Characterization of iron oxide nanoparticles in an Fe2O3-SiO2 composite prepared by a sol-gel method

An Fe2O3-SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials. The dried gel was treated at increasing temperatures, and the samples were characterized by XRD, TEM, magnetic susceptibility measurements, and EPR and Mossbauer spectroscopies. Nanometer size (3-4 nm) X-ray-amorphous iron(III) oxide particles are observed in the samples treated at low temperature. These particles display superparamagnetic behavior in the Mossbauer spectra and susceptibility measurements, and their magnetic moments indicate antiferromagnetic clustering. The occurrence of two sites for iron ions, one in the bulk and one on the surface of nanoparticles,is suggested by EPR and Mossbauer spectroscopies. Heating of the samples to higher temperatures (T > 700 degrees C) gives rise to a small increase of the particle size. Simultaneously XRD and TEM exhibit the formation of gamma-Fe2O3 crystalline particles, Mossbauer spectra reveal a large change in the magnetization, magnetic measurements show the transition from antiferro- to ferrimagnetic behavior, and the signals of ferrimagnetic resonance appear in the EPR spectra. Superparamagnetic relaxation in these particles was investigated by ac susceptibility measurements and paramagnetic resonance experiments at variable temperatures. Further increase in temperature gives rise to the formation of alpha-Fe2O3. Atomic reorganization processes taking place during these transitions are discussed.