Magnetic properties of single crystal Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8+y</SUB>: experimental evidence for a dimensional crossover

Magnetization curves of high-quality superconducting Bi2Sr2CaCu2O8+y single crystals in fields oriented parallel to the c axis have been reported at various temperatures. The irreversibility line and magnetic relaxation of this single crystal have been measured in magnetic fields parallel to the c axis of the crystal. We have observed a low-field anomaly in the magnetization together with a sharp rise of the irreversibility line and a nonlogarithmic magnetic relaxation at 25 K. We argue that 25 K is a boundary temperature where the system undergoes a dimensional crossover from the three-dimensional (3D) to the 2D regime. The relaxation rate also predicts that although the flux creep above this crossover temperature follows the conventional Kim-Anderson thermally activated flux-creep model, below this crossover temperature it follows that predicted by the collective flux pinning or vortex-glass models