Origin of nonmonotonic Tc behavior in ferromagnet/superconductor structures

Abstract

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-Ovchinnikov-Fulde-Ferrell (LOFF) states is developed. It is shown that the oscillatory dependence of critical temperature Tc on the FM layer thickness df is due to oscillations of the Cooper pairs flux at the FM/S boundary. This effect is possible not only in the FM/S multilayers, but in the FM/S bilayers as well. The phenomena of reentrant and periodically reentrant superconductivity in the FM/S bilayers and superlattices are predicted. The competition between ID and 3D LOFF states leads to significant smoothing of the Tc(df) 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. © 2000 Elsevier Science B.V. All rights reserved.