Spin fluctuations in coupled two-dimensional magnetic trilayers

Magnetic trilayer systems consisting of two ultrathin ferromagnetic films of Co and Ni separated by a non-magnetic spacer of Cu are investigated in order to analyze the effect of spin fluctuations in the ultrathin film limit (2D). The element specificity of the X-ray magnetic circular dichroism technique is applied to study the temperature dependence of the Co and Ni magnetization separately. The effect of (i) the interlayer exchange coupling and (ii) the enhanced 2D spin fluctuations on the shift of the critical temperature of the Ni films is determined by varying both the Cu and the Ni thicknesses. For a detailed understanding of these effects the experimental results are compared to a microscopic many-body Green's function theory. Both experiment and theory provide clear indications that for nanostructured magnets a static mean field description is insufficient.