Do exchange coupled ferromagnetic monolayers show different Curie temperatures?

The ac-susceptibility chi(ac) of ultrathin Co and Ni layers coupled indirec tly across a Cu spacer layer is measured as a function of temperature in si tu in UHV. Depending on the strength of the interlayer exchange interaction one or two maxima of the susceptibility are recorded near the respective C urie temperatures of the bare films. Using the element specificity of X-ray magnetic circular dichroism (XMCD) two separate temperature-dependent magn etization curves for Co and Ni are measured vanishing at different temperat ures. The higher one of the two is the Curie temperature of the coupled sys tems. Due to the interlayer interaction the lower characteristic temperatur e cannot be interpreted as a thermodynamic phase transition.