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

摘要

Strong pinning of superconducting flux quanta by a square array of 1$\mu$m-sized ferromagnetic dots in a magnetic-vortex state was visualized bylow-temperature magnetic force microscopy (LT-MFM). A direct correlation of thesuperconducting flux lines with the positions of the dots was derived. Theforce that the MFM tip exerts on the individual vortex in the depinning processwas used to estimate the spatial modulation of the pinning potential. It wasfound, that the superconducting vortices which are preferably located on top ofthe Py dots experience about 15 times stronger pinning forces as compared tothe pinning force in the pure Nb film. The strong pinning exceeds the repulsiveinteraction between the superconducting vortices and allows the vortex clustersto be located at each dot. Our microscopic studies are consistent with globalmagnetoresistace measurements on these hybrid structures.