Nanocrystalline spinel ferrites by solid state reaction route

摘要

Nanostructured NiFe₂O₄, MnFe₂O₄ and (NiZn)Fe₂O₄ were synthesized by aliovalent ion doping using conventional solid-state reaction route. With the doping of Nb₂O₅, the size of NiFe₂O₄ is reduced down to 33 nm. Similarly, nanostructured manganese ferrites (MnFe₂O₄) with diameters in the range of 45–30 nm were synthesized by Ti4+ ion doping. Particle diameters in all the specimens are found to decrease with increasing dopant content. The substitution of Nb5+ or Ti3+ ions essentially breaks up the ferrimagnetically active oxygen polyhedra. This created nanoscale regions of ferrites. Saturation magnetization and coercive field show a strong dependence on the size of the ferrite grains. Superparamagnetic behaviour is observed from the Mössbauer spectra of nanostructured NiFe₂O₄, if the particle size is reduced to 30 nm. Zero field cooled and field cooled curves from 30 nm sized MnFe₂O₄ particles showed a peak at T _B (∼ 125 K), typical of superparamagnetic blocking temperature. These results are explained in terms of core/shell structure of the materials. The d.c. resistivity of the doped specimens decreases by atleast five orders of magnitude compared to pure sample. This is ascribed to the presence of an interfacial amorphous phase between the sites.