Tunable Magnon Polaritons via Eddy-Current-Induced Dissipation in Metallic-Banded YIG Spheres

Abstract: We demonstrate a robust method to dynamically tune magnon dissipation in yttrium iron garnet spheres by equipping a metallic band around the sphere's equator, enabling precise control over magnon-photon coupling states. The collective magnetization dynamics in the YIG sphere induce circular eddy currents in the metallic band, whose magnitude can be systematically varied by adjusting the angle between the metallic band plane and an external static magnetic field. This angular dependence yields a pronounced modulation of the ferromagnetic resonance (FMR) linewidth, facilitating seamless transitions between the Purcell and strong coupling regimes without altering photon cavity parameters. Systematic FMR and cavity spectroscopy measurements confirm that eddy-current-induced losses govern the primary mechanism behind the observed tunable damping. By achieving extensive periodic-angular dependence of magnon relaxation rate, we precisely control the magnon-photon coupling state, approaching the critical coupling condition. These results establish the YIG-metallic-band platform as a versatile and practical approach for engineering tunable magnon-polariton systems and advancing magnonic applications, including those exploring non-Hermitian magnonics.