MAGNETISM IN TRANSITION-METAL CLUSTERS FROM THE ATOM TO THE BULK

Molecular beam deflection measurements of small iron, cobalt, and nick el clusters show how magnetism develops as the cluster size is increas ed from several fens to several hundreds of atoms for temperatures fro m 80 and 1000 K. Cluster magnetization is found to be superparamagneti c for rotationally warm clusters, where it follows the Langevin functi on. The magnetization of rotationally cold clusters is anomalous: it i s strongly reduced and nonlinear with the applied field. For superpara magnetic clusters, the magnetic moments can be determined from the mag netization. We find that ferromagnetism occurs even for the smallest s izes: for clusters with less than about 30 atoms the magnetic moments are atom-like and as the size is increased up to 700 atoms they approa ch the bulk limit, with oscillations probably caused by surface-induce d spin-density waves. The trends are explained in a magnetic shell mod el. The magnetic properties of iron cluster show anomalies, suggesting that a high moment to low moment crystallographic phase transition in Fe clusters occurs at relatively low temperatures.