Critical susceptibility exponent measured from Fe/W(110) bilayers

The critical phase transition in ferromagnetic ultrathin Fe/W(110) films has been studied using the magnetic ac susceptibility. A statistically objective, unconstrained fitting of the susceptibility is used to extract values for the critical exponent γ, the critical temperature Tc, the critical amplitude χo, and the range of temperature that exhibits power-law behavior from individual experimental measurements of χ(T). This avoids systematic errors and generates objective fitting results. An ensemble of 25 measurements on many different films are analyzed. Those which permit a fitting range in reduced temperature extending lower than ≈4.75×10-3 give an average value γ=1.76±0.01. Bilayer films give a weighted average value of γ=1.75±0.02. These results are in agreement with the two-dimensional Ising exponent γ=74. Measurements that do not exhibit power-law scaling as close to Tc (especially films of thickness 1.75 monolayer) show a value of γ higher than the Ising value. Several possibilities are considered to account for this behavior.