Re-entrant phenomenon in superconductor-ferromagnet hybrids and superconducting spin-switch

Full text: Superconductor-ferromagnet metallic hybrids (S/F) based on thin films are the object for intense investigations for recent decade as a base element for superconducting electronics. In (S/F) bilayers the superconducting condensate penetrates through the S/F interface into a ferromagnetic layer, and the pairing wave function not only decays deep into the F metal, but simultaneously oscillates on the magnetic coherence length, ξm. Based on these oscillations various new physical effects were predicted, some of them have been observed experimentally: a non-monotonous and re-entrant superconducting Tc behavior as a function of the F metal thickness, Josephson junctions with intrinsic π-shift across the junction, capsized differential I-V characteristics. The investigations of proximity effect at (S/F) interfaces require technological approach which yields high quality superconducting films with constant thicknesses and enhanced superconducting properties. A special technological process was developed and patented, giving a significant improvement of superconducting properties of Nb films for nanoscale thickness, in comparison with common methods of vacuum deposition. As a main result of study of the prepared Nb/CuNi hybrids we report on the first experimental observation of the double re-entrante superconductivity in Nb/Cu41Ni59 bilayers as a function of the ferromagnetic layer thickness dCuNi. Our experiments give a convincing evidence for the pairing function oscillations associated with a quasi-one dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state establishing in the ferromagnetic layer. Detected phenomenon opens the perspectives for the design on it basis a new generation of spintronic devices, superconducting logic element - spin switch. (author)