Two Susceptibility Maxima and Element Specific Magnetizations in Indirectly Coupled Ferromagnetic Layers

The ac susceptibility χ of ultrathin Co and Ni layers coupled indirectly across a Cu spacer is measured as a function of temperature. Depending on the strength of the interlayer exchange interaction a single χ maximum (singularity) or two χ maxima (one singularity, one resonantlike signal) appear near the respective Curie temperatures of bare layers. Using the element specificity of x-ray magnetic circular dichroism two separate temperature-dependent magnetization curves for Co and Ni are recorded, vanishing at different temperatures. These results support theoretical predictions for the onset of long-range order in model superlattices.