Josephson Junctions With Centered Step and Local Variation of Critical Current Density

Superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson tunnel junctions based on Nb/Al₂O₃/NiCu/Nb stacks with a thickness step in the metallic NiCu interlayer were fabricated. The step height of a few 0.1 nm was defined by optical lithography and controlled etching of both Nb and NiCu layers. Experimentally determined junction parameters by current-voltage characteristics and Fraunhofer pattern indicate a uniform NiCu thickness and similar interface transparencies for etched and non-etched parts. The critical current diffraction pattern was calculated and measured for stepped junctions having the same ground phase difference but different critical current densities in both halves. The measured data show a good agreement with simulations.