We have used the Stern-Gerlach deflection technique to study magnetism in nickel clusters containing 5 to 740 atoms. These superparamagnetic particles are highly magnetic, with magnetic moments per atom that decrease toward the bulk value as their size increases. This approach toward the bulk is not monotonic; we observe magnetization minima for clusters with closed geometrical shells and maxima for relatively open clusters. This result is consistent with enhanced magnetization of the cluster surface. We also observe a decrease in magnetization with increasing cluster vibrational temperature. The implications of various cluster geometries are discussed in conjunction with existing chemical probe studies and theoretical calculations.

• Received 26 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1441