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 3μB per atom, decreasing to about the bulk value (2.2μB per atom) near N=500. For all sizes, {μ} decreases with increasing temperature, and is approximately constant above a temperature TC(N). For example, TC(130) is about 700 K, and TC(550) is about 550 K (TC bulk =1043 K). Limitations of the superparamagnetic model due to rotational effects are discussed.