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/spl times/10m m/spl times/1 mm) was field-cooled in liquid nitrogen. After the applied field was turned off, a field distribution in the area of 10 mm/spl times/10 mm was mapped by scanning the sample surface at intervals of 0.5 mm 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/sub c/ is independent of the field strength, a field distribution generated by square current loops was calculated numerically. The intragranular J/sub c/ was estimated to be 2.6/spl times/10/sup 4/ A/cm/sup 2/ (77 K, OT) 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 intragranular J/sub c/ for a superconductor without destroying the samples.<
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