Numerical calculation of magnetic form factors of complex shape nano-particles coupled with micromagnetic simulations

We investigate the calculation of the magnetic form factors of nano-objects with complex geometrical shapes and non homogeneous magnetization distributions. We describe a numerical procedure which allows to calculate the 3D magnetic form factor of nano-objects from realistic magnetization distributions obtained by micromagnetic calculations. This is illustrated in the canonical cases of spheres, rods and platelets. This work is a first step towards a 3D vectorial reconstruction of the magnetization at the nanometric scale using neutron scattering techniques.Comment: 7 pages, 5 figures. To appear in Physics Procedi

Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations

The magnetic properties of Co (=15 nm, =130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.Comment: 10 pages, 10 figures, submitted to Phys. Rev.

Dipolar interactions in magnetic nanowires aggregates

We investigate the role of dipolar interactions on the magnetic properties of nanowires aggregates. Micromagnetic simulations show that dipolar interactions between wires are not detrimental to the high coercivity properties of magnetic nanowires composites even in very dense aggregates. This is confirmed by experimental magnetization measurements and Henkel plots which show that the dipolar interactions are small. Indeed, we show that misalignment of the nanowires in aggregates leads to a coercivity reduction of only 30%. Direct dipolar interactions between nanowires, even as close as 2 nm, have small effects (maximum coercivity reduction of ~15%) and are very sensitive to the detailed geometrical arrangement of wires. These results strenghten the potential of magnetic composite materials based on elongated single domain particles for the fabrication of permanent magnetic materials.Comment: 7 pages, 8 figures, submitted to Journal of Applied Physic

High temperature structural and magnetic properties of cobalt nanowires

We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and Ms the saturation magnetization. Hc decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300 - 500 K in good agreement with the absence of particle alteration. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation.Comment: 8 pages, 7 figures, submitted to Journal of Solid State Chemistr

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

The origins of the superior catalytic activity of poorly crystallized Ir-based oxide material for the OER in acid is still under debate. Here, authors synthesize porous IrMo oxides to deconvolute the effect of Ir oxidation state from short-range ordering and show the latter to be a key factor

Characterization of a cinnamoyl-CoA reductase 1 (CCR1) mutant in maize: effects on lignification, fibre development, and global gene expression

Cinnamoyl-CoA reductase (CCR), which catalyses the first committed step of the lignin-specific branch of monolignol biosynthesis, has been extensively characterized in dicot species, but few data are available in monocots. By screening a Mu insertional mutant collection in maize, a mutant in the CCR1 gene was isolated named Zmccr1–. In this mutant, CCR1 gene expression is reduced to 31% of the residual wild-type level. Zmccr1– exhibited enhanced digestibility without compromising plant growth and development. Lignin analysis revealed a slight decrease in lignin content and significant changes in lignin structure. p-Hydroxyphenyl units were strongly decreased and the syringyl/guaiacyl ratio was slightly increased. At the cellular level, alterations in lignin deposition were mainly observed in the walls of the sclerenchymatic fibre cells surrounding the vascular bundles. These cell walls showed little to no staining with phloroglucinol. These histochemical changes were accompanied by an increase in sclerenchyma surface area and an alteration in cell shape. In keeping with this cell type-specific phenotype, transcriptomics performed at an early stage of plant development revealed the down-regulation of genes specifically associated with fibre wall formation. To the present authors’ knowledge, this is the first functional characterization of CCR1 in a grass species

Synthèse de nanoparticules d'aluminium et enrobage par des polymères pour des applications énergétiques

Une méthode de synthèse chimique de nanoparticules d'aluminium pour des applications énergétiques a été mise au point et optimisée. Elle consiste à former un hydrure d'aluminium et à le décomposer en présence d'un catalyseur. Il a été montré la nécessité de travailler en atmosphère inerte pour minimiser l'état d'oxydation des particules. Le contrôle de la morphologie des particules a été obtenu (i) via le contrôle de la phase de nucléation par ajout de germes. Des particules moins agrégées et beaucoup plus petites (50nm) ont été ainsi obtenues. (ii) via le contrôle de la croissance par ajout de dodécylamine. Un complexe se forme entre A1H3 et l'agent organique avec départ de H2, permettant d'obtenir des particules de tailles plus homogènes, d'environ 60nm. Par contre, absorbée en très faible quantité à la surface des particules, la dodécylamine ne permet pas de limiter le vieillissement (oxydation) des poudres. L'enrobage des particules par des polymères a ensuite été réalisé par différentes approches : (i) "grafting onto" en 1 étape, consistant à introduire in situ un polymère préformé ; elle a permis d'obtenir des particules hybrides mais leur taille reste supérieure à 100nm et elles sont très oxydées : (ii) "grafting onto" en 2 étapes, les particules ont une taille réduite et sont peu oxydées, mais elles sont pyrophoriques ; (iii) "grafting from" en 2 étapes, elle ne permet pas de contrôler la taille des cristallites, puisqu'il s'agit d'une post-fonctionnalisation mais il a été possible d'obtenir des particules ayant un revêtement polymère dense qui devrait permettre une passivation des particules et dont l'épaisseur peut être contrôlée par le temps de polymérisation.A chemical synthesis to produce aluminum nanoparticles for energetic applications has been developed and optimized. An aluminum hydride is formed and then catalytically decomposed. It has been shown that it is necessary to work under inert atmosphere to minimize the oxidation state of the particles. The control of the morphology of the particles has been obtained through (i) the control of the nucleation phase by addition of seeds. The particles obtained are less aggregated and much smaller (50nm). (ii) the control of the growth phase by addition of dodecylamine. A complex between A1H3 and the organic agent accompanied by a release of H2 is formed, allowing to obtain more homogeneous particlels, qith a mean diameter of 60nm. However, adsorbed in small quantities at the particle surface, dodecylamine do not allow protecting the powders against ageing (oxidation). The particle's coating by polymers is then realized by different approaches : (i) grafting onto in 1 step, consisting in introducing in sity a preformed polymer ; hybrid particles could be obtained by this strategy but they are very oxidized ; (ii) grafting onto in 2 steps, the particles obtained are smaller and they are not much oxidized, but they are pyrophoric ; (iii) grafting from in 2 steps ; it does not allow a control of the crystellites' size as it is a post-fonctionnalization but it was possible to obtain particles covered by a polymeric coating. The thickness of the coating can be tuned by the polymerization duration. It could permit to passivate the particles.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Elaboration de nanoparticules ferromagnétiques par la voie polyol (de la formation en solution vers la préparation d'aimants liés)

