Figure 3: Mechanism of the vortex dipole formation.

(a) Scanning Hall probe microscopy images (I–IV) taken at different locations of the sample as schematically indicated in the left panel. The arrows highlight the direction of the dipoles. Red and blue colours correspond to magnetic field pointing outward and inward, respectively. As follows from the histograms of dipole direction, which are shown below images I to IV, within one and the same scanned location all the dipoles have similar orientations. Scale bar, 100μm; inset, 4μm. (b) Schematic view of a supercurrent flowing through the area around a pinning centre, which is shown by the dashed circle. The clockwise and counterclockwise current flows result in the formation of a vortex dipole. The current distribution in b is equivalent to a uniform current flow (c) superimposed with two current loops with opposite polarities (c). The arrows in c indicate the current density vector distributions, while the colour maps represent the magnetic field B_z, induced by the corresponding currents. The simulated dipole demonstrates a very good qualitative resemblance with experimentally observed vortex dipole shown in d. Scale bar, 2μm.