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

TL;DR

This study investigates the magnetic susceptibility of ultrathin Fe/W(110) films, revealing mixed anisotropy effects near the Curie transition caused by inhomogeneities like step edges, which may influence critical magnetic behavior.

Contribution

It provides evidence of mixed anisotropy in ultrathin magnetic films and links inhomogeneities to deviations from ideal 2D magnetic models.

Findings

Presence of a smaller, paramagnetic hard axis response

Inhomogeneities create mixed anisotropy in the films

Inhomogeneities may alter critical properties from 2D Ising model

Abstract

Measurements of the magnetic susceptibility of Fe/W(110) films with thickness in the range of 1.6 to 2.4 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 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 the 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.