Low field flux dynamics of Pr-doped Bi-2212 single crystals

Magnetic hysteresis M(H,T) and magnetic relaxation M(t) results on various Pr-doped single crystals Bi₂Sr₂Ca_1-x Pr_xCu₂O_8+δ 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_c = 88 K) to underdoped single crystal (x > 0.11), all characteristic field lines in H-T diagram shift to lower reduced temperature T/T_c and lower field region with narrower second peak (H_2p) window. Low field magnetic relaxation data near H_2p indicates a distinct change of flux creep behavior from collective creep model for field smaller than second peak onset field (H_on) to vortex glass model for H > H_on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-latrice to entangled vortex glass.