EFFECTIVE PINNING CENTERS FOR FLUX LINES IN OXIDE SUPERCONDUCTORS WITH YBCO STRUCTURE

The magnetic hysteresis loops of YBa2Cu3Oy, Ln1+xBa2-xCu3O, (Ln=Sm, Nd ) and YBa2 (Cu1-xFex)3Oy systems were measured by a superconducting qu antum interference device (SQUID) magnetometer, and the relationship b etween the microscopic structure and effectiveness of pinning centers for the flux lines is discussed. The oxygen vacancies and disordered o xygens in the Cu1-O planes cannot act as effective pinning centers, wh ile the iron cluster in the Cu1-O planes in the YBa2(Cu1-Fex)3Oy syste m, which is produced during the reduction in flowing N2 gas at high te mperature, is effective for flux pinning.