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