The sequence illustrates what happens when a magnet moves in the vicinity of a layer of iron filings :It attracts the magnetic particles, and a trace is left in the layer of filings. One can thus see the trajectory of the magnet by observing only the iron filings.

The second sequence shows the adaptation made by researchers from Liège of this physical principle, but on a microscopic scale, in a superconductor (Nb) in the vicinity of a magnetic layer (Py). A superconducting vortex moves (it plays the role of the magnet in the previous sequence) and leaves a trace of its passage in the magnetic layer (which plays the role of the iron filings), by reversing the direction of the polarization.
It is then possible to determine the trajectory of the vortex by observing only the trace left in the magnetic layer.

Source :  Imprinting superconducting vortex footsteps in a magnetic layer, Brisbois Jérémy et al., Scientific Reports 6, 27159 (2016).