Z-, Y- and M-type hexagonal ferrites for high-frequency inductive multilayer devices

Hexagonal ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications up to 3 GHz. We report on the preparation, thermal stability and magnetic properties of Z-, Y- and M-type hexagonal ferrites. Co2Z-type ferrite Ba3Co 2Fe24O41 and iron excess Ba3Co 2-yFe24+yO41 (0 y 0.8) were prepared by the mixed oxide route. Single phase Z-type ferrites were obtained after sintering at 1300°C. The permeability of a sample with y = 0.6 sintered at 1300°C is = 30 with a resonance frequency of 1 GHz. The addition of Bi2O3 as sintering aid shifts the maximum shrinkage down to 950°C and dense samples were sintered at 950°C; however, their permeability is only =3...5. It is shown that Co2Z ferrites are not stable under LTCC conditions. Cu-substituted Zrtype ferrites Ba 3Co2-xCuxFe24O41 exhibit enhanced sintering ability, but sintering at 950°C also results in low permeability. Y-type hexagonal ferrites Ba2Co2-x-yZn xCuyFe12O22 were prepared a t 1100°C and a permeability of = 20 was found for x - 1.2 and y = 0.8. Samples with Bi2O3 addition were sintered at 950 and 900°C exhibiting a permeability of = 10. Similar values were measured for M-type ferrites BaFe12-2yCoyTiyO 19 with y = 1.2 which were sintered at 900°C using additives. Co/Ti co-substitution is an essential prerequisite for tailoring the magneto-crystalline anisotropy. Co/Ti- substituted M-type materials are stable under LTCC conditions. Ferrite multilayer devices were fabricated by screen printing coils onto ferrite tapes, stacking and lamination. Firing was performed between 1300°C and 900°C, i.e. at HTCC and LTCC conditions, respectively. The inductance behavior of the devices was evaluated and modeled. It is shown that hexagonal ferrites are suitable materials for the high-frequency multilayer inductors; however, Y- and M-type ferrites are preferred for LTCC- type inductors cofired at 900°C with Ag metallization.