Gauge Field Formulation of Adiabatic Spin Torques

TL;DR

This paper extends the gauge field formulation of spin torques to finite-amplitude magnetization dynamics, providing a new physical picture of Gilbert damping and enabling first-principles derivations without symmetry assumptions.

Contribution

It introduces an adiabatic spin frame and SU(2) gauge field to analyze finite-amplitude spin dynamics, advancing the theoretical understanding of spin torques.

Findings

Gilbert damping arises from time variation of spin-relaxation sources.

The method offers a first-principles approach to derive spin torques.

Provides a new physical interpretation of damping in spin systems.

Abstract

Previous calculation of spin torques for small-amplitude magnetization dynamics around a uniformly magnetized state [J. Phys. Soc. Jpn. {\bf 75} (2006) 113706] is extended here to the case of finite-amplitude dynamics. This is achieved by introducing an `` adiabatic'' spin frame for conduction electrons, and the associated SU(2) gauge field. In particular, the Gilbert damping is shown to arise from the time variation of the spin-relaxation source terms in this new frame, giving a new physical picture of the damping. The present method will allow a `` first-principle'' derivation of spin torques without any assumptions such as rotational symmetry in spin space.