Influence of mechanical milling and thermal annealing on electrical and magnetic properties of nanostructured Ni-Zn and cobalt ferrites

The present article reports some of the interesting and important electrical and magnetic properties of nanostructured spinel ferrites such as Ni0.5Zn0.5Fe2O4 and CoFe2O4. In the case of Ni0.5Zn0.5Fe2O4, d.c. electrical conductivity increases upon milling, and it is attributed to oxygen vacancies created by high energy mechanical milling. The real part of dielectric constant (epsilon') for the milled sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The increase in Neel temperature from 538 K in the bulk state to 611 K on the reduction of grain size upon milling has been explained based on the change in the cation distribution. The dielectric constant is smaller by an order of magnitude and the dielectric loss is three orders of magnitude smaller for the milled sample compared to that of the bulk. In the case of cobalt ferrite, the observed decrease in conductivity, when the grain size is increased from 8-92 nm upon thermal annealing 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. The dielectric loss (tan delta) is an order of magnitude smaller for the nano sized particles compared to that of the bulk counterpart.