An effective quantum parameter for strongly correlated metallic ferromagnets

TL;DR

This paper introduces a new quantum parameter for multi-orbital metallic ferromagnets, revealing how Hund's coupling and orbital degeneracy stabilize ferromagnetism and aligning well with experimental data.

Contribution

It proposes a non-perturbative scheme to quantify quantum corrections in correlated ferromagnets using an effective quantum parameter, linking microscopic interactions to magnetic properties.

Findings

Quantum parameter decreases with Hund's coupling J, especially for large orbital degeneracy.

Calculated spin stiffness and Curie temperature match experimental values for iron.

Long wavelength modes reduce the Curie temperature by about 25%.

Abstract

The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.