Effect of inter-layer spin diffusion on skyrmion motion in magnetic multilayers

TL;DR

This paper investigates how inter-layer spin diffusion influences skyrmion motion in magnetic multilayers, revealing an additional spin torque that affects skyrmion dynamics and Hall angles, especially in ultra-thin films.

Contribution

It introduces the role of inter-layer spin diffusion as a significant factor in skyrmion motion, complementing spin-orbit torques, and explains experimental observations of small Hall angles.

Findings

Inter-layer spin diffusion generates a significant spin torque in multilayers.

This diffusive torque can oppose electron flow, affecting skyrmion velocity.

Surface roughness impacts threshold currents and skyrmion Hall angles.

Abstract

It is well known that skyrmions can be driven using spin-orbit torques due to the spin-Hall effect. Here we show an additional contribution in multilayered stacks arises from vertical spin currents due to inter-layer diffusion of a spin accumulation generated at a skyrmion. This additional interfacial spin torque is similar in form to the in-plane spin transfer torque, but is significantly enhanced in ultra-thin films and acts in the opposite direction to the electron flow. The combination of this diffusive spin torque and the spin-orbit torque results in skyrmion motion which helps to explain the observation of small skyrmion Hall angles even with moderate magnetisation damping values. Further, the effect of material imperfections on threshold currents and skyrmion Hall angle is also investigated. Topographical surface roughness, as small as a single monolayer variation, is shown to be an important contributing factor in ultra-thin films, resulting in good agreement with experimental observations.