Force between a superconductor and a permanent magnet due to trapped flux

Type-II and multiply connected type-I superconductors may experience magnetic forces that are significantly different from the well-known repulsive Meissner effect. In particular flux trapped by a superconductor may result in a force which has a stable position. We have calculated the magnetic force acting on a superconductor, based on a trapped flux model; the results have been compared with experiments at 4.2 K on (1) a block of Nb3Sn (2), a disk of YBa2Cu3Ox, and (3) a hollow cylinder of Pb. Most of the experimental results are in reasonably good agreement with the calculations. However, an interesting discrepancy exists in the frequency of small oscillations about the stable position