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The dynamic response of the levitation force between a permanent magnet and a bulk YBCO superconductor when subjected to a sudden motion-induced step in the magnetic field has been measured. It is found that the levitation force relaxes logarithmically with time. Assuming that force and magnetization behave similarly, the rate of decay can be interpreted in terms of a thermally activated flux creep with a depinning activation energy of U0=0.20 eV. A replot of previously published results for melt-quenched YBCO also demonstrates that also in this material the force relaxation is characterized by a logarithmic decay.

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The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-T c superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.