Microstructure and magnetic properties of Ni-Zn ferrites doped with MnO2

To improve the performance of Ni-Zn ferrites for power field use, the influence of MnO2 additive on the properties of Ni-Zn ferrites was investigated by the conventional powder metallurgy. The results show that MnO2 does not form a visible second phase in the doping mass fraction range of (0-2.0%). The average grain size, sintering density and real permeability gradually decrease with the increase of the MnO2 content. And the DC resistivity continuously increases with the increase of MnO2 content. The saturation magnetization (magnetic moment in unit mass) first increases slightly when mass fraction of MnO2 is less than 0.4% MnO2, and then gradually decreases with increasing the MnO2 mass fraction due to the exchange interaction of the cations. When the excitation frequency is less than 1 MHz, the power loss (Pcv) continuously increases with increasing the MnO2 content due to the decrease of average grain size. However, when the excitation frequency exceeds 1 MHz, eddy current loss gradually becomes the predominant contribution to Pcv. And the sample with a higher resistivity favors a lower Pcv, except for the sample with 2.0% MnO2. The sample without additive has the best Pcv when worked at frequencies less than 1 MHz; and the sample with 1.6% MnO2 additive has the best Pcv when worked at frequencies higher than 1 MHz.