  Magnon self energy due to spin-charge coupling is calculated for the
correlated ferromagnetic Kondo lattice model using a diagrammatic expansion
scheme. Systematically incorporating correlation effects in the form of
self-energy and vertex corrections, the expansion scheme explicitly preserves
the continuous spin rotation symmetry and hence the Goldstone mode. Due to a
near cancellation of the correlation-induced quantum correction terms at
intermediate coupling and optimal band filling relevant for ferromagnetic
manganites, the renormalized magnon energies for the correlated FKLM are nearly
independent of correlation term. Even at higher band fillings, despite
exhibiting overall non-Heisenberg behavior, magnon dispersion in the \Gamma-X
direction retains nearly Heisenberg form. Therefore, the experimentally
observed doping dependent zone-boundary magnon softening must be ascribed to
spin-orbital coupling effects.
