Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.