Abstract
Magnetic hysteresis M(H) and magnetic relaxation M(t) results on Pr-doped single-crystal Bi <sub>2</sub> Sr <sub>2</sub> Ca <sub>0.89</sub> Pr <sub>0.11</sub> Cu <sub>2</sub> O <sub>8+δ</sub> with optimum T <sub>c</sub> = 88 K are reported. Superconducting H-T phase diagram with applied field parallel to single-crystal c-axis indicates that in the mixed state the anomalous second peak field and its onset field H <sub>2p</sub> /H <sub>on</sub> varies from 900/800 G at 15 K to 210/85 G at 40 K with a plateau around 20-30 K. Low-field magnetic relaxation M(t) data near H <sub>2p</sub> /H <sub>on</sub> indicates a distinct change of flux creep behavior. For H<H <sub>on</sub> , the relaxation mechanism follows the collective creep model M(t) = M <sub>0</sub> [1+(μk <sub>B</sub> T/U <sub>0</sub> )ln(t/t <sub>0</sub> )] <sup>-1/μ</sup> and crosses to the vortex glass model M(t) = M <sub>0</sub> (t/t <sub>0</sub> ) <sup>1/[1-(U(0)/k(B)T)]</sup> when H>H <sub>on</sub> . Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.