Size dependence of the properties of hematite nanoparticles

Samples of nanoparticles of hematite (alpha -Fe2O3) with average size between 6 and 27 nm have been studied by use of Mossbauer spectroscopy. The superparamagnetic relaxation was analysed on the basis of the Neel-Brown expression for the relaxation time, tau = tau (0) exp[KV/kT]. It was found that the value of tau (0) increases with increasing particle volume, V, whereas the magnetic anisotropy energy constant, K, decreases. The electric quadrupole interaction, the isomer shift and the magnetic hyperfine field extrapolated to 0 K were found to be essentially independent of particle size.