Electrical and magnetic properties of chemically derived nanocrystalline cobalt ferrite

Nanocrystalline cobalt ferrite particles of 8 nm grain size were synthesized by coprecipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The experimentally observed changes in the dc electrical conductivity and Curie temperature with heat treatment have been attributed to the changes in the cation distributions as obtained from the Mossbauer and extended x-ray absorption fine structure (EXAFS) measurements and to the grain size. The activation energies for conduction as determined from the Arrhenius plots suggest that the conductivity is due to hopping of both electrons and holes. The observed decrease in conductivity when the grain size is increased from 8 to 92 nm is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mossbauer and EXAFS measurements. But the higher conductivity of the 102 and 123 nm particles compared to that of the 92 nm particles is attributed to the higher grain size, since the cation distribution is found to be the same for all these three samples. The Neel temperature increases from 709 K for the as-prepared particles (8 nm) to 809 K for the 92 nm particles because of the change in the cation distribution and it remains almost the same for the higher grain sizes as there is no further change in the cation distribution. (c) 2007 American Institute of Physics.