Magnon excitation by spin-polarized direct currents in magnetic nanostructures

Abstract

The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized direct current is studied. In such a system, spin waves (magnons) may be critically driven out of equilibrium by an effective spin-injection field that is proportional to the current density. A direct comparison between the predicted critical current and previous experimental results sheds light on the nature of the excited mode. Beyond the threshold, it is assumed that the spin waves are coupled through nonlinear interactions arising from dipolar and surface anisotropy energies. It is shown that the magnon-magnon interactions play two major roles in the dynamics: (i) They govern and put a limit to the growth in the population of the unstable mode from the thermal level, and (ii) directly contribute to the renormalization of the magnon energy, which manifests itself through a shift in the precession frequency of the magnetic moments with varying current intensity. Numerical results are presented in remarkable quantitative agreement with recent experiments in nanometric magnetic multilayers, where microwave oscillations generated by direct currents have been observed in the postthreshold regime.