Coupling of two superconductors through a ferromagnet : evidence for a   pi-junction
  We report measurements of the temperature dependence of the critical current in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a Cu$_x$Ni$_{1-x}$ 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 only be explained by assuming that the junction changes from a 0-phase state at high temperatures to a $\pi$-phase state at low temperatures. 
V. V. Ryazanov, V. A. Oboznov, A. Yu. Rusanov, A. V. Veretennikov, A.   A. Golubov and J. Aarts