# Linking Magnon-Cavity Strong Coupling to Magnon-Polaritons through Effective Permeability

## TL;DR

This paper establishes a theoretical and experimental link between magnon-cavity strong coupling and magnon-polaritons by analyzing dispersion relations and effective permeability, clarifying their relationship in spintronics applications.

## Contribution

It explicitly connects magnon-cavity coupling with magnon-polaritons through dispersion analysis and effective permeability modeling, enhancing understanding of magnon-photon interactions.

## Key findings

- Identified a polariton gap dependent on the system's filling factor.
- Demonstrated the gap equals the MP polariton gap when filling factor is 1.
- Provided a model linking cavity and MP coupling via effective permeability.

## Abstract

Strong coupling in cavity-magnon systems has shown great potential for use in spintronics and information processing technologies due to the low damping rates and long coherence times. Although such systems are conceptually similar to those coupled by magnon-polaritons (MPs), the link between magnon-cavity coupling and MPs has not been explicitly defined. In this work we establish such a connection by studying the frequency-wavevector dispersion of a strongly coupled magnon-cavity system, using a height-adjustable microwave cavity, and by modelling the observed behaviour through the system's effective permeability. A polariton gap between the upper and lower coupled modes of the magnon-cavity system is defined, and is seen to be dependent on the system's effective filling factor. This gap is equal to the MP polariton gap in the limit where filling factor = 1, corresponding to the removal of the microwave cavity. Thus, our work clarifies the connection between magnon-cavity and MP coupling, improving our understanding of magnon-photon interactions in coupled systems.