Magnetism of Fe, Co and Ni clusters in molecular beams

The magnetic properties of molecular beams of Fe, Co and Ni clusters deflected with a Stern-Gerlach magnet are reviewed. Single-sided cluster deflections are observed indicating the occurrence of spin relaxation processes within the isolated clusters. We show that under strong supersonic expansion resonant spin-rotation coupling phenomena affect the spin dynamics. On the other hand, for weak supersonic expansion the cluster magnetization follows a superparamagnetic behavior. Cluster magnetic moments are determined as a function of size N (20 less than or equal to N less than or equal to 700) at low temperature. Measurements show that clusters are more magnetic than the bulk for sizes up to a few hundred atoms. Oscillations superimposed to the magnetic moment decrease are observed which cannot be entirely explained by geometrical structure effects. Cluster magnetic moments are also measured as a function of temperature T (78 K less than or equal to T less than or equal to 1000 K) for several cluster size ranges. Progressive convergence of the magnetic moment curve toward the bulk saturation magnetization curve is observed for Ni and Co clusters. However the magnetic moment curves are smeared out due to finite size effects. For Fe clusters, indications of a crystallographic phase transition from high to low moment are given.