Direct observation of superconducting vortex clusters pinned by a periodic array of magnetic dots in ferromagnetic/superconducting hybrid structures

Strong pinning of superconducting flux quanta by a square array of $1\text{\ensuremath{-}}\ensuremath{\mu}\text{m}$-sized ferromagnetic dots in a magnetic-vortex state was visualized by low-temperature magnetic force microscopy. A direct correlation of the superconducting flux lines with the position of the dots was derived. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience stronger pinning forces as compared to the pinning force in the pure Nb film. This pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located at each dot. Our microscopic studies are consistent with global magnetoresistance measurements on the hybrid structures but suggest a modified picture of the pinning mechanism.