Novel solid-state synthesis of α-Fe and Fe₃O₄ nanoparticles embedded in a MgO matrix

Thermally induced reduction of amorphous Fe₂O₃ nanopowder (2–3 nm) with nanocrystalline Mg (∼20 nm) under a hydrogen atmosphere is presented as a novel route to obtain α-Fe and Fe₃O₄ magnetic nanoparticles dispersed in a MgO matrix. The phase composition, structural and magnetic properties, size and morphology of the nanoparticles were monitored by x-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy at temperatures of 24–300 K, transmission electron microscopy and magnetic measurements. Spherical magnetite nanoparticles prepared at a reaction temperature of 300 °C revealed a well-defined structure, with a ratio of tetrahedral to octahedral Fe sites of 1/2 being common for the bulk material. A narrow particle size distribution (20–30 nm) and high saturation magnetization (95 ± 5 A m² kg⁻¹) predispose the magnetite nanoparticles to various applications, including magnetic separation processes. The Verwey transition of Fe₃O₄ nanocrystals was found to be decreased to about 80 K. The deeper reduction of amorphous ferric oxide at 600 °C allows α-Fe (40–50 nm) nanoparticles to be synthesized with a coercive force of about 30 mT. They have a saturation magnetization 2.2 times higher than that of synthesized magnetite nanoparticles, which corresponds well with the ratio usually found for the pure bulk phases. The magnetic properties of α-Fe nanocrystals combined with the high chemical and thermal stability of the MgO matrix makes the prepared nanocomposite useful for various magnetic applications.