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

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 (π/2,π/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.

25 pages, 7 figures

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Cited by in corpus (6)

• Magnon dressing by orbital excitations in ferromagnetic planes of K${}_2$CuF${}_4$ and LaMnO${}_3$
• Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy
• Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites
• Magnon self energy in the correlated ferromagnetic Kondo lattice model: spin-charge coupling effects on magnon excitations in manganites
• Effect of strain-induced orbital splitting on the magnetic excitations in undoped cuprates
• On the origin of CE-type orbital fluctuations in the ferromagnetic metallic La${}_{1.2}$Sr${}_{1.8}$Mn${}_2$O${}_7$