Étude du magnétisme de composites métal-oxyde et métal-diélectrique nanostructurés pour composants passifs intégrés.

This survey deals with the development of nanostructured magnetic composites with innovative magnetic and electric properties. For many applications, in power electronics, in electromagnetic devices for telecommunications, in magnetic data storage technology and in biomedical Microsystems, the composite must present a high magnetic polarization and a good insulation behavior to enhance the frequency limits and minimize dynamic losses. Composite materials developed in this work consist on a coating matrix - magnetic (spinel ferrite) or non-magnetic (dielectric = silica) - in which metallic nanoparticles (iron-nickel or cobalt) are dispersed. These nanocomposites are inventive because the final material may benefit from the coupling of the magnetic properties of the two phases. Electron holography in transmission mode highlighted spins configurations, particularly vortex structure, in Fe30Ni70 nanoparticles. Using the holographic measurements, the validity of a micromagnetic model was checked under quite general conditions. The physical and chemical analyses have confirmed topologies expected for each composite : for the metal-dielectric composite, the layer thickness of the coating was monitored at the nanoscale. The metal-oxide composite, synthesized by direct growth of ferrite on the metallic phase, revealed a good dispersion of metal in the ferrite matrix. The structural and magnetic properties of the different elaborated composites, as a powder or compacted by SPS (Spark Plasma Sintering), were characterized and discussed. The functional properties have been also studied and are promising for the intended applications. The silica coating of nanoparticles led to their surface preparation in order to be functionalized by biological entities.