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Nature of "Disorder'' in the Ordered Double Perovskite Sr2FeMoO6

Wed, 31 Dec 2008 23:00:00 GMT

Nature of "Disorder'' in the Ordered Double Perovskite Sr2FeMoO6 Meneghini, Carlo; Ray, Sugata; Liscio, Fabiola; Bardelli, Fabrizio; Mobilio, Settimio; Sarma, D. D. 2009 Physical Review Letters 103 046403 The degree of B/B' alternate cation order is known to heavily influencethe magnetic properties of A(2)BB'O-6 double perovskites although thenature of such disorder has never been critically studied. Our detailedx-ray absorption fine structure studies in conjunction with synchrotronradiation x-ray diffraction experiments on polycrystalline Sr2FeMoO6samples with various degrees of disorder reveal that a very high degreeof short range order is preserved even in samples with highly reducedlong range chemical order. Based on these experimental results and withthe help of detailed structural simulations, we are able to model thenature of the disorder in this important class of materials and discussthe consequent implications on its physical properties.

Antiferromagnetic-paramagnetic insulating transition in Cr-doped V2O3 investigated by EXAFS analysis

Wed, 31 Dec 2008 23:00:00 GMT

Antiferromagnetic-paramagnetic insulating transition in Cr-doped V2O3 investigated by EXAFS analysis Meneghini, Carlo; Di Matteo, S.; Monesi, Claudia; Neisius, T.; Paolasini, L.; Mobilio, Settimio; Natoli, C. R.; Metcalf, P. A.; Honig, J. M. 2009 Journal of Physics: condensed matter 21 355401 We have performed extended x-ray absorption fine-structure (EXAFS) spectroscopy on a 2.8% Cr-doped V2O3 sample, with the aim of studying its structural evolution in a wide temperature range across the paramagnetic-antiferromagnetic insulating phase transition at T-c. The data were registered with two different set-ups in fluorescence and transmission geometries, for polarized and unpolarized spectra, respectively. Our idea, based on previous experiments reported in the literature, is that extended structural modifications of the nominal trigonal symmetry are present in the paramagnetic insulating phase for several tens of degrees above T-c, involving further-nearest-neighbor vanadium ions. Our data confirm that the paramagnetic insulating phase is not structurally homogeneous in a temperature range of about 30 K around T-c, where local distortions of monoclinic symmetry involving further-nearest neighbors are present. Moreover, the analysis of the absorption profile at Cr K-edge suggests that Cr ions enter the lattice randomly. We finally analyze our findings in light of current theoretical models.

Influence of the interface on the electronic channel switching of a Fe-Ag thin film on a Si substrate

Mon, 24 Aug 2009 22:00:00 GMT

Influence of the interface on the electronic channel switching of a Fe-Ag thin film on a Si substrate Alonso, J.; Fdez-Gubieda, M. L.; Sarmiento, G.; Barandiaran, J. M.; Svalov, A.; Orue, I.; Chaboy, J.; Fernandez Barquin, L.; Meneghini, Carlo; Neisius, T.; Kawamura, N. 2009-08-25 Applied Physics Letters 95 082103 FexAg100-x granular thin films, being 20 < x < 70, were prepared by pulsed laser deposition. The thermal evolution of the electric resistance shows a dramatic drop in the range of 200 < T < 300 K, completely reversible with temperature and associated to a current switching between the granular thin film and its Si substrate. High resolution transmission electron microscopy measurements have revealed an amorphous interface between the thin film and the substrate, and x-ray absorption spectroscopy studies have demonstrated an electronic localization associated to the Fe atoms in this interface, which is intrinsically responsible for the current switching.

