Metal to insulator transition and local structure in Sr2FeMo1-xWxO6 ,

Sr2FeMoO6 oxides1 and several related compounds of general formula A2B’B”O6 (A: alkaline earth, B’, B”: heterovalent transition metals) exhibit half-metallic magnetism and high spin polarization at the Fermi level. They have recently stimulated a large interest in the field of heavily correlated electron systems while they attracted the applied research as promising innovative materials in magnetic tunnel junctions and spintronic devices. Nevertheless a theory completely understanding these special properties is still lacking. The Sr2FeMoO6, for example, is half metallic ferromagnetic (FM) with elevated Curie temperature (Tc > 400 K) and presents large magnetoresistance at room temperature2. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been proposed to explain the metallic state and the magnetoresistance. The strangeness of this model is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply a huge exchange interaction despite the very long distance among Fe ions (~8 Å)3. Half metallic properties on Sr2FeMoO6 sensitively depends on Fe-Mo cation order on the B site of the A2B’B”O6 double perovskite structure. Doping with W substituting for Mo ions in Sr2FeMo1-xWxO6 improves the B-site Fe-Mo/W ordering4. The Sr2FeWO6 is an antiferromagnetic (AFM) insulator, thus the doped system undergoes to a metal to insulator (MI) transition and AFM-FM transition as a function of composition. The doped compounds remain (half-) metallic with high Tc >> RT (Room temperature) on a wide range of W concentrations (0<x<0.7). The nature of the transition is still matter of debate and different possible scenarios are proposed: i) a collective valence transition from Fe3+-(W,Mo)5+ in the metallic state to Fe2+-(W,Mo)6+ in the insulating state; ii) a percolation process in which the nominal valence for W and Mo remain 6+ and 5+ respectively for every compositions and conductivity is given by metallic Mo-rich clusters embedded in the insulating W-rich regions. Finally a combination of the two effects has been suggested5.Detailed knowledge of the microstructure across the metal to insulator transition must help in understanding the origin of magnetotransport properties of these compounds. This study reports the evolution of Sr2FeMo1-xWxO6 local structure as a function of composition as resulting from accurate analysis of Sr, Fe, Mo, and W x-ray absorption fine structure (XAFS) data. XAFS experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. The comparative analysis of near edge regions (XANES) of Fe (K-edge), Mo (K-edge) demonstrates that the local structure around the Fe and Mo ions changes steeply across the M-I transition. On the contrary the local structure around W remains almost unchanged as a function of x. The analysis of the extended regions of the spectra (EXAFS) was performed taking into account for single and multiple scattering contributions till about 5 Å from the absorber. Accurate microstructure evolution as a function of composition is reported. The structural results suggest a step-like structural transition accompanying the MI transition as a function of W doping.