Nonlinear frequency shift and bistability of magnon-polarons

TL;DR

This paper studies the nonlinear behavior of coupled surface acoustic and spin waves in a magnetoacoustic resonator, revealing frequency shifts and bistability that could enable advanced wave-based information processing.

Contribution

It demonstrates the nonlinear dynamics and bistability of magnon-phonon hybrids in a YIG/ZnO resonator, highlighting their potential for nonlinear magnetoacoustic applications.

Findings

Hybridization between SAW modes and SWs causes avoided crossings.

Elevated drive powers induce a positive frequency shift of SW modes.

Nonlinear response leads to bistable behavior and enhanced magnon populations.

Abstract

We investigate the nonlinear dynamics of strongly coupled surface acoustic waves (SAWs) and spin waves (SWs) in a magnetoacoustic resonator based on a YIG/ZnO heterostructure by combining microwave reflection measurements with microfocused Brillouin light scattering spectroscopy. In the linear regime, the electrical response reveals clear hybridization between standing SAW cavity modes and finite-wave-vector SWs, resulting in pronounced avoided crossings. At elevated drive powers, the hybrid system exhibits a strongly field-dependent nonlinear response characterized by a positive frequency shift of the driven SW mode. Using the vector Hamiltonian formalism for nonlinear spin-wave dynamics, we show that this shift is dominated by a cross-shift term. In our resonator geometry, this contribution becomes significant because the standing SAW cavity mode simultaneously excites counterpropagating SWs with wave vectors $+k$ and $-k$. For suitable field detuning, the nonlinear shift drives the SW mode into resonance with the SAW excitation, leading to a strong enhancement of the magnon population, broadband nonlinear scattering, and bistable foldover behavior. Beyond the foldover threshold, both the magnon and phonon responses stabilize. These results establish SAW-driven $k \neq 0$ magnon-phonon hybrids as a promising platform for nonlinear magnetoacoustics and wave-based information processing.