Substitution site and effects on magnetism in Sr-for-Ca substituted CaBaCo4O7

Under a Creative Commons Attribution (CC BY) license.-- et al.Cationic substitutions in the novel magnetoelectric compound CaBaCo4O7 lead to profound changes in its magnetic and electric behaviors. In this work, we present a structural study of the isovalent substitution Sr-for-Ca in CaBaCo4O7. X-ray diffraction, as well as neutron powder diffraction experiments, are reported for a series of samples Ca1- xSrxBaCo4O7 with 0 ≤ x ≤ 0.10. Special emphasis is given to the identification of the substitution site, as Sr has also been reported to substitute for Ba in this crystal structure. The solubility limit for Sr at the Ca site is shown to be at x ≃ 0.08. The variation of lattice constants with Sr-doping firmly supports the Sr-for-Ca substitution. Rietveld refinements of the Sr-substituted samples are presented, and used as starting point to analyse the local structure around Sr by means of X-ray absorption spectroscopy at the Sr K-edge. Both the near-edge absorption and the extended absorption fine-structure confirm the substitution of Sr for Ca, giving definite support to the proposed nominal formula. In addition, macroscopic magnetization measurements are presented which reveal the striking effects of Sr-substitution over the magnetic landscape of this puzzling compound.This work is part of a research project supported by Agencia Nacional de Promoción Científica y Tecnológica (Argentina), under Grant No. PICT-2011-0752, CONICET under Grant No. PIP 490 2012-2014, and Universidad Nacional de Cuyo. We acknowledge Soleil synchrotron for the beamtime allocation and the MinCyT-Soleil collaboration project for the financial support. We also acknowledge ILL and its staff for the beamtime allocation and technical assistance as well as the MinCyT of Argentina for funding the trip to perform the experiments. G.A. and J.C. thank the CONICET-CSIC program for scientific visits and Grant No. MAT2011-27233-C02-02.Peer Reviewe

Microsecond time-resolved X-ray diffraction for the investigation of fatigue behavior during ultrasonic fatigue loading

International audienceA new method based on time-resolved X-ray diffraction is proposed in order to measure the elastic strain and stress during ultrasonic fatigue loading experiments. Pure Cu was chosen as an example material for the experiments using a 20 kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. A two-dimensional hybrid pixel X-ray detector (XPAD3.2) was triggered by the strain gage signal in a synchronous data acquisition scheme (pump–probe-like). The method enables studying loading cycles with a period of 50 µs, achieving a temporal resolution of 1 µs. This allows a precise reconstruction of the diffraction patterns during the loading cycles. From the diffraction patterns, the position of the peaks, their shifts and their respective broadening can be deduced. The diffraction peak shift allows the elastic lattice strain to be estimated with a resolution of ∼10−5. Stress is calculated by the self-consistent scale-transition model through which the elastic response of the material is estimated. The amplitudes of the cyclic stresses range from 40 to 120 MPa and vary linearly with respect to the displacement applied by the ultrasonic machine. Moreover, the experimental results highlight an increase of the diffraction peak broadening with the number of applied cycles

Substitution site and effects on magnetism in Sr-for-Ca substituted CaBaCo4O7

Cationic substitutions in the novel magnetoelectric compound CaBaCo4O7 lead to profound changes in its magnetic and electric behaviors. In this work, we present a structural study of the isovalent substitution Sr-for-Ca in CaBaCo4O7. X-ray diffraction, as well as neutron powder diffraction experiments, are reported for a series of samples Ca1- xSrxBaCo4O7 with 0 = x = 0.10. Special emphasis is given to the identification of the substitution site, as Sr has also been reported to substitute for Ba in this crystal structure. The solubility limit for Sr at the Ca site is shown to be at x ¿ 0.08. The variation of lattice constants with Sr-doping firmly supports the Sr-for-Ca substitution. Rietveld refinements of the Sr-substituted samples are presented, and used as starting point to analyse the local structure around Sr by means of X-ray absorption spectroscopy at the Sr K-edge. Both the near-edge absorption and the extended absorption fine-structure confirm the substitution of Sr for Ca, giving definite support to the proposed nominal formula. In addition, macroscopic magnetization measurements are presented which reveal the striking effects of Sr-substitution over the magnetic landscape of this puzzling compound

