“Pumping” of phonons by a dynamic magnetization promises to extend the range and functionality of magnonic devices. We explore the impact of phonon pumping on room-temperature ferromagnetic resonance (FMR) spectra of bilayers of thin yttrium iron garnet films on thick gadolinium gallium garnet substrates over a wide frequency range. At low frequencies the Kittel mode hybridizes with standing ultrasound waves across the layer stack that acts as a bulk acoustic resonator to form magnon polarons with rapid oscillations in the magnetic susceptibility, as reported before. At higher frequencies, the individual phonon resonances overlap due to their increasing acoustic attenuation, leading to an additional slowly oscillating phonon pumping contribution to the FMR line shape. The broadband frequency dependence of the magnetoelastic coupling strength follows the predictions from phonon pumping theory in the thick substrate limit. In addition, we find substantial magnon-phonon coupling of a perpendicular standing spin wave mode. This evidences the importance of the mode overlap between the acoustic and magnetic modes and provides a route towards engineering the magnetoelastic mode coupling.

• Received 4 February 2022
• Revised 14 June 2022
• Accepted 21 June 2022

DOI:https://doi.org/10.1103/PhysRevB.106.014407