The 0–π Phase Transition in Epitaxial NbN/Ni₆₀Cu₄₀/NbN Josephson Junctions

We fabricate high quality superconductor/ferromagnet/superconductor (SFS) Josephson junctions using epitaxial NbN/Ni₆₀Cu₄₀/NbN trilayer heterostructures. Both experimental measurements and theoretical calculations of the ferromagnet layer thickness dependence of the Josephson critical current are performed. We observe the damped oscillation behavior of the critical current as a function of the ferromagnetic layer thickness at 4.2 K, which shows a 0–π phase transition in this type of magnetic Josephson junction. Clear 0–π and reverse π–0 phase transitions occur around the Ni₆₀Cu₄₀ thicknesses of 3.2 and 6.7 nm. Numerical calculations based on the quasi-classical Usadel equation and the Green function fit well with the experimental results. Compared with the dirty limit, the intermediate regime without the dead layer gives better fit for our SFS Josephson junctions because of the epitaxial structure. Both of the 0- and π-phase junctions show the ideal magnetic field dependence with a Fraunhofer-like pattern at 4.2 K. (paper)