  Tailored magnon-phonon hybrid systems, where high overtone bulk acoustic
resonators couple resonantly to the magnonic 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
\ch{Co25Fe75} thin film and the transverse acoustic phonons in a 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 $C\approx 1$ at cryogenic temperatures. Our
experiments also showcase the potential of strongly coupled magnon-phonon
systems for strain sensing applications.
