TRANSITION-TEMPERATURE AND CRITICAL FIELDS OF NB GD LAYERS/

The transition temperature and the critical fields of e--beam evaporat ed Nb/Gd/Nb trilayers and Nb/Gd multilayers have been determined resis tively and by dc-susceptibility. For constant thickness of the Gd laye rs we observe with decreasing thickness d(Nb)of Nb layers a decrease o f T(c) and H(c2 perpendicular-to) down to a critical thickness d(c), b elow which superconductivity is completely destroyed. This is due to p air breaking by the exchange field within the ferromagnetic Gd layers. The parallel critical fields mostly show the square-root like tempera ture dependence near T(c) typical for 2D-superconductors. The interpla y of orbital pair breaking by the external field and the exchange-fiel d induced pair breaking leads to a nonmonotonic dependence of H(c para llel-to) on d(Nb) as predicted