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. Junctions fabricated with a thin (~ 22 nm) barrier of Cu0.47Ni0.53 sandwiched between Nb electrodes exhibit a re-entrant critical current with temperature, vanishing at T =T_pi ~ 2-4 K. We find that the critical current is negative for T < T_pi, indicating that the junction is a pi-Josephson junction. We find no evidence for second-order Josephson tunneling near T_pi in the CPR predicted by several theories.
@article{arxiv.cond-mat/0402434,
title = {Measurement of the Current-Phase Relation of SFS pi-Josephson junctions},
author = {S. M. Frolov and D. J. Van Harlingen and V. A. Oboznov and V. V. Bolginov and V. V. Ryazanov},
journal= {arXiv preprint arXiv:cond-mat/0402434},
year = {2007}
}