torque-driven antiferromagnetic resonance

The intrinsic fast dynamics make antiferromagnetic spintronics a promising avenue for faster data processing. Ultrafast antiferromagnetic resonance-generated spin current provides valuable access to antiferromagnetic spin dynamics. However, the inverse effect, spin-torque-driven antiferromagnetic resonance (ST-AFMR), which is attractive for practical utilization of fast devices but seriously impeded by difficulties in controlling and detecting Néel vectors, remains elusive. We observe ST-AFMR in Y3Fe5O12/α-Fe2O3/Pt at room temperature. The Néel vector oscillates and contributes to voltage signal owing to antiferromagnetic negative spin Hall magnetoresistance-induced spin rectification effect, which has the opposite sign to ferromagnets. The Néel vector in antiferromagnetic α-Fe2O3 is strongly coupled to the magnetization in Y3Fe5O12 buffer, resulting in the convenient control of Néel vectors. ST-AFMR experiment is bolstered by micromagnetic simulations, where both the Néel vector and the canted moment of α-Fe2O3 are in elliptic resonance. These findings shed light on the spin current-induced dynamics in antiferromagnets and represent a step toward electrically controlled antiferromagnetic terahertz emitters.

SdH Oscillations from the Dirac Surface State in the Fermi-Arc Antiferromagnet NdBi

The recent progress in CuMnAs and Mn3X (X = Sn, Ge, Pt) shows that antiferromagnets (AFMs) provide a promising platform for advanced spintronics device innovations. Most recently, a switchable Fermi-arc is discovered by the ARPES technique in antiferromagnet NdBi, but the knowledge about electron-transport property and the manipulability of the magnetic structure in NdBi is still vacant to date. In this study, SdH oscillations are successfully verified from the Dirac surface states (SSs) with 2-dimensionality and nonzero Berry phase. Particularly, it is observed that the spin-flop transition only appears when the external magnetical field is applied along [001] direction, and features obvious hysteresis for the first time in NdBi, which provides a powerful handle for adjusting the spin texture in NdBi. Crucially, the DFT shows the Dirac cone and the Fermi arc strongly depend on the high-order magnetic structure of NdBi and further reveals the orbital magnetic moment of Nd plays a crucial role in fostering the peculiar SSs, leading to unveil the mystery of the band-splitting effect and to manipulate the electronic transport, high-effectively, in the thin film works in NdBi. It is believed that this study provides important guidance for the development of new antiferromagnet-based spintronics devices based on cutting-edge rare-earth monopnictides.

Intrinsic Néel Antiferromagnetic Multimeronic Spin Textures in Ultrathin Films

Topological antiferromagnetism is a vibrant and captivating research field, generating considerable enthusiasm with the aim of identifying topologically protected magnetic states of key importance in the hybrid realm of topology, magnetism, and spintronics. While topological antiferromagnetic (AFM) solitons bear various advantages with respect to their ferromagnetic cousins, their observation is scarce. Utilizing first-principles simulations, here we predict new chiral particles in the realm of AFM topological magnetism, exchange-frustrated multimeronic spin textures hosted by a Néel magnetic state, arising universally in single Mn layers directly grown on an Ir(111) surface or interfaced with Pd-based films. These nanoscale topological structures are intrinsic; i.e. they form in a single AFM material, can carry distinct topological charges, and can combine in various multimeronic sequences with enhanced stability against external magnetic fields. We envision the frustrated Néel AFM multimerons as exciting highly sought after AFM solitons having the potential to be utilized in novel spintronic devices hinging on nonsynthetic AFM quantum materials, further advancing the frontiers of nanotechnology and nanophysics.

Terahertz Néel spin-orbit torques drive nonlinear magnon dynamics in antiferromagnetic Mn2Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In materials such as Mn2Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs). However, these torques have not yet been observed on ultrafast time scales. Here, we excite Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150 ± 50) cm2 A-1 s-1. At incident terahertz electric fields above 500 kV cm-1, we find pronounced nonlinear dynamics with massive Néel-vector deflections by as much as 30°. Our data are in excellent agreement with a micromagnetic model. It indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach.

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.

Antiferromagnetic half-skyrmions electrically generated and controlled at room temperature

Topologically protected magnetic textures are promising candidates for information carriers in future memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. These textures-nanoscale whirls in the magnetic order-include skyrmions, half-skyrmions (merons) and their antiparticles. Antiferromagnets have been shown to host versions of these textures that have high potential for terahertz dynamics, deflection-free motion and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin-film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The merons and antimerons are localized on 180° domain walls, and move in the direction of the current pulses. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high-density, high-speed magnetic memory devices.

