Spin-orbit torque is a current-induced transfer of angular momentum from an atomic lattice to magnetic order. It is a promising mechanism to write magnetic memories and drive spin torque oscillators. Since its inception, the list of spin-orbit torque mechanisms has grown beyond the conventional spin Hall and Rashba-Edelstein mechanisms to include “unconventional” mechanisms, arising from spin and orbital current generation in ferromagnetic layers, nonmagnetic layers, and their interfaces. In this talk, we use micromagnetic, semiclassical, and first principles calculations to show that unconventional spin-orbit torques are potentially important for devices, from causing nonlocal spin torques in ferromagnetic trilayers to enabling large amplitude, easy-plane spin-orbit torque oscillators.
We will discuss first-principles calculations of spin transport and spin-orbit torques in disordered films and multilayers within the nonequilibrium Green's function technique. For a nonmagnetic Pt film, the behavior of the spin accumulation and the transverse spin current deviate significantly from the conventional spin-diffusion model. The effective transverse spin-diffusion length is much shorter than the longitudinal spin-diffusion length. For ferromagnetic trilayers, we find dampinglike and fieldlike torques with unconventional spin polarizations. The large rotated fieldlike component in Co/Cu/Co trilayers is inconsistent with diffusive transport in the spacer layer but can be explained by nonlocal interactions between the ferromagnetic layers when the mean-free path is not small compared to the spacer thickness.
In this talk, we explore the variety of ways in which ferromagnets electrically generate spin currents. We present first principles transport calculations giving the strength and magnetization dependence of these various mechanisms in transition metal ferromagnets. We compute full spin current tensors to probe all spin directions and show how each mechanism contributes to the spin currents allowed by symmetry. To help guide the interpretation of experiments, we also present calculations of spin current generation in magnetic heterostructures to quantify their transmission through interfaces and the spin torques they exert. Finally, we discuss experiments that have probed these novel mechanisms of spin current generation. Unlocking the full potential of ferromagnets as spin current sources will help create a powerful tool for spintronic devices such as magnetic memories.
In magnetic heterostructures, an in-plane electrical current can manipulate the magnetization direction of the free ferromagnetic layers through spin-orbit torques. Spin-orbit torques are transfers of angular momentum from the crystal lattice to the magnetization using conduction electrons as the medium for transfer. Traditionally, spin-orbit torques have been attributed to two causes: the spin Hall effect and the Rashba-Edelstein effect. However, this framework is incomplete because 1) experiments cannot reproducibly distinguish these mechanisms and 2) theory predicts other torques of comparable strength. Using the Boltzmann equation and first principles calculations, we have investigated the allowed interfacial contributions to spin-orbit torque. These investigations led to the discovery of a novel effect, interface-generated spin currents, which first principles calculations reveal are strong in relevant heavy metal/ferromagnet bilayers (such as Pt/Co). This work has also resulted in simple drift-diffusion models that can capture interfacial spin-orbit effects through the appropriate boundary conditions. Following this work, our group has performed first principles calculations showing that ferromagnets generate spin currents via the intrinsic mechanism that could exert spin-orbit torques at interfaces. In this talk, we discuss how both novel interfacial spin-orbit effects and spin current generation in ferromagnets could play an important role in spin-orbit torque. Recent experiments done in heterostructures, including those with a single ferromagnetic layer, provide evidence that ferromagnetic layers and interfaces cause the spin-orbit torques discussed here. Shedding light on these mechanisms will help clarify the nature of spin-orbit torque, which is crucial for realizing its potential for magnetic memories.