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In this study, we report on how surface-passivated and nonpassivated cobalt ferrite nanoparticles (8 nm diameter), suspended as ionic magnetic fluids and aged under low pH conditions, revealed different behavior as far as the time evolution of the iron/cobalt cation distribution, crystal quality, coercivity, and saturation magnetization are concerned. Different techniques were used to perform a detailed study regarding the chemical stability, structural stability, and surface and magnetic properties of the suspended nanoparticles as a function of the aging time. Properties of surface-passivated and nonpassivated nanoparticles were investigated by transmission electron microscopy, X-ray diffraction, atomic absorption spectrometry, magnetic measurements, Raman spectroscopy, and Mössbauer spectroscopy. Our data showed that the employed nanoparticle surface passivation process, besides the formation of an iron-rich surface layer, modifies the nanoparticle core as well, improving the crystal quality while modifying the Fe/Co cation distribution and the nanoparticle dissolution rate profile. Magnetic data showed that the saturation magnetization increases for surface-passivated nanoparticles in comparison to the nonpassivated ones, though coercivity decreases after passivation. These two observations were associated to changes in the cation distribution among the available tetrahedral and octahedral sites.

Preparation of size-controllable magnetite (Fe 3 O 4 ) nanoparticles by alkaline oxidation of ferrous ion adsorbed in sulfonated mesoporous styrene− divinylbenzene copolymer is described. It was observed that the magnetite nanoparticle size increases by ion-charging the sulfonated polymeric template with ferrous aqueous solution at increasing iron concentration. A simple model describing the amount of iron incorporation in the polymeric template is proposed. The magnetite-based composite was investigated by atomic absorption, transmission electron microscopy, Mo 1ssbauer spectroscopy, X-ray diffraction, and magnetization data.

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Samples of ZnxMg1−xFe2O4 (0≤x≤1) synthesized by the combustion reaction method were investigated by x-ray diffraction, Mössbauer spectroscopy, and Raman spectroscopy. All the samples are found to have a cubic spinel structure and the lattice parameter increases linearly with increasing Zn-content (x). The Mössbauer data showed that the replacement of Mg2+ ions for Zn2+ ions changes substantially the hyperfine parameter. Moreover, it was verified the presence of Fe3+ ions both in A and B sites. The Raman spectra showed five predicted Raman bands for the spinel structure and it was observed the splitting of the A1g Raman mode into tree branches, where each one have been attributed to peaks belonging to each ion (Zn, Mg, and Fe) in the tetrahedral positions.

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