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

TL;DR

This study visualized and analyzed how ferromagnetic dots in a hybrid structure strongly pin superconducting vortices, forming clusters at dot locations, with pinning forces significantly exceeding those in pure superconducting films.

Contribution

It provides direct microscopic evidence of vortex pinning by magnetic dots and quantifies the pinning force enhancement in hybrid structures.

Findings

Vortices are pinned at magnetic dots with 15 times stronger force.

Vortex clusters form at each dot due to strong pinning.

Microscopic observations agree with magnetoresistance measurements.

Abstract

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.