Strong non-equilibrium effects in spin torque systems

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 $U(1)$).