Oscillations and confluence in three-magnon scattering of ferromagnetic resonance

The ferromagnetic resonance (FMR) magnon mode can become unstable above a threshold magnon population, returning to this threshold through three-magnon scattering; this is referred to as the first-order Suhl instability. We have investigated this instability in the time domain over a wide range of excitation powers through homodyning spectroscopy and micromagnetic simulations. We observe a regime that hosts nonequilibrium oscillations of the FMR magnon population. We have developed a model that predicts these oscillations and shows strong qualitative agreement with our results. Notably, these oscillations generate 180° phase shifts of the FMR magnons at high powers. Furthermore, we find that these phase shifts correspond to reversals in the three-magnon scattering direction, between splitting and confluence. These reversals also occur after turning off the microwave excitation, generating prolonged oscillations in the magnon populations. These findings shed new light on the nonequilibrium behavior of three-magnon scattering and the essential role of phase dynamics in magnon scattering processes.