Magnetic flux profiles inBi1.2Pb0.3Sr1.5Ca2Cu3OyandNdBa2Cu3O7−δsuperconductors and a simulation by critical-state models

Abstract

The features of the flux profiles recorded for the sintered and press-sintered ${\mathrm{Bi}}_{1.2}{\mathrm{Pb}}_{0.3}{\mathrm{Sr}}_{1.5}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (BSCCO), and sintered and melt-textured ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (NdBCO) superconducting samples with widely different microstructures are analyzed. From the slope of the flux profiles (at ${H}_{\mathrm{ac}}\ensuremath{\rightarrow}0$) recorded for various dc fields, the intergranular ${J}_{\mathrm{ci}}$ ${(H}_{\mathrm{dc}})$ for the BSCCO samples is determined, whose field dependence is found to follow Kim's model. Using the fit parameters thus obtained, the flux profiles for different ${H}_{\mathrm{dc}}$ could be simulated very well including features like nonlinearity at low fields, the presence of a peak, and the oscillatory behavior. The flux profile of the sintered NdBCO recorded with zero ${H}_{\mathrm{dc}}$ could also be simulated well using Kim's critical-state model. An analysis of the intragranular flux profile determined from the measured profile at high fields suggests a wide distribution of ${H}_{g}^{*},$ (the full penetration field of the grain) and thereby of the ${J}_{\mathrm{cg}}.$ This result is in agreement with the broad transition observed in the ac susceptibility of this sample, attributed to the presence of ${\mathrm{Nd}}_{1+x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ precipitates that modify the local ${J}_{\mathrm{cg}}$ values drastically. All the sintered specimen show nearly 45% of flux penetration (into the sample) at low enough fields ${(H}_{\mathrm{dc}}<40\mathrm{Oe}),$ while the flux entry into the melt-textured NdBCO is $<3%.$ At high dc fields (4--8 KOe) the profile exhibits two distinct slopes that are associated with a flux entry initially into the microcracks and then into the grains, from the microstructural considerations.