Anisotropic order-disorder vortex transition in La_2-x Sr_xCuO₄

We report on magnetization measurements in (La_0.937Sr_0.063)₂CuO₄crystals with the field either parallel or perpendicular to the c axis. A second magnetization peak ("fishtail"), interpreted as indicating a vortex order-disorder transition, is observed in both directions. Differences in the details of the fishtail anomaly in the two directions (width, temperature and time dependence, and history effects) are attributed to the anisotropy and the twin boundaries in the ab plane. The transition fields in both directions, although different in magnitude, exhibit similar qualitative behavior, namely, a strong decrease with temperature in the entire measured range. This unique behavior is explained postulating that both thermally and disordered induced fluctuations contribute to the destruction of the vortex lattice. The resulting "pinned liquid" disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior.