Relation between the second peak in the magnetization curves and magnetic relaxation ofLa2−xSrxCuO4single crystals

Abstract

A large distinct second peak in magnetization has been observed in La{sub 2{minus}x}Sr{sub x}CuO{sub 4} single crystals with x=0.14, 0.15, and 0.18, while a small second peak has been observed in the samples with x=0.115, 0.12, and 0.13. The large second peak has been observed only in the overdoped region (x{ge}0.14). To clarify the flux-pinning mechanism associated with these second peaks, magnetic relaxation has been investigated for x=0.115 and x=0.15. We have evaluated the flux-pinning energy U{sub 0} based on the Anderson-Kim model and a glassy exponent {mu} based on the collective creep model. In the sample having the large second peak, U{sub 0} showed a maximum at a little lower field than the field where the second peak was observed and {mu} changed from positive to negative values with increasing applied field around the field where U{sub 0} took the maximum. On the other hand, the exponent {mu} indicated negative values for the sample having a small second peak. The field dependence of the flux-pinning force above the second peak was found to fit very well to that of a catastrophic depinning model. The maximum in U{sub 0} appears at the field where the pinning character changes from collectivemore » creep to catastrophic depinning; the second peak is related to a crossover in flux dynamics from elastic to plastic behavior. Since both the collective creep and the catastrophic depinning models assume weak inhomogeneous pinning centers, there is a possibility that the pinning centers originate from oxygen-deficient regions which change into efficient pinning centers with increasing field and/or normal metallic states which fluctuatingly appear in the overdoped superconducting region. {copyright} {ital 1997} {ital The American Physical Society}« less