Oscillatory Curie temperature in ultrathin ferromagnets: experimental evidence

We determined the Curie temperature TC for 2-3 monolayers of Co/Cu(1 0 0) as a function of Cu-cap thickness by means of temperature-dependent measurements of the initial zero-field AC susceptibility. We see a strong reduction of TC that we discuss in terms of hybridization effects and modifications of the electronic structure of Cu. In addition, we found a small oscillatory contribution that reflects an indirect exchange interaction Jcap in the ultrathin Co films mediated by quantum-well states in the nonmagnetic Cu overlayer. We estimate that Jcap is of the order of 2 μeV/atom which corresponds to an oscillation amplitude of 4 K as observed in our experiment.