Spin-orbit torques from interfacial spin-orbit coupling for various interfaces

Switching of Perpendicular Magnetization by Spin-Orbit Torque

Magnetic materials with strong perpendicular magnetic anisotropy are of great interest for the development of nonvolatile magnetic memory and computing technologies due to their high stabilities at the nanoscale. However, electrical switching of such perpendicular magnetization in an energy-efficient, deterministic, scalable manner has remained a big challenge. This problem has recently attracted enormous efforts in the field of spintronics. Here, recent advances and challenges in the understanding of the electrical generation of spin currents, the switching mechanisms and the switching strategies of perpendicular magnetization, the switching current density by spin-orbit torque of transverse spins, the choice of perpendicular magnetic materials are reviewed, and the progress in prototype perpendicular SOT memory and logic devices toward the goal of energy-efficient, dense, fast perpendicular spin-orbit torque applications is summarized.

Classification tasks using input driven nonlinear magnetization dynamics in spin Hall oscillator

The inherent nonlinear magnetization dynamics in spintronic devices make them suitable candidates for neuromorphic hardware. Among spintronic devices, spin torque oscillators such as spin transfer torque oscillators and spin Hall oscillators have shown the capability to perform recognition tasks. In this paper, with the help of micromagnetic simulations, we model and demonstrate that the magnetization dynamics of a single spin Hall oscillator can be nonlinearly transformed by harnessing input pulse streams and can be utilized for classification tasks. The spin Hall oscillator utilizes the microwave spectral characteristics of its magnetization dynamics for processing a binary data input. The spectral change due to the nonlinear magnetization dynamics assists in real-time feature extraction and classification of 4-binary digit input patterns. The performance was tested for the classification of the standard MNIST handwritten digit data set and achieved an accuracy of 83.1% in a simple linear regression model. Our results suggest that modulating time-driven input data can generate diverse magnetization dynamics in the spin Hall oscillator that can be suitable for temporal or sequential information processing.

Sign Change of Spin-Orbit Torque in Pt/NiO/CoFeB Structures

Antiferromagnetic insulators have recently been proved to support spin current efficiently. Here, we report the dampinglike spin-orbit torque (SOT) in Pt/NiO/CoFeB has a strong temperature dependence and reverses the sign below certain temperatures, which is different from the slight variation with temperature in the Pt/CoFeB bilayer. The negative dampinglike SOT at low temperatures is proposed to be mediated by the magnetic interactions that tie with the "exchange bias" in Pt/NiO/CoFeB, in contrast to the thermal-magnon-mediated scenario at high temperatures. Our results highlight the promise to control the SOT through tuning the magnetic structure in multilayers.

Spin accumulation and spin Hall effect in a two-layer system with a thin ferromagnetic layer

Spin accumulation and spin current are phenomena that enhance the functionality of the devices operating with charge and spin. We calculated them for the system consisting of a ferroelectric barrier and a thin ferromagnetic layer when the current flows parallel to the interface. We assume Dresselhaus and Rashba spin-orbit coupling linear in electron wave number. We demonstrate that spin accumulation and spin current can be manipulated by changing the direction of the magnetization of the FM layer with respect to the crystallographic axes of the ferroelectric barrier.

Strong Interfacial Coupling of Tunable Ni-NiO Nanocomposite Thin Films Formed by Self-Decomposition

The next-generation spintronic devices including memristors, tunneling devices, or stochastic switching exert surging demands on magnetic nanostructures with novel coupling schemes. Taking advantage of a phase decomposition mechanism, a unique Ni-NiO nanocomposite has been demonstrated using a conventional pulsed laser deposition technique. Ni nanodomains are segregated from NiO and exhibit as faceted "emerald-cut" morphologies with tunable dimensions affected by the growth temperature. The sharp interfacial transition between ferromagnetic (002) Ni and antiferromagnetic (002) NiO, as characterized by high-resolution transmission electron microscopy, introduces a strong exchange bias effect and magneto-optical coupling at room temperature. In situ heating-cooling X-ray diffraction (XRD) study confirms an irreversible phase transformation between Ni and NiO under ambient atmosphere. Synthesizing highly functional two-phase nanocomposites with a simple bottom-up self-assembly via such a phase decomposition mechanism presents advantages in terms of epitaxial quality, surface coverage, interfacial coupling, and tunable nanomagnetism, which are valuable for new spintronic device implementation.

