Magnetic moments of Ni clusters

Abstract

We have calculated the average magnetic moments per atom $\overline{\ensuremath{\mu}}$ of nickel clusters $({\mathrm{Ni}}_{N})$ as a function of cluster size in the range $5<~N<~60$. The cluster geometries have been taken from molecular-dynamics simulations with a semiempirical many-body potential for $N<~20$, and for $N>20$ the same potential was used for a steepest-descent relaxation of icosahedral structures. The spin-polarized electronic structure has been calculated for those optimized geometries with a self-consistent tight-binding method considering the $3d$, $4s$, and $4p$ valence electrons. The results are discussed in comparison with recent experimental data and other theoretical calculations. The theory explains perfectly the magnetic moments up to $N\ensuremath{\approx}16$ and the decreasing tendency of $\overline{\ensuremath{\mu}}$ with cluster size. With respect to the oscillations of the measured magnetic moment for larger $N$, the theory gives an explanation for the minima, which appear to be associated with an icosahedral growth pattern. The observed maxima are more difficult to reproduce and we discuss some plausible alternatives.