Planar SFS Josephson Junctions Made by Focused Ion Beam Etching

TL;DR

This paper reports the fabrication of superconductor-ferromagnet-superconductor Josephson junctions using focused ion beam etching, achieving high precision and demonstrating their electrical properties and uniformity.

Contribution

It introduces a novel method for creating planar S-F-S Josephson junctions with high spatial resolution using FIB, enabling control over junction parameters.

Findings

Junctions show clear Fraunhofer modulation, indicating uniformity.

Achieved a range of $I_c R_n$ products from 0.5 mV to 1 μV.

Successfully fabricated both S-F-S and S-S'-S junctions.

Abstract

Superconductor-Ferromagnet-Superconductor (S-F-S) Josephson junctions were fabricated by making a narrow cut through a S-F double layer using direct writing by Focused Ion Beam (FIB). Due to a high resolution (spot size smaller than 10 nm) of FIB, junctions with a small separation between superconducting electrodes ($\leq$ 30 nm) can be made. Such a short distance is sufficient for achieving a considerable proximity coupling through a diluted CuNi ferromagnet. We have successfully fabricated and studied S-F-S (Nb-CuNi-Nb) and S-S'-S (Nb-Nb/CuNi-Nb) junctions. Junctions exhibit clear Fraunhofer modulation of the critical current as a function of magnetic field, indicating good uniformity of the cut. By changing the depth of the cut, junctions with the $I_c R_n$ product ranging from 0.5 mV to $\sim 1\mu $V were fabricated.