Measurement of the Spin Absorption Anisotropy in Lateral Spin Valves

The spin absorption process in a ferromagnetic material depends on the spin orientation relative to the magnetization. Using a ferromagnet to absorb the pure spin current created within a lateral spin valve, we evidence and quantify a sizable orientation dependence of the spin absorption in Co, CoFe, and NiFe. These experiments allow us to determine the spin-mixing conductance, an elusive but fundamental parameter of the spin-dependent transport. We show that the obtained values cannot be understood within a model considering only the Larmor, transverse decoherence, and spin diffusion lengths, and rather suggest that the spin-mixing conductance is actually limited by the Sharvin conductance.

Rashba Effect in Functional Spintronic Devices

Exploiting spin transport increases the functionality of electronic devices and enables such devices to overcome physical limitations related to speed and power. Utilizing the Rashba effect at the interface of heterostructures provides promising opportunities toward the development of high-performance devices because it enables electrical control of the spin information. Herein, the focus is mainly on progress related to the two most compelling devices that exploit the Rashba effect: spin transistors and spin-orbit torque devices. For spin field-effect transistors, the gate-voltage manipulation of the Rashba effect and subsequent control of the spin precession are discussed, including for all-electric spin field-effect transistors. For spin-orbit torque devices, recent theories and experiments on interface-generated spin current are discussed. The future directions of manipulating the Rashba effect to realize fully integrated spin logic and memory devices are also discussed.

Generalized Spin Drift-Diffusion Formalism in the Presence of Spin-Orbit Interaction of Ferromagnets

We generalize the spin drift-diffusion formalism by considering spin-orbit interaction of a ferromagnet, which generates transverse spin currents in the ferromagnet. We consider quantum-mechanical transport of transverse spins in a spin-orbit coupled ferromagnet and develop a generalized drift-diffusion equation and boundary condition. By combining them, we identify previously unrecognized spin transport phenomena in heterostructures including ferromagnets. As representative examples, we show self-generated spin torque and self-generated charge pumping in ferromagnet-normal metal bilayers. The former is a torque exerting on a ferromagnet, originating from a transverse spin current leaving from the ferromagnet itself, whereas the latter is the Onsager reciprocity of the former. Our work not only provides a concise formalism for the effects of nondephased transverse spins in ferromagnets but also enables to design spintronic devices without an external spin source.

Large Damping-Like Spin-Orbit Torque in a 2D Conductive 1T-TaS2 Monolayer

A damping-like spin-orbit torque (SOT) is a prerequisite for ultralow-power spin logic devices. Here, we report on the damping-like SOT in just one monolayer of the conducting transition-metal dichalcogenide (TMD) TaS₂ interfaced with a NiFe (Py) ferromagnetic layer. The charge-spin conversion efficiency is found to be 0.25 ± 0.03 in TaS₂(0.88)/Py(7), and the spin Hall conductivity (14.9×105ℏ2eΩ-1m-1) is found to be superior to values reported for other TMDs. We also observed sizable field-like torque in this heterostructure. The origin of this large damping-like SOT can be found in the interfacial properties of the TaS₂/Py heterostructure, and the experimental findings are complemented by the results from density functional theory calculations. It is envisioned that the interplay between interfacial spin-orbit coupling and crystal symmetry yielding large damping-like SOT. The dominance of damping-like torque demonstrated in our study provides a promising path for designing the next-generation conducting TMD-based low-powered quantum memory devices.

Negative spin Hall magnetoresistance of normal metal/ferromagnet bilayers

Interconversion between charge and spin through spin-orbit coupling lies at the heart of condensed-matter physics. In normal metal/ferromagnet bilayers, a concerted action of the interconversions, the spin Hall effect and its inverse effect of normal metals, results in spin Hall magnetoresistance, whose sign is always positive regardless of the sign of spin Hall conductivity of normal metals. Here we report that the spin Hall magnetoresistance of Ta/NiFe bilayers is negative, necessitating an additional interconversion process. Our theory shows that the interconversion owing to interfacial spin-orbit coupling at normal metal/ferromagnet interfaces can give rise to negative spin Hall magnetoresistance. Given that recent studies found the conversion from charge currents to spin currents at normal metal/ferromagnet interfaces, our work provides a missing proof of its reciprocal spin-current-to-charge-current conversion at same interface. Our result suggests that interfacial spin-orbit coupling effect can dominate over bulk effects, thereby demanding interface engineering for advanced spintronics devices.

Perspectives of Electrically generated spin currents in ferromagnetic materials

Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.

