Electronic structure and magnetic moments of Co-4 and Ni-4 clusters supported on the MgO(001) surface

We have studied the interaction of various isomers of Co-4 and Ni-4 cluster s with the regular (0 0 1) surface of MgO. First principles gradient-correc ted density functional calculations have been performed on cluster models e mbedded in point charges to represent the oxide surface. The magnetic and g eometric structure of the free and supported clusters have been studied to identify the modifications induced by the interaction with the substrate. G as-phase Co-4 is a non-planar square very close in energy to rhombic or dis torted rhombic structures. On MgO the most stable isomer is a tetrahedron b ut the rhombus is only slightly higher in energy. Adsorbed Ni-4 assumes tet rahedral or distorted tetrahedral shapes similar to the gas-phase structure . Only a partial magnetic quenching is observed, restricted to the metal at oms in direct contact with the surface. Ni forms stronger bonds than Co wit h 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 Co-4 and 2.4 eV for Ni-4) the pert urbation induced on the cluster electronic structure is small. (C) 2001 Els evier Science B.V. All rights reserved.