Mossbauer studies of magnetite and Al-substituted maghemites

This paper reviews a systematic Mossbauer study of maghemite gamma-Fe2O3 and Al-substituted maghemites gamma-(Fe1-yAly)(2)O-3. Three series off samples prepared from different methods and having different morphological characteristics and aluminum contents were investigated. It was found that both the cation distribution and the solubility limit depend on the preparation method, and no general conclusion in that respect could be inferred. From the temperature dependence of the hyperfine fields the exchange integrals could be calculated, and were found as: J(AB) = -25 K, J(AA) = -18 K, and J(BB) = -3 K. The hyperfine fields show a crossing in the vicinity of 300 K, for both substituted and unsubstituted samples, as a result of the relatively strong A-A interaction. The Curie temperatures were found to be in the range of 948-730 K, the lower value referring to the sample containing 22 mole% Al. The influence of maghemite on the Mossbauer spectra (MS) of magnetite was explored in some detail. It was demonstrated on the basis of the MS recorded for a variety of reference mixtures, that it is not possible to resolve the ferric A-site components due to maghemite and magnetite, even with the absorber subjected to a strong external field.