Transition temperature and critical fields of Nb/Gd layers

The transition temperature and the critical fields of e--beam evaporated Nb/Gd/Nb trilayers and Nb/Gd multilayers have been determined resistively and by dć-susceptibility. For constant thickness of the Gd layers we observe with decreasing thickness dNb of Nb layers a decrease of Tc and Hc2⊥ down to a critical thickness dc, below which superconductivity is completely destroyed. This is due to pair breaking by the exchange field within the ferromagnetic Gd layers. The parallel critical fields mostly show the square-root like temperature dependence near Tc typical for 2D-superconductors. The interplay of orbital pair breaking by the external field and the exchange-field induced pair breaking leads to a nonmonotonic dependence of Hc{\textbar} on dNb as predicted by recent theories.