Currents passing through bilayer nanowires with a ferromagnetic layer and a non-magnetic layer with strong spin-orbit coupling, excite magnetic dynamics which cannot be explained with standard models. The dynamics appear to require two torques which can be characterized as damping-like and field-like. Typically very different models yield the different torques. A model based on the Boltzmann equation can unify these approaches. It combines both torques due to the bulk spin Hall effect and the spin transfer torque and torques due to the interfacial spin-orbit coupling. An approximation to the Boltzmann equation, the drift-diffusion model, qualitative reproduces the behavior, but quantitatively fails to reproduce the results. While the Boltzmann equation with appropriate parameters can match the torques for any particular sample, it fails to describe the experimentally observed thickness dependences in many cases. This disagreement suggests that the parameters describing the system vary as the thicknesses do.
Citation: Physical Review B
Pub Type: Journals
spin current, spin Hall Effect, spin transfer torque, spin-orbit coupling, magnetodynamics, current-induced domain wall motion, nanowires, Boltzmann equation