Showing papers by "Marc Respaud published in 2005"

Multimillimetre-large superlattices of air-stable iron–cobalt nanoparticles

Abstract: Self-organization of nanoparticles into two- and three-dimensional superlattices on a large scale is required for their implementation into nano- or microelectronic devices1,2. This is achieved, generally after a size-selection process3,4, through spontaneous self-organization on a surface5,6,7,8,9,10,11, layer-by-layer deposition12 or the three-layer technique of oversaturation3,14, but these techniques consider superlattices of limited size. An alternative method developed in our group involves the direct formation in solution of crystalline superlattices, for example of tin nanospheres, iron nanocubes or cobalt nanorods, but these are also of limited size15,16,17. Here, we report the first direct preparation in solution of multimillimetre-sized three-dimensional compact superlattices of nanoparticles. The 15-nm monodisperse FeCo particles adopt an unusual short-range atomic order that transforms into body-centred-cubic on annealing at 500 ∘C. The latter process produces an air-stable material with magnetic properties suitable for radiofrequency applications.

Magnetism in the low-doping regime (x<0.50) of Bi1−xSrxMnO3 perovskites

Abstract: The magnetic properties of low-doped Bi1−xSrxMnO3 manganites (x<0.50) are investigated. Although a progressive dilution by Sr doping of the ferromagnetic ground state of pure BiMnO3 was previously proposed, it is found that compounds with x=0.25, 0.33, and 0.5 present an antiferromagnetic ground state. However, it is confirmed that a strong ferromagnetic component is easily induced under field in the compounds with x=0.25 and 0.33, both presenting the same type of charge and polaron ordering.

Si nanocrystals by ultra-low energy ion implantation for non-volatile memory applications

Abstract: In nanocrystal (nc) metal-oxide-semiconductor (MOS) memory structures, a fine control of nc location and population is required for pinpointing the optimal device architectures. In this work, we show how to manipulate and control the depth-position, size and surface density of two dimensional (2D) arrays of Si ncs embedded in thin (<10 nm) SiO2 layers, fabricated by ultra-low energy (typically 1 keV) ion implantation and subsequent annealing. The influence of implantation and annealing conditions on the nc characteristics (e.g. size, density) and the charge storage properties of associated MOS structures is reported with particular emphasis upon the effect of annealing in N2-diluted–O2 gas mixture. The latter annealing conditions restore the integrity of the oxide and allow for the fabrication of non-volatile memory devices operating at low-gate voltages. Annealing in diluted oxygen has also an effect on the population of silicon ncs. Their evolution has been studied as a function of the annealing duration under N2 + O2 at 900 °C. An extended spherical Deal–Grove model for the self-limiting oxidation of embedded Si ncs has been carried out. It shows that stress effects, due to the deformation of the oxide, slows down the chemical oxidation rate and leads to a self-limiting oxide growth. The model predictions are in agreement with the experimental results.

Synthèse organométallique de nano-objets magnétiques aux propriétés physiques contrôlées

Abstract: Le potentiel d'utilisation des nanoparticules magnetiques (stockage de l'information, ferrofluides, inducteurs...) dans divers secteurs industriels et les limitations des methodes physiques courantes d'elaboration motive la recherche de voies de synthese alternatives. Nous allons voir que la synthese organometallique permet l'obtention de nanoparticules isolees ideales pour l'etude fondamentale des proprietes physiques emergeant a l'echelle du nanometre (aimantation et anisotropie accrues, structure cristalline originale) et de la relation proprietes physiques/environnement chimique. Ces etudes, en retour, ont permis le developpement de nouveaux objets nanometriques. Notamment, des nanobâtonnets de cobalt, materiaux bien adaptes pour l'enregistrement magnetique haute densite, ont pu etre synthetises.