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

We investigated the upper critical magnetic field ${H}_c$ of a superconductor-ferromagnet (S/F) bilayer of Nb/${\mathrm{Cu}}₄₁{\mathrm{Ni}}₅₉$ and a Nb film (as reference). We obtained the dependence of ${H}_{c\ensuremath{\perp}}$ and ${H}_{c\ensuremath{\parallel}}$ (perpendicular and parallel to the film plane, respectively) on the temperature $T$ by measurements of the resistive transitions and the dependence on the inclination angle $\ensuremath{\theta}$ of the applied field to the film plane, by nonresonant microwave absorption. Over a wide range, ${H}_{c\ensuremath{\perp}}$ and ${H}_{c\ensuremath{\parallel}}$ show the temperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and close to the critical temperature, deviations are observed. While ${H}_c(\ensuremath{\theta})$ of the Nb film follows the Tinkham prediction for thin superconducting films, the Nb/${\mathrm{Cu}}₄₁{\mathrm{Ni}}₅₉$-bilayer data exhibit deviations when $\ensuremath{\theta}$ approaches 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 structure in S/F bilayers, adopting the segmentation approach from high-temperature superconductors.