ENHANCED FLUX-PINNING AND CRITICAL CURRENTS BY HF AND HF-CA SUBSTITUTION IN YBA2CU3O7-DELTA

Hf and Ca substituted samples with stoichiometric compositions (Y1-xHf x)Ba2Cu3Oz with x = 0.0, 0.1, 0.2 and (Y0.85-yCayHf0.15)Ba2Cu3Oz with y = 0.1, 0.2, 0.4, respectively, prepared under identical conditions h ave been characterized by d.c. magnetization and thermogravimetric ana lysis measurements. Detailed magnetization measurements on randomly or ientated powdered samples as a function of temperature and field exhib it pronounced M(H) hysteresis loops, indicative of large flux pinning, even as T approaches T(c). It is believed that the flux pinning arise s due to the preferential substitution of Hf or Hf-Ca on the Y site, w hich acts as a pinning centre. The critical current density J(c) has b een deduced from hysteresis data which is discussed in the light of th e flux creep model.