Effect of cobalt substitution on the structure, electrical, and magnetic properties of nanorcrystalline Ni_(0.5)Zn_(0.5)Fe_2O_4 prepared by the polyol process

摘要

Highly crystalline Co_xNi_(0.5-x)Zn_(0.5)Fe_2O_4 (x=0.0, 0.2, 0.4) nanoparticles of about 5 nm in size were synthesized by the polyol method and subsequently sintered at 700 °C for 4 h. The structural, electrical, and magnetic properties of the sintered ferrites were investigated using various techniques including X-ray diffraction, thermal analysis, infrared and energy dispersive X-ray spectroscopies, transmission electron microscopy, and vibrating sample magnetometry. The lattice parameter increased linearly with Co content in agreement with increasing substitution of Ni~(2+) for Co~(2+). The sintered powders are composed of nanograms with average size ranging from ~30 to ~40 nm. The electrical study showed a resistivity decrease with increasing temperature indicating a semiconducting behavior. In addition, the dc conductivity was found to follow the Arrhenius law with a slope change observed at a critical temperature, the Curie temperature. The variation of the dielectric properties (dielectric constant and the dielectric loss) with frequency and temperature was explained on the basis of Maxwell-Wagner type of interfacial polarization. At room temperature, the best characteristics (high resistivity, high permittivity, and low loss factor) were found with the Co composition x=0.2. They were found to be considerably better than those of the unsubstituted Ni-Zn bulk ferrite. Magnetic investigation showed that all the sintered particles exhibit superparamagnetic behavior at room temperature. Additionally, the saturation magnetization and the Curie temperature showed similar trend; they increased slightly from x=0.0 to x=0.2 and then they decreased notably for the composition x=0.4. This can be attributed to the changes of superexchange interactions between cations in the spinel-type sublattices on Ni substitution.