Magnetization of ultrathin ferromagnetic films at finite temperatures

The in-plane magnetization of a two-dimensional film may be stabilized by either dipolar interactions or by an in-plane anisotropy in the absence of an external field. We have calculated the conditions for either of these effects to dominate and show that there is only a restricted range of q values in which dipolar effects are important before the exchange dominates. We report measurements of all relevant anisotropy fields for an epitaxial Ag/2 ML Co/Ag(001) film structurally characterized by angle-resolved Auger spectroscopy and low-energy electron diffraction, together with a measurement of the ground state moment per Co atom (enhanced from the bulk value) using polarized neutron reflection. The results allow us to extend our previous study of the temperature dependence of the magnetization by including the perpendicular fields and the field dependence of the magnetization at 300 K. We show that the field dependence of the magnetization is not consistent with a spin wave gap due to dipolar interactions but is consistent with an anisotropy-induced spin wave gap, confirming the results of a preliminary study.