Structure and magnetism of cobalt clusters

Abstract

The lowest-energy geometric structures and isomers of freestanding ${\mathrm{Co}}_{N}$ clusters $(4<~N<~60)$ and their corresponding magnetic moments are calculated using an evolutive algorithm based on a many-body Gupta potential and a self-consistent $\mathrm{spd}$ tight-binding method, respectively. We found an icosahedral growth pattern for the global minimum with some hcp and fcc structures for some particular sizes, whereas for the second isomer, distorted icosahedral structures are obtained in general. With the aim to study the possible coexistence of isomers within the experimental cluster beam we assumed an equilibrium distribution and calculated for each cluster size the different coexistent structures and the relative populations at room temperature. Our results show that the coexistence is present only at some particular sizes, in agreement with chemical-adsorption and photoionization experiments. Our self-consistent tight-binding calculations considering $3d,$ $4s,$ and $4p$ valence electrons for the magnetic properties show that the magnetic moments for the global minima and the second isomers are in general very similar except in a small region of $20<N<40$ atoms where the magnetic moment of the global minimum is smaller than that of the second isomer. We compare our results for the magnetic behavior of the global minimum with theoretical calculations available in the literature as well as with experimental results.