Origin of nonmonotonic T-c behavior in ferromagnet/superconductor structures

The original theory of proximity effect in the layered ferromagnetic metal/ superconductor (FM/S) structures taking into account a finite transparency of FM/S interface as well as a competition between ID and 3D Larkin-Ovchin- nikov-Fulde-Ferrell (LOFF) states is developed. It is shown that the oscill atory dependence of critical temperature T-c on the FM layer thickness di i s due to oscillations of the Cooper pairs flux at the FM/S boundary. This e ffect is possible not only in the FM/S multilayers, but in the FM/S bilayer s as well. The phenomena of reentrant and periodically reentrant supercondu ctivity in the FM/S bilayers and superlattices are predicted. The competiti on between 1D and 3D LOFF states leads to significant smoothing of the T-c( d(f)) dependence, so that in the most cases one maximum of this dependence is realized as this has been observed in Gd/Nb and Fe/Nb/Fe. (C) 2000 Elsev ier Science B.V. All rights reserved.