Mössbauer spectra of single-domain fine particle systems described using a multiple-level relaxation model for superparamagnets

A multilevel relaxation model has been developed to describe the dynamic behavior of a single-domain particle from blocked through to superparamagnetic. When combined with an accurate expression for the relaxation time and a log-normal particle size distribution, this model successfully describes the M\"ossbauer spectra of real fine particle systems at all temperatures of interest, and yields consistent values for anisotropy and blocking temperature. Spectra of two Fe₃O₄ ferrofluids and a polysaccharide iron complex have been fitted. Blocking temperatures (TB) determined with our model agree with those extrapolated from frequency dependent χac data. Anisotropy energies $(K)$ are in the range of 1--3×10⁴J/m³ and superparamagnetic relaxation times are ~10^-8 s. Interparticle interactions are shown to reduce both K and TB.