Ferromagnetic Josephson junctions with steplike interface transparency

Within the framework of the quasiclassical Usadel equations we study the Josephson effect in superconductor-insulator-ferromagnet-superconductor (SIFS) and SIFNS (N is a normal metal) structures with a steplike transparency of the FS or NS interface. At certain parameters the steplike transparency leads to the formation of a region, where the critical current-density distribution J_C(y) along the junction exhibits a damped oscillation with a sign change. This results in the formation of a 0-π nanojunction with the characteristic length of 0 and π regions of the order of the coherence length ξ_F for SIFS and ξ_N for SIFNS junctions, respectively. Using several transparency steps one can create an array of nanojunctions. Such structures exhibit an unusual behavior in an external magnetic field H. The total critical current grows with increasing H up to a certain value, which depends on the size of a single nanojunction, and has multiperiodic oscillations in the case of an array.