Fluctuation model of current-driven magnon excitation

A model is suggested that explains the stationary high-frequency magnetization oscillations observed previously by M. Tsoi et al. when passing dc current through a silver tip mounted on a magnetic Co/Cu multilayer. At the interface between nonmagnetic and ferromagnetic ($N/F$) metals, Aronov's gap $\Delta_A$ (difference in the electrochemical potentials of electrons with opposite spins) arises upon the passage of electric current, thereby energetically promoting magnon creation. Electrons flowing from the nonmagnetic to the ferromagnetic metal become spin-polarized. In a magnetic field, magnetization fluctuations in a mesoscopic ferromagnet give rise to magnetization precession around the magnetic field. The precession is damped due to the viscous losses. In the presence of spin-polarized electron flow, the fluctuations also produce a current-induced torque that compensates for the dissipative torque, leading to stationary high-frequency magnetization oscillations.