FUNDAMENTAL AND HARMONIC AC SUSCEPTIBILITY RESPONSE OF MOCVD YBA2CU3O7 THIN-FILMS - MODEL OF FLUX LINE BEHAVIOR

A.c. susceptibility is widely used as a non-destructive method to char acterize the basic properties of thin film superconductors. In this st udy we present a model that describes the a.c. susceptibility in thin films, in the region near T(c), where high T(c) superconducting materi als have a non-linear response to even very small applied magnetic a.c . fields. Our model is an extension of the critical state model of Bea n and takes into account the existence of thermally activated flux flo w of vortices in the transition region. Measurements of the a.c. susce ptibility as a function of temperature up to the ninth harmonic were c arried out on YBa2Cu3O7 thin films prepared by metal-organic chemical vapour deposition (MOCVD). The a.c. field (1.6-800 A m-1) was applied perpendicular to the film plane. The experimental results show very go od agreement with the proposed theoretical model and suggest that pinn ing and thermally activated flux flow play an essential role in the ma gnetic response of single crystal YBa2Cu3O7 thin films.