Magnetic states and spin-glass properties in Bi0.67Ca0.33MnO3: macroscopic ac measurements and neutron scattering

We report on the magnetic properties of the manganite Bi_{1-x}Ca_{x}MnO_3 (x=0.33) at low temperature. The analysis of the field expansion of the ac susceptibility and the observation of aging properties make clear that a spin glass phase appears below T = 39K, in the presence of magnetic order. Neutron scattering shows both magnetic Bragg scattering and magnetic diffusion at small angles, and confirms this coexistence. In contrast to Pr_{1-x}Ca_{x}MnO_3 (x=0.3-0.33) which exhibits a mesoscopic phase separation responsible for a field driven percolation, the glassy and short range ferromagnetic order observed here does not cause colossal magnetoresistance (CMR).Comment: accepted in Phys Rev

Etudes structurales et magnétiques de manganites<br />BixCa1-xMnO3 présentant des mises en ordre complexes.

Charge ordering phenomena and magnetism in BixCa1-xMnO3 manganites.The structural and magnetic study was carried out on a Bi-rich part of the BixCa1-xMnO3 system, 0.7 " x " 0.5, working on ceramics and single crystals. We combine the results of different diffraction techniques – X Rays, electrons and neutrons- leaning on the transmission electron microscopy observations and correlating our results with the magnetic property. The compound x = 0,5 differs others composed by the evolution of its modulation vector as a function of the temperature. A "unique" model of the charge localization is presented for x = 0,55, 0,6 and 0,64. The charge localization is done, not on a Mn site, but on a Mn-O-Mn entity. The evolution of the magnetic structure withBi/Ca ratio study was performed on powder diffraction data. The data can be refined with different physicals models, in particular the classical CE-type model is one of the solutions for BixCa1-xMnO3 manganites with x=0.5. The moments tend to establish a ferromagnetic coupling with increasing x. The composite Bi0.67Ca0.33MnO3 appears as a key point for which the majority of the charge localization is destabilized. The study of the BT magnetic state of this compound confirmed the coexistence of a spin glass state and of a long range magnetic order. The origin of the spin glass state in Bi0.67Ca0.33MnO3 is interpreted as a dilution of FM domains in an AFM matrix induced by the random occupation by Bi/Ca of the A site. A similar magnetic state is shown for the Bi0.71Ca0.24MnO3 compounds when it present a type surstructure [111]p (close to the multiferroic BiMnO3 surstructure). A frustration model was proposed while basing itself on the refined orbital ordering.Indexation Rameau: Manganite, Pérovskite, Structure cristalline (solides), Structure magnétique,Neutrons**diffraction, Rayons X**diffraction, Microscopie électronique en transmission, Verres de spin.L'étude structurale et magnétique de la partie riche en bismuth, 0,7 ≥ x ≥ 0,5, du système BixCa1-xMnO3 est présentée dans ce manuscrit. Les résultats des techniques de diffraction des rayons X, des neutrons, des électrons, de microscopie électronique en transmission haute résolution et de mesures des propriétés macroscopique ont été confrontés afin de produire une interprétation la plus juste possible expliquant les relations entre propriétés magnétique et structure. Des échantillons poly-cristallins et des monocristaux ont été synthétisé à cette fin. Le composé x = 0.5 semble à part. Un modèle de localisation des charges « unique » est présenté pour x = 0,55, 0,6 et 0,64. La localisation des charges se fait, non pas sur un site Mn, mais sur une entité MnOMn (polaron de Zener). Plusieurs modèles de structures magnétiques, dont le modèle de type CE, permettent de modéliser les diffractogrammes BT de ces composés.Quand x augmente les couplages ferromagnétiques tendent à augmenter. Le composé Bi0.67Ca0.33MnO3 apparaît comme un composé charnière pour lequel la localisation des charges est déstabilisée. L'étude de son état magnétique BT a confirmé la coexistence d'un état verre de spin et d'un ordre magnétique à longue distance. L'origine de l'état verre de spins dans Bi0.67Ca0.33MnO3 est interprétée comme une dilution de domaines FM dans une matrice AFM induite par l'occupation aléatoire du site A par le Bi et le Ca. Bien que le composé Bi0.71Ca0.24MnO3 présente le même état BT, une surstructure nucléaire différente de type [111]p (proche de celle du multiferroïque BiMnO3) a été affinée. Un modèle de frustration magnétique a été proposé en se basant sur l'ordre orbitalaire affiné dans ce composé

Etudes structurales et magnétiques de manganites BixCa1-xMnO3 présentant des mises en ordre complexes

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Un dispositif pédagogique de mesure de la résistivité électrique : Illustrer par l’expérience l’apport des méthodes géophysiques pour la caractérisation des propriétés du sol

La méthode géophysique de mesure de la résistivité électrique est utilisée en science du sol pour aider à la caractérisation des propriétés des sols. L’expérience ludique conçue à l’occasion du Salon International de l’Agriculture de Paris 2009 par l’Inra d’Orléans permet aux visiteurs de découvrir cette technologie. Par le biais d’une manipulation d’un dispositif de mesure miniaturisé sur une maquette de sol, le public acquière des notions sur la variabilité spatiale de la couverture pédologique et prend conscience de l’intérêt d’utiliser la mesure de résistivité électrique pour détecter des états différenciés du sol

Identifying the characteristic scales of soil structural recovery after compaction from three in-field methods of monitoring

International audienceHighlights • Soil structural changes were assessed in-field from one-year soil monitoring. • Methods of description were invasive (coring, visual) and geophysical (ERT). • The study shows a characteristic time-scale of soil structural recovery longer than 1year. • Some regenerative processes were identified at seasonal scale from temporal ERT. • Cracking occurred in the topsoil, abruptly in the dry period, still visible in the wet and cool. Abstract The impact of compaction by traffic on agricultural soils is not strict and irreversible. After compaction by machine traffic, soil structure changes both spatially and temporally as different generative processes occur. These are generally well-described in the literature. However, the preferential periods of occurrence and the soil depths affected by changes – thus, the characteristic scales of soil structural recovery – remain hypothetical. Further investigation through precise spatial and temporal monitoring under real in-field soil conditions is needed. In this paper, the structural changes of a locally trafficked silt–loam soil were assessed under both cropped and bare areas conventionally tilled from one-year soil monitoring. The monitoring was performed in-field by three methods at a low temporal resolution with the standard methods of visual description and soil coring in pits, and at a high temporal resolution with the non-destructive Electrical Resistivity Tomography (ERT) method. The specific use of ERT for this purpose is discussed. Compaction by traffic affected the overall tilled soil layer and was shown to be time-persistent. This suggested a characteristic time-scale of a complete structural recovery longer than one year, regardless of soil management. At the finest temporal scale, the results also highlighted some seasonal processes that potentially affect the long-term recovery, such as bio-drilling and soil cracking. The processes were related to the soil management, the wetting/drying cycles and the freeze/thaw effect. They likewise induced the start of structure fragmentation in the first centimetres of the soil and acted abruptly in the dry period, preferentially under the area initially cropped, with persistent effects on the soil structure in the rainy and cool season

ACOUSTISOL : potentiel des émissions acoustiques pour l’étude dynamique de la structure des sols

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