Coherent antiferromagnetic spintronics

Antiferromagnets have attracted extensive interest as a material platform in spintronics. So far, antiferromagnet-enabled spin-orbitronics, spin-transfer electronics and spin caloritronics have formed the bases of antiferromagnetic spintronics. Spin transport and manipulation based on coherent antiferromagnetic dynamics have recently emerged, pushing the developing field of antiferromagnetic spintronics towards a new stage distinguished by the features of spin coherence. In this Review, we categorize and analyse the critical effects that harness the coherence of antiferromagnets for spintronic applications, including spin pumping from monochromatic antiferromagnetic magnons, spin transmission via phase-correlated antiferromagnetic magnons, electrically induced spin rotation and ultrafast spin-orbit effects in antiferromagnets. We also discuss future opportunities in research and applications stimulated by the principles, materials and phenomena of coherent antiferromagnetic spintronics.

In-Plane Anomalous Hall Effect in PT-Symmetric Antiferromagnetic Materials

The anomalous Hall effect (AHE), a protocol of various low-power dissipation quantum phenomena and a fundamental precursor of intriguing topological phases of matter, is usually observed in ferromagnetic materials with an orthogonal configuration between the electric field, magnetization, and the Hall current. Here, based on the symmetry analysis, we find an unconventional AHE induced by the in-plane magnetic field (IPAHE) via the spin-canting effect in PT-symmetric antiferromagnetic (AFM) systems, featuring a linear dependence of magnetic field and 2π angle periodicity with a comparable magnitude to conventional AHE. We demonstrate the key findings in the known AFM Dirac semimetal CuMnAs and a new kind of AFM heterodimensional VS_{2}-VS superlattice with a nodal-line Fermi surface and, also, briefly discuss the experimental detection. Our Letter provides an efficient pathway for searching and/or designing realistic materials for a novel IPAHE that could greatly facilitate their application in AFM spintronic devices. National Science Foundation.

Time-Reversal-Even Nonlinear Current Induced Spin Polarization

We propose a time-reversal-even spin generation in second order of electric fields, which dominates the current induced spin polarization in a wide class of centrosymmetric nonmagnetic materials, and leads to a novel nonlinear spin-orbit torque in magnets. We reveal a quantum origin of this effect from the momentum space dipole of the anomalous spin polarizability. First-principles calculations predict sizable spin generations in several nonmagnetic hcp metals, in monolayer TiTe_{2}, and in ferromagnetic monolayer MnSe_{2}, which can be detected in experiment. Our work opens up the broad vista of nonlinear spintronics in both nonmagnetic and magnetic systems.

Generation of third-harmonic spin oscillation from strong spin precession induced by terahertz magnetic near fields

The ability to drive a spin system to state far from the equilibrium is indispensable for investigating spin structures of antiferromagnets and their functional nonlinearities for spintronics. While optical methods have been considered for spin excitation, terahertz (THz) pulses appear to be a more convenient means of direct spin excitation without requiring coupling between spins and orbitals or phonons. However, room-temperature responses are usually limited to small deviations from the equilibrium state because of the relatively weak THz magnetic fields in common approaches. Here, we studied the magnetization dynamics in a HoFeO₃ crystal at room temperature. A custom-made spiral-shaped microstructure was used to locally generate a strong multicycle THz magnetic near field perpendicular to the crystal surface; the maximum magnetic field amplitude of about 2 T was achieved. The observed time-resolved change in the Faraday ellipticity clearly showed second- and third-order harmonics of the magnetization oscillation and an asymmetric oscillation behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic vector L plays an important role in the nonlinear dynamics of spin systems far from equilibrium.

Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin-phonon interactions

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave-induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

Topological Hall Effect in Thin Films of an Antiferromagnetic Weyl Semimetal Integrated on Si

Since the large room-temperature anomalous Hall effect was discovered in noncollinear antiferromagnets, Mn₃Sn has received immense research interest as it exhibits abundant exotic physical properties including Weyl points and enormous potential for antiferromagnetic spintronic device applications. In this work, we report the emergence of the topological Hall effect in Mn₃Sn films grown on Si that is the workhorse for the modern highly integrated information technology. Importantly, through a series of systematic comparative experiments, the intriguing topological Hall effect phenomenon related to the appearance of the noncoplanar chiral spin structure is found to be induced by the Mn₃Sn/SiO₂ interface. Furthermore, it was found that the current injection to a Pt/Mn₃Sn bilayer Hall bar device can effectively manipulate the chiral spin structure of Mn₃Sn, which demonstrates the feasibility of Si-based noncollinear antiferromagnetic spintronics.

