A thermal reduction method has been developed to prepare magnetite/hematite nanocomposites and pure magnetite nanoparticles targeted for specific applications. The relative content of hematite α-Fe2O3 and magnetite Fe3O4 nanoparticles in the product was ensured by maintaining proper conditions in the thermal reduction of α-Fe2O3 powder in the presence of a high boiling point solvent. The structural, electronic, and magnetic properties of the nanocomposites were investigated by F57e-Mössbauer spectroscopy, x-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at every step of the chemical and thermal treatment. It is established that not all iron ions in the octahedral B-sites of magnetite nanoparticles participate in the electron hopping Fe2+⇄Fe3+ above the Verwey temperature TV, and that the charge distribution can be expressed as (Fe3+)tet[Fe1.852.5+Fe0.153+]octO4. It is shown that surface effects, influencing the electronic states of iron ions, dominate the vacancy effect, and thus govern the observed specific features of the Verwey transition and magnetic properties. The sharp increase in coercivity observed in magnetite nanoparticles below TV is much stronger than for bulk magnetite.
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Lyubutin et al. (2009) studied this question.
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