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.

Handedness anomaly in a non-collinear antiferromagnet under spin-orbit torque.

Non-collinear antiferromagnets are an emerging family of spintronic materials because they not only possess the general advantages of antiferromagnets but also enable more advanced functionalities. Recently, in an intriguing non-collinear antiferromagnet Mn3Sn, where the octupole moment is defined as the collective magnetic order parameter, spin-orbit torque (SOT) switching has been achieved in seemingly the same protocol as in ferromagnets. Nevertheless, it is fundamentally important to explore the unknown octupole moment dynamics and contrast it with the magnetization vector of ferromagnets. Here we report a handedness anomaly in the SOT-driven dynamics of Mn3Sn: when spin current is injected, the octupole moment rotates in the opposite direction to the individual moments, leading to a SOT switching polarity distinct from ferromagnets. By using second-harmonic and d.c. magnetometry, we track the SOT effect onto the octupole moment during its rotation and reveal that the handedness anomaly stems from the interactions between the injected spin and the unique chiral-spin structure of Mn3Sn. We further establish the torque balancing equation of the magnetic octupole moment and quantify the SOT efficiency. Our finding provides a guideline for understanding and implementing the electrical manipulation of non-collinear antiferromagnets, which in nature differs from the well-established collinear magnets.

Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers.

Realization of novel topological phases in magnonic band structures represents a new opportunity for the development of spintronics and magnonics with low power consumption. In this work, we show that in antiparallelly aligned magnetic multilayers, the long-range, chiral dipolar interaction between propagating magnons generates bulk bands with nonzero Chern integers and magnonic surface states carrying chiral spin currents. The surface states are highly localized and can be easily toggled between nontrivial and trivial phases through an external magnetic field. The realization of chiral surface spin currents in this dipolarly coupled heterostructure represents a magnonic implementation of the coupled wire model that has been extensively explored in electronic systems. Our work presents an easy-to-implement system for realizing topological magnonic surface states and low-dissipation spin current transport in a tunable manner.

Control of Néel Vector with Spin-Orbit Torques in an Antiferromagnetic Insulator with Tilted Easy Plane.

Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque switching represents a challenging topic. Because of the diminishing magnetic susceptibility, current-induced antiferromagnetic dynamics remain poorly characterized, complicated by spurious effects. Here, by growing a thin film antiferromagnet, α-Fe_{2}O_{3}, along its nonbasal plane orientation, we realize a configuration where the spin-orbit torque from an injected spin current can unambiguously rotate and switch the Néel vector within the tilted easy plane, with an efficiency comparable to that of classical ferrimagnetic insulators. Our study introduces a new platform for quantitatively characterizing switching and oscillation dynamics in antiferromagnets.

Nonreciprocal Transmission of Incoherent Magnons with Asymmetric Diffusion Length.

Unequal transmissions of spin waves along opposite directions provide useful functions for signal processing. So far, the realization of such nonreciprocal spin waves has been mostly limited at a gigahertz frequency in the coherent regime via microwave excitation. Here we show that, in a magnetic bilayer stack with chiral coupling, tunable nonreciprocal propagation can be realized in spin Hall effect-excited incoherent magnons, whose frequencies cover the spectrum from a few gigahertz up to terahertz. The sign of nonreciprocity is controlled by the magnetic orientations of the bilayer in a nonvolatile manner. The nonreciprocity is further verified by measurements of the magnon diffusion length, which is unequal along opposite transmission directions. Our findings enrich the knowledge on magnetic relaxation and diffusive transport and can lead to the design of a passive directional signal isolation device in the diffusive regime.

Magnetic Domain Wall Based Synaptic and Activation Function Generator for Neuromorphic Accelerators.

Magnetic domain walls are information tokens in both logic and memory devices and hold particular interest in applications such as neuromorphic accelerators that combine logic in memory. Here, we show that devices based on the electrical manipulation of magnetic domain walls are capable of implementing linear, as well as programmable nonlinear, functions. Unlike other approaches, domain-wall-based devices are ideal for application to both synaptic weight generators and thresholding in deep neural networks. Prototype micrometer-size devices operate with 8 ns current pulses and the energy consumption required for weight modulation is ≤16 pJ. Both speed and energy consumption compare favorably to other synaptic nonvolatile devices, with the expected energy dissipation for scaled 20 nm devices close to that of biological neurons.

