Vortex-Antivortex Pair Unbinding Driven by the Spin Texture of a Ferromagnet-Superconductor Bilayer

We report the observation of a pronounced dip in the in-plane field (H//) dependence of the critical current density Jc(H) and a peak in resistance R(H) of a NbN-HoNi5 bilayer at temperatures below the magnetic ordering temperature (TCurie ~ 5.5 K) of HoNi5, which is lower than the onset temperature (~ 9 K) of superconductivity in the NbN layer. The extrema in Jc(H) and R(H) appear at fields much below the upper critical field of NbN. It is suggested that the origin of this feature is linked with the non-collinear magnetization of HoNi5 whose normal component assists in depairing of Pearl vortex-antivortex pairs in the system. A spin reorientation transition driven by the H// diminishes this normal component and consequently the excess dissipation.