Do Exchange Coupled Ferromagnetic Monolayers Show Different Curie Temperatures

The ac-susceptibility X ac of ultrathin Co and Ni layers coupled indirectly across a Cu spacer layer is measured as a function of temperature in situ in UHV. Depending on the strength of the interlayer exchange interaction one or two maxima of the susceptibility are recorded near the respective Curie temperatures of the bare films. Using the element specificity of X-ray magnetic circular dichroism (XMCD) two separate temperature-dependent magnetization curves for Co and Ni are measured vanishing at different temperatures. The higher one of the two is the Curie temperature of the coupled systems. Due to the interlayer interaction the lower characteristic temperature cannot be interpreted as a thermodynamic phase transition.