SURFACE-ENHANCED MAGNETISM IN NICKEL CLUSTERS

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 de crease toward the bulk value as their size increases. This approach to ward the bulk is not monotonic; we observe magnetization minima for cl usters with closed geometrical shells and maxima for relatively open c lusters. This result is consistent with enhanced magnetization of the cluster surface. We also observe a decrease in magnetization with incr easing cluster vibrational temperature. The implications of various cl uster geometries are discussed in conjunction with existing chemical p robe studies and theoretical calculations.