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 Mössbauer 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 Mössbauer 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 Mössbauer spectroscopies. Heating of the samples to higher temperatures (T > 700 °C) gives rise to a small increase of the particle size. Simultaneously XRD and TEM exhibit the formation of γ-Fe2O3 crystalline particles, Mössbauer 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 α-Fe2O3. Atomic reorganization processes taking place during these transitions are discussed.

No abstract

The structural and magnetic properties of two nanocomposites, containing respectively 16.9 and Fe 2 O 3 ÈSiO 2 28.5 wt.% were investigated. The samples were synthetized by a solÈgel method, using ethylene glycol Fe 2 O 3 , as a solvent, and heating the gels gradually to 900 ¡C. The procedure allowed us to obtain c-Fe 2 O 3 nanoparticles homogeneously dispersed in the amorphous silica matrix. The particles have a narrow size distribution and mean sizes from 3 to 6 nm depending on the iron oxide content. The magnetic properties of the samples were investigated by static and dynamic susceptibility measurements. All the samples showed superparamagnetic behaviour. The superparamagnetic relaxation was investigated also by Mo ssbauer spectroscopy. Hysteresis loops were measured at 2.5 K and both samples showed high values of coercive Ðeld. The role of magnetic interparticle interactions on the magnetic properties is discussed.