# Exchange-enhanced Ultrastrong Magnon-Magnon Coupling in a Compensated Ferrimagnet

## TL;DR

This paper reports the discovery of ultrastrong magnon-magnon coupling in a compensated ferrimagnet, enabled by exchange interactions, with tunable coupling strength near the compensation temperature, advancing understanding of spin dynamics.

## Contribution

It demonstrates exchange-enhanced magnon-magnon coupling in a ferrimagnet, a novel mechanism distinct from electromagnetic interactions, with experimental and theoretical analysis.

## Key findings

- Magnon-magnon coupling exceeds 30% of mode frequency.
- Coupling strength is tunable by magnetic field direction.
- Ultrastrong coupling observed near the compensation temperature.

## Abstract

The ultrastrong coupling of (quasi-)particles has gained considerable attention due to its application potential and richness of the underlying physics. Coupling phenomena arising due to electromagnetic interactions are well explored. In magnetically ordered systems, the quantum-mechanical exchange-interaction should furthermore enable a fundamentally different coupling mechanism. Here, we report the observation of ultrastrong intralayer exchange-enhanced magnon-magnon coupling in a compensated ferrimagnet. We experimentally study the spin dynamics in a gadolinium iron garnet single crystal using broadband ferromagnetic resonance. Close to the ferrimagnetic compensation temperature, we observe ultrastrong coupling of clockwise and anticlockwise magnon modes. The magnon-magnon coupling strength reaches more than 30% of the mode frequency and can be tuned by varying the direction of the external magnetic field. We theoretically explain the observed phenomenon in terms of an exchange-enhanced mode-coupling mediated by a weak cubic anisotropy.