Influence of a uniform current on collective magnetization dynamics in a ferromagnetic metal

Abstract

We discuss the influence of a uniform current $\stackrel{\ensuremath{\rightarrow}}{j}$ on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $\ensuremath{\epsilon}(\stackrel{\ensuremath{\rightarrow}}{q})$ has a current-induced contribution proportional to $\stackrel{\ensuremath{\rightarrow}}{q}\ensuremath{\cdot}\mathcal{J}\ensuremath{\rightarrow},$ where $\mathcal{J}\ensuremath{\rightarrow}$ is the spin current, and predict that collective dynamics will be more strongly damped at finite $\stackrel{\ensuremath{\rightarrow}}{j}.$ We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for $j\ensuremath{\gtrsim}{10}^{9}{\mathrm{A}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.