In situ characterisation of Sr₂FeMoO₆ films prepared by pulsed laser deposition

Abstract
Thin epitaxial films of the ferromagnetic double perovskite Sr₂FeMoO₆ (SFMO) have been prepared by pulsed laser deposition on SrTiO₃(100) substrates under different oxygen pressures. The surface electronic structure of the as-deposited films has been analysed by XPS without exposure to the atrnosphere. The films consist mainly of the c-axis oriented SFMO phase over a wide range of oxygen pressure from 10^-8 to 10^-4 mbar. However, the analysis of the films by XPS and XRD revealed the presence of different secondary phases depending on the preparation conditions. The films prepared under ultra-high vacuum conditions show the presence of metallic Fe. The films prepared under pure oxygen gas flow at pressures above 10^-6 mbar did not show any trace of metallic Fe, but Fe₃O₄ phase. In these films XPS analysis revealed the formation of the SrFeO3 phase (SFO), or another non reported Sr-containing phase, which is not observed by XRD. At higher oxygen pressures above 10^-4 mbar there is a clear formation of SrMoO₄ phase (SMO). In conclusion, a ultra-high vacuum atmosphere favours the formation of metallic Fe, whereas the increase in oxygen pressure during deposition induces the decomposition of the SFMO phase to SFO and SM0 phases. The conditions for the preparation of pure SFMO phase have not been attained so far, which would lie within a very narrow oxygen pressure window. Our best films (with no any other ferromagnetic phase than SFMO) show a saturation magnetisation of about 1 µ_B/f.u. at 10 K, and T_c close to 200 K. The resistivity of the films has a metallic-like behaviour and remains almost constant in the temperature range fiom 10 to 350 K to about 5 to 8 10^-4 Ω cm. It was measured a magnetoresistance of about -10% at 10 T and 10 K, which coincides with the values reported in the literature for SFMO films.