Ferromagnetic Relaxation. III. Theory of Instabilities

Abstract
In this paper a quantum-mechanical model is used to investigate various instability processes in ferromagnetic insulators. Time-dependent perturbation theory is employed to find the condition for which the rate of energy flowing into a magnon or phonon is equal to the rate at which it relaxes. This leads to a general instability criterion for various magnon-magnon, photon-magnon, and magnon-phonon processes.