Rigorous Electrodynamic Approach to Ferromagnetic Resonance in Cavity‐Coupled Ferrimagnetic Films

Abstract

A rigorous electrodynamic approach to the modeling of ferromagnetic resonance (FMR) in ferrimagnetic films of arbitrary thickness placed in a rectangular microwave cavity is presented in this letter. It is shown that the coupling of the cavity mode with the FMR mode of an in‐plane magnetized ferrimagnetic film occurs when the real part of its intrinsic diagonal permeability component µ approaches zero. The obtained theoretical prediction of frequency vs. magnetic bias is confirmed by experiments performed on a 3 µm thin yttrium iron garnet (YIG) layer grown on a 500 µm thick gadolinium gallium garnet (GGG) substrate for both even and odd cavity modes. It is also shown that the proposed rigorous electrodynamic approach agrees well with perturbation theory in the high damping regime, however, leads to qualitatively different conclusions for low damping. Finally, it is demonstrated that the experimentally observed higher‐order FMR modes can be attributed to the longitudinal modes distributed across the thickness of the film at frequencies corresponding to negative values of µ approaching zero. This letter paves the way for further study of, among others, the magnon–microwave photon coupling strength and the development of ferromagnetic linewidth characterization methods.