Chiral phonons and phononic birefringence in ferromagnetic metal - bulk acoustic resonator hybrids*

Magnetoelastic coupling between excitation modes of the spin system (spin waves) and the lattice (phonons) is of interest from a fundamental perspective and can enable mode hybridization. For quantum sensing and transduction protocols, excitation transfer between the magnetic and elastic systems is of importance, but typically requires strong coupling between the magnetic and elastic modes. Here, we present our current results on coupling the magnetization dynamics in a ferromagnetic thin film to a high-overtone bulk acoustic resonator. We show that the typically weak coupling affects the magnetization dynamics of the magnetic layer and can thereby be characterized with high sensitivity using broadband ferromagnetic resonance (bbFMR) spectroscopy [1]. In our experiments [2], we investigate the magnetoelastic coupling of polycrystalline metallic thin films deposited on silicon and sapphire substrates via DC sputter deposition as a function of microwave excitation frequency and substrate material by performing bbFMR experiments. Utilizing a model based on coherent magnetoelastic coupling and the phononic properties of the substrate material, we obtain a full description of the observed changes in ferromagnetic resonance as a function of microwave frequency. Furthermore, we will discuss the implications of this model for the phononic angular momentum transport.

[1] R. Schlitz et al., Phys. Rev. B 106, 014407 (2022).

[2] M. Müller et al., arXiv:2303.08429 (2023).