Superconductor - ferromagnet - superconductor (SFS) π-junctions [1] were investigated experimentally in detail. The transitions to π-state and back to the conventional 0-state were found to result from temperature decrease or ferromagnetic interlayer thickness increase. Thickness dependence of the critical current was studied and nonmonotonic behavior related to the spatial oscillations of the induced superconducting order parameter in a ferromagnet was observed. Thickness dependences of the SFS-junctions (Nb-Cu_1-xNi_x-Nb) were measured for several contents of Cu/Ni-alloy. The following results were obtained:
i) the larger magnetic content of F-layer the smaller the period of the order parameter spatial oscillations;
ii) there is quite good quantitative agreement between values of the oscillation period extracted from the SF-bilayer and SFS junction measurements for ferromagnetic layers prepared by the same rf-sputtering method with using the same target;
iii) value of the imaginary part of the coherence length in the ferromagnetic Cu/Ni is found to be much larger than the order parameter decay length due to existence extra mechanisms of the order parameter decay in Cu/Ni alloy, mainly the strong spin-flip scattering on nickel-rich clusters nucleated in the alloy.
We present also measurements of the current-phase relation (CPR) of SFS Josephson junctions as a function of temperature. The CPR is determined by incorporating the junction in an superconducting loop coupled to a dc SQUID, allowing measurement of the junction phase difference. We find that the critical current is negative for T smaller than temperature of 0-π-transition, indicating that the junction is a π-Josephson junction.
[1] V.V. Ryazanov, V.A. Oboznov, A.Yu. Rusanov, A.V. Veretennikov, A.A. Golubov, and J. Aarts. Phys. Rev. Letters 86, 2427 (2001).