Sr2FeMoO6 oxides with double perovskite structure1 are half metallic ferromagnetic with elevated Curie temperature (Tc > 400 K) and present large magnetoresistance at room temperature2. In the last few years these compounds stimulated large interest as potential electrodes in magnetic tunnel junction and innovative spintronic materials. On the other hand these peculiar magnetic and electronic properties attracted the fundamental research in the field of heavily correlated electron systems. Their crystallographic structure derives from the ideal cubic perovskites: it is made of weakly tetragonal distorted cubic units with the Sr ions located at the center and the Fe(Mo)O6 sharing the cube edges. The regular alternation of Fe and Mo along the lattice edges (chemical order) strengthens the magnetoresistance while increasing the miss-site defects (chemical disorder) weakens the magnetoresistance. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been suggested to explain the metallic state. Chemical disorder reduces the DE coupling and gives rise to tunnel type conductibility. The most striking features in this picture is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply huge exchange interactions despite the very long distance among Fe ions (~8 Å)3. This work proposes a detailed EXAFS study on two Sr2FeMoO6, a fully chemically ordered and a fully chemically disordered sample, in order to get insights on the micro-structural origin of their magnetotransport properties. X-ray absorption experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. In order to have a complete and exhaustive description of local order Fe K-edge (~7112 eV), Mo-K edge (~20000 eV) and W-LIII (~10200 eV) edge EXAFS data were analysed. The complete EXAFS spectra were refined taking into account for single as well multiple scattering contributions till about 6 Å. This permitted to probe the relative arrangement of Fe and Mo ions. The coherence of the structural models, as resulting from the three edges analysis strengthens and gives confidence on the results. The main result is that either ordered and disordered compounds show very similar chemically ordered local structure, i.e. the presence of Fe-Fe and Mo-Mo coordination is weak (almost negligible) in both the samples. These finding contrasts with the information from X-ray diffraction analysis, i.e. a high ordered (order parameter ~ 90%) and a completely disordered sample. Our results suggest that the long-range chemically disordered sample is made of small (20-30 Å) chemically ordered clusters. Such a small size would produce weak and broad superlattice reflections easily confused with the background in standard diffraction patterns.
Liscio, F., Bardelli, F., Meneghini, C., Mobilio, S., Ray, S., Sarma, D.D. (2004). Local structure and magnetotransport in Sr2FeMoO6 double perovskite compounds: an EXAFS study.
Local structure and magnetotransport in Sr2FeMoO6 double perovskite compounds: an EXAFS study
MENEGHINI, CARLO;MOBILIO, Settimio;
2004-01-01
Abstract
Sr2FeMoO6 oxides with double perovskite structure1 are half metallic ferromagnetic with elevated Curie temperature (Tc > 400 K) and present large magnetoresistance at room temperature2. In the last few years these compounds stimulated large interest as potential electrodes in magnetic tunnel junction and innovative spintronic materials. On the other hand these peculiar magnetic and electronic properties attracted the fundamental research in the field of heavily correlated electron systems. Their crystallographic structure derives from the ideal cubic perovskites: it is made of weakly tetragonal distorted cubic units with the Sr ions located at the center and the Fe(Mo)O6 sharing the cube edges. The regular alternation of Fe and Mo along the lattice edges (chemical order) strengthens the magnetoresistance while increasing the miss-site defects (chemical disorder) weakens the magnetoresistance. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been suggested to explain the metallic state. Chemical disorder reduces the DE coupling and gives rise to tunnel type conductibility. The most striking features in this picture is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply huge exchange interactions despite the very long distance among Fe ions (~8 Å)3. This work proposes a detailed EXAFS study on two Sr2FeMoO6, a fully chemically ordered and a fully chemically disordered sample, in order to get insights on the micro-structural origin of their magnetotransport properties. X-ray absorption experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. In order to have a complete and exhaustive description of local order Fe K-edge (~7112 eV), Mo-K edge (~20000 eV) and W-LIII (~10200 eV) edge EXAFS data were analysed. The complete EXAFS spectra were refined taking into account for single as well multiple scattering contributions till about 6 Å. This permitted to probe the relative arrangement of Fe and Mo ions. The coherence of the structural models, as resulting from the three edges analysis strengthens and gives confidence on the results. The main result is that either ordered and disordered compounds show very similar chemically ordered local structure, i.e. the presence of Fe-Fe and Mo-Mo coordination is weak (almost negligible) in both the samples. These finding contrasts with the information from X-ray diffraction analysis, i.e. a high ordered (order parameter ~ 90%) and a completely disordered sample. Our results suggest that the long-range chemically disordered sample is made of small (20-30 Å) chemically ordered clusters. Such a small size would produce weak and broad superlattice reflections easily confused with the background in standard diffraction patterns.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
