Using first-principles density-functional-theory–based calculations, we analyze the structural stability of small clusters of 3$d$ late transition metals. We consider the relative stability of the two structures: layer-like structures with hexagonal closed packed stacking and more compact structures of icosahedral symmetry. We find that the Co clusters show an unusual stability in hexagonal symmetry compared to the small clusters of other members, which are found to stabilize in icosahedral-symmetry–based structure. Our study reveals that this is driven by the interplay between the magnetic-energy gain and the gain in covalency through the $s$-$d$ hybridization effect. Although we have focused our study primarily on clusters with 19 atoms, we find this behavior to be general for clusters with between 15 and 20 atoms.

• Received 25 March 2011

DOI:https://doi.org/10.1103/PhysRevB.84.075429