NbN-based Josephson junction with PdNi-ferromagnetic barrier*

Superconducting π-phase shift by ferromagnetic Josephson junctions (superconductor/ferromagnet/superconductor junctions) is an attractive feature for various applications in the superconducting electronics. Although CuNi alloys have been often used as a ferromagnetic barrier in the junctions so far, they are known to contain magnetic Ni clusters and show the strong spin-flip scattering. This is a disdvantage in the device application from the point view of device uniformity and reproducibility. In this study, we developed NbN-based Josephson junctions with a Pd₈₉Ni₁₁ ferromagnetic barrier, which is expected to show uniform ferromagnetisim and smaller spin-flip scattering. NbN electrode has advantages of the higher critical temperature or the compatibility with oxide-free Josephson junctions for quantum bits as compared to the conventional Nb electrodes. We fabricated the junctions with various PdNi thicknesses from 15 to 50 nm and obtained clear overdamped current-voltage characteristics. This is an important step toward the practical applications of NbN-based SFS Josephson junction for superconducting electronics.