Finite-Size Scaling in Thin Fe/Ir(100) Layers

The critical temperature of thin Fe layers on Ir(100) is measured through M\"ossbauer spectroscopy as a function of the layer thickness. From a phenomenological finite-size scaling analysis, we find an effective shift exponent λ0ex0ex=0ex0ex3.15±0.15, which is twice as large as the value expected from the conventional finite-size scaling prediction λ0ex0ex=0ex0ex1/ν, where ν is the correlation length critical exponent. Taking corrections to finite-size scaling into account, we derive the effective shift exponent λ0ex0ex=0ex0ex(1+2Δ₁)/ν, where Δ₁ describes the leading corrections to scaling. For the 3D Heisenberg universality class, this leads to λ0ex0ex=0ex0ex3.0±0.1, in agreement with the experimental data. Earlier data by Ambrose and Chien on the effective shift exponent in CoO films are also explained.