LOCAL-FIELD-DEPENDENT CRITICAL-CURRENT DENSITY FOR HIGH-TC YBA2CU3O7-DELTA SUPERCONDUCTORS

The intergranular critical-current density J(c) as a function of the local internal field H(i) has been studied by AC susceptibility measurements for a sintered 123-YBCO superconductor at 77 K. Two samples cut from the same pallet are used for this purpose. Our results show that instead of Bean's, Kim's and the power-law models, the exponential-law and the triangular-pulse (linear) critical-state models are appropriate to fit field-dependent AC susceptibility data, so that they can be a good basis for J(c) (H(i)) determination from AC susceptibility measurements. The estimated J(c)(H(i)) curve has a linear low-field part, and the excellent exponential fit in a wide field range reflects a transition from the linear to a power-law-type J(c). This behavior is also found in low-field AC-and DC-magnetic measurements of many other sintered samples. Vortex pinning quantities obtained from Kim's model fit appear to be meaningless.