Evolution of the structure and magnetic properties of FeCo nanoparticles in an alumina aerogel matrix

Nanocrystalline γ-Al₂O₃ and FeCo-Al ₂O₃ nanocomposite aerogels with high surface areas and pore volumes were prepared by high-temperature supercritical drying of alcogels obtained by a fast sol-gel procedure. The formation of γ-Al ₂O₃ occurs via a sequence of stages: in the parent aerogel an alkyl derivative of boehmite is observed whose calcination gives rise to a disordered phase and finally to γ-Al₂O₃ which is stable up to 1000 °C. In the presence of iron and cobalt, calcination of the aerogel gives rise to a spinel phase similar to γ-Al₂O ₃ where metal ions partially fill the vacancies. Nanocomposites constituted of FeCo alloy nanoparticles dispersed into γ-Al ₂O₃ matrix are obtained via reduction in hydrogen flow of the aerogels containing iron and cobalt. The amount and average size of the nanoparticles depends both on the temperature and time of the reduction treatment and affects the resulting magnetic properties. All the reduced aerogels show superparamagnetic behavior, but the collective magnetic properties are strongly dependent on the amount and mean size of the alloy nanoparticles.