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 mmx10mmx1mm) was field-cooled in liquid nitrogen. After the applied field was turned off, a field distribution in the area of 10mmx10mm 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 J(c) is independent of the field strength, a field distribution generated by square current loops was calculated numerically. The intragranular J(c) was estimated to be 2.6x10(4) A/cm(2) (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 J(c) for a superconductor without destroying the samples.