Theory of the Magnetic Moment in Iron Pnictides

We show that the combined effects of spin orbit, monoclinic distortion, and p-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-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 micro_B-0.35 micro_B per Fe and a z projection of 0.06 micro_B, both of which are in agreement with experiment.