Low spin moment due to hidden multipole order from spin-orbital ordering in LaFeAsO

Abstract

An antiferromagnetic (AF) low-moment solution, $0.35{\ensuremath{\mu}}_{\text{B}}/\text{Fe}$, is found in the case of LaOFeAs for an intermediately strong Coulomb interaction $U$ of 2.75 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 understood as a type of spin-orbital ordering. Parallels can be drawn to the stabilization of the AF order in, e.g., ${\text{CaCuO}}_{2}$.