CONTRIBUTION OF IBA TECHNIQUES TO THE STUDY OF YBACUO THIN-FILMS WITHANOMALOUS C-AXIS LATTICE-PARAMETER

In this paper we report further results concerning the expansion of th e c-axis lattice parameter of superconducting YBaCuO thin films. Our f ilms were deposited, on MgO single crystals heated to about 750-degree s-C, by inverted cylindrical magnetron sputtering of a stoichiometric Y1Ba2Cu3O7 target, at relatively high oxygen partial pressure (p(O2) = 0.25 mbar) and then cooled to room temperature at the same p(O2). Com pared with films prepared in the same conditions, but cooled at high o xygen pressure (1 atm), the c-axis lattice parameter shows a significa nt increase DELTAc. The films prepared at low p(O2) have unexpected hi gh critical transition temperatures (T(c) almost-equal-to 88 K). Howev er, according to the relationship between the c-axis lattice parameter and the oxygen stoichiometry, these films would be oxygen deficient. The oxygen content measurements, performed by nuclear reaction analysi s and Rutherford backscattering spectrometry, show that the films are fully oxygenated (i.e. O6.8-O7.0), independently of the oxygen pressur e during cooling. Therefore, for the films cooled at low p(O2), the us ual relationship between the c-axis lattice parameter and the oxygen c ontent does not hold, showing the anomalous expansion of the c-axis la ttice parameter. The c-axis expansion is not due to oxygen deficiencie s at the chain sites, but it seems rather that the films grow with an oxygen content higher than that expected by thermodynamical considerat ions, because of the presence of atomic oxygen during the film formati on. In order to study the effect of the substrate, a series of YBaCuO thin films were prepared in the same conditions, but using LaAlO3 as s ubstrate. The results also show a significant difference DELTAc in the c-axis lattice parameter between the films prepared at low and high p (O2). While DELTAc is the same for films deposited on MgO and LaAlO3, the oxygen content deduced from the c-axis lattice parameter is shifte d to higher oxygen stoichiometries in the case of LaAlO3 substrates. T he results are discussed.