Abstract
Magnetic-flux distributions B(r) below the lower critical field of the grains in a zero-field-cooled disk and a ring of polycrystalline YBa2Cu3O7−δ are measured at 77 K with a Hall probe. The measured distribution in the disk had a steep gradient on the sample surface, and the distribution in the ring, over the applied magnetic-field range of 1.5–4 mT, was ‘‘W’’ shaped. These results can only be explained consistently by considering the intergrain current, the intragrain currents, and their interaction. It is shown that the magnetic-flux density measured by a Hall probe with an active area much larger than the grain size is not the intergranular magnetic-flux density Binter but μBinter, where μ is effective magnetic permeability (intergranular volume fraction), and dB(r)/dr gives μ times the intergrain (transport) critical current density Jct and not Jct as is assumed by many authors.
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