Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness dF of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to π (π junction). When 0 and π segments are joined to form a " 0-π junction," spontaneous supercurrents around the 0-π boundary can appear. Here we report on the visualization of supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS) junctions by low-temperature scanning electron microscopy (LTSEM). We discuss data for rectangular 0, π, 0-π, 0-π-0, and 20× (0-π-) junctions, disk-shaped junctions where the 0-π boundary forms a ring, and an annular junction with two 0-π boundaries. Within each 0 or π segment the critical current density is fairly homogeneous, as indicated both by measurements of the magnetic field dependence of the critical current and by LTSEM. The π parts have critical current densities jcπ up to 35A/ cm2 at T=4.2K, which is a record value for SIFS junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology is capable to produce Josephson devices with a unique topology of the 0-π boundary.
|Physical Review B - Condensed Matter and Materials Physics
|Published - 3 Mar 2010