Low-Temperature Firing of Substituted M-Type Hexagonal Ferrites for Multilayer Inductors

Co/Ti-substituted M-type BaFe_12-2xCo_xTi_xO₁₉ ferrites were investigated for multilayer inductor applications. Single-phase ferrite powders were prepared at 1000°C through the mixed oxide route. After sintering at 1300°C a maximum permeability of μ´ = 37 is observed for x = 1.2. The addition of a glass sintering aid shifts the temperature of maximum shrinkage down to below 900°C and dense samples were obtained after firing at 900°C. For ferrites with x = 1.2 a permeability of μ´ = 13 and a resonance frequency of 1 GHz is observed. Contrary to substituted Zand Y-type hexagonal ferrites, the Co/Ti-substituted M-type ferrites are stable during co-firing at 900°C and show no sign of thermal decomposition, i.e., these materials are compatible with the low-temperature ceramic cofiring (LTCC) technology. It is shown that Co/Ti-M-type ferrites are excellent magnetic materials for multilayer inductors.