Microstructural and magnetic characterization of alumina-iron nanocomposites

Iron-alumina nanocomposite powders containing 10 wt % iron were prepared by selective reduction of alumina-haematite solid solutions. Microstructural study showed three types of metal dispersion in the alumina matrix according to the elaboration process: iron grains that were >70 nm, most of the iron particles were <10 nm and directly epitaxied in the alumina matrix, and iron particles that were surrounded by an interfacial 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 behaviour (larger particles) and a superantiferromagnetic behaviour (smaller particles). Furthermore, analysis of thermoremanent behaviour, coercive field and dissymmetry of hysteresis loops allowed the interfacial phase surrounding some iron particles to be identified as an antiferromagnetic phase, Fe_1+x Al_2–x O₄. 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 consequence the mean magnetic moment of iron in these nanocrystals is larger than that in bulk metallic iron.