We report measurements of the temperature dependence of the critical current in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1−x alloy, with x around 0.5. With decreasing temperature Ic generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which Ic rises again. Such a sharp cusp can only be explained by assuming that the junction changes from a 0-phase state at high temperatures to a π-phase state at low temperatures.
@article{arxiv.cond-mat/0008364,
title = {Coupling of two superconductors through a ferromagnet : evidence for a pi-junction},
author = {V. V. Ryazanov and V. A. Oboznov and A. Yu. Rusanov and A. V. Veretennikov and A. A. Golubov and J. Aarts},
journal= {arXiv preprint arXiv:cond-mat/0008364},
year = {2009}
}