Reentrant superconducting behavior of the Josephson SFS junction. Evidence for the π-phase state

Critical supercurrents, Ic in Nb_Cu1−xNix_Nb Josephson SFS junctions with F-layers prepared from ferromagnetic Cu1−xNix alloys have been studied. For value x=0.52 and particular F-layer thickness we have observed Ic(T) oscillations with Ic vanishing for some values of T. We associate this reentrant superconducting behavior with a crossover of the SFS junction from ‘0’- to ‘π’-state that is related to temperature dependence of spatial oscillation period of induced superconducting order parameter in the weak ferromagnet. We argue this is the first experimental evidence of the π-behavior of a Josephson junction, that is the special feature of superconducting pair flow through a ferromagnet predicted for SFS junctions by Bulaevskii, Buzdin et al [1].