Size quantization and interfacial effects on a novel γ-Fe₂O₃/SiO₂ magnetic nanocomposite via sol-gel matrix-mediated synthesis

Novel isolated magnetic single-domain γ-Fe₂O₃ nanoclusters have been prepared by coprecipitation of ferrous and ferric salts encapsulated within sol-gel derived silica (SiO₂). The nonmagnetic SiO₂ coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of the Fe₂O₃ nanoclusters provides a means for thermally stable dispersion of Fe₂O₃ clusters. The precipitated particles coated with SiO₂ are spherical with 4-5 nm diameters. Surface and strain effects played a critical role in determining the overall magnetic behavior of the spherical single-domain particles. Superparamagnetic behavior was observed by superconducting quantum interference device magnetometry and Mössbauer spectroscopy. Superparamagnetic barrier energies and the low-temperature coercivities were modified through cluster/support interface microstructure manipulation. The optical studies showed the absorption edge of the nanocomposites to be slightly blue shifted in the UV-VIS spectrum range when compared to that of bulk γ-Fe₂O₃. This was attributed to the combined effects of the quantum confinement of the nanocrystalline γ-Fe₂O₃ clusters and the stress present at the particle/support interface. The magnetic properties can be manipulated via the matrix microstructure, synthesis conditions and thermal treatment.