Magnetic field expulsion in superconducting granular ceramics and in polymer/superconductor composites

The magnetic interaction between a permanent magnet and superconducting ceramics such as YBa2Cu3O7-delta and Bi1,6Pb0,4Sr2Cu2Cu3O(10+delta) depend on the superconducting state of each phase and on the junctions between grains. In the case of polymer/superconductor composites, screening effects depend on the volume Fraction of superconductor. Measurements of the evolution of the levitation force (F=A/d(gamma)) as a function of the interaction distance d are used to characterize the effective response of the ceramics or composites to the magnetic flux penetration. Some of the abnormal variations of the exponent gamma and of the term A (in F=A/d(gamma)) could be reinterpreted in terms of a change in superconducting regime. Other observed variations of gamma should be due to the variation of the effective field from the cylindrical magnet.