Room-temperature magnetoresistance in an all-antiferromagnetic tunnel junction

Antiferromagnetic spintronics^1-16 is a rapidly growing field in condensed-matter physics and information technology with potential applications for high-density and ultrafast information devices. However, the practical application of these devices has been largely limited by small electrical outputs at room temperature. Here we describe a room-temperature exchange-bias effect between a collinear antiferromagnet, MnPt, and a non-collinear antiferromagnet, Mn₃Pt, which together are similar to a ferromagnet-antiferromagnet exchange-bias system. We use this exotic effect to build all-antiferromagnetic tunnel junctions with large nonvolatile room-temperature magnetoresistance values that reach a maximum of about 100%. Atomistic spin dynamics simulations reveal that uncompensated localized spins at the interface of MnPt produce the exchange bias. First-principles calculations indicate that the remarkable tunnelling magnetoresistance originates from the spin polarization of Mn₃Pt in the momentum space. All-antiferromagnetic tunnel junction devices, with nearly vanishing stray fields and strongly enhanced spin dynamics up to the terahertz level, could be important for next-generation highly integrated and ultrafast memory devices^7,9,16.

Optically Triggered Néel Vector Manipulation of a Metallic Antiferromagnet Mn2Au under Strain

The absence of stray fields, their insensitivity to external magnetic fields, and ultrafast dynamics make antiferromagnets promising candidates for active elements in spintronic devices. Here, we demonstrate manipulation of the Néel vector in the metallic collinear antiferromagnet Mn₂Au by combining strain and femtosecond laser excitation. Applying tensile strain along either of the two in-plane easy axes and locally exciting the sample by a train of femtosecond pulses, we align the Néel vector along the direction controlled by the applied strain. The dependence on the laser fluence and strain suggests the alignment is a result of optically triggered depinning of 90° domain walls and their motion in the direction of the free energy gradient, governed by the magneto-elastic coupling. The resulting, switchable state is stable at room temperature and insensitive to magnetic fields. Such an approach may provide ways to realize robust high-density memory device with switching time scales in the picosecond range.

Emergence of zero-field non-synthetic single and interchained antiferromagnetic skyrmions in thin films

Antiferromagnetic (AFM) skyrmions are envisioned as ideal localized topological magnetic bits in future information technologies. In contrast to ferromagnetic (FM) skyrmions, they are immune to the skyrmion Hall effect, might offer potential terahertz dynamics while being insensitive to external magnetic fields and dipolar interactions. Although observed in synthetic AFM structures and as complex meronic textures in intrinsic AFM bulk materials, their realization in non-synthetic AFM films, of crucial importance in racetrack concepts, has been elusive. Here, we unveil their presence in a row-wise AFM Cr film deposited on PdFe bilayer grown on fcc Ir(111) surface. Using first principles, we demonstrate the emergence of single and strikingly interpenetrating chains of AFM skyrmions, which can co-exist with the rich inhomogeneous exchange field, including that of FM skyrmions, hosted by PdFe. Besides the identification of an ideal platform of materials for intrinsic AFM skyrmions, we anticipate the uncovered knotted solitons to be promising building blocks in AFM spintronics.

Depinning phase transition of antiferromagnetic skyrmions with quenched disorder

Antiferromagnetic skyrmions are considered to be promising information carriers due to their attractive properties. Therefore, the pinning phenomenon of antiferromagnetic skyrmions is of great significance. With the Landau-Lifshitz-Gilbert equation, we simulate the nonstationary dynamic behaviors of skyrmions driven by currents in a chiral antiferromagnetic thin film with quenched disorder. Based on the dynamic scaling forms, the critical current and static and dynamic critical exponents of the depinning phase transition are accurately determined. A theoretical analysis using Thiele's approach is presented in comparison with the numerical simulation. Unlike the ferromagnetic skyrmions, the critical current of the antiferromagnetic skyrmions is very sensitive to a small nonadiabatic coefficient. This is important for manipulating antiferromagnetic skyrmions and designing novel information processing devices.