Roadmap of spin-orbit torques.

Spin-orbit torque (SOT) is an emerging technology that enables the efficient manipulation of spintronic devices. The initial processes of interest in SOTs involved electric fields, spin-orbit coupling, conduction electron spins and magnetization. More recently interest has grown to include a variety of other processes that include phonons, magnons, or heat. Over the past decade, many materials have been explored to achieve a larger SOT efficiency. Recently, holistic design to maximize the performance of SOT devices has extended material research from a nonmagnetic layer to a magnetic layer. The rapid development of SOT has spurred a variety of SOT-based applications. In this Roadmap paper, we first review the theories of SOTs by introducing the various mechanisms thought to generate or control SOTs, such as the spin Hall effect, the Rashba-Edelstein effect, the orbital Hall effect, thermal gradients, magnons, and strain effects. Then, we discuss the materials that enable these effects, including metals, metallic alloys, topological insulators, two-dimensional materials, and complex oxides. We also discuss the important roles in SOT devices of different types of magnetic layers, such as magnetic insulators, antiferromagnets, and ferrimagnets. Afterward, we discuss device applications utilizing SOTs. We discuss and compare three-terminal and two-terminal SOT-magnetoresistive random-access memories (MRAMs); we mention various schemes to eliminate the need for an external field. We provide technological application considerations for SOT-MRAM and give perspectives on SOT-based neuromorphic devices and circuits. In addition to SOT-MRAM, we present SOT-based spintronic terahertz generators, nano-oscillators, and domain wall and skyrmion racetrack memories. This paper aims to achieve a comprehensive review of SOT theory, materials, and applications, guiding future SOT development in both the academic and industrial sectors.

Strong Coupling between Microwave Photons and Nanomagnet Magnons.

Coupled microwave photon-magnon hybrid systems offer promising applications by harnessing various magnon physics. At present, in order to realize high coupling strength between the two subsystems, bulky ferromagnets with large spin numbers are utilized, which limits their potential applications for scalable quantum information processing. By enhancing single spin coupling strength using lithographically defined superconducting resonators, we report high cooperativities between a resonator mode and a Kittel mode in nanometer thick Permalloy wires. The on-chip, lithographically scalable, and superconducting quantum circuit compatible design provides a direct route towards realizing hybrid quantum systems with nanomagnets, whose coupling strength can be precisely engineered and dynamic properties can be controlled by various mechanisms derived from spintronic studies.

Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film.

A quantitative investigation of the current-induced torque in antiferromagnets represents a great challenge due to the lack of an independent method for controlling Néel vectors. By utilizing an antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction α-Fe_{2}O_{3}, we show that the Néel vector can be controlled with a moderate external field, which is further utilized to calibrate the current-induced magnetic dynamics. We find that the current-induced magnetoresistance change in antiferromagnets can be complicated by resistive switching that does not have a magnetic origin. By excluding nonmagnetic switching and comparing the current-induced dynamics with the field-induced one, we determine the nature and magnitude of current-induced effects in Pt/α-Fe_{2}O_{3} bilayer films.

Gigahertz Frequency Antiferromagnetic Resonance and Strong Magnon-Magnon Coupling in the Layered Crystal CrCl_{3}.

We report broadband microwave absorption spectroscopy of the layered antiferromagnet CrCl_{3}. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Because of the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in the GHz frequency range and a symmetry-protected crossing between them. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be induced between the two resonant modes.

Current-Induced Domain Wall Motion in a Compensated Ferrimagnet.

Owing to the difficulty in detecting and manipulating the magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. By employing heavy-metal-rare-earth-transition-metal alloy bilayers, we experimentally study current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum at the angular momentum compensation point. With experiment and modeling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy-metal-ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.

Room-Temperature Spin-Orbit Torque Switching Induced by a Topological Insulator.

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

Coherent magnon-induced domain-wall motion in a magnetic insulator channel.

