Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness $d_F$ of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to $\pi$ ($\pi$ junction). When 0 and $\pi$ segments are joined to form a “$0\text{−}\pi$ junction,” spontaneous supercurrents around the $0\text{−}\pi$ 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, $\pi$, $0\text{−}\pi$, $0\text{−}\pi \text{−}0$, and $20\times \left(0\text{−}\pi \text{−}\right)$ junctions, disk-shaped junctions where the $0\text{−}\pi$ boundary forms a ring, and an annular junction with two $0\text{−}\pi$ boundaries. Within each 0 or $\pi$ 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 $\pi$ parts have critical current densities $j_c^{\pi }$ up to $35\text{A}/{\text{cm}}^2$ at $T=4.2\text{K}$, 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 $0\text{−}\pi$ boundary.

• Received 23 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.094502