A complete set of the generalized drift-diffusion equations for a coupled charge and spin dynamics in ferromagnets in the presence of extrinsic spin-orbit coupling is derived from the quantum kinetic ...approach, covering major transport phenomena, such as the spin and anomalous Hall effects, spin swapping, spin precession, and relaxation processes. We argue that the spin swapping effect in ferromagnets is enhanced due to spin polarization, while the overall spin texture induced by the interplay of spin-orbital and spin precession effects displays a complex spatial dependence that can be exploited to generate torques and nucleate or propagate domain walls in centrosymmetric geometries without the use of external polarizers, as opposed to the conventional understanding of spin-orbit mediated torques.
Modern magnetic-memory technology requires all-electric control of perpendicular magnetization with low energy consumption. While spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) ...heterostructures
holds promise for applications in magnetic random access memory, until today, it has been limited to the in-plane direction. Such in-plane torque can switch perpendicular magnetization only deterministically with the help of additional symmetry breaking, for example, through the application of an external magnetic field
, an interlayer/exchange coupling
or an asymmetric design
. Instead, an out-of-plane SOT
could directly switch perpendicular magnetization. Here we observe an out-of-plane SOT in an HM/FM bilayer of L1
-ordered CuPt/CoPt and demonstrate field-free switching of the perpendicular magnetization of the CoPt layer. The low-symmetry point group (3m1) at the CuPt/CoPt interface gives rise to this spin torque, hereinafter referred to as 3m torque, which strongly depends on the relative orientation of the current flow and the crystal symmetry. We observe a three-fold angular dependence in both the field-free switching and the current-induced out-of-plane effective field. Because of the intrinsic nature of the 3m torque, the field-free switching in CuPt/CoPt shows good endurance in cycling experiments. Experiments involving a wide variety of SOT bilayers with low-symmetry point groups
at the interface may reveal further unconventional spin torques in the future.
Current-driven spin torques in metallic spin valves composed of antiferromagnets are theoretically studied using the nonequilibrium Green's function method implemented on a tight-binding model. We ...focus our attention on G-type and L-type antiferromagnets in both clean and disordered regimes. In such structures, spin torques can either rotate the magnetic order parameter coherently (coherent torque) or compete with the internal antiferromagnetic exchange (exchange torque). We show that, depending on the symmetry of the spin valve, the coherent and exchange torques can either be in the plane, is proportional to n x (q x n) or out of the plane is proportional to n x q, where q and n are the directions of the order parameter of the polarizer and the free antiferromagnetic layers, respectively. Although disorder conserves the symmetry of the torques, it strongly reduces the torque magnitude, pointing out the need for momentum conservation to ensure strong spin torque in antiferromagnetic spin valves.
Insulating compensated ferrimagnets, especially hosting room-temperature compensation points, are considered promising candidates for developing ultra-high-density and ultrafast magnonic devices ...owing to combining the characteristics of both ferromagnets and antiferromagnets. These intriguing features become outstanding close to their compensation points. However, their spin–orbit torque (SOT)-induced magnetization switching, particularly in the vicinity of the compensation points, remains unclear. Herein, we systematically investigated the SOT in insulating compensated ferrimagnetic Gd3Fe5O12/Pt heterostructures with perpendicular magnetic anisotropy. A nearly room-temperature compensation point (T comp ∼ 297 K) was consistently identified by the magnetization curves, spin Hall-induced anomalous Hall effect, and spin Hall magnetoresistance measurements. Moreover, using 100 ns duration pulsed current, deterministic current-induced magnetization switching below and above T comp, even at 294 and 301 K, was achieved with opposite switching polarity. It is found that a large current is required to switch the magnetization in the vicinity of T comp, although the effective SOT field increases close to T comp. Our finding provides alternative opportunities for exploring ultrafast room-temperature magnon-based devices.
The spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the ...SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/Co20Fe60B20 heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θS) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the BixSe(1-x) was observed at room temperature with a low critical magnetization switching current density of 4.3 × 105 A cm-2. Quantum transport simulations using a realistic sp3 tight-binding model suggests that the high SOT in sputtered BixSe(1-x) is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θS, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1-x) films provide an avenue for the use of BixSe(1-x) as a spin density generator in SOT-based memory and logic devices.
It is well known that moving magnetic textures may pump spin and charge currents along the direction of motion, a phenomenon called electronic pumping. Here, the electronic pumping arising from the ...steady motion of ferromagnetic skyrmions is investigated by solving the time evolution of the Schrödinger equation implemented on a tight-binding model with the statistical physics of the many-body problem. In contrast with rigid one-dimensional magnetic textures, we show that steadily moving magnetic skyrmions are able to pump large dc currents. This ability arises from their nontrivial magnetic topology, i.e., the coexistence of the spin-motive force and the topological Hall effect. Based on an adiabatic scattering theory, we compute the pumped current and demonstrate that it scales with the reflection coefficient of the conduction electrons against the skyrmion. In other words, in the semiclassical limit, reducing the size of the skyrmion and the width of the nanowire enhances this effect, making it scalable. We propose that such a phenomenon can be exploited in the context of racetrack devices, where the electronic pumping enhances the collective motion of the train of skyrmions.
We predict that a lateral electrical current in antiferromagnets can induce nonequilibrium Néel-order fields, i.e., fields whose sign alternates between the spin sublattices, which can trigger ...ultrafast spin-axis reorientation. Based on microscopic transport theory calculations we identify staggered current-induced fields analogous to the intraband and to the intrinsic interband spin-orbit fields previously reported in ferromagnets with a broken inversion-symmetry crystal. To illustrate their rich physics and utility, we consider bulk Mn(2)Au with the two spin sublattices forming inversion partners, and a 2D square-lattice antiferromagnet with broken structural inversion symmetry modeled by a Rashba spin-orbit coupling. We propose an antiferromagnetic memory device with electrical writing and reading.
The magnitude of spin-orbit torque (SOT), exerted to a ferromagnet (FM) from an adjacent heavy metal (HM), strongly depends on the amount of spin current absorbed in the FM. We exploit the large spin ...absorption at the Ru interface to manipulate the SOTs in HM/FM/Ru multilayers. While the FM thickness is smaller than its spin dephasing length of 1.2 nm, the top Ru layer largely boosts the absorption of spin currents into the FM layer and substantially enhances the strength of SOT acting on the FM. Spin-pumping experiments induced by ferromagnetic resonance support our conclusions that the observed increase in the SOT efficiency can be attributed to an enhancement of the spin-current absorption. A theoretical model that considers both reflected and transmitted mixing conductances at the two interfaces of FM is developed to explain the results.
The Smrcka-Streda version of Kubo's linear response formula is widely used in the literature to compute nonequilibrium transport properties of heterostructures. It is particularly useful for the ...evaluation of intrinsic transport properties associated with the Berry curvature of the Bloch states, such as anomalous and spin Hall currents as well as the dampinglike component of the spin-orbit torque. Here we demonstrate in a very general way that the widely used decomposition of the Kubo-Bastin formula introduced by Smrcka and Streda contains an overlap, which has lead to widespread confusion in the literature regarding the Fermi surface and Fermi sea contributions. To remedy this pathology, we propose a decomposition of the Kubo-Bastin formula based on the permutation properties of the correlation function and derive a set of formulas, without an overlap, that provides direct access to the transport effects of interest. We apply these formulas to selected cases and demonstrate that the Fermi sea and Fermi surface contributions can be uniquely addressed with our symmetrized approach.
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