Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and \pi segments

TL;DR

This paper visualizes supercurrents in various ferromagnetic Josephson junction configurations using LTSEM, revealing homogeneous critical current densities and demonstrating the fabrication of complex 0-$pi$ boundary topologies.

Contribution

It introduces LTSEM visualization of supercurrents in diverse 0-$pi$ Josephson junction arrangements, including novel topologies and record critical current densities.

Findings

Homogeneous critical current density within each segment.

Record critical current density of 35 A/cm² at 4.2 K.

Successful fabrication of complex 0-$pi$ boundary topologies.

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 $\pi$ ($\pi$ junction). When 0 and $\pi$ segments are joined to form a "0-$\pi$ junction", spontaneous supercurrents around the 0-$\pi$ 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-$\pi$, 0-$\pi$-0 and 20 \times 0-$\pi$ junctions, disk-shaped junctions where the 0-$\pi$ boundary forms a ring, and an annular junction with two 0-$\pi$ 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 $35\units{A/cm^2}$ 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-$\pi$ boundary.