Enhanced pinning of superconducting vortices at circular magnetic dots in the magnetic-vortex state

Low-temperature magnetic force microscopy (LT-MFM) was used to study the distribution of superconducting vortices in Nb above a square array of 1 μm-sized circular ferromagnetic dots in a magnetic-vortex state. 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 a pinning force, about 15 times stronger as compared to the pinning force in the pure Nb film. This strong pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located above the dots.