Magnetic hysteresis and relaxation in Bi2212 single crystals doped with Fe and Pb

Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviors for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Unlike pure and Fe doped Bi2212 crystals, heavily Pb doped crystal showed strong vortex pinning behavior. We interpret the strong pinning in heavily Pb doped Bi2212 single crystals as arising from the improved Josephson coupling in Bi2212 single crystal after heavy Pb doping. In heavily Pb doped single Bi2212 crystals, H_dis(T) was observed to decrease with increasing T. Here, H_dis(T) is an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. However, in pure and Fe doped Bi2212 single crystals, H_dis(T) was observed to be temperature independent. We also report a significant shift of T_CR, a crossover temperature separating two pinning regimes, toward higher temperatures with heavy Pb doping of Bi2212 single crystals. On the other hand T_CR did not shift with Fe doping of Bi2212 single crystals. It is argued that the temperature dependence of H_dis(T) and the shift of T_CR in heavily Pb doped Bi2212 crystals was related to the enhanced c-axis conductivity caused by the Pb situated between the CuO₂ layers and imposing a 3D characteristic on the vortex lattice.