Abstract
The magnetic hysteresis loops and the relaxation in Bi 2Sr 2CaCu 2O 8 single crystals of various doses of columnar defects (CDs) are studied mainly focusing on physical nature of anomalous second magnetization peak (SMP). With increasing CD density up to a matching field of 100 G, it is observed that the magnetization around the second peak systematically decreases, while the magnetization around the neck region between two peaks increases. The reduction of the SMP after introducing low-density CD is interpreted as CDs role of suppressing the decoupling transition of neighboring pancake vortices, thus resulting in a less enhanced magnetization compared to that of the unirradiated crystal. Both magnetization loops and the dynamic behavior of vortices seem to change near SMP suggesting a transition of vortex structure in that field region. We also observed that the experimental time window is an important factor for ability to observe the SMP. The experimental results are discussed in terms of both static and dynamic effects.
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