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Procédé de recuit protégé appliqué à des nanoparticules d'oxyde de fer : étude des relations structure / propriétés magnétiques

Charlotte Vichery 1 
1 Chimie du solide
LPMC - Laboratoire de physique de la matière condensée
Abstract : Because of their promising applications, particularly in the field of biomedicine, many research activities are currently focused in understanding and improving the magnetic properties of iron oxide nanoparticles. The large surface/volume ratio, inherent to the size reduction at the nanometer scale, and crystalline defects have a significant influence on the magnetic properties of these particles, because they give rise to weakly coupled and misaligned spins, which are responsible for low saturation magnetization values. In the course of this PhD project, a strategy of protected annealing was investigated to improve the crystallinity and/or to change the local composition of maghemite and cobalt ferrite nanoparticles, without agglomeration or grain growth. A diamagnetic, refractory and inert sol-gel silica matrix was selected. The high-temperature annealing of the composite samples was first used to study the impact of various structural defects on the magnetic properties. We showed that for maghemite nanoparticles, ordering of Fe vacancies and changes in the surface state, as a result of dehydration, had opposite effects. Vacancy ordering predominates for particles of 14 nm and tends to increase magnetization values, while the removal of surface hydroxyl groups, predominant for particles of 7 nm, induces the propagation of the outer shell made of misaligned spins toward the center of the particles, thus decreasing magnetization values. In the case of cobalt ferrite, the Co-Fe ions distribution may also have an impact on spin misalignment. However, an 800°C annealing did not induced a large increase in inversion degree, probably because of the refractory nature of CoFe2O4. This strategy of protected annealing also allowed modulating the magnetic properties of maghemite particles upon cobalt doping. A progressive increase of the anisotropy constant, up to 3.5 times, was observed as a result of the diffusion of Co2+ ions into the spinel structure.
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Submitted on : Friday, November 23, 2012 - 11:58:19 AM
Last modification on : Monday, November 16, 2020 - 4:38:05 PM
Long-term archiving on: : Sunday, February 24, 2013 - 3:48:52 AM


  • HAL Id : pastel-00756567, version 1



Charlotte Vichery. Procédé de recuit protégé appliqué à des nanoparticules d'oxyde de fer : étude des relations structure / propriétés magnétiques. Matériaux. Ecole Polytechnique X, 2012. Français. ⟨pastel-00756567⟩



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