Quantum Energy Current Induced Coherence in a Spin Chain under Non-Markovian Environments

We investigate the time-dependent behaviour of the energy current between a quantum spin chain and its surrounding non-Markovian and finite temperature baths, together with its relationship to the coherence dynamics of the system. To be specific, both the system and the baths are assumed to be initially in thermal equilibrium at temperature Ts and Tb, respectively. This model plays a fundamental role in study of quantum system evolution towards thermal equilibrium in an open system. The non-Markovian quantum state diffusion (NMQSD) equation approach is used to calculate the dynamics of the spin chain. The effects of non-Markovianity, temperature difference and system-bath interaction strength on the energy current and the corresponding coherence in cold and warm baths are analyzed, respectively. We show that the strong non-Markovianity, weak system-bath interaction and low temperature difference will help to maintain the system coherence and correspond to a weaker energy current. Interestingly, the warm baths destroy the coherence while the cold baths help to build coherence. Furthermore, the effects of the Dzyaloshinskii-Moriya (DM) interaction and the external magnetic field on the energy current and coherence are analyzed. Both energy current and coherence will change due to the increase of the system energy induced by the DM interaction and magnetic field. Significantly, the minimal coherence corresponds to the critical magnetic field which causes the first order phase transition.

A comparative study of the domain wall motion in ferrimagnets (Fe,Co)1-x(Gd,Tb)x

Magnetic domain walls (DWs) in rare-earth-transition-metal (RE-TM) ferrimagnetic alloys can be used as information carriers in nonvolatile spintronic devices. Due to the rich combinations of RE-TM elements (such as CoGd, FeGd, CoTb, and FeTb in our case), it is intriguing to reveal the characteristics of DW dynamics in these wide choices of RE-TM compounds. Through a systematic study of the DW motion in thin films with different compositions of stacking order Pt(3 nm)/(Fe,Co)_1-x(Gd,Tb)_x(∼8 nm)/Ta(3 nm), we show that the partially compensated ferrimagnets CoGd and FeGd can exhibit a faster DW motion under various (in-plane and out-of-plane) magnetic fields driven by current-induced spin-orbit torques. In stark contrast with the fast motion of domain walls in Gd-based ferrimagnets, we find that the CoTb system exhibits much slower DW dynamics, and the FeTb system shows no motion, but evolved into a multi-domain state upon applying current pulses. Our results demonstrate that ferrimagnets CoGd and FeGd are more suitable candidates for achieving ultrafast DW motion, which could be useful for developing spintronic memory and logic devices.

Intrinsic Nonlinear Electric Spin Generation in Centrosymmetric Magnets

We propose an intrinsic nonlinear electric spin generation effect, which can dominate in centrosymmetric magnets. We reveal the band geometric origin of this effect and clarify its symmetry characters. As an intrinsic effect, it is determined solely by the material's band structure and represents a material characteristic. Combining our theory with first-principle calculations, we predict sizable nonlinear spin generation in single-layer MnBi_{2}Te_{4}, which can be detected in experiment. Our theory opens a new route for all-electric controlled spintronics in centrosymmetric magnets which reside outside of the current paradigm based on linear spin response.

Two-dimensional antiferromagnetic semiconductor T'-MoTeI from first principles

Two-dimensional intrinsic antiferromagnetic semiconductors are expected to stand out in the spintronic field. The present work finds the monolayer T'-MoTeI is intrinsically an antiferromagnetic semiconductor by using first-principles calculation. Firstly, the dimerized distortion of the Mo atoms causes T'-MoTeI to have dynamic stability, which is different from the small imaginary frequency in the phonon spectrum of T-MoTeI. Secondly, T'-MoTeI is an indirect-bandgap semiconductor with 1.35 eV. Finally, in the systematic study of strain effects, there are significant changes in the electronic structure as well as the bandgap, but the antiferromagnetic ground state is not affected. Monte Carlo simulations predict that the Néel temperature of T'-MoTeI is 95 K. The results suggest that the monolayer T'-MoTeI can be a potential candidate for spintronics applications.

