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Nondestructive inductive measurement of local critical current densities in Bi-2223 thick films

The local critical current density Jc of Bi-2223 thick films (thickness mm) was nondestructively measured using an ac inductive technique previously developed for measuring Jc of large bulk superconductors (Mawatari et al 2003 Appl. Phys. Lett. 83 3972). In this method an ac magnetic field is generated by an ac drive current (I0cosωt) in a small flat coil placed just above the superconducting specimen. When the magnetic fluxes penetrate the superconductor, a third-harmonic voltage (V3cos3ωt) is generated in the coil, and V3 can then be calculated as V3 = (G3ω/Jc)I02, where G3 is a coil coefficient calculated from the size and configuration of the coil. Thus, Jc can be measured in the surface region (typically down to mm beneath the surface) of the bulk superconductor by measuring V3 as a function of I0. In the measurement of Jc of Bi-2223 thick films, in regions with small I0, V3 followed the expected behaviour, , and thus the surface Jc can be measured from the slope of the V3–I0 curves. With larger I0,V3 gradually deviated from this behaviour. With further increase in I0, V3 suddenly changed its curvature, reaching a plateau and then increasing again, due to magnetic-field penetration below the bottom of the film. The threshold current Ith at the change in curvature is nearly proportional to Jcd, and thus the total Jc can be estimated as the average critical current density for the entire thickness. The distribution of Jc in a large superconducting thick film can be measured by scanning the measuring coil over the entire sample surface.