MEASUREMENT OF THE DISTRIBUTION OF TRAPPED FLUX IN A JOSEPHSON-COUPLED MEDIUM BY NEUTRON POLARIMETRY

The patterns of the distribution of internal fields in a ceramic sample of Y-Ba-Cu-O are obtained by means of neutron-polarized beam scanning. Information about the Josephson screening currents is extracted from these patterns for fields smaller than the lower critical field of the grains. Dependences of the screening currents with respect to the internal field B near the sample boundary and in its depth are distinguished from one another for field levels lying below a certain field. The maximum values of screening current densities correspond to critical current densities. They depend linearly on the field B. A zero-field critical density J0 and a characteristic length scale of the trapped flux are extracted from this dependence. The quantities of the above-mentioned parameters at 60 K are approximately 70 A/cm2 and 200-600 mum, respectively.