Observation of Purcell Effect in Electrically Coupled Cavity-Magnet System

We report the observation of the Purcell effect in a cavity-metallic magnet hybrid system using electric-field-mediated coupling. In this configuration, microwave-induced axial currents in the microwire induce circular magnetic fields that drive the ferromagnetic resonance (FMR) of the magnetized microwire. Field-dependent transmission and reflection spectroscopies reveal a clear cavity perturbation consistent with the Purcell regime, in which the magnetic loss rate exceeds the light-matter coupling strength. Despite the small magnetic volume ($\sim 10^{-13}\,\text{m}^3$), measurements performed at both room temperature and $T = 7$ mK show coupling rates as high as $g/2pi = 56$ MHz, one order of magnitude stronger than the one expected from conventional coupling at the magnetic antinode. Time-domain ringdown measurements directly show the magnetic-field-dependent modification of the cavity photon lifetime, in agreement with theoretical predictions. These results establish a versatile approach for coupling microwave fields to metallic magnets via geometric and electric-field-mediated interactions, opening new opportunities for hybrid cavity-magnet systems.