Viewpoints on resonance in ideal ferrite slab loaded rectangular waveguides

When measurements of ferromagnetic resonance are made with one or more ferrite slabs in a waveguide, the reaction of the ferrite on the driving source is of such magnitude that a small perturbation viewpoint is inadequate to describe observed results. In marked contrast to treating the ferrite material as a perturbation, it would appear that these resonances are more appropriately studied by treating the waveguide as a perturbation on the ferrite slab, assuming the slab thickness to be sufficiently large with respect to the magnetic charge skin depth. The type of perturbation study made in this paper identifies the resonance in a uniformly loaded guide with the appearance of a propagation constant singularity. An operator formalism, geared to the study of such singularities, is employed to deduce the resonance characteristics of a stratified slab geometry. Relatively simple relationships for the appearance of resonance are obtained in terms of the permeability components, which correlate well with experimental results. The theory, as developed, demonstrates resonance of thick slabs to be shifted significantly from the Kittel value for thin slabs, and accounts for the appearance of "spurious" for "forbidden" resonances. The ferrite model chosen is one following an ideal magnetization precession description, with losses entering phenomenologically through a Landau Liwschitz damping term. This assumes no microscopic structure for the material and we may, therefore, not seek to deduce the intrinsic characteristics of the resonance line. Nevertheless, we may show the appearance of line width broadening and asymmetry effects produced by purely macroscopic factors.