Search for second-order Josephson tunneling in Superconductor-Ferromagnet-Superconductor junctions

We have fabricated Nb-CuNi-Nb SFS (Superconductor-Ferromagnet-Superconductor) $\pi $-Josephson junctions in a geometry that allows us to perform transport measurements to determine the uniformity of the sample and subsequent current-phase measurements to look for period doubling on the same junction. Junctions of this type exhibit transitions between the 0-junction and the $\pi $-junction state as a function of the ferromagnetic barrier thickness and temperature. Near the 0-$\pi $ transition points, it is predicted that first-order tunneling will be suppressed and the second-order term, proportional to sin(2$\phi )$, may dominate. The interpretation of past experiments to measure this term have been ambiguous due to concerns that non-uniformity in the ferromagnetic layer could mimic second-order Josephson behavior by producing half-integer Shapiro steps.