Use of RBS and Raman spectroscopy to study oxygen mobility in YBaCuO thin films by 18O tracing experiments

Abstract

The 18O tracer technique is particularly well adapted to study the mobility of oxygen and to evidence the presence of oxygen sites of different energies. The annealing experiments in a pure oxygen atmosphere enriched to > 98% 18O were performed for 19 h at 200 mbar and at three different temperatures: 350, 450 and 650°C. At 350, and at 450°C the orthorhombic phase was totally preserved (Y 1Ba 2Cu 3O 7), whereas at 650°C the nominal oxygen composition was O 6,4 and corresponds to the orthorhombic-te-tragonal transition. All the experiments were performed using YBaCuO samples deposited on MgO substrate by laser ablation or cathodic sputtering. The samples were characterized by X-rays and T c measurements. Their composition and structure were studied by RBS, NRA and ion channeling. The 18O distribution was studied using the resonant nuclear reaction at 629 keV. The localization in the YBaCuO structure of the 18O atoms was studied using Raman spectroscopy. It was found that the 18O uptake at 350°C leads to the formation of a thin surface layer (15 nm) enriched to about 30% whereas the volume enrichment appears to be low (3.5%) and quasi-constant. This result suggests that the 18O exchange at 350°C is controlled by the diffusion. The reoxidation in 18O atmosphere of partially reduced sample is shown to be much quicker than that observed by direct exchange. It was found that the 18O exchange at 450°C leads to a quasi-constant 18O profile (enrichment to 70% was observed after annealing during 19 h). The Raman spectroscopy showed that the annealing at 450°C induced equivalent exchange in CuO 2 planes and of the bridging oxygen. The annealing at 650°C leads to a total exchange between 16O and 18O.