Flux creep in cation deficient Bi2(Sr,Ca)3Cu 2O8+d

Flux pinning in Bi₂(Sr,Ca)_3-xCu₂O _8+d was investigated as a function of cation deficiency and thermal history. A comparison of polycrystalline samples of cation stoichiometric and highly alkaline earth deficient composition (x =0.3) shows a marked difference in the magnetization hysteresis (ΔM) and time-dependent decay of magnetization at temperatures below 40 K. The pinning energy (95-140 meV at 10-20 K) derived from flux creep data for an x=0.3 cation-deficient composition is 4-7 times greater than that for a stoichiometric composition. The difference in ΔM between stoichiometric and nonstoichiometric compositions increases with increasing field out to 5.5 T and is greatest at 20 K, but still significant at 40 K. The intragranular J_c of an x=0.3 composition exceeds that of the stoichiometric by >10² at a field of 1 T. The dependence of pinning characteristics in cation-deficient samples on thermal history suggests that defect clustering or incipient precipitation plays an important role in improving pinning