Co/Ti-substituted M-type hexagonal ferrites for high-frequency multilayer inductors

• Saturation magnetization and coercivity of Co–Ti substituted M-type hexaferrites BaFe12−2xCoxTixO19 decrease with x.
• Ferrites were sintered at low-temperature of T=900 °C using a BBSZ glass additive.
• Large initial permeability of µ′=17 was obtained at x=1.3 for low-temperature sintered Co/Ti-substituted ferrites.
• Monolithic ferrite multilayer inductor with Co/Ti substituted M-type ferrite layers and Ag metallization was cofired at 900 °C.
• Demonstrates, that monolithic ferrite multilayer based on Co/Ti substituted hexagonal ferrites can be operated up to high frequency of 1 GHz.

The sintering behavior, microstructure and permeability of Co/Ti-substituted M-type hexagonal ferrites BaCoxTixFe12−2xO19 (1.0≤x≤1.4) was studied for applications as multilayer inductors in the high-frequency range up to 2 GHz. Single-phase M-type ferrites were obtained after calcination at 1000 °C. The saturation magnetization and coercivity decrease with x and hysteresis measurements illustrate a gradual transition of the anisotropy from uniaxial to planar. Addition of 5 wt% of a BBSZ glass allows densification at 900 °C through liquid-phase sintering. The permeability of samples sintered at 900 °C increases with the Co/Ti substitution and reaches its maximum of µ′=16 at 1 MHz at x=1.3 with a resonance frequency fr≥1 GHz. Monolithic ferrite multilayer inductors were fabricated with printed Ag coil patterns by co-firing at 900 °C. It is shown that Co/Ti-substituted hexagonal M-type ferrite is an excellent material for the high-frequency multilayer inductors.