An Electromagnetic Approach to Cavity Spintronics

TL;DR

This paper develops a precise electromagnetic perturbation theory to understand and predict the coupling between electromagnetic resonators and magnetic samples in cavity spintronics, aiding device development.

Contribution

It introduces a highly accurate electromagnetic perturbation framework that predicts hybrid mode frequencies without fitting parameters, considering shape and field configuration effects.

Findings

Coupling depends on excitation fields, magnetic properties, and sample shape.

The theory accurately predicts resonant frequencies in different cavity geometries.

Application to experiments demonstrates the model's versatility and predictive power.

Abstract

The fields of cavity quantum electrodynamics and magnetism have recently merged into \textit{`cavity spintronics'}, investigating a quasiparticle that emerges from the strong coupling between standing electromagnetic waves confined in a microwave cavity resonator and the quanta of spin waves, magnons. This phenomenon is now expected to be employed in a variety of devices for applications ranging from quantum communication to dark matter detection. To be successful, most of these applications require a vast control of the coupling strength, resulting in intensive efforts to understanding coupling by a variety of different approaches. Here, the electromagnetic properties of both resonator and magnetic samples are investigated to provide a comprehensive understanding of the coupling between these two systems. Because the coupling is a consequence of the excitation vector fields, which directly interact with magnetisation dynamics, a highly-accurate electromagnetic perturbation theory is employed which allows for predicting the resonant hybrid mode frequencies for any field configuration within the cavity resonator, without any fitting parameters. The coupling is shown to be strongly dependent not only on the excitation vector fields and sample's magnetic properties but also on the sample's shape. These findings are illustrated by applying the theoretical framework to two distinct experiments: a magnetic sphere placed in a three-dimensional resonator, and a rectangular, magnetic prism placed on a two-dimensional resonator. The theory provides comprehensive understanding of the overall behaviour of strongly coupled systems and it can be easily modified for a variety of other systems.