Electric-field control of nonlinear THz spintronic emitters

Energy-efficient spintronic technology holds tremendous potential for the design of next-generation processors to operate at terahertz frequencies. Femtosecond photoexcitation of spintronic materials generates sub-picosecond spin currents and emission of terahertz radiation with broad bandwidth. However, terahertz spintronic emitters lack an active material platform for electric-field control. Here, we demonstrate a nonlinear electric-field control of terahertz spin current-based emitters using a single crystal piezoelectric Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) that endows artificial magnetoelectric coupling onto a spintronic terahertz emitter and provides 270% modulation of the terahertz field at remnant magnetization. The nonlinear electric-field control of the spins occurs due to the strain-induced change in magnetic energy of the ferromagnet thin-film. Results also reveal a robust and repeatable switching of the phase of the terahertz spin current. Electric-field control of terahertz spintronic emitters with multiferroics and strain engineering offers opportunities for the on-chip realization of tunable energy-efficient spintronic-photonic integrated platforms.

Spintronic terahertz (THz) emitters are a class of magnetic heterostructure where femtosecond laser excitations generate THz radiation emission. While they have great potential, electric field control of spintronic emitter remains a challenge. Here, by combining a spintronic emitter with a piezoelectric substrate, Agarwal et al. demonstrate electric field control of THz emission through induced piezostrain.

Piezoelectric Strain-Controlled Magnon Spin Current Transport in an Antiferromagnet

As the core of spintronics, the transport of spin aims at a low-dissipation data process. The pure spin current transmission carried by magnons in antiferromagnetic insulators is natively endowed with superiority such as long-distance propagation and ultrafast speed. However, the traditional control of magnon transport in an antiferromagnet via a magnetic field or temperature variation adds critical inconvenience to practical applications. Controlling magnon transport by electric methods is a promising way to overcome such embarrassment and to promote the development of energy-efficient antiferromagnetic logic. Here, the experimental realization of an electric field-induced piezoelectric strain-controlled magnon spin current transmission through the antiferromagnetic insulator in the Y₃Fe₅O₁₂/Cr₂O₃/Pt trilayer is reported. An efficient and nonvolatile manipulation of magnon propagation/blocking is achieved by changing the relative direction between the Néel vector and spin polarization, which is tuned by ferroelastic strain from the piezoelectric substrate. The piezoelectric strain-controlled antiferromagnetic magnon transport opens an avenue for the exploitation of antiferromagnet-based spin/magnon transistors with ultrahigh energy efficiency.

Current-induced Néel order switching facilitated by magnetic phase transition

Terahertz (THz) spin dynamics and vanishing stray field make antiferromagnetic (AFM) materials the most promising candidate for the next-generation magnetic memory technology with revolutionary storage density and writing speed. However, owing to the extremely large exchange energy barriers, energy-efficient manipulation has been a fundamental challenge in AFM systems. Here, we report an electrical writing of antiferromagnetic orders through a record-low current density on the order of 10⁶ A cm⁻² facilitated by the unique AFM-ferromagnetic (FM) phase transition in FeRh. By introducing a transient FM state via current-induced Joule heating, the spin-orbit torque can switch the AFM order parameter by 90° with a reduced writing current density similar to ordinary FM materials. This mechanism is further verified by measuring the temperature and magnetic bias field dependences, where the X-ray magnetic linear dichroism (XMLD) results confirm the AFM switching besides the electrical transport measurement. Our findings demonstrate the exciting possibility of writing operations in AFM-based devices with a lower current density, opening a new pathway towards pure AFM memory applications.

Electrical manipulation of antiferromagnetic order is crucial for future memory devices, but existing switching schemes require a large current density. Here, the authors achieve record low current density switching in FeRh by taking advantage of its antiferromagnetic to ferromagnetic phase transition.

Highly Tunable Magnetic and Magnetotransport Properties of Exchange Coupled Ferromagnet/Antiferromagnet-Based Heterostructures

Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical film thickness and measurement temperature, bimetallic Mn-based alloys and transition-metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM γ-IrMn₃ thin films with coexisting AFM phases into complex exchange coupled MgO(001)/γ-Ni₃Fe/γ-IrMn₃/γ-Ni₃Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the thermally disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetotransport properties of the exchange coupled heterostructures. We further reveal that the persistence of thermally disordered and reversible AFM moments is crucial for achieving highly tunable magnetic properties and multilevel magnetoresistance states. We anticipate that the presented approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM moments at the nanoscale.

