| Abstract
| - NiFe2O4 nanocrystals were dispersed in silica by a sol−gel route. The dried gel wasamorphous, in which isolated Fe3+ ions had a weak interaction with silica matrix, ascharacterized by a weak IR absorption at ca. 580 cm-1. Heat treatment at 400 °C resultedin nickel ferrite clusters being partially formed, and these clusters were observed to interactwith the matrix through Si−O−Fe bonds. This interaction reached its maximum with thecomplete formation of NiFe2O4 clusters as the temperature was raised to 600 °C. Above thistemperature, NiFe2O4 clusters grew larger into nanocrystals, while the interaction betweenthe nanocrystals and silica matrix disappeared with breakage of Si−O−Fe bonds. The graingrowth for magnetic nanoparticles was accompanied with rearrangement of amorphous silicanetwork. The preference of forming NiFe2O4 nanocrystals eliminated the possibility ofprecipitation of crystallite component oxides, e.g., NiO, γ-Fe2O3, or Fe3O4 in amorphous silicamatrix, or crystalline silica, e.g., cristobalite or quartz, even when the treatment temperaturewas 1100 °C. Fe ions in silica glasses were determined by Mössbauer spectroscopy to bepresent exclusively as Fe3+ ions in a high-spin state at octahedral coordination, and thechemical environment of the Fe3+ ions seemed to remain unchanged until the nickel ferriteclusters crystallized. The formation mechanism for NiFe2O4 nanocrystals can be explainedin terms of Ni2+ ions shifting from the tetrahedral centers to undistorted octahedral sites inthe spinel lattice and the partial transformation of FeO6 octahedron to FeO4 tetrahedron.The critical dimension for the NiFe2O4 nanocrystals in silica was detected as ca. 9 nm. Belowthe critical size, NiFe2O4 nanocrystals had a superparamagnetic single-domain structure,while the nanocrystals with particle sizes larger than the critical size exhibited bulklikebehavior.
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