Single-domain particles of Fe3O4 have been observed to exhibit superparamagnetic behavior with the Mössbauer effect. Using a simple theory for the spin relaxation, it was possible to make an approximate curve fit of the spectra, taking into account the particle-size distribution of the samples. For a sample prepared by a grinding method, the values obtained by the fitting procedure were Ku=(6.0±1.0)×104 erg/cm3 for the energy barrier and τ0=(9.5±1.5) × 10−10 sec for the frequency factor. The value of Ku was interpreted as being due to shape anisotropy; analysis of the hyperfine spectra of both bulk and fine-particle Fe3O4 at T=4.2°K supported this view. The flipping frequencies of the particle moments were altered by the application of a magnetic field, so that the average internal field in a relaxed spectrum could be determined. Other samples of Fe3O4, prepared by a precipitation method, gave the values Ku=6.0×104 and 8.0×104 erg/cm3. The latter value could be partially accounted for by the existence of a small oxygen excess in those particles prepared at temperatures close to 100°C.
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