Non-Heisenberg spin dynamics of double-exchange ferromagnets with Coulomb repulsion

With a variational three-body calculation we study the role of the interplay between the on-site Coulomb, Hund's rule, and superexchange interactions on the spin-wave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron-hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spin-wave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to recent experiments in the manganites.