Coupling of Two Superconductors through a Ferromagnet: Evidence for aπJunction

Abstract

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 ${\mathrm{Cu}}_x{\mathrm{Ni}}_{1-x}$ 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 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.