Magnetic properties of clusters of transition metal atoms

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Abstract
Using a newly proposed calculational scheme that combines the Hubbard approx- imation with the tight-binding molecular-dynamics method, we obtain the magnetic moments of Fen and Nin clusters with cluster size up ton = 55 in a systematic way. Our results indicate that the average magnetic moment per atom is signiflcantly higher in the cluster than in the bulk, in agreement with recent experimental data for Fe and Ni clusters. Furthermore, it is found that magnetic efiects stabilize the clusters in geometries that were found to be completely unstable when magnetism is ignored. In general, magnetic efiects drive Fe (and to a lesser extent Ni) clusters into geometries of higher symmetry than that of the corresponding singlet states. Magnetic properties of clusters of transition metal atoms (CTMAs) are the outcome of a delicate interplay among various factors such as the bond length, the coordination number, the fllling of the d-band, the number of atoms in the cluster, N, and the symmetry of the cluster. Each of these factors is an independent variable (parameter) contributing to the total energy, EN , of the cluster. The determination of the ground state of the cluster involves flnding the minimum of the total energy with respect to all these variables, making the theoretical study of the CTMAs a very di-cult subject. The task is computationally formidable, even for intermediate size clusters, i.e. for 10 < N < 70. Thus ab initio results, which include a global symmetry-unconstrained geometry relaxation and an unrestricted total-spin value for the cluster, are very rare and are restricted to cluster sizes of N • 7 (1)-(4). For cluster sizes with N > 7 the common practice is one in which variation of only a limited number of the independent parameters of the system is allowed while performing a restricted (constrained) minimization of the total energy of the cluster to determine the ground state (4)-(13). Even c