Direct observation of a sin(2$\phi )$ component in the current-phase relation of superconductor-ferromagnet-superconductor (SFS) Josephson junctions

We present direct measurements of the current-phase relation (CPR) of SFS Josephson junctions in an rf-SQUID geometry. The junctions are fabricated from Nb-Cu$_{47}$Ni$_{53}$-Nb trilayers with a junction area of 2x2 $\mu $m$^{2}$ and a CuNi thickness of 7 nm. By measuring the magnetic flux through the rf-SQUID as a function of applied current, we observe transitions between an ordinary 0-Josephson junction state and a $\pi $-junction state characterized by a phase difference of $\pi $ in the ground state occurring at temperatures between 1.5 K and 3.5 K. Near this temperature crossover, we observe period-doubling of the CPR indicating the existence of a term proportional to sin(2$\phi )$. Work is underway to determine if this signifies an intrinsic second-order tunneling mechanism or is the result of junction inhomogeneities.