Phase transition and flux pinning of oxygen defects in Y1Ba2Cu3O7-x bulk materials

A systematic study of oxygen defect related anomalous magnetization or so called open-quotes fish tailclose quotes phenomenon in Y1Ba2Cu3O7-x(YBCO) bulk materials has been conducted. Flux pinning behaviors of a weak center or a weak superconductor region in not fully oxygenated YBCO (single crystals and melt-textured materials) have been observed. It is believed that the phase transition of the weak centers, caused by external magnetic fields, transforms the weak centers into a normal state or the pinning centers. The maximum magnetic relaxation of an oxygen deficient YBCO sample, corresponding to the state transition onset, occurs at a temperature dependent magnetic field which the authors define as the upper critical field or Hc2 of the weak center. A technique to distinguish the pinning forces from one another, when several pinning mechanisms coexist in the superconductors, will be mentioned