Magnon Excitation Spectrum in Ferromagnetic Quasicubic Manganites

The magnon excitation spectrum of ferromagnetic quasicubic manganites is studied by using the linearized spin-wave approximation for the double-exchange and the superexchange interactions. Two situations are distinguished: 1) a rigid coupling of the t
_2g
and e
_g
spins via a large Hund's rule coupling and 2) a softened coupling between these spins (e.g., due to coupling of the e
_g
electrons to Jahn-Teller modes). Case 2) leads to a softening of the magnon dispersion (compared to the Heisenberg model) close to the zone boundary.