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

Nanocrystalline gamma-Al2O3 and FeCo-Al2O3 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 gamma-Al2O3 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 gamma-Al2O3 which is stable up to 1000 degreesC. In the presence of iron and cobalt, calcination of the aerogel gives rise to a spinel phase similar to gamma-Al2O3 where metal ions partially fill the vacancies. Nanocomposites constituted of FeCo alloy nanoparticles dispersed into gamma-Al2O3 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.