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