High quality ferromagnetic 0 and pi Josephson tunnel junctions

Abstract

We fabricated high quality $\Nb/\Al_2Ø_3/\Ni_{0.6}\Cu_{0.4}/\Nb$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic $\Ni_{0.6}\Cu_{0.4}$ 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 $\Al_2Ø_3$ tunnel barrier allows 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 $β_c\approx700$ at $T=2.1 {\rm K}$. The critical current density in the $π$ state was up to $5\:\rm{A/cm^2}$ at $T=2.1 {\rm K}$, resulting in a Josephson penetration depth $λ_J$ as low as $160\:\rm{μm}$. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the $\Ni_{0.6}\Cu_{0.4}$ layer and different transparencies of the interfaces.