Size dependence of the properties of hematite nanoparticles

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