Theory of Current-Induced Angular Momentum Transfer Dynamics in Spin-Orbit Coupled Systems

Motivated by the importance of understanding various competing mechanisms to the current-induced spin-orbit torque on magnetization in complex magnets, we develop a theory of current-induced spin-orbital coupled dynamics in magnetic heterostructures. The theory describes angular momentum transfer between different degrees of freedom in solids, e.g., the electron orbital and spin, the crystal lattice, and the magnetic order parameter. Based on the continuity equations for the spin and orbital angular momenta, we derive equations of motion that relate spin and orbital current fluxes and torques describing the transfer of angular momentum between different degrees of freedom, achieved in a steady state under an applied external electric field. We then propose a classification scheme for the mechanisms of the current-induced torque in magnetic bilayers. We evaluate the sources of torque using density functional theory, effectively capturing the impact of the electronic structure on these quantities. We apply our formalism to two different magnetic bilayers, Fe/W(110) and Ni/W(110), which are chosen such that the orbital and spin Hall effects in W have opposite sign and the resulting spin- and orbital-mediated torques can compete with each other. We find that while the spin torque arising from the spin Hall effect of W is the dominant mechanism of the current-induced torque in Fe/W(110), the dominant mechanism in Ni/W(110) is the orbital torque originating in the orbital Hall effect of the non-magnetic substrate. Thus the effective spin Hall angles for the total torque are negative and positive in the two systems. Our prediction can be experimentally identified in moderately clean samples, where intrinsic contributions dominate. This clearly demonstrates that our formalism is ideal for studying the angular momentum transfer dynamics in spin-orbit coupled systems as it goes beyond the "spin current picture" by naturally incorporating the spin and orbital degrees of freedom on an equal footing. Our calculations reveal that, in addition to the spin and orbital torque, other contributions such as the interfacial torque and self-induced anomalous torque within the ferromagnet are not negligible in both material systems.

A single layer spin-orbit torque nano-oscillator

Spin torque and spin Hall effect nano-oscillators generate high intensity spin wave auto-oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate highly coherent field and current tunable microwave signals from nano-constrictions in single 15–20 nm thick permalloy layers with oxide interfaces. Using a combination of spin torque ferromagnetic resonance measurements, scanning micro-Brillouin light scattering microscopy, and micromagnetic simulations, we identify the auto-oscillations as emanating from a localized edge mode of the nano-constriction driven by spin-orbit torques. Our results pave the way for greatly simplified designs of auto-oscillating nano-magnetic systems only requiring single ferromagnetic layers with oxide interfaces.

Spin torque nano-oscillatiors promise novel microwave applications but the functioning relies on the spin current from additional ferromagnetic or metal layers. The authors here achieved in a single ferromagnetic layer sandwiched by nonmagnetic insulators the spin wave auto-oscillations due to a localized edge mode of the nano-constriction.

Large spin-orbit torque efficiency enhanced by magnetic structure of collinear antiferromagnet IrMn

Spin-orbit torque (SOT) offers promising approaches to developing energy-efficient memory devices by electric switching of magnetization. Compared to other SOT materials, metallic antiferromagnet (AFM) potentially allows the control of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spin-torque ferromagnetic resonance experiments, we show that the magnetic structure of epitaxially grown L1₀-IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of twin domains, with the spin axes orienting toward [111] and [-111], respectively. This unconventional magnetic structure is responsible for much larger SOT efficiencies up to 0.60 ± 0.04, compared to 0.083 ± 0.002 for the polycrystalline IrMn. Furthermore, we reveal that this magnetic structure induces a large isotropic bulk contribution and a comparable anisotropic interfacial contribution to the SOT efficiency. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.

Spin-Orbit Torques in Heavy-Metal-Ferromagnet Bilayers with Varying Strengths of Interfacial Spin-Orbit Coupling

Despite intense efforts it has remained unresolved whether and how interfacial spin-orbit coupling (ISOC) affects spin transport across heavy-metal (HM)-ferromagnet (FM) interfaces. Here we report conclusive experiment evidence that the ISOC at HM/FM interfaces is the dominant mechanism for "spin memory loss". An increase in ISOC significantly reduces, in a linear manner, the dampinglike spin-orbit torque (SOT) exerted on the FM layer via degradation of the spin transparency of the interface for spin currents generated in the HM. In addition, the fieldlike SOT is also dominated by the spin Hall contribution of the HM and decreases with increasing ISOC. This work reveals that ISOC at HM/FM interfaces should be minimized to advance efficient SOT devices through atomic layer passivation of the HM/FM interface or other means.