Superluminal-like magnon propagation in antiferromagnetic NiO at nanoscale distances

Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices^1,2. Owing to their low energy dissipation, antiferromagnetic magnons can propagate over micrometre distances³. However, direct observation of their high-speed propagation has been elusive due to the lack of sufficiently fast probes². Here we measure the antiferromagnetic magnon propagation in the time domain at the nanoscale (≤50 nm) with optical-driven terahertz emission. In non-magnetic-Bi₂Te₃/antiferromagnetic-insulator-NiO/ferromagnetic-Co trilayers, we observe a magnon velocity of ~650 km s^-1 in the NiO layer. This velocity far exceeds previous estimations of the maximum magnon group velocity of ~40 km s^-1, which were based on the magnon dispersion measurements of NiO using inelastic neutron scattering^4,5. Our theory suggests that for magnon propagation at the nanoscale, a finite damping makes the dispersion anomalous for small magnon wavenumbers and yields a superluminal-like magnon velocity. Given the generality of finite dissipation in materials, our results strengthen the prospects of ultrafast nanodevices using antiferromagnetic magnons.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.

Current-induced manipulation of exchange bias in IrMn/NiFe bilayer structures

The electrical control of antiferromagnetic moments is a key technological goal of antiferromagnet-based spintronics, which promises favourable device characteristics such as ultrafast operation and high-density integration as compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments by electric current has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report current-induced manipulation of the exchange bias in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the exchange bias is gradually modulated up to ±22 degrees by an in-plane current, which is independent of the NiFe thickness. This suggests that spin currents arising in the IrMn layer exert SOTs on uncompensated antiferromagnetic moments at the interface which then rotate the antiferromagnetic moments. Furthermore, the memristive features are preserved in sub-micron devices, facilitating nanoscale multi-level antiferromagnetic spintronic devices.

Antiferromagnets have great promise for spin-based information processing, offering both high operation speed, and an immunity to stray fields. Here, Kang et al demonstrate electrical manipulation of the exchange-bias, without the need for a heavy metal layer.

Generation of circular spin current in an AB magnetic ring with vanishing net magnetization: a new prescription

In this work we report for the first time the appearance of non-decaying circular spin current in a magnetic ring with vanishing net magnetization, even in absence of any spin chirality. Breaking the symmetry in hopping integrals we can misalign up and down spin electronic energy levels which yields a net spin current in the magnetic quantum ring, threaded by an Aharonov-Bohm flux. Along with spin current, a net charge current also appears, and we compute both these currents using the second quantized approach. A tight-binding framework is employed to describe the magnetic ring where each site of the ring contains a finite magnetic moment. Itinerant electrons get scattered from the localized magnetic moments at different lattice sites, and the moments are arranged in such a way that the net magnetization vanishes. The interplay between magnetic moments and asymmetric hopping integrals leads to several atypical features in energy spectra, especially the existence of vanishing current carrying energy eigenstates together with the current carrying ones. The formation of such states those do not contribute any current is the artifact of different kinds of on-site energies and/or hopping integrals in different segments of the magnetic ring. The atypical signatures of energy levels are directly reflected into the charge and spin currents, and here we critically investigate the behaviors of circular currents as functions of electron filling, hopping integrals, strength of spin-moment interaction and ring size. Finally, we discuss briefly the possible experimental realization to implement our proposed magnetic system. The present analysis may provide a new route of generating persistent spin current in magnetic quantum rings with vanishing net magnetization, circumventing the use of spin-orbit coupled systems.

Spin Pumping of an Easy-Plane Antiferromagnet Enhanced by Dzyaloshinskii-Moriya Interaction

Recently, antiferromagnets have received revived interest due to their significant potential for developing next-generation ultrafast magnetic storage. Here, we report dc spin pumping by the acoustic resonant mode in a canted easy-plane antiferromagnet α-Fe_{2}O_{3} enabled by the Dzyaloshinskii-Moriya interaction. Systematic angle and frequency-dependent measurements demonstrate that the observed spin-pumping signals arise from resonance-induced spin injection and inverse spin Hall effect in α-Fe_{2}O_{3}-metal heterostructures, mimicking the behavior of spin pumping in conventional ferromagnet-nonmagnet systems. The pure spin current nature is further corroborated by reversal of the polarity of spin-pumping signals when the spin detector is switched from platinum to tungsten which has an opposite sign of the spin Hall angle. Our results reveal the intriguing physics underlying the low-frequency spin dynamics and transport in canted easy-plane antiferromagnet-based heterostructures.

