Josephson coupling and Fiske dynamics in ferromagnetic tunnel junctions

We report on the fabrication of Nb/AlO<Subscript> x </Subscript>/Pd<Subscript>0.82</Subscript>Ni<Subscript>0.18</Subscript>/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to π-coupling is observed for a thickness d <Subscript>F</Subscript> <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\simeq$</EquationSource> </InlineEquation> 6 nm of the ferromagnetic Pd<Subscript>0.82</Subscript>Ni<Subscript>0.18</Subscript> interlayer. The magnetic field dependence of the π-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd<Subscript>0.82</Subscript>Ni<Subscript>0.18</Subscript> has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010