Size and structural dependence of the magnetic properties of rhodium clusters

The size and structural dependence of the magnetic properties of Rh-N clusters (9 less than or equal to N less than or equal to 55) are studied by using a d-electron tight-binding Hamiltonian including Coulomb interactions in the unrestricted Hartree-Fock approximation. Three main different types of cluster geometries are considered (viz., fee, bcc. and icosahedral). In each case the equilibrium bond length R is optimized by maximizing the cohesive energy E-coh(N). The geometries yielding the largest E-coh(N) alternate as a function of N. These structural changes, together with the variation of R, play a crucial role in the determination of the average magnetic moment <(mu)over bar>(N) of Rh-N. The calculated size dependence of <(mu)over bar>(N) corresponding to the most stable geometries presents oscillations which are in good qualitative agreement with experiment. The magnetic properties of Rh-N clusters show a remarkable structural dependence which is characteristic of weak (unsaturated) itinerant ferromagnetism. The relation between the observed <(mu)over bar>(N) and the cluster geometry is analyzed. The role of nonuniform geometry relaxation, sp electrons. and sp-d hybridization effects art: quantified for representative examples. Perspectives of extensions of this study are also discussed.