Orbital correlation effects in transition metal microclusters

Within the framework of a self-consistent tight-binding LDA + U approach established in the present work, it is argued that orbital correlation (OC) has crucial effects to very small transition metal microclusters (with number of atoms N typically less than 10) by leading to orbital polarized ground states. Numerical calculations on Fe, Co and Ni diatomic pairs have revealed that inclusion of OC changes magnetic properties fundamentally: the inter-atomic exchange couplings are at least one order smaller than the bulk values and change with respect to electron filling from antiferromagnetic (for Fe) to ferromagnetic (for Ni), and the magnetic easy axis is usually completely different after the OC effect is considered.