  We realize a cavity magnon-microwave photon system in which magnetic dipole
interaction mediates strong coupling between collective motion of large number
of spins in a ferrimagnet and the microwave field in a three-dimensional
cavity. By scaling down the cavity size and increasing number of spins, an
ultrastrong coupling regime is achieved with a cooperativity reaching 12600.
Interesting dynamic features including classical Rabi oscillation, magnetically
induced transparency, and Purcell effect are demonstrated in this highly
versatile platform, highlighting its great potential for coherent information
processing.
