$\hbox {Mg}_{1-{\rm x}}\hbox {Co}_{\rm x}\hbox {Fe}_{1.98}\hbox {O}_{4}$ Ceramics With Promising Magnetodielectric Properties for Antenna Miniaturization

We have studied the effects of concentration of cobalt and sintering temperature on the complex relative permittivity and permeability of MgFe_1.98O₄ ferrite ceramics. All samples were doped with 3 wt% Bi₂O₃ to improve the sintering properties of the Mg_1-xCo_xFe_1.98O₄ ceramics and thus attain a low dielectric loss tangent. X-ray diffraction data indicated that the ferrite samples all have a single-phase spinel structure and that there was no reaction between the ferrite and Bi₂O₃. In terms of microstructure development and grain growth, the effect of Bi₂O₃ is much more pronounced than that of Co. At a given concentration of Co, the magnetic properties can be fairly explained by a magnetic circuit model. The relationship between static permeability and the concentration of Co can be understood by using the single-ion model combined with the local uniaxial anisotropic model. Among the samples we prepared, Mg_0.96Co_0.04Fe_1.98O₄ sintered at 1000degC for 2 h has almost equal real permeability and permittivity-both approximately 10 - as well as sufficiently low dielectric and magnetic loss tangent over a range 3 to 30 MHz [the high-frequency (HF) band]. These properties that should be useful for miniaturization of HF antennas.