Hyperfine characterization of the crystallization of nanocrystalline NiFe2O4 from the amorphous silica gel

Nanocrystalline NiFe₂O₄ was in‐situ prepared in amorphous silica using tetramethylor‐thosilicate and nickel (iron) nitrate hydrate as the starting materials in a sol‐gel reaction. The magnetic nanocrystals in the amorphous silica glasses grew slowly with increasing temperature. Above 600^○C, nickel ferrite nanoparticles began to precipitate from the amorphous silica matrix. Mössbauer spectroscopy of the nanocomposites suggested that in the silica glasses, Fe ions were present exclusively as Fe³⁺ in octahedral coordination, and the chemical environment of the Fe³⁺ ions appeared to remain unchanged until the crystallization of nickel ferrite nanocrystals. The formation of NiFe₂O₄ nanocrystals was the result of partial transformation of the FeO₆ octahedra to FeO₄ tetrahedra. The nanocrystalline NiFe₂O₄ are characterized by super‐paramagnetic behaviour at room temperature.