SELECTIVE O-18 LABELING IN A-AXIS ORIENTED YBACUO THIN-FILMS

Superconducting YBaCuO thin films were deposited on SrTiO3 substrate b y inverted cylindrical magnetron sputtering of a stoichiometric target . The composition and the structure of the films were studied by Ruthe rford Backscattering Spectrometry (RBS) and Nuclear Reaction Analysis (NRA), in random and channeling geometries. Annealing treatments, allo wing the selective O-18 --> O-16 exchange in the CuO chain-planes, wer e performed in O-18(2) atmosphere. In order to accelerate the oxygen d iffusion during the annealing, we used, on the one hand, a-axis orient ed films and, on the other hand, we adopted thermodynamical conditions corresponding to an oxygen composition of O6.6 (p(O2) = 0.13 mbar and T = 400-degrees-C). The full oxygenation was accomplished by increasi ng the oxygen partial pressure to p(O2) = 200 mbar at T = 400-degrees- C, during 30 minutes. The absolute O-18 content, determined by NRA, sh owed a partial and a non uniform O-18 --> O-16 exchange. In spite of t he fact that the RBS spectra were identical for different points on th e sample, the NRA results confirmed that the absolute O-18 content var ies from point to point. Moreover, Raman spectroscopy revealed some in homogeneities concerning the oxygen site occupation by the O-18 tracer atoms in the YBaCuO lattice: In the center of the sample, O-18 atoms are present only in die CuO chain-planes, whereas few mm away of the c enter, we found the O-18 atoms also in the CuO2 planes. These results are discussed.