Relativistic density functional study of gold coated magnetic nickel clusters

Abstract

Coating of magnetic clusters by gold atoms is becoming an experimental technique of increasing interest for passivation and stabilization of these small metal particles. To computationally investigate the effect of gold coating, we have studied the magnetic clusters Ni6 and Ni13 employing an all-electron scalar-relativistic density functional method. We examine two series of octahedral clusters with increasing gold coverage of up to a monolayer: Ni6Aun (n=0,8,32) and Ni13Aun (n=0,6,8,14,24,30,42). Structural features, binding energies, and gold adsorption energies are determined and discussed. The different atomic radii of Au and Ni lead to rather short Au–Au and relatively long Ni–Ni distances in these clusters. The Au–Ni contacts are found to be the longest nearest-neighbor distances; a detailed analysis indicates these bonds to be the strongest in these Au-covered Ni clusters. The atomization energies change only slightly with increasing Au coverage. Also, the effect of gold adsorption on the magnetic properties of the Ni cores is analyzed. For the Ni6Aun series the magnetism decreases with n, while for Ni13Aun a maximum cluster magnetization is calculated for incomplete gold coverage. This different behavior of the two cluster series can be traced to differing numbers of unpaired electrons per atom in the pure Ni clusters and to an increased magnetic moment due to the adsorption of isolated Au atoms. Both series exhibit a residual magnetism at full monolayer coverage of the Ni cores.