Local and chemical environment dependence of the magnetic properties of CoRh core-shell nanoparticles

The ground-state magnetic properties of Co sub(x)Rh sub(1-x) nanoparticles having sizes in the range of 0.8-2 nm (N = 43 and 273 atoms) and Co concentrations x [asymptotically =] 0, 0.25,0.5, 0.75, and 1 are investigated in the framework of density-functional theory by using a fixed-moment method. Electron correlation effects are explored by comparing the results of the local spin-density and generalized-gradient approximations to the exchange and correlation functional. The role of chemical order on the magnetic behavior is investigated by considering a variety of core-shell atomic arrangements with nearly spherical CoRh interfaces. A local relaxation of the cluster geometry is performed by taking face-centered cubic structures as starting configurations. All considered Co sub(x) Rh sub(1-x) clusters are found to be magnetic with an average spin moment per CoRh unit that is larger than in macroscopic alloys having similar concentrations. This is a consequence of both, the enhancement of the Co moments and the occurrence of important induced Rh moments, which couple parallel to the Co moments. The distribution of the local magnetic moments within the clusters is found to depend strongly on the local and chemical environment of the atoms. In particular, the Rh moments show a nontrivial dependence as a function of the distance to the CoRh interface. The results for the local magnetic moments are correlated to the electronic densities of states, which reflect the concentration and chemical-order dependence of the cluster electronic structure. Finally, the effects of coating and of the 3d-4d interface are analyzed by comparing the magnetic behaviors of core-shell particles with those of the corresponding pure Co and Rh cores.