Comment on the Two-Ion Anisotropy in the Heavy-Rare-Earth Metals

The importance of anisotropic two-ion couplings in the rare-earth metals, Er and Tb, as deduced from measurements of the spin-wave energies, has recently been questioned by Lindgård. I find that this objection is based on an improper expansion of the excitation energies, and that the $q$-dependent single-ion contributions to the spin-wave energies are unimportant within a self-consistent Hartree-Fock approximation.