Size dependence of the properties of hematite nanoparticles

Samples of nanoparticles of hematite $(\alpha$-$\ab{Fe_2O_3})$ with average size between 6 and 27 nm have been studied by use of Mössbauer spectroscopy. The superparamagnetic relaxation was analysed on the basis of the Néel-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.