Pronounced Tc enhancement and magnetic memory effects in hybrid films

Combined magnetic and magnetoresistance measurements are employed for the study of magnetic memory effects on the nucleation of superconductivity in a hybrid film consisting of randomly distributed CoPt ferromagnetic nanoparticles embedded in a high quality Nb layer. Our results show that at constant external magnetic field the nucleation of superconductivity stronglyg depends on the macnetic history of the hybrid film. More specifically, since pronounced enhancement of T-c (DeltaT(c)(5 kOe) similar or equal to 0.1 K) is observed only when the saturation field is exceeded, we believe that it is mainly the collective action of the ferromagnetic nanoparticles that drives the observed effect. A simple explanation is proposed for our experimental findings.