Vortex-Antivortex coexistence in Nb based Superconductor/Ferromagnet heterostructures

Superconductor/Ferromagnet thin film heterostructures, based on Niobium/Permalloy (Nb/Py), have been studied by low temperature Magnetic Force Microscopy. The experimental observation of spontaneous Vortex-Antivortex in these systems depends on the Nb penetration depth and thickness, on the intensity of the Py stray field induced by the small, alternating up-and-down, out-of-plane component $M_0$, and the width of the Py stripe-like magnetic domains. Comparison with the available theoretical models allows to estimate the threshold of the local $M_0$ for different Py thicknesses.