Direct Observation of Magnon-Phonon Strong Coupling in Two-Dimensional Antiferromagnet at High Magnetic Fields

We report the direct observation of strong coupling between magnons and phonons in a two-dimensional antiferromagnetic semiconductor FePS_{3}, via magneto-Raman spectroscopy at magnetic fields up to 30 Tesla. A Raman-active magnon at 121  cm^{-1} is identified through Zeeman splitting in an applied magnetic field. At a field-driven resonance with a nearby phonon mode, a hybridized magnon-phonon quasiparticle is formed due to strong coupling between the two modes. We develop a microscopic model of the strong coupling in the two-dimensional magnetic lattice, which enables us to elucidate the nature of the emergent quasiparticle. Our polarized Raman results directly show that the magnons transfer their spin angular momentum to the phonons and generate phonon spin through the strong coupling.

The 2021 Magnonics Roadmap

Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

There is accelerating interest in developing memory devices using antiferromagnetic (AFM) materials, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects. Recent studies have shown, however, that high current densities create non-magnetic contributions to resistive switching signals in AFM/heavy metal (AFM/HM) bilayers, complicating their interpretation. Here we introduce an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures, and demonstrate it in IrMn₃/Pt devices. A six-terminal double-cross device is constructed, with an IrMn₃ pillar placed on one cross. The differential voltage is measured between the two crosses with and without IrMn₃ after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn₃ pillar, eliminating any possibility of non-magnetic switching artifacts. Micromagnetic simulations support our findings, indicating a complex domain-mediated switching process.

Anti-ferromagnetic based memories have a wide range of advantages over their ferromagnetic counterparts, however, their electrical signatures of switching are complicated by spurious signals. Here, Arpaci et al demonstrate an experimental method to distinguish between anti-ferromagnetic switching, and such spurious signatures.

Frequency-Independent Terahertz Anomalous Hall Effect in DyCo5 , Co32 Fe68 , and Gd27 Fe73 Thin Films from DC to 40 THz

The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion phenomena at terahertz (THz) frequencies. Here, it is revealed that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo₅ , Co₃₂ Fe₆₈ , and Gd₂₇ Fe₇₃ . The frequency-dependent conductivity-tensor elements σ_xx and σ_yx  are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of σ_yx for CoFe and highlight the role of the large Drude scattering rate (≈100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co₃₂ Fe₆₈ sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni₈₁ Fe₁₉ , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

Cr-Doped Ge-Core/Si-Shell Nanowire: An Antiferromagnetic Semiconductor

An antiferromagnet offers many important functionalities such as opportunities for electrical control of magnetic domains, immunity from magnetic perturbations, and fast spin dynamics. Introducing some of these intriguing features of an antiferromagnet into a low dimensional semiconductor core-shell nanowire offers an exciting pathway for its usage in antiferromagnetic semiconductor spintronics. Here, using a quantum mechanical approach, we predict that the Cr-doped Ge-core/Si-shell nanowire behaves as an antiferromagnetic semiconductor. The origin of antiferromagnetic spin alignments between Cr is attributed to the superexchange interaction mediated by the p_z orbitals of the Ge atoms that are bonded to Cr. A weak spin-orbit interaction was found in this material, suggesting a longer spin coherence length. The spin-dependent quantum transport calculations in the Cr-doped nanowire junction reveals a switching feature with a high ON/OFF current ratio (∼41 times higher for the ON state at a relatively small bias of 0.83 V).

Complex magnetism of the two-dimensional antiferromagnetic Ge2F: from a Néel spin-texture to a potential antiferromagnetic skyrmion

Based on density functional theory combined with low-energy models, we explore the magnetic properties of a hybrid atomic-thick two-dimensional (2D) material made of germanene doped with fluorine atoms in a half-fluorinated configuration (Ge₂F). The Fluorine atoms are highly electronegative, which induces magnetism and breaks inversion symmetry, triggering thereby a finite and strong Dzyaloshinskii–Moriya interaction (DMI). The magnetic exchange interactions are of antiferromagnetic nature among the first, second and third neighbors, which leads to magnetic frustration. The Néel state is found to be the most stable state, with magnetic moments lying in the surface plane. This results from the out-of-plane component of the DMI vector, which seems to induce an effective in-plane magnetic anisotropy. Upon application of a magnetic field, spin-spirals and antiferromagnetic skyrmions can be stabilized. We conjecture that this can be realized via magnetic exchange fields induced by a magnetic substrate. To complete our characterization, we computed the spin-wave excitations and the resulting spectra, which could be probed via electron energy loss spectroscopy, magneto-Raman spectroscopy or scanning tunneling spectroscopy.

