Coupling of two superconductors through a ferromagnet:: Evidence for a π junction

We report measurements of the temperature dependence of the critical current, I-c, in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature I-c generally increases,but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I-c rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a pi phase state at low temperatures.