The Ferromagnetism of Iron and Nickel Clusters in a Molecular Beam

The magnetic properties of small FeN and NiN 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 Tvib (≈︁ 200‐1300 K). The intrinsic magnetic moments per atom μ at T ≈︁ 300 K are 2.2 μB for Fe130 and 0.5 μB for Ni560. A bulk‐like magnetization curve is found for Ni560 and for Fe130 at Tvib ≈︁ 1000 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 FeN 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.