Re-entrant spin glass and magnetoresistance in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide

We have investigated the static and dynamic response of magnetic clusters in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide, where a sequence of magnetic phase transitions, i.e., paramagnetic (PM) to ferromagnetic at T_{C} $\leq$ 270K and ferromagnetic to canted spin glass state at T_f$$\leq$ 125K is observed

Influence of substitutional disorder on the electrical transport and the superconducting properties of Fe1+z_{1+z}Te1−x−y_{1-x-y}Sex_{x}Sy_{y}

We have carried out an investigation of the structural, magnetic, transport and superconducting properties of Fe$_{1+z}$Te$_{1-x-y}$Se$_x$S$_y$ ceramic compounds, for $z=0$ and some specific Se (0$\leq$ x $\leq$ 0.5) and S (0 $\leq$ y $\leq$0.12) contents. The incorporation of Se and S to the FeTe structure produces a progressive reduction of the crystallographic parameters as well as different degrees of structural disorder associated with the differences of the ionic radius of the substituting cations. In the present study, we measure transport properties of this family of compounds and we show the direct influence of disorder in the normal and superconductor states. We notice that the structural disorder correlates with a variable range hopping conducting regime observed at temperatures $T >$ 200 K. At lower temperatures, all the samples except the one with the highest degree of disorder show a crossover to a metallic-like regime, probably related to the transport of resilient-quasi-particles associated with the proximity of a Fermi liquid state at temperatures below the superconducting transition. Moreover, the superconducting properties are depressed only for that particular sample, in accordance to the condition that superconductivity is affected by disorder when the electronic localization length $\xi_L$ becomes smaller than the coherence length $\xi_{SC}$.Comment: 23 pages, 9 figure

Antiphase boundaries in truncated octahedron-shaped Zn-doped magnetite nanocrystals

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Evidence for the verwey transition in highly nonstoichiometric nanometric fe-based ferrites

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Apparent magic numbers in embedded Ti-O clusters

International audienceStable clusters constituted of four TiO2 entities have been evidenced in nanometer-sized spinel oxides by a combination of diffraction and x-ray-absorption expts. Contrary to free clusters, the size of the embedded clusters is strongly detd. by the constraints imposed by the surrounding matrix so that the magic no. of four is in fact specific to the matrix and only then apparent

Apparent magic numbers in embedded Ti-O clusters

International audienceStable clusters constituted of four TiO2 entities have been evidenced in nanometer-sized spinel oxides by a combination of diffraction and x-ray-absorption expts. Contrary to free clusters, the size of the embedded clusters is strongly detd. by the constraints imposed by the surrounding matrix so that the magic no. of four is in fact specific to the matrix and only then apparent

Evidence for the verwey transition in highly nonstoichiometric nanometric fe-based ferrites

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Suivi par diffraction de rayons X “in situ” de l'évolution du paramètre de maille du ferrite nanométrique γ\gamma-Fe2_2O3_3 lors de l'isotherme d'adsorption d'eau

The soft chemistry method has been used to synthesize maghemite ($\gamma$-Fe$_2$O$_3$) nanoparticles in the 9-14 nm range. These nanometric powders are crystallized in the spinel structure. The control of both Oxygen stoichiometry and elimination of the remaining impurities leads to obtain maghemite model material. In the crystallite size range studied here, cell parameter (0.8346 nm) appears to be not linked to crystallite size. Nevertheless, “in-situ” X-Ray Diffraction studies demonstrate that there are very small changes in cell parameter with surface modification under water vapour atmosphere. Chemisorption and Physisorption phenomenons takes an important part at nanometric scale.La méthode de synthèse par chimie douce a été utilisée pour synthétiser des nanoparticules de maghémite $\gamma$-Fe$_2$O$_3$ dans une gamme de taille allant de 9 à 14 nm. Ces poudres, parfaitement cristallisées en phase spinelle, exemptes d'impuretés, avec une staechiométrie en oxygène parfaitement contrôlée, ont servi de matériaux modèles pour cette étude. Le paramètre de maille de ce matériau (0,8346 nm) s'avère être non dépendant de la taille. Cependant, le suivi en Diffraction de Rayons X “in-situ” révèle des variations de ce paramètre avec la modification de la surface en présence de vapeur d'eau. Les phénomènes de chimisorption et de physisorption jouent un rôle prépondérant dans les évolutions observées à l'échelle nanométrique

Segregation phenomena in nanometric titanium ferrites: Complementarity of different experimental techniques (DRX, XPS, EXAFS...)

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Phénomènes de ségrégation dans les ferrites de titane nanométriques : apports complémentaires de différentes techniques expérimentales (DRX, XPS, EXAFS ...)

Thanks to the complementarily of average and local techniques two segregation phenomenon have been evidenced in nanostructured ferrites: a segregation related to kinetic effects and an other related to thermodynamical effects. The combination of local (MET, XPS) and averring techniques (DRX...) demonstrate that some heterogeneities exist in nanometric powders with Fe$^{2+}$ cations when they are studied ex situ. These heterogeneities are related to kinetic effects linked to the difference of mobility between Fe$^{2+}$ and Ti$^{4+}$ cations during the partial oxidation of Fe$^{2+}$. Rietveld refinements of X ray and neutron diffraction patterns have shown that both titanium cations and vacancies (created during the oxidation reaction) are exclusively located in octahedral coordination. Furthermore, for titanium ferrites totally oxidized during a long time (they only contained Fe$^{3+}$ cations), the Rietveld refinements lead to a titanium content lower than that obtained by classical chemical analyses (ICP, EDX, EELS...). A complementarily EXAFS study has evidenced that the titanium lack is in fact located in TiO$_2$ clusters in the nanometric spinel particles, whose size are lower than 4 Å.Grâce à la complémentarité de diverses techniques deux phénomènes de ségrégation ont été mis err évidence dans les ferrites de titane nanostructurés : une ségrégation cinétique (dite dynamique) et une ségrégation thermodynamique. Par des techniques d'analyse locales (MET, XPS) et statistiques (DRX...), des hétérogénéités au sein des grains de poudre contenant des cations Fe$^{2+}$ ont été mises en évidence lors de leur étude à l'air. Leur origine est cinétique et découle d'une oxydation incomplète des cations Fe$^{2+}$. Des affinement de Rietveld de diagrammes de diffraction des rayons X et diffraction de neutrons ont permis de montrer quant à eux que le titane ainsi que toutes les lacunes créées lors (le l'oxydation des cations Fe$^{2+}$, se localisent uniquement dans les sites octaédriques de la structure spinelle. Cependant, pour certaines compositions ne contenant que des cations Fe à la valence 3, alors (tue. les techniques d'analyse chimique classiques (ICP, EDX, EELS...) donnent bien le taux de titane attendu, les affinements structuraux conduisent à une teneur en titane bien inférieure. Une étude complémentaire en EXAFS a permis de montrer que le déficit en titane était en fait localisé dans des clusters de TiO$_2$ de taille inférieure à 4 Å, à l'intérieur même des grains nanométriques de ferrite de structure spinelle