Effect of oxygen chemisorption on the magnetism of small nickel clusters

Abstract

Local-spin-density calculations have been performed for three nickel clusters interacting with oxygen atoms. These clusters model the local environment of oxygen adsorbed on the reconstructed Ni(110) surface. For the smallest (4-atom) nickel cluster the position of the oxygen atom was optimized and was found to be 0.35 Å above the long bridge with a lateral displacement of 0.43 Å perpendicular to the bridge. It is found that the presence of oxygen leaves the total spin magnetic moment of the two larger clusters (10 atoms) unchanged. However, it induces appreciable variations in the local atomic moments. In both cases, the nickel atoms nearest the adatom have their moment strongly reduced. The remaining atoms show comparatively small, globally positive, changes of moment. The three oxygen 2p orbitals interact with the cluster orbitals and give rise to three pairs of bonding (B) and anti-bonding (AB) orbitals. The oxygen p orbital parallel to the surface and parallel to the long bridge site (p x ) has a strong overlap with the orbitals of the topmost Ni atoms. This gives rise to the formation of a high lying AB orbital. The contribution of the B-AB pair to the bond order is 1.0. The O-2p orbital perpendicular to the surface has a smaller overlap. The AB level lies very close to the Fermi level and this gives a contribution of approximately 0.5 to the bond order. The O-2p parallel to the surface and perpendicular to the bridge is almost nonbonding and does not contribute to the bond order. We thus assign a bond order of 1.5 to the O-Ni bond. A simple counting scheme shows that this number is consistent with the result that the global magnetic moment is unaffected by the oxygen. Part of the spin-up d local density of states (LDOS) on the topmost Ni atoms is pushed above the Fermi energy E F through the formation of the O-2p x derived AB orbitals. This, together with the strong itinerant ferromagnetism of Ni, is the origin of the decrease of moment on these atoms. Our calculated DOS are in line with results of spin-resolved inverse photoemission spectroscopy in that: (1) the presence of oxygen causes a diminution of spin-down LDOS at E F on the topmost Ni atoms, (2) the Ni 10 clusters have an spd hybrid spin-up level just above E F which remains when O is added, and (3) the oxygen induces a small increase of spin-up LDOS around + 2 eV.