Orbital degeneracy, Hund's coupling, and band ferromagnetism: Effective quantum parameter, suppression of quantum corrections, and enhanced stability

An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degeneracy and Hund's coupling. This quantum parameter determines, in analogy with 1/N for the generalized Hubbard model and 1/S for quantum spin systems, the strength of quantum corrections to spin stiffness and spin-wave energies. Quantum corrections are obtained by incorporating correlation effects in the form of self-energy and vertex corrections within a spin-rotationally-symmetric approach in which the Goldstone mode is explicitly preserved order by order. It is shown that even a relatively small Hund's coupling is rather efficient in strongly suppressing quantum corrections, especially for large N, resulting in strongly enhanced stability of the ferromagnetic state. This mechanism for the enhancement of ferromagnetism by Hund's coupling implicitly involves a subtle interplay of lattice, dimensionality, band dispersion, spectral distribution, and band filling effects.