Abstract
Both absorption ( 57Fe) and emission ( 57Co) Mössbauer spectra of frozen samples containing acicular γ Fe 2O 3 particles with coprecipitated cobalt and iron ions have been measured in order to investigate the early stages of the coating process. It is shown that a rapid oxidation of Fe 2+ into Fe 3+ during the coprecipitation of ferrous and cobaltous ions is induced by a process involving the surface of γ Fe 2O 3 particles. The surface is also shown to be responsible for the magnetic ordering observed at the level of precipitated Fe 3+ ions since the first steps of the process. A treatment of γ Fe 2O 3 powder based on the coprecipitation of ferric and cobaltous ions results in the segregation of a large amount of these ions in the form of ultrafine particles with para- or superparamagnetic behaviour. A different distribution of coprecipitated cations associated with different oxidation states of iron in the starting solution is interpreted in terms of the solubility product values for Fe(OH) 3, Fe(OH) 2 and Co(OH) 2.
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