Microstructure and Electromagnetic Properties of Ni-Zn-Co Ferrite up to 20 GHz

The present paper examines the relation between different developed microstructures and the microwave electromagnetic properties in Ni-Zn-Co ferrite. To this end, the Ni0.25Zn0.25Co0.5Fe2O4 composition has been prepared with the conventional ceramic process with varied prefiring T-P (750 degrees C, 1000 degrees C) and sintering T-S top temperatures (1200 degrees C, 1250 degrees C). When lower temperatures are applied in these production stages, incomplete microstructures with low density, higher porosity, or finer grains are achieved. On account of these features, the contributions of domain wall motion and spin rotation to the complex permeability mu*(f) move to higher frequencies, whereas microwave dielectric permittivity epsilon*(f) is decreased. In particular in conjunction with the high Co content, the wall relaxation and spin resonance are interestingly forced to occur at 850 MHz and 8.05 GHz, respectively. Regarding the electromagnetic wave attenuation, the ferrite annealed at lower temperatures exhibits strong return loss peaks at higher frequencies, but without other performance improvement. We should notice that the variations in sintering temperature T-S yield the maximum changes in the recorded parameters, including the coercive field, mu*(f), and epsilon*(f), indicating the inferior role of prefiring T-P in Ni-Zn ferrite.