Magnetic flux profiles in Bi1.2Pb0.3Sr1.5Ca2Cu3Oy and NdBa2Cu3O7-δ superconductors and a simulation by critical-state models

The features of the flux profiles recorded for the sintered and press-sintered Bi1.2Pb0.3Sr1.5Ca2Cu3Oy (BSCCO), and sintered and melt-textured NdBa2Cu3O7-delta (NdBCO) superconducting samples with widely different microstructures are analyzed. From the slope of the flux profiles (at H-ac-->0) recorded for various dc fields, the intergranular J(ci) (H-dc) for the BSCCO samples is determined, whose held dependence is found to follow Kim's model. Using the fit parameters thus obtained, the flux profiles for different H-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-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(cg). This result is in agreement with the broad transition observed in the ac susceptibility of this sample, attributed to the presence of Nd1+xBa2-xCu3O7-delta precipitates that modify the local J(cg) values drastically. All the sintered specimen show nearly 45% of flux penetration (into the sample) at low enough fields (H-dc<40 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.