Ultrahigh cooperativity interactions between magnons and resonant photons in a YIG sphere

Abstract

Resonant photon modes of a 5-mm-diameter yttrium iron garnet (YIG) sphere loaded in a cylindrical cavity in the 10-30-GHz frequency range are characterized as a function of applied dc magnetic field at millikelvin temperatures. The photon modes are confined mainly to the sphere and exhibited large mode filling factors in comparison to previous experiments, allowing ultrastrong coupling with the magnon spin-wave resonances. The largest observed coupling between photons and magnons is 2g/2π=7.11 GHz for a 15.5-GHz mode, corresponding to a cooperativity of C=1.51±0.47×107. Complex modifications, beyond a simple multioscillator model, of the photon mode frequencies were observed between 0 and 0...