Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H-c2 perpendicular to and parallel H-c2 parallel to to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconductine 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 oil the critical fields, and the dimensional crossover behavior of H-c2 parallel to(T) are studied in details. For the multilayered structure, the effects of the pi-phase state oil both the superconducting transition temperature T-c and the upper critical fields (H-c2 perpendicular to, and H-c2 parallel to) are also considered. The nonmonotonic T-c behaviors are predicted. The interplay between 0- and pi-phases leading to the strong oscillations of Tc as well as the temperature dependence of the zero temperature critical fields oil the ferromagnetic layer thickness are investigated theoretically. (c) 2006 Elsevier B.V. All rights reserved.