Structural features of magnetic materials containing γ-Fe₂O₃ particles (2-5 nm) dispersed over amorphous porous SiO₂ matrix have been studied by TEM, EPR, XRD, IR- and Moessbauer spectroscopy. γ-Fe₂O₃-SiO₂ nanocomposites were prepared by mixing Fe₃O₄ nH₂O and SiO₂ nH₂O sols. Thermo-stimulated structural transformations in the nanocomposites were studied in the temperature range of 100-1100°C depending on Fe₂O₃ content (1-8 wt. %). High thermal stability of γ-Fe₂O₃ clusters in the pores of SiO₂ matrix was revealed. Partial transformation of γ-Fe₂O₃ into α-Fe₂O₃ and the particle growth to 20 nm occur only under high-temperature (1100°C) treatment. High thermal stability of small γ-Fe₂O₃ grains in the γ-Fe₂O₃-SiO₂ nanocomposites can be explained by the tendency of Fe³⁺ ions to have high-symmetric cubic coordination of ions like in Fe₃O₄ nH₂O particles in the corresponding sol. Additional stabilization of cubic symmetry is connected with OH-groups occurring in Fe³⁺ coordination environment. These groups are difficult to remove from the mesa-porous SiO₂ structure. Magnetically active nanoclusters dispersed over various matrixes can be potentially used to obtain materials demonstrating magnetoresistive effect

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