The low spin moment in LaOFeAs is due to a hidden multipole order caused by spin orbital ordering

An antiferro-magnetic (AF) low moment solution, 0.4 ${\mu }_{\mathrm{B}}$/Fe, is found in the case of LaOFeAs for an intermediately strong Coulomb interaction $U$ of 2.5–3.0 eV. This solution is stabilized over a large moment solution due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. The multipoles are of rank four and can be understand as a kind of spin-orbital ordering. Parallels can be drawn to the stabilization of the AF order in e.g. CaCuO${}_2$.