Synthesis, sintering behavior and magnetic properties of Cu-substituted Co₂Z hexagonal ferrites

Cu-substituted Z-type Ba₃Co_2−x Cu _x Fe₂₄O₄₁ hexagonal ferrites (0 ≤ x ≤ 1) were prepared by the standard oxide route. Almost single phase Z-type ferrite powders were observed after calcination at 1,260 °C. The permeability of samples sintered at 1,260 °C varies with the Cu content; a maximum permeability at 10 MHz of μ′ = 17 was found for x = 0.6. It is shown that the permeability behavior is determined by the concentration of Z-type ferrite in the sample and the microstructure. Addition of 3 wt% Bi₂O₃ leads to almost complete densification of Co₂Z samples under low temperature ceramic cofiring (LTCC) conditions at 900 °C. However, the permeability of samples sintered at 900 °C is reduced to μ′ = 5 with resonance frequency at 2 GHz. It is shown that Cu-substituted Co₂Z ferrites are not stable at T < 1,260 °C. Partial decomposition into other hexagonal ferrites causes a reduction of the permeability. In spite of this severe drawback, Co₂Z hexagonal ferrites with Cu modifications are suitable materials for multilayer inductors for high frequency applications.