Magnetic hysteresis and flux creep in Bi2Sr2CaCu2Oδ+2 single crystals doped with Fe and Pb

Full text: A comparative study of critical current density Jc and irreversible field Hirr is made from hysteresis loop measurements in pure, iron and lead doped Bi2212 single crystals. A strong field and temperature dependence of Jc and Hirr is observed for pure and iron doped samples, whereas weak field and temperature dependence is observed for lead doped samples. The normalised relaxation rate S as a function of temperature has been calculated from the magnetic relaxation measurements for the pure, lead and iron doped samples. A large shift of TCR, a cross over temperature from single vortex pinning region to vortex bundle pinning region, has been observed for the lead doped samples. The value of TCR for the pure Bi2212 single crystal is 20K. A similar shift of TCR has been reported for the irradiated Bi2212 single crystals, where the samples had strong pinning. This indicates that an enhanced pinning of vortex lines has been observed in the lead doped samples. From X ray measurements, a reduction of c axis parameter has been observed in lead doped samples, as compared to pure and iron doped samples. Additionally, an increased c axis conductivity has been reported for the lead doped samples. Therefore, the increased pinning is due to the strong coupling of pancake vortices which arises from the enhanced c axis conductivity in the lead doped samples