Theory of the Magnetic Moment in Iron Pnictides

We show that the combined effects of spin orbit, monoclinic distortion, and $p\mathrm{\text{−}}d$ hybridization in tetrahedrally coordinated Fe in LaFeAsO invalidate 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 differing $p\mathrm{\text{−}}d$ hybridizations, the upper level is more itinerant, while electrons in the lower level are more localized. The resulting magnetic moment is highly anisotropic with an in-plane value of $0.25{\mu }_B–0.35{\mu }_B$ per Fe and a $z$ projection of $0.06{\mu }_B$, both of which are in agreement with experiment.

Figure 1

Evolution of the energy levels of the Fe $3d$ and As $4p$ levels after the inclusion of spin-orbit coupling, $p\mathrm{\text{−}}d$ hybridization $V_{\mathrm{pd}}$, and the monoclinic crystal field distortion.

Figure 2

Energy levels, hoping matrix elements, on-site interactions, intraband interaction, and spin components as a function of hybridization $M$. Shown in the inset of (c) is the on-site interaction $U_2$ as a function of the monoclinic distortion.