Magnetic anisotropy of FeO and CoO: the influence of gradient corrections on exchange and correlation

We study the formation of local magnetic moments and the magnetic anisotropy of the antiferromagnetic transition-metal compounds FeO and CoO in the framework of the density functional theory. Exchange and correlation are described within two (semi-)local approximations, the local spin-density approximation (LDA) and the generalized gradient approximation (GGA), in order to investigate the role of gradient corrections. An additional on-site Coulomb interaction U is considered in an LDA+U/GGA+U approach and spin–orbit interaction is taken into account. We find that the orbital magnetization is strongly quenched in the GGA+U approach compared to LDA+U results and experimental findings. The quenching is explained in terms of an overestimated exchange enhancement for the partially filled 3d shells due to the gradient corrections inherent in GGA+U. (paper)