Influence of the FFLO-like State on the Upper Critical Field of a S/F Bilayer: Angular and Temperature Dependence

Abstract

We investigated the upper critical magnetic field, $H_c$, of a superconductor-ferromagnet (S/F) bilayer of Nb/Cu${}_{41}$Ni${}_{59}$ and a Nb film (as reference). We obtained the dependence of $H_{c\perp }$ and $H_{c\parallel }$ (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, $\theta$, of the applied field to the film plane, by non-resonant microwave absorption. Over a wide range, $H_{c\perp }$ and $H_{c\parallel }$ show the temperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and close to the critical temperature deviations are observed. While $H_c(\theta )$ of the Nb film follows the Tinkham prediction for thin superconducting films, the Nb/Cu${}_{41}$Ni${}_{59}$-bilayer data exhibit deviations when $\theta$ 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.

Accepted to Phys. Rev. B