STATIC AND DYNAMIC MECHANISMS OF THE ANOMALOUS FIELD-DEPENDENCE OF MAGNETIZATION IN BI-SR-CA-CU-O AND BI-PB-SR-CA-CU-O SINGLE-CRYSTALS

We have measured and analyzed both the static and dynamic contribution to the anomalous nonmonotonic dependence of the irreversible magnetization, M(B), for underdoped Bi2Sr2CaCu2O8+y and overdoped (Bi1.6Pb0.4)Sr2CaCu2O8+y single crystals. We extracted the "unrelaxed" magnetization Mc(B) and traced the time evolution of M(B,t) from short to long time scales using combined magnetization and flux-creep measurements. Despite their large differences in anisotropy, pinning force, and flux-creep rate s(B)=dln M/dln t, both samples showed the anomaly over a wide range of T, the anomaly being most pronounced in the "unrelaxed" state. This suggests that the primary cause of the anomaly lies in the flux-pinning defect structure of the material. However, M(B,E) also essentially depends on the induced electric field E in the sample, so the shape of the M(B) curve can strongly change with time due to the relaxation of E(t) and the nonmonotonic dependence of s(B). © 1994 The American Physical Society.