Strong Coupling of 3D Cavity Photons to Travelling Magnons At Low Temperatures

TL;DR

This paper demonstrates strong coupling between travelling magnons in YIG films and 3D microwave cavity photons at very low temperatures, revealing significant coupling strength and complex spectral features.

Contribution

It reports the first observation of strong magnon-photon coupling in a travelling magnon system at milli-Kelvin temperatures with detailed spectral analysis.

Findings

Coupling strength of 350 MHz or 7.3% of resonance frequency.

Observation of asymmetry in the photon-magnon avoided crossing.

Identification of higher order magnon modes in dual YIG film structures.

Abstract

We demonstrate strong coupling between travelling magnons in an Yttrium Iron Garnet film and 3D microwave cavity photons at milli-Kelvin temperatures. The coupling strength of $350$MHz or $7.3$\% of resonance frequency is observed. The magnonic subsystem is represented by the Damon-Eshbach magnetostatic surface wave with a distribution of wave numbers giving the linewidth of 15MHz. The ways to improve this parameter are discussed. The energy gap in the spectrum given by the Zeeman energy and the shape-anisotropy energy in the film geometry give rise to a significant asymmetry of the double peak structure of the photon-magnon avoided level crossing. A structure of two parallel YIG films is investigated using the same re-entrant magnetostatic surface wave transducer revealing a higher order magnon modes existing in both films. Combination of a multi-post re-entrant cavity and multiple films is a potential base for engineering both magnon and photon spectra.