Magnetic behaviour of chemically disordered YBa2Cu3O7-delta Fx thin films

We measured the ac magnetic permeability of an oxygen-deficient YBa2Cu3O7-d elta thin film epitaxially grown by Pulsed Laser Deposition on a (100) SrTi O3 substrate, before and after fluorination under diluted NF3, as a functio n of temperature T and amplitude of the applied field. Fluorine apparently occupies empty sites on a and b axes of the perovskite structure, with very different reaction rates. For each fluorine content we determined the widt h of the transition, the field H-1(T) of first flux penetration in the samp le and the peak field H-p(T) giving maximum dissipation at temperature T. T he transition width appears to be related to the difference between fluorin e populations in the two axes. H-p(T) obeys the same power law as in YBa2Cu 3 O(7-delta)A(x) (A = F, Cl) ceramics, but this is not the case of H-1(T). Our results are compatible with a model of chemical disorder generating a r andom distribution of superconducting critical temperatures T-s in the samp le. (C) 2000 Elsevier Science Ltd. All rights reserved.