Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

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Abstract--Several samples of large-and small-particle magnetite (Fe304), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mrssbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly-or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400*(2, transforms to a mixture of wellcrystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300"C in a controlled 02 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mrssbauer spectra of the decomposition products at T < 200"C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

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