Abstract
Magnetic hysteresis M(H,T) and magnetic relaxation M(t) results on various Pr-doped single crystals Bi <sub>2</sub> Sr <sub>2</sub> Ca <sub>1-x</sub> Pr <sub>x</sub> Cu <sub>2</sub> O <sub>8+δ</sub> are reported. Superconducting H-T phase diagram with applied field parallel to single crystal c-axis indicates that with progressive Pr doping from optimum composition (x = 0.11, T <sub>c</sub> = 88 K) to underdoped single crystal (x > 0.11), all characteristic field lines in H-T diagram shift to lower reduced temperature T/T <sub>c</sub> and lower field region with narrower second peak (H <sub>2p</sub> ) window. Low field magnetic relaxation data near H <sub>2p</sub> indicates a distinct change of flux creep behavior from collective creep model for field smaller than second peak onset field (H <sub>on</sub> ) to vortex glass model for H > H <sub>on</sub> . Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-latrice to entangled vortex glass. © 2000 Elsevier Science B.V. All rights reserved.