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

TL;DR

This paper investigates how orbital fluctuations and spin-orbital coupling influence magnetic excitations in orbitally degenerate ferromagnets, revealing mechanisms behind magnon softening and anomalies observed in manganites.

Contribution

It introduces a theoretical framework for understanding the impact of orbital fluctuations on magnon spectra, especially near orbital ordering instability, highlighting non-Heisenberg effects.

Findings

Orbital fluctuations cause strong magnon self-energy corrections.

Zone-boundary magnon energies are suppressed near orbital ordering.

Magnon softening increases with hole doping and narrow bandwidths.

Abstract

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. 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 CE-type orbital correlations), 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.