The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn1−xNixFe2O4 ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn0.5Ni0.5Fe2O4 for the bulk, the maximum Ms is found in the composition of Zn0.3Ni0.7Fe2O4 for nanoparticles. The Hc of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn0.3Ni0.7Fe2O4 annealed at different temperatures indicate that the maximum Ms (83.2 emu/g) appears in the sample annealed at 900 °C. The Hc of Zn0.3Ni0.7Fe2O4 firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn0.3Ni0.7Fe2O4 nanoparticles is found to be about 30 nm.