Processing and properties of cobalt-substituted lithium ferrite in the GHz frequency range

Polycrystalline cobalt-substituted lithium ferrites with general formula Li0.5−x/2CoxFe2.5−x/2O4 (x = 0.0, 0.2, 0.4 and 0.5) were prepared by a non-conventional citrate precursor method, which is advantageous over conventional (solid state sintering) method because of low temperature (600 °C) processing. Small amounts of Bi2O3 (0.5 wt.%) were added to reduce the porosity and dielectric loss tangent at sintering temperatures (1000-1200 °C). The complex magnetic permeability (μ′ − jμ″) and complex dielectric permittivity (ϵ′ − jϵ″) of cobalt-substituted lithium ferrites were measured in the 8-13 GHz microwave frequency range. The systematic variations of the real and imaginary parts of both permeability and permittivity with frequency and composition have been analyzed. The minimum value of ϵ′ ∼ 11.3 measured for x = 0.4 substituted lithium ferrite is observed for a sample sintered at 1000 °C. The highest values of ϵ′ and ϵ″ were observed for samples sintered at 1200 °C. μ′ varies between 1.84 and 3.90 for pure and cobalt-substituted lithium ferrite bulk ceramics. The spinel structure of the samples was confirmed by X-ray diffraction (XRD).