Experimental and theoretical approaches on magnetoresistivity of Lu-Doped Y-123 superconducting ceramics

摘要

This study discusses the change of the flux pinning mechanism, electrical and superconducting properties of Lu added YBa_2Cu_3O_(7-δ) bulk superconducting ceramics prepared by the liquid ammonium nitrate and derivatives at 970 ℃ for 20 h by means of magnetotrans-port measurements conducted in the magnetic filed range from 0 to 6 kG. The critical transition (both T_c~(onset) and T_c~(offset)) temperatures, residual resistivity (ρ_0), residual resistivity ratios (RRR), irreversibility fields (μ_0H_(irr)), upper critical fields (μ_0H_(c2)), penetration depths (λ) and coherence lengths (ξ) of the Yba_2Lu_xCu_3O_(7-δ) materials are evaluated from the magnetoresistivity curves. The resistivity criteria of 10 and 90 % normal-state resistivity serve as the important parameters for the description of the irreversibility and upper critical fields, respectively. Moreover, ρ_0, μ_0H_(irr)(0) and μ_0H_(c2)(0) values of the bulk samples are theoretically calculated using the extrapolation method at absolute zero temperature (T = 0 K). Likewise, the ξ And λ. Values are inferred from μ_0H_(irr)(0) and μ_0H_(c2)(0) values obtained, respectively. At the same time, activation energies of the samples studied are determined from thermally activated flux creep (TAFC) model. The results obtained indicate that the pinning mechanism, electrical and superconducting properties of the samples enhance with the increment of the Lu addition up to level of 0.1 wt% beyond which these properties start to deteriorate systematically and in fact reach the local minimum points for the sample doped with 0.9 wt% Lu due to the degradation of pinning ability, density, crystallinity and connectivity between grains. Similarly, the presence of the magnetic field results in the reduction of these properties as a consequence of the decrement in the flux pinning in the samples prepared. Namely, the maximum T_c~(onset) of 94.6 K and T_c~(offset) of 92.5 K are observed for the sample doped with 0.1 wt% Lu whereas the minimum temperature values are obtained to be about 71.2 and 50.3 K for the sample doped with 0.9 wt% Lu. In fact, the T_c~(offset) value decreases to 20.5 K with the increment in the applied magnetic field up to 6 kG. Besides, the flux pinning energies of the samples decrease with ascending applied magnetic field. The pure sample has the flux pinning energy of 15,211 K at zero field while the maximum and minimum values are found to be about 16,722 K and 2,058 K for the samples added with 0.1 and 0.9 wt% Lu, respectively. The U_0 of 158 K at 6 kG applied magnetic field is obtained for the latter sample, showing that this sample exhibits much weaker flux pinning, lesser crystallinity and connectivity between grains compared to the other samples produced as a result of the stronger pair-breaking mechanism. The dissipation mechanism is also discussed by means of the magnetic field dependence of the activation energy.