Multimode excitation of exchange spin waves in epitaxial ferrite-dielectric structures

Microwave transmission through an epitaxial yttrium iron garnet film shows multiple oscillatory contributions within the surface-magnetostatic-wave (MSSW) band and an additional response below its lower frequency boundary, near the uniform FMR frequency. We compare these observations with spin-wave-resonance (SWR) spectra and an exchange-inclusive waveguide model. The resolved SWR resonances are consistent with a family of volume exchange spin-wave modes described by the pinned/free boundary-value problem. The below-cutoff response is an unusual feature outside the reference MSSW band and motivates an exchange-based interpretation, but the transmission data alone do not uniquely establish a surface-mode assignment. Its frequency range is represented by phenomenological evanescent curves, not eigenmodes of the pinned/free problem. The model is used for a qualitative, experiment-led interpretation; because it omits dissipative losses and crystalline anisotropy, it is not intended to quantitatively reproduce the full transmission spectra.