Advancing the development of spin-wave devices requires high-quality low-damping magnetic materials where magnon spin currents can efficiently propagate and effectively interact with local magnetic textures. Here we show that magnetic domain walls can modulate spin-wave transport in perpendicularly magnetized channels of Bi-doped yttrium iron garnet. Conversely, we demonstrate that the magnon spin current can drive domain-wall motion in the Bi-doped yttrium iron garnet channel device by means of magnon spin-transfer torque. The domain wall can be reliably moved over 15-20 µm distances at zero applied magnetic field by a magnon spin current excited by a radio-frequency pulse as short as 1 ns. The required energy for driving the domain-wall motion is orders of magnitude smaller than those reported for metallic systems. These results facilitate low-switching-energy magnonic devices and circuits where magnetic domains can be efficiently reconfigured by magnon spin currents flowing within magnetic channels.

Hybrid magnonics in hybrid perovskite antiferromagnets.

Hybrid magnonic systems are a newcomer for pursuing coherent information processing owing to their rich quantum engineering functionalities. One prototypical example is hybrid magnonics in antiferromagnets with an easy-plane anisotropy that resembles a quantum-mechanically mixed two-level spin system through the coupling of acoustic and optical magnons. Generally, the coupling between these orthogonal modes is forbidden due to their opposite parity. Here we show that the Dzyaloshinskii-Moriya-Interaction (DMI), a chiral antisymmetric interaction that occurs in magnetic systems with low symmetry, can lift this restriction. We report that layered hybrid perovskite antiferromagnets with an interlayer DMI can lead to a strong intrinsic magnon-magnon coupling strength up to 0.24 GHz, which is four times greater than the dissipation rates of the acoustic/optical modes. Our work shows that the DMI in these hybrid antiferromagnets holds promise for leveraging magnon-magnon coupling by harnessing symmetry breaking in a highly tunable, solution-processable layered magnetic platform.

Magnetic oscillations driven by the spin Hall effect in 3-terminal magnetic tunnel junction devices.

We show that a direct current in a tantalum microstrip can induce steady-state magnetic oscillations in an adjacent nanomagnet through spin torque from the spin Hall effect (SHE). The oscillations are detected electrically via a magnetic tunnel junction (MTJ) contacting the nanomagnet. The oscillation frequency can be controlled using the MTJ bias to tune the magnetic anisotropy. In this 3-terminal device, the SHE torque and the MTJ bias therefore provide independent controls of the oscillation amplitude and frequency, enabling new approaches for developing tunable spin torque nano-oscillators.

Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect.

We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.

Resonance measurement of nonlocal spin torque in a three-terminal magnetic device.

A pure spin current generated within a nonlocal spin valve can exert a spin-transfer torque on a nanomagnet. This nonlocal torque enables new design schemes for magnetic memory devices that do not require the application of large voltages across tunnel barriers that can suffer electrical breakdown. Here we report a quantitative measurement of this nonlocal spin torque using spin-torque-driven ferromagnetic resonance. Our measurement agrees well with the prediction of an effective circuit model for spin transport. Based on this model, we suggest strategies for optimizing the strength of nonlocal torque.

Spin-torque ferromagnetic resonance induced by the spin Hall effect.

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance dynamics. The Oersted field from the current also generates a ferromagnetic resonance signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.

Resonant Spin Transmission Mediated by Magnons in a Magnetic Insulator Multilayer Structure.

While being electrically insulating, magnetic insulators can behave as good spin conductors by carrying spin current with excited spin waves. So far, magnetic insulators are utilized in multilayer heterostructures for optimizing spin transport or to form magnon spin valves for reaching controls over the spin flow. In these studies, it remains an intensively visited topic as to what the corresponding roles of coherent and incoherent magnons are in the spin transmission. Meanwhile, understanding the underlying mechanism associated with spin transmission in insulators can help to identify new mechanisms that can further improve the spin transport efficiency. Here, by studying spin transport in a magnetic-metal/magnetic-insulator/platinum multilayer, it is demonstrated that coherent magnons can transfer spins efficiently above the magnon bandgap of magnetic insulators. Particularly the standing spin-wave mode can greatly enhance the spin flow by inducing a resonant magnon transmission. Furthermore, within the magnon bandgap, a shutdown of spin transmission due to the blocking of coherent magnons is observed. The demonstrated magnon transmission enhancement and filtering effect provides an efficient method for modulating spin current in magnonic devices.

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 range1,2. The coupling of insulating AFMs to spin-orbit materials3-7 enables spin transport via AFM magnons. In particular, spin transmission over several micrometres occurs in some AFMs with easy-axis anisotropy8,9. 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 momentum9-11. 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.