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

TL;DR

This study explores how an FFLO-like state influences the upper critical magnetic field in a superconductor-ferromagnet bilayer, revealing deviations from standard models and proposing a new vortex structure based on experimental angular and temperature dependence data.

Contribution

It provides new insights into the impact of FFLO-like states on the critical field behavior and introduces a novel vortex structure model for S/F bilayers.

Findings

$H_{c}$ depends on temperature as predicted by Ginzburg-Landau theory.

Deviations occur near critical temperature and at small angles, indicating additional anisotropy.

Proposes a new vortex structure in S/F bilayers based on segmentation approach.

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.