Effect of Deoxygenation on the Flux-Pinning Properties of Superconducting Y-123

Pinning properties were investigated for bulk Y-123 superconductors in an underdoped condition by heat treatment at a high temperature. The critical current density was drastically decreased. It is ascribed to the weakened pinning strength of 211 phase particles resulting from a degradation of the superconductivity in the block layer. A decrease of g2, the number of flux lines in the flux bundle, was found to be stronger than that of the pinning strength. This indicates that an additional disorder because of the thermal activation of flux lines also makes g2 smaller. The present result shows that the disorder transition that causes the peak effect is more strongly affected by the elastic properties than by the pinning strength.