Electrical conductivity and dielectric behaviour of nanocrystalline NiFe2O4 spinel

Electrical conductivity and dielectric measurements have been performed for nanocrystalline NiFe2O4 spinel for four different average grain sizes, ranging from 8 to 97 nm. The activation energy for the grain and grain boundary conduction and its variation with grain size have been reported in this paper. The conduction mechanism is found to be due to the hopping of both electrons and holes. The high-temperature conductivity shows a change of slope at about 500 K for grain sizes of 8 and 12 nm and this is attributed to the hole hopping in tetrahedral sites of NiFe2O4. Since the activation energy for the dielectric relaxation is found to be almost equal to that of the de conductivity, the mechanism of electrical conduction must be the same as that of the dielectric polarization. The real part epsilon' of the dielectric constant and the dielectric loss tandelta for the 8 and 12 nm grain size samples are about two orders of magnitude smaller than those of the bulk NiFe2O4. The anomalous frequency dependence of epsilon' has been explained on the basis of hopping of both electrons and holes. The electrical modulus analysis shows the non-Debye nature of the nanocrystalline nickel ferrite.