Critical States of Superconductivity and Their Crossover in Multilayer Superconductor/Ferromagnet Structures

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/N-bl(S/F) structure has been obtained in the single-mode approximation. Eigenfunctions describing the spatial distribution of superconducting correlations in the direction per-pendicular 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 pi-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. DOI: 10.1134/S0021364011090086