SUPERCONDUCTIVITY IN LAYERED NB GD FILMS

The transition temperature and the critical fields of electron-beam ev aporated Nb/Gd/Nb triple layers and Nb/Gd multilayers have been determ ined by measurements of the electrical resistivity and the dc suscepti bility. For constant thickness d(Gd) of the Gd layers, we observe a de crease of T(c) and H(c2 perpendicular-to) with decreasing thickness d( Nb) of Nb layers down to a critical thickness d(c), below which superc onductivity is completely destroyed. The parallel critical fields most ly show the square-root temperature dependence near T(c), typical for two-dimensional superconductors. As predicted theoretically, competing pair-breaking mechanisms lead to a nonmonotonic dependence of H(c par allel-to) on d(Nb). We have also studied the dependence T(c)(d(Gd)) wi th constant d(Nb) and find a decrease of the T(c)(d(Gd)) curve with in creasing d(Cd) and a steplike structure at d(Gd) almost-equal-to 20 an gstrom. To clarify the nature of this step, the ferromagnetic transiti on of the Gd films is determined with the transverse magneto-optical K err effect. Long-range magnetic order is found only above d(Gd) almost -equal-to 20 angstrom, which is attributed to the formation of a disco ntinuous film below this thickness. These results indicate a change in the underlying pair-breaking mechanism.