Andrey Geim of the Universities of Nijmegen and Manchester, and co-workers from Russia, Belgium and the UK, measured the amount of flux entering micron-sized aluminium discs, where edge effects are important. The measurements were made using a so-called ballistic Hall magnetometer. The samples were first cooled to 0.5 K in the absence of a magnetic field. As the field was increased, changes in the magnetic flux led to changes in the Hall voltage, which can easily be measured. The technique has a resolution of a tiny fraction of a flux quantum.

The team found that the magnetic flux increased in steps as individual vortices penetrated the disc. On closer examination, they noted that the height of some of the steps was as small as 0.001f. In other cases the jumps were negative, indicating that magnetic flux may be expelled.

Geim and co-workers repeated the experiments using discs with slightly different shapes and found that the amount of flux carried by the vortices depends crucially on the roughness of the edge of the disk. This effect arises due to changes in the structure of vortices near the edge. The team concludes that the effect will be important in the majority of experiments with thin films.