AC Magnetic Flux Profile and Critical Current Density Obtained by Campbell\´s Method in Polycrystalline ${\rm MgB}_{2}$

In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB₂ bulks prepared by a modified PIT (powder in tube) method. Campbell´s method was applied for the purpose of acquiring the penetrating AC magnetic flux profile and the characteristic of AC magnetic field vs. penetration depth from the specimen´s surface. A computer simulation on the penetrating AC flux profile in MgB₂ bulks with certain amount of randomly distributed voids, oxidized grains and other faults was also carried out. The global critical current density obtained by Campbell´s method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various kinds of faults and the lack of the electrical connectivity between grains. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB₂ specimens can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results.