Superconducting transitions of YBa2Cu3Ox superconductors in high fields

Superconducting transitions were measured in field-cooled cooling and field -cooled warming modes in external magnetic fields ranging between 1 mT and 7 T on YBa2Cu3Ox (Y123) single crystals with and without the fishtail effec t. The Y123 crystal with the fishtail effect exhibited a two-step transitio n when bring cooled in high fields, while the Y123 crystal without the fish tail showed a single step transition. Furthermore, an upturn in magnetic mo ment was observed at lower temperatures for Y123 with the fishtail effect. Such an upturn is ascribed to the local Cu magnetic moment due to oxygen va cancies. The fishtail effect is thus caused by oxygen vacancies.