Experimental Observation of Superparamagnetism in Manganese Clusters

A molecular beam of manganese clusters, $\mathrm{Mn}{}_n$ $(n=11\mathrm{}–99)$, produced at 68 K is deflected toward high field by a gradient field magnet. These results indicate that $\mathrm{Mn}{}_n$ clusters in this size range are superparamagnetic species whose intrinsic moments can be determined within the framework of the Langevin model of paramagnetism. Local minima in per-atom magnetic moments are observed for $\mathrm{Mn}{}_{13}$ and $\mathrm{Mn}{}_{19}$, suggestive of an icosahedral growth sequence for the smaller size range. For larger clusters, broad oscillations in the per-atom moments are observed, with a minimum near $\mathrm{Mn}{}_{32–37}$ and a maximum around $\mathrm{Mn}{}_{50–56}$.