Influence of the FFLO-like state on the upper critical field of a superconductor/ferromagnet bilayer: Angular and temperature dependence

Abstract

We investigated the upper critical magnetic field,Hc, of a superconductor-ferromagnet (S/F)bilayer of Nb/Cu41Ni59and a Nb film (as reference). We obtained the dependence ofHc⊥andHc‖(perpendicular and parallel to the film plane, respectively) on the temperature,T, by measurementsof the resistive transitions and the dependence on the inclination angle,θ, of the applied field tothe film plane, by non-resonant microwave absorption. Over a wide range,Hc⊥andHc‖show thetemperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and closeto the critical temperature deviations are observed. WhileHc(θ) of the Nb film follows the Tinkhamprediction for thin superconducting films, the Nb/Cu41Ni59-bilayer data exhibit deviations whenθapproaches zero. We attribute this finding to the additional anisotropy induced by the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state and propose a new vortex structurein S/F bilayers, adopting the segmentation approach from high-temperature superconductors.