MATERIAL PARAMETERS OF YBA2CU3OY IN THE KIM-ANDERSON CRITICAL-STATE MODEL

The magnetic response of a melt-processed YBa2Cu3Ogamma under an exter nal ac magnetic field has been investigated in the framework of the Ki m-Anderson critical-state model, J(c)(B-i)= k/(B-0 + \B\)(i), where k and B-0 are the material parameters, and B-i is the local internal mag netic-flux density. The magnetization curves obtained from direct meas urements using a superconducting quantum interference device and from the Fourier series of higher-harmonic complex susceptibilities were si mulated with the magnetization equations based on the model and the te mperature-dependence of k and B-0 were studied. The k(T) behavior indi cates that at lower temperatures, weak point defects (oxygen vacancies ) play a predominant role as the pinning center, but at higher tempera tures below T-c, a contribution from the normal-state Y2BaCuO5 particl es becomes important. The temperature-dependence of B-0 was found to b e B-0 alpha (1 - T/T-c)(3/2), suggesting that B-0 is the quantity clos ely correlated to the irreversibility field. (C) 1998 Elsevier Science B.V. All rights reserved.