Rare-earth magnetism and adiabatic magnon spectra

The low-lying magnetic excitation spectrum of rare-earth metals is derived within an adiabatic approach on the basis of spin-density-functional theory. The treatment accounts for conduction-electron spin intersite exchange, on-site exchange between conduction and f electrons, on-site spin-orbit interaction, and crystal-field interaction. It is shown that the finiteness of the $f-d$ interaction, that is a possible noncollinearity of conduction electron and f spins at the same site, plays an essential role for the magnon dispersion, and a simple Heisenberg model for the total site spin does not apply. The obtained magnon spectra are in semiquantitative agreement with experimental data, where the latter are available. Main sources of discrepancy might be incomplete knowledge of the crystal-field parameters and the neglect of intersite correlations of the fast conduction-electron motion.