Potential asymmetry in antiferromagnetic 3d transition metal monoxides

Anisotropic behavior has been studied for some 3d transition metal monoxides (MnO, FeO, CoO, and NiO) in the framework of classical and density functional theories. The calculations suggest that deviation of potential and energy from spherical symmetry around the metal and oxygen atoms is required to describe the behavior, which corresponds to symmetry breaking of the system. As for the symmetry breaking along the antiferromagnetic order, it is confirmed only for MnO that superexchange interaction between the nearest-neighboring metal atoms induces the symmetry breaking. In contrast, for NiO, this symmetry breaking is mainly caused by charge transfer from metal to oxygen ions. For FeO, it is predominantly caused by Jahn-Teller effects. The calculations suggest that the anisotropic behavior of CoO is affected by two mechanisms, namely, the Jahn-Teller and charge-transfer effects.