Josephson interferometry and Shapiro step measurements of superconductor-ferromagnet-superconductor 0-π junctions

We report the observation of the 0-pi junction state in superconductor-ferromagnet-superconductor Josephson junctions arising from variations in the effective barrier thickness d/xi(F). By varying the temperature, the junctions can be tuned from the 0- to the pi-state through an intermediate regime in which parts of the junction are in each state. A signature of the 0-pi regime is that the usual peak in the critical current diffraction pattern at zero magnetic field becomes a dip, evidence for sign changes in the critical current density. In some junctions, the zero-field critical current does not vanish at any temperature and half-integer Shapiro steps are observed, indicating the presence of spontaneous supercurrents circulating within the junctions around the 0-pi interfaces.