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Mossbauer spectra of 57Fe in ultrafine NiFe2O4 and CoFe2O4 powders were studied, as a function of crystallite size and temperature. The hyperfine field was found to collapse at a temperature, lower then the Curie points which decreased with particle size. This collapse of the hyperfine field is interpreted as motional narrowing due to the onset of superparamagnetism.
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Schuele et al. (1965) studied this question.
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