Magnon-magnon coupling in synthetic ferrimagnets

TL;DR

This paper investigates the hybridization of magnon modes in synthetic ferrimagnets with dissimilar layers, combining theoretical models and experiments to understand spectral gaps and mode coupling.

Contribution

It provides a combined theoretical and experimental analysis of magnon-magnon coupling in synthetic ferrimagnets with dissimilar layers, highlighting the role of symmetry and exchange interactions.

Findings

Spectral gaps are sizable across all conditions.

Spectra match Landau-Lifshitz-Gilbert predictions.

Mode hybridization depends on exchange coupling and layer properties.

Abstract

Magnetic multilayers with interlayer exchange coupling have been widely studied for both static and dynamic regimes. Their dynamical responses depend on the exchange coupling strength and magnetic properties of individual layers. Magnetic resonance spectra in such systems are conveniently discussed in terms of coupling of acoustic and optical modes. At a certain value of applied magnetic field, the two modes come close to being degenerate and the spectral gap indicates the strength of mode hybridisation. In this work, we theoretically and experimentally study the mode hybridisation of interlayer-exchange-coupled moments with dissimilar magnetisation and thickness of two ferromagnetic layers. In agreement with symmetry analysis for eigenmodes, our low-symmetry multilayers exhibit sizable spectral gaps for all experimental conditions. The spectra agree well with the predictions from the Landau-Lifshitz-Gilbert equation at the macrospin limit whose parameters are independently fixed by static measurements.