DETERMINATION OF CRITICAL CURRENT DENSITIES OF YBA2CU3O7-DELTA THIN-FILMS FROM AC SUSCEPTIBILITY MEASUREMENTS

The temperature dependent AC susceptibilities chi(T)=chi' (T) -ichi'' (T) of laser ablated YBa2Cu3O7-delta thin films were measured for different AC fields aligned perpendicular to the film surface. According to the critical-state model calculations of Sun et al. [Phys. Rev. B 44 (1991) 5275], the maximum of the imaginary part of the AC susceptibility chi'' (T) occurs when the amplitude of the applied field H(AC) equals approximately the full penetration field H*. With the relation H* is-proportional-to J(c)l, where l is the thickness of the film, an estimate of the values of the critical current density J(c) at the chi'' peak temperatures has been made. The J(c) values obtained in this way are in good agreement with those determined by AC screening measurements also performed in this study. For a temperature range of about 10 K below T(c), the critical current density J(c)(T) exhibited a temperature dependence J(c)(T) is-proportional-to (1 - T/T(c))n, with n approximately 2.5. By taking J(c)(T) as an input value, we could calculate AC susceptibilities of the films studied. The calculated chi' (T) and chi'' (T) closely resemble the measured curves. The present work has therefore demonstrated that the theoretical calculations of Sun et al. can be used to explain the observed AC susceptibilities of high-T(c) thin films.