Strong non-equilibrium effects in spin torque systems

TL;DR

This paper investigates the instability of persistent magnetization precession in ferromagnetic nanoparticles driven by spin-transfer torque, predicting abrupt switching of spin orientation due to non-equilibrium effects and deriving an effective action for the coupled dynamics.

Contribution

It introduces a novel analysis of non-equilibrium effects in spin torque systems and derives an effective action describing magnetization and voltage dynamics.

Findings

Adjustment of electronic distribution causes instability in precession state

Abrupt switching of spin orientation occurs with increased spin-transfer current

Effective action for coupled magnetization and voltage dynamics is derived

Abstract

We consider a problem of persistent magnetization precession in a single domain ferromagnetic nano particle under the driving by the spin-transfer torque. We find that the adjustment of the electronic distribution function in the particle renders this state unstable. Instead, abrupt switching of the spin orientation is predicted upon increase of the spin-transfer torque current. On the technical level, we derive an effective action of the type of Ambegaokar-Eckern-Sch\"on action for the coupled dynamics of magnetization (gauge group $SU(2)$) and voltage (gauge group $U(1)$).