The magnetic properties of composites that consisted of γ-Fe 2 O 3 nanocrystals (4−10 nm) dispersed in submicrometer spherical silica particles at different packing fractions have been studied. The spherical composites were prepared by aerosol pyrolysis of a methanol solution containing tetraethoxysilane and iron nitrate and further annealing in a conventional furnace. Hysteresis loops registered at room temperature indicated that all composites exhibited superparamagnetic behavior. Studies of the thermal evolution of magnetization indicated that these composites could be considered as an adequate experimental system to determine the influence of particle size and interparticle interactions on the magnetic properties. It was observed that the blocking temperature strongly depends on particle size. Also, we have found that interparticle interactions cause frustration of the moments, and we have determined a dependence of the effective anisotropy with the particle size probably associated with the existence of surface anisotropy.
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Tartaj et al. (2002) studied this question.
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