Paper EM+MI-MoA4
X-ray Photoemission Spectroscopy of Sr₂FeMoO₆ Film Stoichiometry and Valence State

Monday, October 18, 2010, 3:00 pm, Room Dona Ana

We have used a combination of monochromatized X-ray photoemission spectroscopy and Rutherford backscattering spectrometry to determine bulk compound stoichiometry and charge state of Sr₂FeMoO₆ epitaxial films. Complex oxides are an attractive class of materials for the creation of novel electronic devices due to their diverse properties. Sr₂FeMoO₆ (SFMO) is one such material due to the discovery of low field colossal magnetoresistance at room temperature and its half metallic character making it suitable in applications from magnetic read heads and nonvolatile magnetic random access memory to spin injectors in spintronic devices. But in order to create functional devices from such materials, it is necessary to have complete control over the crystal stoichiometry and ordering. Complicating this is that many of these materials, SFMO included, are not point compounds and small changes in stoichiometry can lead to the formation of other material phases which would strongly influence the materials properties.

Using standard samples with known stoichiometries of Fe₂O₃ and SrMoO₃ we were able to obtain sensitivity factors relative to oxygen for Sr, Fe, and Mo. In conjunction with RBS measurements, these sensitivities allowed us to determine sensitivity factors specific to SFMO for measuring stoichiometry. These studies also identify an optimum ion sputtering process for removing surface contaminants and an SrMoO4 overlayer while avoiding preferential sputtering of film constituents or alteration of their characteristic valence states. For Sr₂FeMoO₆, low energy (500 eV), glancing incidence Ar⁺ sputtering for short (tens of seconds) periods is successful in achieving stoichiometric compositions and characteristic charge states of the film constituents. Furthermore, the adventitious surface carbon content dropped from 10% to less than 2% in the first 15s of sputtering. With longer sputtering times we observed the oxygen content to fall below its stoichiometric value (60%) and continue to fall with further sputtering indicating the preferential removal of oxygen from the film. We also observed the emergence of a metallic component to the iron and molybdenum valence states after 120s of sputtering indicating damage to the crystal lattice. Thus XPS reveals both a different surface overlayer stoichiometry as well as a ‘window’ of sputter time that provides bulk stoichiometry. The evolution of composition and valence state with sputtering provides a guide to measure stoichiometry and charge state of SFMO and complex oxide thin films in general.