Electrical and magnetic properties of magnesium ferrite ceramics doped with Bi2O3

The effects of concentration of Bi2O3 and sintering temperature on DC resistivity, complex relative permittivity and permeability of MgFe1.98O4 ferrite ceramics were studied. The objective of the study was to develop magneto-dielectric materials, with almost equal values of permeability and permittivity, as well as low magnetic and dielectric loss tangent, for the design of antennas with reduced physical dimensions. It was found that the poor densification and slow grain growth rate of MgFe1.98O4 can be greatly improved by the addition of Bi2O3, because liquid phase sintering was facilitated by the formation of a liquid phase layer due to the low melting point of Bi2O3. It was found that 3% Bi2O3 can result in fully sintered MgFe1.98O4 ceramics. The DC resistivities of the MgFe1.98O4 ceramics were increased as a result of the addition of Bi2O3, except for 0.5%. The exceptionally low resistivities of the 0.5% samples were explained by a 'cleaning' effect of the small amount of liquid phase at the samples' grain boundaries. The electrical and magnetic properties of the MgFe1.98O4 ceramics exhibited a strong dependence on the concentration of Bi2O3. The 0.5% samples were found to have the highest dielectric loss tangents, which can be understood similarly to the DC resistivity results. The 2-3% Bi2O3 is required to attain low dielectric loss MgFe1.98O4 ceramics for antenna application. Low concentration of Bi2O3 increased the static permeability of the MgFe1.98O4 ceramics owing to the improved densification and grain growth, while too high a concentration led to decreased permeability owing to the incorporation of the non-magnetic component (Bi2O3) and retarded grain growth. However, the addition of Bi2O3 alone is not able to produce magneto-dielectric materials based on MgFe1.98O4 ceramics, and further work is necessary to modify the permeability using cobalt (Co).