The Structure And Magnetic Properties Of Ni-Zn Ferrite Nanoparticles

The Zn_1-xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol-gel auto-combustion in order to investigate their microstructure and magnetoelectric properties by meas of Differential thermal analysis (DTA), Scanning electron microscope (SEM), X-ray diffraction (XRD), Vibrating sample magnetometer (VSM), and LCR automatic measurement systerm. The results of DTA indicate that the decomposition characteristic of dried gels is independent of Ni content x, the thermal decomposition temperature is about 210 oC and Ni-Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. It is observed from XRD that Zn_1-xNi_xFe₂O₄ nanoparticles with pure spinel cubic structure could be abtained only by once heat-treating at low temperature. The average grain size of samples heat-treated at 700 oC is about 35 nm. The a of Zn_1-xNi_xFe₂O₄ nanoparticles declines gradually with increasing x, and is larger than that of the bulk. VSM redults show that different from the composition of Zn0.5Ni0.5Fe2O4 for the bulk, the maximum Ms (69.1 emu/g) of Zn_1-xNi_xFe₂O₄ nanoparticles is found in the composition of Zn_0.3Ni_0.7Fe₂O₄. The Ms of samples with a lower Ni content is smaller than the bulk while is larger than the bulk for the samples with a higher Ni content. The Hc of Zn_1-xNi_xFe₂O₄ nanoparticles is much larger than the bulk and increases with enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ heat-treated at different temperatures indicate that the maximum Ms (83.2 emu/g) appears in the sample with an average grain size of 100 nm which could be considered as the critical diameter of nano-magnetic properties of particles (The nano-magnetic properties of particles disapear gradually when nanoparticles are larger than 100 nm). The Hc of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as grain size increases, and presents a maximum value of 114.92 Oe when grains grow up to about 30 nm, and then declines to that of the bulk rapidly with grains further growing. The single-domain critical diameter (under the critical diameter grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm. SEM shows that naniparticles are spherical and quite uniform. The grains grow up with enhancing heat-treating temperature, and the grains annealed at temperatures belower than 900℃are single crystalline grains. The results of LCR show that the resistivity is up to 10⁷ MΩ·cm.