First-principles study of binary transition-metal clusters and alloys

Small CoMRhN clusters have been studied by performing first-principles calculations in the framework of the density functional theory. In order to understand the interplay between their structural order, chemical order and magnetic properties, the corresponding bulk alloys in various hypothetically ordered phases were also calculated. In the bulk alloys and in most of the clusters there is an induced magnetic moment on the Rh atoms with increasing Co concentration which results in an increase in the average magnetic moment per atom. This effect, when present, may be twice as large in the clusters than in the corresponding macroscopic alloys. For low-symmetry clusters structures, the substitution of Rh by Co atoms results in a remarkable enhancement of the local magnetic moment at the Rh neighbours even at low Co concentration. On the other hand, the Co doping effect can be masked in cluster geometries of higher symmetry in which pure Rh clusters already show a high magnetic moment. In addition it is shown that local coordination and magnetic energy are the leading parameters for the determination of the stability and magnetic enhancement, whereas chemical order seems less important.