Upper Critical Fields of Nb/Pd Multilayers

We measured critical temperature and critical magnetic fields of Nb/Pd multilayers where the Nb thickness is held constant (d_Nb=250 Å) while the Pd thickness, d_Pd, is systematically varied from 10 to 200 Å. The critical temperature shows a monotonic decrease as a function of the Pd thickness, which can be tentatively described using the classical de Gennes-Werthamer theory for proximity coupled systems. The critical magnetic field measurements reveal unusual behavior, like a positive curvature of H_c2(T) close to T_c and a dimensional 3D-2D crossover, already present in multilayers with very thin Pd layers. In particular the angular behavior of the upper critical field, in the case of small d_Pd values, confirm the 2D nature of all the samples at T=4.2 K, giving also indication of the columnar nature of the Nb layers. These unusual behaviors might be related to the strong paramagnetic nature of the Pd layers. On the other hand also the proximity theory for S/N systems describes the perpendicular critical field data, but some inconsistencies are found quantitatively.