Orbital fluctuations, spin-orbital coupling, and anomalous magnon softening in an orbitally degenerate ferromagnet

The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet within the framework of a nonperturbative Goldstone-mode-preserving approach based on a systematic inverse-degeneracy expansion scheme. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital-ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (pi/2, pi/2,0) (corresponding to period 4a orbital correlations as in charge-exchange (CE) phase of manganites involving staggered arrangement of nominally Mn(3+)/Mn(4+) ions and staggered ordering of occupied 3x(2)-r(2)/3y(2)-r(2) orbitals on alternating Mn(3+) sites) are shown to generically yield strong intrinsically non-Heisenberg (1-cos q)(2) magnon self-energy correction, resulting in no spin stiffness reduction but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.