Stern-Gerlach deflection of ferromagnetic cluster beams

Abstract

The magnetic moments and heat capacities of small iron, cobalt and nickel clusters in a molecular beam have been measured as a function of their size and temperature using Stern-Gerlach deflections. Single-sided deflections are found indicating that spin relaxation occurs within the isolated clusters. Superparamagnetic behaviour is observed when all the degrees of freedom of the clusters are in equilibrium. Marked decreases in the magnetic moment are observed with increasing temperature and increasing size, slowly converging to the bulk magnetization curve as the cluster size increases. At a given temperature, oscillation of the moment is observed as a function of size, which suggests the filling of layers of atoms in the cluster. Heat capacity measurement of nickel and cobalt clusters in the beam reveals that the magnetic contribution to the heat capacity is well approximated using mean-field concepts, combined with measured magnetic properties. In contrast, iron clusters show anomalies both in the magnetic moments and in the heat capacities.