High quality ferromagnetic 0 and π Josephson tunnel junctions

The authors fabricated high quality Nb /Al₂O₃/Ni_0.6Cu_0.4/Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni_0.6Cu_0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al₂O₃ tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter β_c increases exponentially with decreasing temperature and reaches β_c≈700 at T =2.11K_{. The critical current density in the π state was up to 5A/cm² at T =2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6}Cu_0.4 layer and different transparencies of the interfaces.