Multiple order parameter configurations in superconductor/ferromagnet multilayers

Abstract

he coupling of two superconductors (S) through a ferromagnet (F) can lead to either a zero- or a π-phase difference between the superconducting banks. Most research in this area is performed on trilayer S/F/S film structures, in which two-order parameter configurations are possible. Increasing the number of layers and junctions leads to a larger number of possible configurations with, in principle, different properties such as the superconducting transition temperature Tc. Here we study the behavior of a series of multilayers made of superconducting Nb and ferromagnetic Pd81Ni19. We find that for the individual layer thicknesses used, the transition width ⵼Tc increases with increasing number of bilayers in the multilayer, in a well-defined manner. That the broadening is not simply due to increased disorder in the larger stacks, it is shown from x-ray diffraction, which finds very sharp interfaces for all samples; and from the effect of the magnetic field on the transition, which shows a considerable sharpening. We can make a connection with the various order parameter configurations using a matrix formulation of quasiclassical theory based on the Usadel equations and show that these different configurations take part in the Josephson networks, which are building up in the transition to the superconducting state.