Low-field flux creep behavior near second peak H_2p in Pr-doped Bi-2212 single crystal

Magnetic hysteresis M(H) and magnetic relaxation M(t) results on Pr-doped single-crystal Bi₂Sr₂Ca_0.89Pr_0.11Cu₂O_8+δ with optimum T_c = 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_2p/H_on 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_2p/H_on indicates a distinct change of flux creep behavior. For H<H_on, the relaxation mechanism follows the collective creep model M(t) = M₀[1+(μk_BT/U₀)ln(t/t₀)]^-1/μ and crosses to the vortex glass model M(t) = M₀(t/t₀)^{1/[1-(U(0)/k(B)T)]} when H>H_on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.