Anatomy of Spin-Transfer Torque

TL;DR

This paper elucidates the microscopic mechanisms behind spin-transfer torques in magnetic heterostructures, showing that the transverse spin current component is effectively absorbed at interfaces, leading to a torque proportional to the incoming transverse spin current.

Contribution

It explicitly demonstrates the absorption processes of transverse spin currents using models and first principles calculations, clarifying the origin of spin-transfer torque.

Findings

Transverse spin currents are nearly fully absorbed at interfaces.

The torque is proportional to the transverse component of the incoming spin current.

Three processes contribute to spin absorption: reflection, rotation, and precession.

Abstract

Spin-transfer torques occur in magnetic heterostructures because the transverse component of a spin current that flows from a non-magnet into a ferromagnet is absorbed at the interface. We demonstrate this fact explicitly using free electron models and first principles electronic structure calculations for real material interfaces. Three distinct processes contribute to the absorption: (1) spin-dependent reflection and transmission; (2) rotation of reflected and transmitted spins; and (3) spatial precession of spins in the ferromagnet. When summed over all Fermi surface electrons, these processes reduce the transverse component of the transmitted and reflected spin currents to nearly zero for most systems of interest. Therefore, to a good approximation, the torque on the magnetization is proportional to the transverse piece of the incoming spin current.