MAGNETIC-FLUX PROFILES IN BI1.2PB0.3SR1.5CA2CU3OY AND NDBA2CU3O7-DELTA 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-textur ed NdBa2Cu3O7-delta (NdBCO) superconducting samples with widely differ ent 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 f ound to follow Kim's model. Using the fit parameters thus obtained, th e flux profiles for different H-dc could be simulated very well includ ing features like nonlinearity at low fields, the presence of a peak, and the oscillatory behavior. The flux profile of the sintered NdBCO r ecorded with zero H-dc could also be simulated well using Kim's critic al-state model. An analysis of the intragranular flux profile determin ed from the measured profile at high fields suggests a wide distributi on of H-g, (the full penetration field of the grain) and thereby of t he J(cg). This result is in agreement with the broad transition observ ed in the ac susceptibility of this sample, attributed to the presence of Nd1+xBa2-xCu3O7-delta precipitates that modify the local J(cg) val ues drastically. All the sintered specimen show nearly 45% of flux pen etration (into the sample) at low enough fields (H-dc<40 Oe), while th e 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 w ith a flux entry initially into the microcracks and then into the grai ns, from the microstructural considerations.