MOSSBAUER-EFFECT IN OXIDIZED IRON FINE PARTICLES AND EXPLANATION OF THE SPECTRA BY MAGNETIC FLUCTUATION

Mossbauer measurements were made on iron fine particles of various degrees of oxidation, and the spectra due to the oxidized part were studied with varying temperature (78 K - room temperature) and applied magnetic field (0-1.05 T). All samples had similar dependence of the spectral shape on temperature below 200 K. The spectra for less oxidized samples were only slightly affected by an applied magnetic field of 0.71 T at room temperature. For largely oxidized samples, strong dependence on temperature and applied field was observed above 200 K and below 0.36 T. The dependence of the spectral shape on temperature and applied field was discussed by fitting a magnetic fluctuation model to the experimental spectra. The result showed that there was a thermal fluctuation of the magnetization vector of the microcrystals constituting the oxide surface layer under the influence of various magnetic interactions, and that the magnetic fluctuation of the cluster as a whole on a much longer time scale was responsible for the rapid change of the spectra with temperature and applied field above 200 K and below 0.36 T observed for largely oxidized samples.