Microscopic derivation of Spin-transfer torques in ferromagnets

TL;DR

This paper provides a microscopic derivation of spin-transfer torques in ferromagnets, confirming macroscopic theories and clarifying the physics and validity of adiabatic and non-adiabatic contributions.

Contribution

It offers a unified microscopic derivation of both adiabatic and non-adiabatic spin-transfer torques, matching previous macroscopic results and explaining recent experimental measurements.

Findings

Confirmed macroscopic spin-transfer torque coefficients

Established validity conditions for the theoretical model

Explained recent experimental observations of non-adiabatic STT

Abstract

Spin-transfer torque (STT) provides key mechanisms for current-induced phenomena in ferromagnets. While it is widely accepted that STT involves both adiabatic and non-adiabatic contributions, their underlying physics and range of validity are quite controversial. By computing microscopically the response of conduction electron spins to a time varying and spatially inhomogeneous magnetic background, we derive the adiabatic and non-adiabatic STT in a unified fashion. Our result confirms the macroscopic theory [Phys. Rev. Lett. \textbf{93},~127204 (2004)] with all coefficients matched exactly. Our derivation also reveals a benchmark on the validity of the result, which is used to explain three recent measurements of the non-adiabatic STT in quite different settings.