Nonlinear magnon polaritons

TL;DR

This paper investigates how nonlinear spin-wave interactions in yttrium iron garnet films suppress magnon polaritons at high microwave powers, combining experimental observations with a theoretical model involving three-magnon decay processes.

Contribution

It provides the first experimental demonstration and theoretical explanation of nonlinear suppression of magnon polaritons in magnetic materials.

Findings

Strong coupling observed at low powers with avoided crossing.

Complete gap closure at high powers due to nonlinear effects.

Theoretical model explains the saturation and gap closing phenomena.

Abstract

We experimentally and theoretically demonstrate that nonlinear spin-wave interactions suppress the hybrid magnon-photon quasiparticle or "magnon polariton" in microwave spectra of an yttrium iron garnet film detected by an on-chip split-ring resonator. We observe a strong coupling between the Kittel and microwave cavity modes in terms of an avoided crossing as a function of magnetic fields at low microwave input powers, but a complete closing of the gap at high powers. The experimental results are well explained by a theoretical model including the three-magnon decay of the Kittel magnon into spin waves. The gap closure originates from the saturation of the ferromagnetic resonance above the Suhl instability threshold by a coherent back reaction from the spin waves.