Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and πsegments

Abstract

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 $π$ ($π$ 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 \times 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 $j_c^π$ up to $35\units{A/cm^2}$ at $T = 4.2\units{K}$, 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-$π$ boundary.

29 pages, 8 figures