MAGNETIC-PROPERTIES OF SINGLE-CRYSTAL BI2SR2CACU2O8- EXPERIMENTAL-EVIDENCE FOR A DIMENSIONAL CROSSOVER(Y )

Magnetization curves of high-quality superconducting Bi2Sr2CaCu2O3+y s ingle crystals in fields oriented parallel to the c axis have been rep orted at various temperatures. The irreversibility line and magnetic r elaxation of this single crystal have been measured in magnetic fields parallel to the c axis of the crystal. We have observed a low-field a nomaly in the magnetization together with a sharp rise of the irrevers ibility line and a nonlogarithmic magnetic relaxation at 25 K. We argu e that 25 K is a boundary temperature where the system undergoes a dim ensional crossover from the three-dimensional (3D) to the 2D regime. T he relaxation rate also predicts that although the flux creep above th is crossover temperature follows the conventional Kim-Anderson thermal ly activated flux-creep model, below this crossover temperature it fol lows that predicted by the collective flux pinning or vortex-glass mod els.