Structure, bonding, and magnetism of small Fe-n, Co-n, and Ni-n, clusters, n<=5

Small clusters of iron, cobalt, and nickel were studied with all-electron density functional calculations using both local and generalized gradient approximations. The highly spin-polarized 3d4s electrons give rise to directional bonding, magnetism, and Jahn-Teller distortions, yielding structures where the bonding is maximized but high magnetic moments still remain, coupled ferromagnetically. The magnetization is enhanced relative to the bull;, in qualitative agreement with experimental data for clusters larger than fifty atoms. Short d-type bond lengths dominate for dimers and trimers. Both short (d-type) and longer 4s bonds occur for the tetramers. For the pentamers there an many single s bonds and the R-e approach the bulk values.