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 $\gamma ,$ the critical temperature $T_c,$ the critical amplitude ${\chi }_o,$ and the range of temperature that exhibits power-law behavior from individual experimental measurements of $\chi \mathrm{}\left(T\right).$ 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 $\approx 4.75\times {10}^{-3}$ give an average value $\gamma =1.76\mathrm{}\pm \mathrm{0.01.}$ Bilayer films give a weighted average value of $\gamma =1.75\mathrm{}\pm \mathrm{0.02.}$ These results are in agreement with the two-dimensional Ising exponent $\gamma =\frac{7}{4}.$ Measurements that do not exhibit power-law scaling as close to $T_c$ (especially films of thickness 1.75 monolayer) show a value of $\gamma$ higher than the Ising value. Several possibilities are considered to account for this behavior.

• Received 29 July 2003

DOI:https://doi.org/10.1103/PhysRevB.69.094411