Iron−Cobalt Alloy Nanoparticles Embedded in an Alumina Xerogel Matrix

Nanocrystalline γ-Al 2 O 3 and FeCo−Al 2 O 3 nanocomposite xerogels with high surface areas and pore volumes were prepared from alcogels obtained by a fast sol−gel procedure. The formation of γ-Al 2 O 3 occurs via a sequence of stages starting from a disordered pseudo-bohemite phase which around 700 °C gives rise to amorphous allumina; this progressively cristallizes as γ-Al 2 O 3, which is stable up to 1100 °C, when microcrystalline α-Al 2 O 3 becomes the dominant phase; in the range 1000−1200 °C minor traces of δ-Al 2 O 3 and θ-Al 2 O 3 are present. Xerogels containing iron and cobalt are amorphous up to 700 °C; calcination at 800 °C gives rise to a spinel phase similar to γ-Al 2 O 3 where metal ions partially fill the vacancies; at 1000 °C γ-Al 2 O 3 progressively disappears to form α-Al 2 O 3 and Co(Fe)Al 2 O 4, which are the only phases present at 1200 °C. Reduction in hydrogen flow of the xerogel, previously calcined at 450 °C, leads to a nanocomposite constituted of FeCo alloy nanoparticles around 10 nm dispersed into α-Al 2 O 3 nanocrystalline matrix. Zero-field-cooled (ZFC) and FC magnetic curves are typical of superparamagnetic materials and indicate the occurrence of high-strength particle interactions.