Microscopic origin of magnetism and magnetic interactions in ferropnictides

TL;DR

This paper clarifies the magnetic nature of ferropnictides, showing they are moderately correlated systems with local moments driven by Hund's coupling, and long-range interactions not explained by superexchange.

Contribution

It challenges existing theories by demonstrating that ferropnictides are moderately correlated with Hund's rule coupling, and their magnetic interactions are long-range and driven by electronic structure.

Findings

Moderate correlations driven by Hund's rule, not Hubbard U.

Iron moments are largely local and influenced by intra-atomic exchange.

Magnetic interactions are long-range and not due to superexchange.

Abstract

One year after their initial discovery, two schools of thought have crystallized regarding the electronic structure and magnetic properties of ferropnictide systems. One postulates that these are itinerant weakly correlated metallic systems that become magnetic by virtue of spin-Peierls type transition due to near-nesting between the hole and the electron Fermi surface pockets. The other argues these materials are strongly or at least moderately correlated, the electrons are considerably localized and close to a Mott-Hubbard transition, with the local magnetic moments interacting via short-range superexchange. In this paper we argue that neither picture is fully correct. The systems are moderately correlated, but with correlations driven by Hund's rule coupling rather than by the on-site Hubbard repulsion. The iron moments are largely local, driven by Hund's intra-atomic exchange. Superexchange is not operative and the interactions between the Fe moments are considerably long-range and driven mostly by one-electron energies of all occupied states.