Measurement of the current-phase relation of superconductor/ferromagnet/superconductor π Josephson junctions

We present measurements of the current-phase relation (CPR) of superconductor-ferromagnet-superconductor (SFS) Josephson junctions as a function of temperature. The CPR is determined by incorporating the junction into a superconducting loop coupled to a dc SQUID, allowing measurement of the junction phase difference. We find that the critical current of Nb-Cu_0.47Ni_0.53-Nb Josephson junctions with barrier thickness ∼22nm changes sign at T<T_π∼2-4K , indicating that the junction becomes a π Josephson junction. We find no evidence for second-order Josephson tunneling near T_π in the CPR predicted by several theories.