Based on density functional theory combined with low-energy models, we explore the magnetic properties of a hybrid atomic-thick two-dimensional (2D) material made of germanene doped with fluorine atoms in a half-fluorinated configuration (Ge₂F).

Magnon Landau Levels and Spin Responses in Antiferromagnets

We study gauge fields produced by gradients of the Dzyaloshinskii-Moriya interaction and propose a model of an AFM topological insulator of magnons. In the long wavelength limit, the Landau levels induced by the inhomogeneous Dzyaloshinskii-Moriya interaction exhibit relativistic physics described by the Klein-Gordon equation. The spin Nernst response due to the formation of magnonic Landau levels is compared to similar topological responses in skyrmion and vortex-antivortex crystal phases of AFM insulators. Our studies show that AFM insulators exhibit rich physics associated with topological magnon excitations.

Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle Effect

We report on experiments demonstrating coherent control of magnon spin transport and pseudospin dynamics in a thin film of the antiferromagnetic insulator hematite utilizing two Pt strips for all-electrical magnon injection and detection. The measured magnon spin signal at the detector reveals an oscillation of its polarity as a function of the externally applied magnetic field. We quantitatively explain our experiments in terms of diffusive magnon transport and a coherent precession of the magnon pseudospin caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. This experimental observation can be viewed as the magnonic analog of the electronic Hanle effect and the Datta-Das transistor, unlocking the high potential of antiferromagnetic magnonics toward the realization of rich electronics-inspired phenomena.

Birefringence-like spin transport via linearly polarized antiferromagnetic magnons

Antiferromagnets (AFMs) possess great potential in spintronics because of their immunity to external magnetic disturbance, the absence of a stray field or the resonance in the terahertz range^1,2. The coupling of insulating AFMs to spin-orbit materials^3-7 enables spin transport via AFM magnons. In particular, spin transmission over several micrometres occurs in some AFMs with easy-axis anisotropy^8,9. Easy-plane AFMs with two orthogonal, linearly polarized magnon eigenmodes own unique advantages for low-energy control of ultrafast magnetic dynamics². However, it is commonly conceived that these magnon modes are less likely to transmit spins because of their vanishing angular momentum^9-11. Here we report experimental evidence that an easy-plane insulating AFM, an α-Fe₂O₃ thin film, can efficiently transmit spins over micrometre distances. The spin decay length shows an unconventional temperature dependence that cannot be captured considering solely thermal magnon scatterings. We interpret our observations in terms of an interference of two linearly polarized, propagating magnons in analogy to the birefringence effect in optics. Furthermore, our devices can realize a bi-stable spin-current switch with a 100% on/off ratio under zero remnant magnetic field. These findings provide additional tools for non-volatile, low-field control of spin transport in AFM systems.

Electric-Field-Controlled Antiferromagnetic Spintronic Devices

In recent years, the field of antiferromagnetic spintronics has been substantially advanced. Electric-field control is a promising approach for achieving ultralow power spintronic devices via suppressing Joule heating. Here, cutting-edge research, including electric-field modulation of antiferromagnetic spintronic devices using strain, ionic liquids, dielectric materials, and electrochemical ionic migration, is comprehensively reviewed. Various emergent topics such as the Néel spin-orbit torque, chiral spintronics, topological antiferromagnetic spintronics, anisotropic magnetoresistance, memory devices, 2D magnetism, and magneto-ionic modulation with respect to antiferromagnets are examined. In conclusion, the possibility of realizing high-quality room-temperature antiferromagnetic tunnel junctions, antiferromagnetic spin logic devices, and artificial antiferromagnetic neurons is highlighted. It is expected that this work provides an appropriate and forward-looking perspective that will promote the rapid development of this field.

Controlling magnetic domain wall velocity by femtosecond laser pulses

Using the technique of double high-speed photography, we find that a femtosecond laser pulse is able to change the velocity of a moving domain wall in an yttrium iron garnet. The change depends on the light intensity and the domain wall velocity itself. To explain the results we propose a model in which the domain wall velocity is controlled by photo-induced generation of vertical Bloch lines.

Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.