Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Uprety, Krishna Kamal, Magnetic hysteresis and relaxation in Bi2212 single crystals doped with iron and lead, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2002. http://ro.uow.edu.au/theses/1896
Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviour for Bi2.iSr1.9Ca1.0(Cu1-yFey)2O8 + δ single crystals with Fe concentration y = 0, 0.005, 0.016 and 0.022 and for Bi(2-x)PbxSr2CaCu2O8 + δ single crystals with Pb content x = 0.34. Here y = 0 is pure Bi2212 single crystal. The main objective of this thesis was to study mechanisms for the dramatically improved vortex pinning behaviour in Bi2212 single crystals with heavy Pb doping. It is argued that the strong vortex pinning behaviour in the heavily Pb doped Bi2212 single crystal came from improved c-axis conductivity i. e. a reduction in the resistivity anisotropy parameter. In heavily Pb doped Bi2212 single crystals, Pb resides between CuO2 planes and thus reduces the anisotropy parameter significantly. Two microstructures such as Pb rich and Pb poor lamellar plates are also observed in heavily Pb doped Bi2212 single crystals. The Pb content in our crystal was relatively low. Therefore, these lamellar plates are not likely to dominate the vortex dynamics in our heavily Pb doped Bi2212 single crystal. However, no significant improvement in the vortex pinning behaviour has been observed in the Bi2212 single crystals with Fe doping. In iron doped Bi2212 single crystals, Fe substitutes Cu in the CuO2 planes and does not decrease the anisotropy parameter.
A comparative study of the temperature dependence of the field Hpeak(T), at which the second magnetization peak occurs in |M(H)|, is made for pure, Fe doped and heavily Pb doped Bi2212 single crystals. The second magnetization peak, persisting close to the critical temperature Tc is observed for heavily Pb doped Bi2212 single crystals. The second peak is not enhanced in Bi2212 single crystals after Fe doping. Pure and Fe doped Bi2212 single crystals have second peaks between temperatures 20 and 40 K. The peak field Hpeak for all the crystals is observed to decrease with increasing temperatures, T. Comparative studies are also made on the temperature dependence of fields Hmin(T) and Hinfl(T) where M(H) has a minimum between the first and second peaks at Hmin(T), and it has an inflection point on the low-field-side of the second peak at Hinfl(T). In pure and Fe doped Bi2212 single crystals, a pronounced peak in the derivative |dM/dH| is observed corresponding to Hinfl(T), andHinfl(T) is independent of temperature T. We relate this peak to the field Hdis(T), an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. A minimum in the normalized relaxation rate S (H) is observed at Hinfl, indicating two different flux-creep mechanisms above and below that field, and two different solid vortex phases. In heavily Pb doped single Bi2212 crystals, Hinfl(T) was observed to decrease with increasing T. However, in pure and Fe doped Bi2212 single crystals, Hinfl(T) was observed to be temperature independent. It is concluded that the negative slope of Hinfl(T) in heavily Pb doped Bi2212 crystals is related to the enhanced c-axis conductivity caused by the Pb sitting between the CuO2 layers and causing 3D vortex lines, while in Fe doped Bi2212 crystals the defects sit on the CuO2 planes and thus do not enhance the coupling between pancake vortices. The field Hmin is observed to increase with temperature for pure and Fe doped Bi2212 single crystals. However, in heavily Pb doped Bi2212 single crystals, Hmin is observed to decrease with T.
An increase in the crossover temperature TCR, a temperature separating two different pinning regimes, has been observed with heavy Pb doping in Bi2212 single crystals. Discussion on these pinning regimes is presented in section 5.3. The increase of TCR may be interpreted as due to improved interlayer Josephson coupling of 2 D vortices in Bi2212 crystal after heavy Pb doping. Heavily Pb doped crystal has TCR = 35K for the field that is within the second peak in its magnetic hysteresis loop. The TCR in Fe doped crystal, however, did not change with Fe doping. For pure Bi2212 and Fe doped Bi2212 crystals, TCR =19K. The observed different results above or below TCR in the current-voltage curve and in the effective activation energy have also suggested the different pinning regimes.
Comparative studies of the field dependence of the normalized critical current density, Jc|Jc0 have also been made for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Below 20 K, pure and heavily Pb doped crystals showed a weak field dependence of Jc|Jc0 indicating strong vortex pinning behaviour. Above 20 K, all other crystals, except heavily Pb doped crystals show a strong field dependence of Jc|Jc0.