Planar S–F–S Josephson junctions made by focused ion beam etching

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 (⩽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 ∼1 μV were fabricated.