Co 2 Z hexagonal ferrites for multilayer inductors

Z type Ba₃Co_1·4Fe_24·6O₄₁ for multilayer inductor applications at high frequency (up to 2 GHz) was prepared by a standard ceramic route. Single phase Z type ferrite is obtained at 1300°C. Dense samples sintered at 1300°C display a permeability of μ=30 throughout the megahertz range and a maximum μ″ at 0·5 GHz. The addition of Bi₂O₃ shifts the temperature of maximum shrinkage down to 900°C. Dense samples were obtained after sintering at 900°C; however, their permeability is only μ=5, but the maximum μ″ is at 2 GHz. It is shown that Co₂Z ferrite is not stable at T<1300°C and that partial decomposition causes a reduction of permeability. Ferrite tapes were prepared by tape casting, and multilayer structures were fabricated by screen printing, stacking, lamination and cofiring. Firing was performed at 1300, 1000 and 900°C, i.e. at high and low temperature ceramic cofiring conditions respectively. It is shown that Co₂Z hexagonal ferrites are suitable materials for multilayer inductors, although those prepared at 900°C suffer from lower permeability.