Exponential critical state model applied to ac susceptibility data for the superconductor YBa₂Cu₃O_7−δ

We derive new expressions for the average magnetization loops, M(H), based on the exponential critical state model. The components χ^′ and χ^″ of the complex susceptibility are calculated and an algorithm to fit ac susceptibility data is discussed. This algorithm is employed to study the intergranular response χ^′(H_m) and χ^″(H_m) measured for two samples of YBa₂Cu₃O_7−δ as a function of the ac field amplitude H_m. One sample is a porous sintered cylinder and the other is a very dense melt‐textured bar. In both cases good fits of the calculated components χ^′ and χ^″ are obtained using an algorithm that involves two free parameters, the full penetration field, H_p, and the sample quality factor, p. An interesting result for the melt‐textured sample is the observation of a step in χ^′(H_m) curves at very low H_m, possibly associated with grain clustering.