Abstract
Superconducting YBaCuO thin films with various oxygen contents were prepared by inverted cylindrical magnetron sputtering of a stoichiometric YBaCuO target on a MgO single crystal heated to about 750°C. The samples were cooled to room temperature either (i) at p o 2 = 0.25 mbar or (ii) at p o 2 = 1 atm.As-grown samples were characterized by ϱ( T) measurements ( T c, Δ T c). The film composition was determined by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, nuclear reaction analysis (NRA) and X-ray diffraction (XRD). The oxygen content, directly measured by RBS and NRA, were compared to that deduced from the c-axis lattice parameter. While for the (i) samples a difference is observed between RBS and XRD results, the latter being inconsistent with both T c and the oxygen content, for the (ii) samples, a good agreement is found. The copper oxidation state was controlled by X-ray absorption spectroscopy (XAS) of the CuL 2,3 edge operated in the total electron yield mode. The results show that for the as-grown YBaCuO films only the Cu(II) state is present. The effect of long temperature annealings in 18O 2 and 16O 2 atmospheres ( T = 650°C, t = 19 h) was also studied. The results are discussed.
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