In-plane direct current probing for spin orbit torque-driven effective fields in perpendicularly magnetized heavy metal/ferromagnet/oxide frames

Electrical manipulation of magnetization states has been the subject of intense focus as it is a long-standing goal in the emerging field of spintronics. In particular, torque generated by an in-plane current with a strong spin-orbit interaction shows promise for control of the adjacent ferromagnetic state in heavy-metal/ferromagnet/oxide frames. Thus, the ability to unlock precise spin orbit torque-driven effective fields represents one of the key approaches in this work. Here, we address an in-plane direct current measurement approach as a generic alternative tool to identify spin orbit torque-driven effective fields in a full polar angle range without adopting the commonly used harmonic analyses. Our experimental results exhibited a strongly polar angular dependency of the spin orbit torque-driven effective fields observed from Ta or W/CoFeM/MgO frames.

Intrinsic Spin-Orbit Torque Arising from the Berry Curvature in a Metallic-Magnet/Cu-Oxide Interface

We report the observation of the intrinsic dampinglike spin-orbit torque (SOT) arising from the Berry curvature in metallic-magnet/CuO_{x} heterostructures. We show that a robust dampinglike SOT, an order of magnitude larger than a fieldlike SOT, is generated in the heterostructure despite the absence of the bulk spin-orbit effect in the CuO_{x} layer. Furthermore, by tuning the interfacial oxidation level, we demonstrate that the fieldlike SOT changes drastically and even switches its sign, which originates from oxygen-modulated spin-dependent disorder. These results provide important information for a fundamental understanding of the physics of the SOTs.

Theory of in-plane current induced spin torque in metal/ferromagnet bilayers

Using a semiclassical approach that simultaneously incorporates the spin Hall effect (SHE), spin diffusion, quantum well states, and interface spin-orbit coupling (SOC), we address the interplay of these mechanisms as the origin of the spin-orbit torque (SOT) induced by in-plane currents, as observed in the normal metal/ferromagnetic metal bilayer thin films. Focusing on the bilayers with a ferromagnet much thinner than its spin diffusion length, such as Pt/Co with  ∼10 nm thickness, our approach addresses simultaneously the two contributions to the SOT, namely the spin-transfer torque (SHE-STT) due to SHE-induced spin injection, and the inverse spin Galvanic effect spin-orbit torque (ISGE-SOT) due to SOC-induced spin accumulation. The SOC produces an effective magnetic field at the interface, hence it modifies the angular momentum conservation expected for the SHE-STT. The SHE-induced spin voltage and the interface spin current are mutually dependent and, hence, are solved in a self-consistent manner. The result suggests that the SHE-STT and ISGE-SOT are of the same order of magnitude, and the spin transport mediated by the quantum well states may be an important mechanism for the experimentally observed rapid variation of the SOT with respect to the thickness of the ferromagnet.

Spin-orbit torques associated with ferrimagnetic order in Pt/GdFeCo/MgO layers

We investigate spin orbit torque (SOT) efficiencies and magnetic properties of Pt/GdFeCo/MgO multilayers by varying the thicknesses of GdFeCo and MgO layers. Our studies indicate that the ferrimagnetism in the GdFeCo alloy is considerably influenced by both thicknesses due to the diffusion of Gd atoms toward the MgO layer. Comparing to conventional Pt/ferromagnet/MgO structures, the Pt/GdFeCo/MgO exhibits a lower efficiency of SOTs associated with ferrimagnetic order and a similar magnitude of magnetic damping. The previous models that have been developed for rigid ferromagnets are inappropriate to analyze our experimental data, leading to an unphysical consequence of spin transmission larger than unity. Our results imply that the heavy-metal/ferrimagnet system is quite different from heavy-metal/ferromagnet systems in terms of magnetic dynamical modes, spin angular momentum transfer, and relaxation processes.

Spin-Orbit Torques in NbSe2/Permalloy Bilayers

We present measurements of current-induced spin-orbit torques generated by NbSe₂, a fully metallic transition-metal dichalcogenide material, made using the spin-torque ferromagnetic resonance (ST-FMR) technique with NbSe₂/Permalloy bilayers. In addition to the out-of-plane Oersted torque expected from current flow in the metallic NbSe₂ layer, we also observe an in-plane antidamping torque with torque conductivity σ_S ≈ 10³ (ℏ/2e)(Ωm)^-1 and indications of a weak field-like contribution to the out-of-plane torque oriented opposite to the Oersted torque. Furthermore, in some samples we also measure an in-plane field-like torque with the form m̂ × ẑ, where m̂ is the Permalloy magnetization direction and ẑ is perpendicular to the sample plane. The size of this component varies strongly between samples and is not correlated with the NbSe₂ thickness. A torque of this form is not allowed by the bulk symmetries of NbSe₂ but is consistent with symmetry breaking by a uniaxial strain that might result during device fabrication.