Direct Observation of Magnetic Flux Behavior in High-Tc Oxide Superconductors

The magnetic flux behavior in YBa
2
Cu
3
O
x
specimens under various applied fields was observed at 10K and 77K using the Faraday effect of an iron garnet film with stripe magnetic domains. The magnetic flux distribution was determined by optical observation of the domain structure in the iron garnet film placed on the sample. The magnetic flux gradients in a crystal fabricated by the Melt-Powder-Melt-Growth (MPMG) method and in a thin film were in good agreement with those predicted by the critical state model at both 10K and 77K. On the other hand, in a sintered sample the flux penetrated through the superconducting weak-link regions, and no flux gradient was observed at 77K.