Critical Current Density Estimated from the Flux Density Distribution in Bi2Sr2CaCu2Ox Polycrystals

The flux density distributions in sintered Bi 2 Sr 2 CaCu 2 O x high-T c superconducting specimens were observed by means of a magnetooptic method using iron garnet films. The samples were formed into discs with thicknesses of 60 to 600 ?m and a diameter of 1.5 mm. SEM images showed that specimens consisted of grains with diameters of order 300 ?m. Flux penetrated inhomogeneously into specimens of thickness smaller than the grain diameters. However, flux penetration became homogeneous as the thickness was increased. Preferential flux penetration at grain boundaries was not observed in all specimens. The value of the critical current density, estimated from the flux penetration distance using Bean's model applied to discshaped specimens, was about 104 A/cm2 at 30K, which is nearly equal to the value estimated from the magnetization assuming that the shielding current flows uniformly through the entire specimen.