Vortices in the presence of a periodic array of pinning centers
A magnetic image, made using scanning Hall probe microscopy, of a sample with a square array of holes which act as pinning centers for vortices. The magnetic field is such that half of the holes contain vortices.Throughout most of the visible field these vortices form a "checkerboard" pattern, with every other hole occupied. Because of the limited resolution of the probe, these uniform checkerboard areas appear faint. But along "grain boundaries", where the black (occupied) squares of the checkboard abut other black squares, a distinct boundary of alternating black and white lines is clearly visible. if these formed a perfectly periodic array themselves the image would be a uniform grey, but defects (in the form of domain walls) are visible in this picture.