Nanocrystalline nickel ferrites prepared by doping with niobium ions

Doping of nickel ferrites by suitable aliovalent ions was explored in order to synthesize nanocrystalline ferrites. With the doping of Nb2O5, the size of NiFe2O4 is reduced down to 33 nm. The substitution of Nb5+ ions essentially break up the ferrimagnetically active oxygen polyhedra and the size of the grains are confined. An increase of coercive field and a decrease of magnetization is observed as the size of NiFe2O4 is reduced. This behavior can be explained introducing core-shell model of magnetic nanoparticles. Mössbauer spectra shows superparamagnetic doublet that supports the presence of smaller sized particles in the specimens. The DC resistivity of the doped specimens decreases by at least five orders of magnitude compared to pure sample. This is ascribed to the presence of an interfacial amorphous phase between the sites.