Structural, electronic, and magnetic properties of 13-, 55-, and 147-atom clusters of Fe, Co, and Ni: A spin-polarized density functional study

Atomic clusters of 13, 55, and 147 atoms of Fe, Co, and Ni with several values of total magnetic moments were scanned, and minimum of total energy with respect to the total magnetic moment was searched using ab initio calculations based on spin-polarized density functional theory. After complete structural relaxation the icosahedral and cuboctahedral clusters retain their initial structural symmetry. However, anticuboctahedral Fe-13, after complete structural relaxation, adopted bcc-like structure while anticuboctahedral Co-13 and Ni-13 finally adopted C-2V symmetry. It was found that icosahedral structures are lower in energy than octahedral structures at all total magnetic moments. The cohesive energy and nearest-neighbor interatomic distance increase toward the bulk value as size of the cluster is increased. The electronic and magnetic structures of a cluster are sensitive to interatomic distance, size, and geometrical symmetry of the cluster. The local magnetic moment of surface atom increases almost linearly while local magnetic moment of center atom increases nonmonotonically as total magnetic moment of cluster is increased. The M-13 and M-55 (M=Fe, Co, Ni) atomic clusters are half metallic in the sense that highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap is very small for minority spin as compared to that for majority-spin component. However, 147-atom clusters are metallic as the HOMO-LUMO energy gap for both spin components is zero or nearly zero.