Observation of Flux Distribution on the Surface of MPMG-processed YBCO

A macroscopic observation of a magnetic flux distribution on the surface of a superconducting bulk material was performed. First, a single-domained thin rectangular YBCO sample (10 mm×10mm×1mm) was field-cooled in liquid nitrogen. After the applied field was turned off, a field distribution in the area of 10mm×10mm was mapped by scanning the sample surface at intervals of 0.5mm using a Hall probe fixed to a XY-stage. The obtained field distribution was visualized into a three-dimensional distribution map and a two-dimensional contour graph. The overall shape of the field distribution had a four-fold symmetry reflected by the sample geometry. Next, assuming that the critical current density Jcis independent of the field strength, a field distribution generated by square current loops was calculated numerically. The intragranular Jcwas estimated to be 2.6×104A/cm2 (77K, 0T) by fitting the calculated profile to the measured profile. Finally, it is found that the measurement of field distributions on the surface of a superconductor using a Hall probe is applicable to an estimation of an ntragranular Jcfor a superconductor without destroying the samples.