Temperature dependence of the magnon-phonon interaction in hybrids of high-overtone bulk acoustic resonators with ferromagnetic thin films

Tailored magnon-phonon hybrid systems, in which high-overtone bulk acoustic resonators couple resonantly to the Kittel mode of a ferromagnetic thin film, are considered optimal for the creation of acoustic phonons with a defined circular polarization. This class of devices is therefore ideal for the investigation of phonon-propagation properties and assessing their capacity to transport angular momentum in the classical, and potentially even in the quantum, regime. Here, we study the coupling between the magnons in a ferromagnetic Co25Fe75 thin film and the transverse acoustic phonons in bulk acoustic wave resonators formed by the sapphire substrate onto which the film is deposited. Using broadband ferromagnetic resonance experiments as a function of temperature, we investigate the strength of the coherent magnon-phonon interaction and the individual damping rates of the magnons and phonons participating in the process. This demonstrates that this coupled magnon-phonon system can reach a cooperativity C≈1 at cryogenic temperatures. Our experiments also showcase the potential of strongly coupled magnon-phonon systems for strain-sensing applications.