NONMONOTONIC BEHAVIOR OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE IN BIMETALLIC FERROMAGNET-SUPERCONDUCTOR STRUCTURES

For layered ferromagnet/superconductor (F/S) structures we develop a t heory of the proximity effect. In contrast to previous approaches, thi s theory allows for a finite transmission coefficient of the interface between the two metals and competition between the diffusion and spin -wave types of quasiparticle motion in the ferromagnet's strong exchan ge field. The superconductivity in F/S systems proves to be a superpos ition of BCS pairing with a constant-sign pair amplitude in the S-laye rs and Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) pairing with an oscilla ting wave function in the F-layers. We show that the oscillatory behav ior of the superconducting transition temperature T-c is due to oscill ations of the Cooper pair flux from the S-layer to the F-layer, which are the result of oscillations of the discontinuity Clump) of the pair amplitude at the F/S boundary as the thickness df of the F-layer incr eases. The presence of nonmagnetic impurities leads to heavy damping o f the oscillations of the LOFF pair amplitude and rapid deterioration of the coherent coupling of the boundaries of the F-layer in which the T-s vs, d(f) dependence reaches a plateau as d(f) increases. In F/S s uperlattices, in contrast to F/S double-layer junctions, there are two forms of the LOFF state, the 0-phase and the pi-phase, which differ i n their symmetry with respect to the center of the F layer. This gives rise to additional oscillations in the T-c (d(f)) dependence due to t he 0-pi transitions. As the most vivid manifestation of LOFF states in F/S-systems, we predict the existence of recurrent and periodically r ecurrent superconductivities, We give a qualitative explanation of the different behavior of the superconducting transition temperature obse rved by different groups of experimenters dealing with the same ferrom agnet-superconductor structures. (C) 1998 American Institute of Physic s. [S1063-7761(98)01105-6].