Transient dynamics and scattering reversals in three-magnon scattering of ferromagnetic resonance

Max Planck Quantum Matter Seminar

  • Date: Feb 9, 2023
  • Time: 04:00 PM - 05:00 PM (Local Time Germany)
  • Speaker: Alex Hamill
  • University of Minnesota
  • Location: online via Zoom
  • Host: Hope Bretscher
Transient dynamics and scattering reversals in three-magnon scattering of ferromagnetic resonance

Magnons are the quanta of collective spin excitations. Their phase degree of freedom, highly nonlinear behavior, and long lifetimes make them an active subject in fundamental research and research towards next-generation microwave technology and computing paradigms. Magnons’ nonlinear behavior primarily manifests through magnon-magnon scattering processes, of which previous studies have largely focused on the steady-state behavior of the magnon populations; to this end, we comprehensively investigated the transient behavior of the magnon populations while also taking their phases into account. In particular, we investigated the first-order Suhl instability, where a magnon mode undergoes three-magnon splitting above a threshold population, thus saturating it at this threshold value. We examined the instability of the zero wavevector mode, corresponding to ferromagnetic resonance (FMR), which has a distinctly low threshold population for this instability. This low threshold, along with our developed measurement technique utilizing homodyning spectroscopy, allowed us to perform a time-resolved and phase-sensitive investigation of this process over several orders of magnitude in excitation power. We observe a regime that hosts transient and power-dependent oscillations of the FMR magnon population, despite higher-order magnon scattering processes at large powers. Also at high powers, we find that the scattering generates 180 degree phase shifts of the FMR magnons and that, furthermore, these phase shifts correspond to reversals in the three-magnon scattering direction, between splitting and confluence. These scattering reversals are most directly observed after removing the microwave excitation, generating coherent oscillations of the FMR magnon population much larger than its steady-state value during the excitation. Our model is in strong agreement with these findings. These findings reveal the transient behavior of this three-magnon scattering process, and the nontrivial interplay between three-magnon scattering and the magnons' phases.

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