Ce travail est en relation avec l'élaboration de nanoparticules ferromagnétiques anisotropes par la voie polyoL Une nouvelle approche de synthèse sous irradiation micro-ondes a notamment été développée et les résultats montrent un gain de temps de Tordre de 50% par rapport à un chauffage convectif classique ainsi qu'un certain contrôle du rapport d'aspect en jouant sur la rampe de température. Afin de mieux comprendre les mécanismes de nucléation-croissance, une approche combinée expérience et théorie, reposant sur l'analyse des systèmes réactionnels et la mise en œuvre des outils de la chimie théorique a été entreprise avec le cobalt. Elle montre que le paramètre clef conduisant à une croissance anisotrope suivant Taxe c du système hexagonal est le potentiel chimique du ligand carboxylate, provenant du précurseur de cobalt. Les propriétés magnétiques des nanobâtonnets de Co mesurées (Ms = 113 uem/g et HC = 9-10 kOe) permettent d'envisager l'utilisation de ces objets pour diverses applications telles que l'enregistrement magnétique haute densité ou l'élaboration d'aimants permanents suivant une approche ascendante. Le greffage de sels de diazonium sur ces particules ferromagnétiques a également été étudié. Ce greffage ouvre la voie à la polymérisation contrôlée à partir de la surface des matériaux et à leur incorporation dans des matrices polymères. Enfin, afin d'augmenter l'aimantation des particules, une approche consistant à synthétiser des particules bimétalliques anisotropes de Fer-Cobalt a été envisagée. Les premiers résultats obtenus sont en faveur de la formation de particules anisotropes présentant un cœur de Co et une coquille riche en fer.This work is related to the elaboration of ferromagnetic anisotropic nanoparticles prepared using the polyol process. A new synthetic method based on the use on microwaves irradiation has been developed and the results show a 50% reduction of synthesis time as well as a control on the aspect ratio with the temperature ramping rate. In order to better understand the nucleation and growth mechanisms accounting for the formation of these objects, a combined experimental and theoretical approach has been performed with cobalt as a case study. The results show that the key parameter for obtaining an anisotropic growth along the c axis of the hexagonal System is the chemical potential of the carboxylate ligand coming from the Co precursor. The magnetic properties of the Co nanorods (Ms =113 emu/g and HC = 9-10 kOe) allow to consider the use of such objects for various applications such as high density magnetic recording and the preparation of permanent magnets following a bottom-up approach. The grafting of diazonium salts on the surface of the ferromagnetic particles has also been studied. It should allow the polymerization form the particle surface which will facilitate their subsequent incorporation in a polymer matrix. Finally, in order to increase the saturation magnetization of the particles, the preparation of Fe-Co bimetallic particles has been studied. The first results show that anisotropic particles with a Co core and a Fe shell are formed.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Electrochemical Active Surface Area Determination of Iridium‐Based Mixed Oxides by Mercury Underpotential Deposition

International audienceThe electrochemical surface area (ECSA) is a critical property to describe, analyze and compare electrocatalysts. The determination of the mass activity of a given catalyst is associated with this parameter which can thus lead to materials benchmarking. Reliable and robust methods to measure ECSA are needed, and those have to accommodate different structures, morphologies and compositions. In this study, we investigate mercury underpotential deposition (HgUPD) as a way to estimate ECSA for ultraporous electrocatalysts based on iridium and iridium-molybdenum electrocatalysts for the oxygen evolution reaction. Results reveal a clear agreement between physisorption measurements and HgUPD with excellent reproducibility. The method shows also that pre- and post-catalysis surface area measurements are not affected by the catalytic process on short timescale, opening the possibility of electrocalyst stability and degradation monitoring