Theory of the Magnetic Moment in Iron Pnictides

TL;DR

This paper develops a theoretical model explaining the highly anisotropic magnetic moment in LaOFeAs by considering spin-orbit coupling, lattice distortion, and hybridization effects, aligning with experimental observations.

Contribution

It introduces a minimal theoretical model that accounts for the anisotropic magnetic moments in LaOFeAs, incorporating effects often neglected in simpler Hund's rule approaches.

Findings

Magnetic moment is highly anisotropic with in-plane and z-components matching experiments.

p-d hybridization causes different on-site repulsions, affecting electron localization.

The model explains antiferromagnetic ordering of the xy spin projection.

Abstract

We show that the combined effects of spin-orbit, monoclinic distortion, and p-d hybridization in tetrahedrally coordinated Fe in LaOFeAs invalidates the naive Hund's rule filling of the Fe d-levels. The two highest occupied levels have one electron each but as a result of the p-d hybridization have very different on-site repulsions. As a result, electrons in the upper level are more itinerant while those in the lower level are more localized. It is the xy-projection of the spin in the lower level that orders antiferromagnetically as the z-components of the spins in the two levels is shown to be vanishingly small in the ground state. The resulting magnetic moment is highly anisotropic with an in-plane value of $0.25-0.35\mu_B$ per Fe and a z-projection of $0.06\mu_B$, both of which are in agreement with experiment. As a consequence, we arrive the minimal model that describes the electronic properties of LaOFeAs.