Noureddine Jouini

Magnetic properties of ultrafine cobalt ferrite particles synthesized by hydrolysis in a polyol medium

Fine CoFe2O4 powders with monodisperse, nalmost equi-axial nanometer-sized particles were synthesised in a npolyol medium by forced hydrolysis of ionic Co(II) and Fe(III) nsalts at 160u2006°C. K(Co) XANES and 57Fe Mossbauer nspectroscopy show that the structure of this ferrite is slightly deviated nfrom an inverse spinel structure: 16% of cobalt atoms are in tetrahedral sites. nThe particles are superparamagnetic above 300xa0K and ferrimagnetic below nthis blocking temperature with, at low temperature, strong coercivity, a saturation nmagnetisation value close to the bulk value and high reduced remanence. The nsaturation magnetisation measured at 5xa0K is clearly enhanced with respect nto CoFe2O4 nanometer-sized particles previously nprepared by other methods. These magnetic characteristics suggest that these nparticles have a high crystallinity which may result from this novel synthesis nroute.

Submicrometer zinc oxide particles: Elaboration in polyol medium and morphological characteristics

A novel and easy route for preparing submicrometer particles of zinc oxide, involving hydrolysis of zinc salt in a polyol medium, is proposed. Zinc acetate dihydrate and diethyleneglycol appear to be the best candidates for obtaining a high yield of particles with well-defined morphological characteristics. Monodisperse spherical particles in the submicrometer range (0.2−0.4 μm) have been obtained for a salt concentration less than 0.1 mol 1 −1 . The particle size depends mainly on the heating rate. The particles are microporous (surface area: 80 m 2 g −1 ) and are formed by aggregation of small crystallites (10 nm). Calcination at moderate temperature drastically reduces this porosity without significant interparticle sintering. At higher concentration, no aggregation occurs and tiny single crystallite particles are obtained.

Nickel ferrite nanoparticles: elaboration in polyol medium via hydrolysis, and magnetic properties

Ultrafine magnetic nickel ferrite NiFe2O4 particles of high crystallinity were directly prepared by forced hydrolysis of ionic iron (III) and nickel (II) solutions in 2-hydroxyethyl ether at about 478xa0K under atmospheric pressure. The resulting nickel ferrite particles exhibit very interesting magnetic properties: they are superparamagnetic at room temperature and have a saturation magnetization close to that of the bulk at low temperature. An in-field Mossbauer study shows clearly that this surprising behaviour is mainly due to: (i) a departure of the cation distribution from the classical distribution encountered in the bulk material and (ii) the absence of spin canting for both tetrahedral and octahedral cations.

Metastable solid solutions in the system ZnOCoO: synthesis by hydrolysis in polyol medium and study of the morphological characteristics

Abstract Metastable solid solutions in the system ZnOue5f8CoO have been synthesized by hydrolysis of ionic zinc and/or cobalt salts in polyol medium. The solubility of Co in the zincite oxide was significantly increased (at.% Co=65) compared to that obtained under thermodynamical equilibrium (at.% Co=6.5 at 800°C). CoO was also obtained via this route. The products are made up of sub-micrometer particles. A detailed study of their morphology along with pure ZnO particle morphology was conducted. Different growth mechanisms are evidenced.

Magnetic properties of zinc ferrite nanoparticles synthesized by hydrolysis in a polyol medium

Highly crystalline, nanometre sized ZnFe2O4 particles with different diameters, 6.6 and 14.8 nm, were prepared by forced hydrolysis in a polyol medium. The DC magnetic properties exhibit a strong dependence on the particle size as a result of the unusual cation distribution. They clearly establish their superparamagnetic character at room temperature and the occurrence of ferrimagnetic or ferromagnetic ordering at low temperature. The magnetization is found to increase with grain size reduction. The 57Fe Mossbauer spectra were recorded at 300 and 4.5 K. There is no evidence for the presence of the Fe2+ charge state, confirming the perfect stoichiometry of the two samples. At 300 K, the Mossbauer spectra consist of doublets due to the superparamagnetic behaviour whereas at 4.5 K they reveal a magnetically blocked state. Mossbauer spectra at 10 K in an external 6 T magnetic field applied parallel to the direction of the gamma rays clearly show a close to collinear Neel-like ferrimagnetic ordering for the 6.6 nm particles and a canted Yafet–Kittel-like ferrimagnetic ordering for the 14.8 nm ones.

Thermodynamics of Nanoparticles: Experimental Protocol Based on a Comprehensive Ginzburg-Landau Interpretation

The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. On the basis of experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Ginzburg-Landau theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.

Co80Ni20 Nanowires-Based Tunable Sintered Materials

This work focuses on the development and study of nanostructured magnetic materials by a bottom-up strategy allowing the modulation of their magnetic behavior (soft to - hard). The main goal of this work is to tune the magnetic bulk coercivity properties of new nanostructured bulk materials based on magnetic nanowires assembly. Spark-Plasma-Sintering (SPS) process allows an organization of magnetic nanowires inside the sintered material and preserving at the same time the nanoscale character of the nanowires in order to take advantage of the confinement peculiar magnetic properties. We show here that changing the temperature and magnetic field parameters during this new sintering process is at the origin of a controlled anisotropy and coercitivity in the as-obtained bulk systems.