MICROSTRUCTURAL AND MAGNETIC CHARACTERIZATION OF ALUMINA IRON NANOCOMPOSITES

Iron-alumina nanocomposite powders containing 10 wt % iron were prepar ed by selective reduction of alumina-haematite solid solutions. Micros tructural study showed three types of metal dispersion in the alumina matrix according to the elaboration process: iron grains that were > 7 0 nm, most of the iron particles were < 10 nm and directly epitaxied i n the alumina matrix, and iron particles that were surrounded by an in terfacial phase. In agreement with transmission electron miscroscopy ( TEM) observations, magnetic study confirmed a distribution of the iron particles size, showing the superposition of a ferromagnetic behaviou r (larger particles) and a superantiferromagnetic behaviour (smaller p articles). Furthermore, analysis of thermoremanent behaviour, coercive field and dissymmetry of hysteresis loops allowed the interfacial pha se surrounding some iron particles to be identified as an antiferromag netic phase, Fe1+xAl2-xO4. Nevertheless, at the interface of metallic iron epitaxied on the alumina matrix some atomic planes always existed where iron was ionic (even if no other phase was detected). As a cons equence the mean magnetic moment of iron in these nanocrystals is larg er than that in bulk metallic iron.