Current-induced torques and interfacial spin-orbit coupling

TL;DR

This paper uses first principles calculations to analyze current-induced torques in Pt-Co bilayers, focusing on interfacial spin-orbit effects and their dependence on layer structure, providing insights into the mechanisms behind these torques.

Contribution

It offers a detailed first-principles analysis of interfacial spin-orbit torque mechanisms in Pt-Co bilayers, emphasizing the role of interface structure and layer thickness.

Findings

The torque depends strongly on layer thicknesses and interface structure.

The primary source of the field-like torque is a spin-orbit effect on Co induced by Pt.

The torque's dependence on magnetization direction resembles a Rashba model.

Abstract

In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here, we calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism using first principles methods. We focus exclusively on the analogue to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the "field-like" torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.