Gauge Field Formulation of Adiabatic Spin Torques

Abstract

Previous calculation of spin torques for small-amplitude magnetization dynamics around a uniformly magnetized state [J. Phys. Soc. Jpn. 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.