Effect of grain size on electromagnetic properties of Ni0.7Zn0.3Fe2O4 ferrite

The electromagnetic properties of polycrystalline Ni0.7Zn0.3Fe2O4 with varying grain size in the range 1-12 μm, were investigated. The grain size was varied by sintering the ferrite at different temperatures/time upto 1300 deg C/240 min. A rapid grain growth was observed at 1300 deg C/240 min. Initial permeability was found to increase linearly with the grain size upto about 5 μm and after that the rate of increase was reduced due to the formation of more number of closed porosity. The permeability and relative loss factor were very stable upto about 10 MHz in small-grained cores. A relatively lower loss factor in the order of 10-5 was found in the material. Resistivity was found to decrease sharply with the grain size. The ferrite with 1250 deg C/60 min firing was best for high-frequency low-loss application