The 0–$\pi$ Phase Transition in Epitaxial NbN/Ni$_{60}$Cu$_{40}$/NbN Josephson Junctions
Feng Li1,2,3, Wei Peng1,2, Zhen Wang1,2,3,4**
1Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050 2CAS Center for Excellence in Superconducting Electronics, Shanghai 200050 3University of Chinese Academy of Sciences, Beijing 100049 4School of Physical Science and Technology, University of ShanghaiTech, Shanghai 200031
Abstract：We fabricate high quality superconductor/ferromagnet/superconductor (SFS) Josephson junctions using epitaxial NbN/Ni$_{60}$Cu$_{40}$/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–$\pi$ phase transition in this type of magnetic Josephson junction. Clear 0–$\pi$ and reverse $\pi$–0 phase transitions occur around the Ni$_{60}$Cu$_{40}$ 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 $\pi$-phase junctions show the ideal magnetic field dependence with a Fraunhofer-like pattern at 4.2 K.
2019, Vol. 36 
