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)=0.48µB . 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-Tc cuprates.
We next discuss the implication of the magnetically frustrated AF2 configuration on the structural distortion recently observed by neutron scattering[12]. It is a common observation that when the system is magnetically frustrated, the frustration is usually lifted by a structural distortion[19]. In the case of LaOFeAs, the frustration is due to the parallel and antiparallel alignment of the spins along the sides of a square. Hence one expects to see a structural distortion which brings the two Fe spins closer to each other while for the other side, they move apart from each other.
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