Electronic structure and magnetic moments of Co₄ and Ni₄ clusters supported on the MgO(0 0 1) surface

We have studied the interaction of various isomers of Co₄ and Ni₄ clusters with the regular (0 0 1) surface of MgO. First principles gradient-corrected density functional calculations have been performed on cluster models embedded in point charges to represent the oxide surface. The magnetic and geometric structure of the free and supported clusters have been studied to identify the modifications induced by the interaction with the substrate. Gas-phase Co₄ is a non-planar square very close in energy to rhombic or distorted rhombic structures. On MgO the most stable isomer is a tetrahedron but the rhombus is only slightly higher in energy. Adsorbed Ni₄ assumes tetrahedral or distorted tetrahedral shapes similar to the gas-phase structure. Only a partial magnetic quenching is observed, restricted to the metal atoms in direct contact with the surface. Ni forms stronger bonds than Co with the oxide surface so that distorted structures compete in energy with the undistorted ones. The results show that despite the relatively strong bond between the cluster and MgO (2.0 eV for Co₄ and 2.4 eV for Ni₄) the perturbation induced on the cluster electronic structure is small.