EXPONENTIAL CRITICAL-STATE MODEL APPLIED TO AC SUSCEPTIBILITY DATA FOR THE SUPERCONDUCTOR YBA2CU3O7-DELTA

We derive new expressions for the average magnetization loops, M(H), b ased on the exponential critical state model. The components chi' and chi '' of the complex susceptibility are calculated and an algorithm t o fit ac susceptibility data is discussed. This algorithm is employed to study the intergranular response chi'(H-m) and chi ''(H-m) measured for two samples of YBa2Cu3O7-delta as a function of the ac field ampl itude 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 calculat ed components chi' and chi '' are obtained using an algorithm that inv olves two free parameters, the full penetration field, H-p, and the sa mple quality factor, p. An interesting result for the melt-textured sa mple is the observation of a step in chi'(H-m) curves at very low H-m, possibly associated with grain clustering. (C) 1996 American Institut e of Physics.