THE FERROMAGNETISM OF IRON AND NICKEL CLUSTERS IN A MOLECULAR-BEAM

The magnetic properties of small Fe(N) and Ni(N) clusters are studied in a molecular beam by their Stern-Gerlach deflections. The magnetization of the particles is measured as a function of applied magnetic field H, size N and vibrational temperature T(vib) (almost-equal-to 200-1300 K). The intrinsic magnetic moments per atom mu at T almost-equal-to 300 K are 2.2 mu(B) for Fe130 and 0.5 mu(B) for Ni560. A bulk-like magnetization curve is found for Ni560 and for Fe130 at T(vib) almost-equal-to 1000 K mu is found to be only slightly reduced compared with its low temperature value. The results are compared with the Heisenberg model. Anomalously low magnetization for strongly cooled Fe(N) clusters (N = 120 - 140) is found to be related to the rotational speeds. The distinct roles of rotational and vibrational temperatures for the ferromagnetism of isolated Fe clusters are rationalized in a model.