SPIN WAVES IN THE FERROMAGNETIC HEAVY RARE EARTHS - Rare Earth Magnetism: Structures and Excitations - page 197

Fig. 5.2. The spin-wave dispersion relations along the symmetry lines
in the Brillouin zone for Tb. In contrast to Gd, the anisotropy gives rise to
an energy gap at the origin, and there are large effects due to interactions
with the phonons. The third branch along, for example, ΓM may also be
due to phonon interactions, or it may be a manifestation of the breaking
of the hexagonal symmetry by the ordered moment in a particular do-
main, in the multi-domain sample.The lifting of the double degeneracy
along the line KH provides evidence for anisotropic two-ion coupling.