Origin of the ~150 K Anomaly in LaOFeAs; Competing Antiferromagnetic Superexchange Interactions, Frustration, and Structural Phase Transition

From first principles calculations we find that the nearest and next nearest neighbor superexchange interactions between Fe ions in LaOFeAs are large, antiferromagnetic (AF), and give rise to a frustrated magnetic ground state which consists of two interpenerating AF square sublattices with M(Fe)=0.48$\mu_B$. The system lowers its energy further by removing the frustration via a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaOFeAs recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions and the frustrated ground state suggest that magnetism and superconductivity in doped LaOFeAs may be strongly coupled, much like in the high-T$_c$ cuprates.