Critical states of superconductivity and their crossover in multilayer superconductor/ ferromagnet structures

Abstract

In order to calculate the critical temperature of multilayer S/F structures (where S is a superconductor and F is a ferromagnet), a matrix method for solving linearized Usadel equations has been proposed. The spectrum of critical temperatures T(k) for the F/Nbl(S/F) structure has been obtained in the single-mode approximation. Eigenfunctions describing the spatial distribution of superconducting correlations in the direction perpendicular to the S–F interfaces have been calculated for each T(k) value. It has been found that dependences of T(k) on the thickness of F layers have a jump near the transition from 0 to π-state; any of the calculated T(k) values can be implemented in the region of jumps. It has been shown that the crossover of eigenstates is characterized by the suppression of superconductivity in outer S layers and by induced countercurrents in F layers. The possibility of the experimental implementation of a state corresponding to a given value from the spectrum of T(k) has been discussed.