Nanocrystalline spinel ferrites by solid state reaction route

摘要

Nanostructured NiFe_2O_4, MnFe_2O_4 and (NiZn)Fe_2O_4 were synthesized by aliovalent ion doping using conventional solid-state reaction route. With the doping of Nb_2O_5, the size of NiFe_2O_4 is reduced down to 33 nm. Similarly, nanostructured manganese ferrites (MnFe_2O_4) with diameters in the range of 45-30 nm were synthesized by Ti~(4+) ion doping. Particle diameters in all the specimens are found to decrease with increasing dopant content. The substitution of Nb~(5+) or Ti~(3+) 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 Mossbauer spectra of nanostructured NiFe_2O_4, if the particle size is reduced to 30 nm. Zero field cooled and field cooled curves from 30 nm sized MnFe_2O_4 particles showed a peak at T_B (approx 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.