Reentrant superconducting behavior of the Josephson SFS junction. Evidencefor the pi-phase state

Critical supercurrents, I-c, 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 I-c(T) oscillations with I, vanishing for some values of T. We associate this ree ntrant superconducting behavior with a crossover of the SFS junction from ' 0'- to 'pi'-state that is related to temperature dependence of spatial osci llation period of induced superconducting order parameter in the weak ferro magnet. We argue this is the first experimental evidence of the pi -behavio r of a Josephson junction, that is the special feature of superconducting p air flow through a ferromagnet predicted for SFS junctions by Bulaevskij, B uzdin et. al [1].