Magnetic field distribution at the surface of field-oriented YBa2Cu3Ox polycrystals and the critical current

Magnetic field distributions at the surface of field-oriented polycrystalline YBa 2 Cu 3 O x disks were observed in a flux penetration process by a magneto-optical method using magnetic films, and intergrain and intragrain critical currents ( J ct and J cg ) were investigated. Three garnet films with various saturation fields were used as magnetic films. The value of J ct was investigated using the garnet film with low H s , from the applied field at which the flux penetrates to the center of a specimen. Using the garnet film with high H s , the magnetic field distribution was observed. The magnetic field has a minimum near the edge and is nearly constant ( H 0 ) at the inner region. It turned out that such a distribution is characteristic of superconductors with weak links at the grain boundary. We prove for the first time that the J cg can be estimated from the difference between the applied field and H 0 .