High quality ferromagnetic 0 and π Josephson tunnel junctions

The authors fabricated high quality Nb/Al2O3/Ni0.6Cu0.4/Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or pi 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 beta(c) increases exponentially with decreasing temperature and reaches beta(c)approximate to 700 at T=2.11 K. The critical current density in the pi state was up to 5 A/cm(2) at T=2.11 K, resulting in a Josephson penetration depth lambda(J) as low as 160 mu m. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces. (c) 2006 American Institute of Physics.