Cutoff Angles of the Backward Spin Wave in a Tangentially Magnetized Ferrite Plate

The cutoff angles for the wave vector and the group velocity vector of a backward spin wave propagating in a tangentially magnetized ferrite plate are calculated. It is established that the expressions for these angles do not depend on the mode number and coincide with the analogous expressions for the corresponding cutoff angles of a spin wave in an unbounded ferromagnet. It is found that the regions corresponding to all possible orientations of the wave vector for the backward and surface spin waves in a ferrite slab and for a spin wave in an unbounded ferromagnet are adjacent and nonintersecting. It is shown that, if the isofrequency dependence of the wave has inflection points, then the range of all possible orientations of the group velocity vector of the wave can be wider than the angular interval enclosed between the cutoff angles for the group velocity vector.