Elaboration et caractérisation de composites métal-ferrite nanostructurés, pour applications en moyennes et hautes fréquences

This work was conducted within the framework of a CNRS program, called the ‘materials program', and specially in the project #154 : ‘nanostructured soft magnetic materials. This study consist in elaboration of magnetic composites from milled commercial ferrite and iron or permalloy nanopowders, and their characterisation. The aim was to take advantage of the resistivity of the spinel ferrite on the one hand, of the strong magnetisation and soft magnetic properties of metallic nanoparticles on the other hand. The dispersion of metallic particles is realised by comilling. First, some basis of the magnetic theory are recalled (chapter 1). Then the peculiar physic and magnetic properties of nanometric materials are presented (chapter 2). The elaborating process of the nanostructured composites –powder preparation, comilling, compaction and sintering –are detailled (chapter 3). Then the structural characterisation techniques used during this work are presented. Finally, results of the different measurements, and their interpretation are presented (chapter 5), showing the influence of the elaboration steps on the structural and magnetic properties of the composites. It is proved that the oxido-reduction reaction between iron and ferrite debase the magnetic properties of both component. For this reason a Fe25Ni75-ferrite composite was tested, which seems to be chemically more stable. It is showed by SEM and TEM that the mixing method succeed in dispersing metallic nanoparticles, which keep their nanometric size after sintering. However, magnetic and structural characterisations have proved that this process introduce relevant strain and porosity, which both deteriorate the permeability and the saturation magnetisation.