In situ synchrotron powder diffraction study of the thermal decomposition of cement-asbestos: preliminary results

Wed, 31 Dec 2008 23:00:00 GMT

In situ synchrotron powder diffraction study of the thermal decomposition of cement-asbestos: preliminary results Gualtieri, A. F.; Lassinantti Gualtieri, Magdalena; Meneghini, Carlo 2009 Zeitschrift für Kristallographie Supplements 30 353-358 The elimination of asbestos-containing materials like cement-asbestos, is an environmental priority. An industrial process for the safe recovery of cement-asbestos slates was recently developed and permits the thermal transformation of asbestos fibres into non-fibrous crystalline phases in a tunnel kiln. Optimisation of the process requires knowledge of the reaction dynamics. Here, time-resolved synchrotron powder diffraction was used to follow the thermal transformation of cement-asbestos. The use of a closed capillary as sample holder allowed to closely resemble the atmospheric conditions found in the industrial reactor. In this preliminary work, we describe the reaction sequence which undergoes cement-asbestos during its thermal decomposition. The excellent time resolution of the collected data allowed the observation of meta-stable phases at non-ambient conditions.

Local structure of Sr2FeMoxW1-xO6 double perovskites across the composition-driven metal to insulator transition

Wed, 31 Dec 2008 23:00:00 GMT

Local structure of Sr2FeMoxW1-xO6 double perovskites across the composition-driven metal to insulator transition Bardelli, Fabrizio; Meneghini, Carlo; Mobilio, Settimio; Ray, Sugata; Sarma, D. D. 2009 Journal of Physics: condensed matter 21 195502 Sr2FeMoO6 oxides exhibit a half-metallic ferromagnetic (HM-FM) ground state and peculiar magnetic and magnetotransport properties, which are interesting for applications in the emerging field of spintronics and attractive for fundamental research in the field of heavily correlated electron systems. Sr2FeWO6 is an insulator with an antiferromagnetic (I-AFM) ground state. The solid solutions Sr2FeMoxW1-xO6 also have peculiar properties-W doping enhances chemical order which allows stabilization of the HM-FM state; as the W content exceeds a certain value a metal to insulator transition (MIT) occurs. The role of W indetermining the physical properties of Sr2FeMoxW1-xO6 systems has been a matter of intense investigation. This work deals with the problem of the structural and electronic changes related to the MIT from a local perspective by means of x-ray absorption spectroscopy (XAS). This technique allows one to probe in detail the local structure and electronic modifications around selected absorber ions (W, Mo, Fe and Sr in our case). The results of XAS analysis in the whole composition range (0 <= x <= 1), in the near edge (XANES) and extended (EXAFS)regions, demonstrate an abrupt change of the local structure around the Fe and Mo sites at the critical composition, x(c). This change represents the microstructural counterpart associated with the MIT. Conversely, the local structure and electronic configuration of W ions remain unaltered in the whole composition range, suggesting indirect participation of W in the MIT.

Diffraction anomalous fine structure study of iron/iron oxidenanoparticles

Wed, 31 Dec 2008 23:00:00 GMT

Diffraction anomalous fine structure study of iron/iron oxidenanoparticles Meneghini, Carlo; Boscherini, Federico; Pasquini, Luca; Renevier, Hubert 2009 Journal of applied crystallography 42 642-648 Diffraction anomalous fine structure is a recently developed techniquewhich can provide a measurement of the local structure of a givenelement in a particular phase or crystallographic site. Most previousinvestigations have applied the technique to bulk solids, thin films,and nano-dots or wires on crystalline substrates. In this paper, thetechnique is applied to highly disordered nanometre-sized Fe/Fe oxidecore-shell nanocrystalline powders, the diffraction patterns of whichexhibit weak and greatly broadened diffraction peaks. Focusing on theoxide shell diffraction peaks, a qualitative analysis of the near-edgespectral region and a quantitative analysis of the extended energyregion are provided; in particular, good quality fittings of theextended range spectra are obtained. The local structure is selectivelyprobed around the tetrahedral and octahedral sites of the oxide shell,finding the presence of a high degree of structural disorder. Thisstudy demonstrates that diffraction anomalous fine structure can now befruitfully applied to nanocrystalline powders.