Non-logarithmic magnetic relaxation in Bi_2.2Sr_1.7CaCu₂O₈ single crystals; evidence for collective flux pinning

High precision magnetic relaxation measurements have been performed on BiSrCaCuO single crystals with the field H applied parallel to the c-axis. At temperatures T/Tc > 0.5, the data exhibit two different field regimes. One for H<H* (H* corresponds to the field where full flux penetration of the sample is achieved) where the ZFC and RM magnetization relaxes logarithmically with time in agreement with the conventional flux creep model; a second one for H > H* where the relaxation follows a [ln(t/t0] -α dependence. This relaxation law is predicted in the collective flux creep theory for J ≪ Jc as well as in the vortex-glass model. In the temperature range 0.25 < T/Tc < 0.4, there is a change in character of the measured decay of the magnetization. This change in character is interpreted to arise from thermal fluctuations of the flux-line lattice giving rise to a considerable reduction in the pinning strength above the depinning temperature. © 1991.