Magnetostatic mode spectra of uniaxial ferromagnets

The Damon-Eshbach dispersion relation for magnetostatic modes in a rectangular platelet is expressed in terms of the permeability tensor of the magnetic medium and the consequences of a uniaxial easy-plane anisotropy are investigated. The anisotropy modifies the critical angle between bulk and surface modes which results in the existence of bulk excitations within the series of DE-surface modes. Two systems are analyzed. In the case of a ferromagnetic layer compound, the anisotropy is large compared with the demagnetizing field and the series of surface modes is replaced by a second series of volume modes. Recent measurements are reported on Ca-doped YIG. Several hundred modes of forward and backward volume modes are in good agreement with the calculated dispersion function using the parameters of pure YIG and Hu = −250 Oe. It is suggested that a measured anisotropy profile along the film normal suppresses a few bulk modes within the series of surface modes and accounts for a gap observed on the low field side of the uniform resonance.