Magnetic Forces in High T c superconductors

Magnetic forces between a permanent magnet and bulk YBCO high T c superconducting specimens prepared in different processes are investigated in this paper. The magnetic forces in specimens prepared by melting growth are larger than those of the sintered specimens. Repulsion forces between magnet and specimens were measured when the superconducting specimens raised toward the magnet. The repulsion force decreased and became an attraction force in some specimens when the specimen was lowered away from the magnet. This attraction force is closely related to the pinning of the specimen. The levitation of a magnet on specimens having attraction force was stable in a continuous range of positions and orientations. The magnetic forces are closely related to the microstructures of the specimens which are discussed in this paper.