Preparation, thermal stability and permeability behavior of substituted Z-type hexagonal ferrites for multilayer inductors

Co₂Z-type hexagonal ferrites with iron excess Ba₃Co_2 − x Fe_24 + x O₄₁ (0 ≤ x ≤ 0.8) and deficiency Ba₃Co_2 + y Fe_24 − y O₄₁ (0 ≤ y ≤ 0.6) were prepared by an oxalate coprecipitation technique. This synthesis route leads to almost single phase Z-type ferrites for x = 0 after calcination and sintering at 1330 °C. The Z-type formation is enhanced for x > 0 and single phase ferrites are obtained for 0.4 ≤ x ≤ 0.8. The permeability of Z-type ferrites varies with composition x: Maximum permeability of μ′ = 28 is observed for 0.4 ≤ x ≤ 0.6 for samples sintered at 1330 °C. The frequency dispersion shows broad peaks of μ″ stretching from 200 MHz to >1 GHz. For iron deficient samples 0 ≤ y ≤ 0.6 multi-phase mixtures were obtained. For Ag-based multilayer inductor applications sintering at 950 °C is required. Co₂Z ferrites with Fe excess are not stable at this temperature as demonstrated by XRD. The permeability of samples sintered at 950 °C is drastically reduced to μ′ = 3. This demonstrates that these materials are not able to provide sufficient permeability for multilayer inductors for high-frequency operations since they are not compatible with the low temperature ceramic cofiring technology.