Some Magnetic Properties of Magnetite (Fe3O4) Microcrystals

Abstract

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 K_u=(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 K_u 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 K_u=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.