Charge-Density-Waves Tuned by Crystal Symmetry

The electronic orders appearing in condensed matter systems are originating from the precise arrangement of atoms constituting the crystal as well as their nature. This teneous relationship can lead to highly different phases in condensed matter, and drive electronic phase transitions. Here, we show that a very slight deformation of the crystal structure of TbTe$_3$ can have a dramatic influence on the electronic order that is stabilized. In particular, we show that the Charge Density Wave (CDW) developping along the $\vec{c}$ axis in the pristine state, switches to an orientation along $\vec{a}$ when the naturally orthorhombic system is turned into a tetragonal system. This is achieved by performing true biaxial mechanical deformation of a TbTe$_3$ sample from 250K to 375K, and by measuring both structural and electronic parameters with x-ray diffraction and transport measurements. We show that this switching transition is driven by the tetragonality parameter $a/c$, and that the transition occurs for $a=c$, with a coexistence region for $0.9985< a/c < 1.002$. The CDW transition temperature $T_c$ is found to have a linear dependence with $a/c$, with no saturation in the deformed states investigated here, while the gap saturates out of the coexistence region. The linear dependence of $T_c$ is accounted for within a tight-binding model. Our results question the relationship between the gap and $T_c$ in RTe$_3$ systems. More generally, our method of applying true biaxial deformation at cryogenic temperatures can be applied to many systems displaying electronic phase transitions, and opens a new route towards the study of coexisting or competing electronic orders in condensed matter

Controlled biaxial deformation of nanostructured W/Cu thin films studied by X-ray diffraction

The deformation behaviour of 150. nm thick W/Cu nanocomposite deposited on polyimide substrates has been analysed under equi-biaxial tensile testing coupled to X-ray diffraction technique. The experiments were carried out using a biaxial device that has been developed for the DiffAbs beamline of SOLEIL synchrotron source. Finite element analysis has been performed to study the strain distribution into the cruciform shape substrate and define the homogeneous deformed volume. X-ray measured elastic strains in tungsten sub-layers could be carried out for both principal directions. The strain field was determined to be almost equi-biaxial as expected and compared to finite element calculations

Diffraction techniques and vibrational spectroscopy opportunities to characterise bones

From a histological point of view, bones that allow body mobility and protection of internal organs consist not only of different organic and inorganic tissues but include vascular and nervous elements as well. Moreover, due to its ability to host different ions and cations, its mineral part represents an important reservoir, playing a key role in the metabolic activity of the organism. From a structural point of view, bones can be considered as a composite material displaying a hierarchical structure at different scales. At the nanometre scale, an organic part, i.e. collagen fibrils and an inorganic part, i.e. calcium phosphate nanocrystals are intimately mixed to assure particular mechanical properties

Biophysical Characterization and Membrane Interaction of the Two Fusion Loops of Glycoprotein B from Herpes Simplex Type I Virus

The molecular mechanism of entry of herpesviruses requires a multicomponent fusion system. Cell invasion by Herpes simplex virus (HSV) requires four virally encoded glycoproteins: namely gD, gB and gH/gL. The role of gB has remained elusive until recently when the crystal structure of HSV-1 gB became available and the fusion potential of gB was clearly demonstrated. Although much information on gB structure/function relationship has been gathered in recent years, the elucidation of the nature of the fine interactions between gB fusion loops and the membrane bilayer may help to understand the precise molecular mechanism behind herpesvirus-host cell membrane fusion. Here, we report the first biophysical study on the two fusion peptides of gB, with a particular focus on the effects determined by both peptides on lipid bilayers of various compositions. The two fusion loops constitute a structural subdomain wherein key hydrophobic amino acids form a ridge that is supported on both sides by charged residues. When used together the two fusion loops have the ability to significantly destabilize the target membrane bilayer, notwithstanding their low bilayer penetration when used separately. These data support the model of gB fusion loops insertion into cholesterol enriched membranes

Diffusion centrale des rayons X sous incidence rasante. Etude de la morphologie d'agrégats

La diffusion centrale des rayons X en incidence rasante est une technique assez récente qui résulte en fait de la combinaison de deux techniques utilisant les rayons X: la réflectométrie et la diffusion centrale. Elle permet de caractériser, et donc d'obtenir des informations assez précises, sur la morphologie d'agrégats déposés ou recueillis sur des surfaces planes (verre ou monocristal de Si par exemple). Cette technique non destmctive permet des mesures précises quand elle est conduite auprès d'une source de rayonnement synchrotron. A l'aide d'Images Plates, il est alors possible d'étudier l'anisotropie des agrégats et d'en déduire leurs dimensions latérale et verticale. Nous donnerons d'abord des exemples pour des agrégats d'or déposées sur un substrat de silicium préalablement recouvert d'une sous-couche de 200 Å de carbone amorphe pour faire la comparaison avec la microscopie électronique, puis nous montrerons les potentialités de cette nouvelle technique pour d'autres dépôts.It is shown that grazing-incidence small-angle X-ray scattering (GISAXS) is a new experimental technique which combines both x-ray reflectometry and scattering at low angles. It allows characterization of the morphology of aggregates deposited or gathered on flat substrate as for example silicon wafer or Coming glass. Full potentialities of this technique are obtained when using a synchrotron source (flux and collimation) and when patterns are recorded with Image Plates (IPs). It is then possible to study the anisotropic shape of the scattering pattern and to determine the dimensions of the aggregates. Results will be shown on gold clusters deposited on a silicon wafer covered by a carbon sublayer in order to made the comparison with electron microscopy. Other examples will also be shown in order to highlight the advantages of such a technique