Observation of mixed anisotropy in the critical susceptibility of an ultrathin magnetic film

Measurements of the magnetic susceptibility of ultrathin Fe/W(110) films with thickness in the range of 1.6–2.4 monolayers (ML) Fe show that, in addition to the large response along the easy axis associated with the Curie transition, there is a much smaller, paramagnetic hard-axis response that is not consistent with the two-dimensional (2D) anisotropic Heisenberg model used to describe homogeneous in-plane ferromagnets with uniaxial anisotropy. The shape, amplitude, and peak temperature of the hard-axis susceptibility, as well as its dependence upon layer completion close to 2.0 ML, indicate that inhomogeneities in the films create a system of mixed anisotropy. A likely candidate for inhomogeneities that are magnetically relevant in the critical region are the closed lines of step edges associated with incomplete layers. According to the Harris criterion, the existence of magnetically relevant inhomogeneities may alter the critical properties of the films from those of a 2D Ising model. Experiments in the recent literature are discussed in this context.