Magnetic moments of iron clusters with 25 to 700 atoms and their dependence on temperature.

Magnetic moments μ(N) of iron clusters in a molecular beam, with temperatures ranging from 100 to 1000 K, are investigated from their Stern-Gerlach deflections. We find that at a temperature of 120 K, μ (25≤N≤130) is 3μ B per atom, decreasing to about the bulk value (2.2μ B per atom) near N=500. For all sizes, μ decreases with increasing temperature, and is approximately constant above a temperature T C (N). For example, T C (130) is about 700 K, and T C (550) is about 550 K (T C bulk=1043 K). Limitations of the superparamagnetic model due to rotational erects are discussed