Arsenic-bridged antiferromagnetic superexchange interactions in LaFeAsO

From the first-principles calculations, we have studied the electronic and magnetic structures of LaFeAsO. It is found that a large magnetic moment of ∼2.6μ_B is located around each Fe ion, embedded in the environment of itinerant electrons. In the ground state, these local Fe moments are in collinearly antiferromagnetic order, resulting from the interplay between the strong nearest- and next-nearest-neighbor superexchange antiferromagnetic interactions bridged by As atoms. The structure transition observed by the neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials.