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).