Multicritical behavior of the phase diagrams of ferromagnet/superconductor layered structures

For ferromagnet/superconductor (F/S) layered structures, new 3D Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states are predicted. In most cases, these states are characterized by a higher critical temperature T-c than the known 1D LOFF states. It is shown that the nonmonotonic behavior of T-c is determined by the oscillations of the Cooper pair flux through the F/S boundary, which occur as a result of the 3D-1D-3D phase transitions at the Lifshits triple points. The appearance of the new 3D LOFF states and the presence of nonmagnetic impurities leads to a strong damping of the 1D oscillations of the LOFF pair amplitude and to a considerable smoothing of the dependence of T-c on the F layer thickness d(f). An interpretation of the behavior of the experimental dependences T-c(d(f)) obtained for F/S structures is proposed. (C) 2000 MAIK "Nauka/Interperiodica".