Effects of base composition and dopants on the properties of hexagonal ferrites

The effects of the base composition and dopants on the saturation magnetization and coercivity of hexaferrites BaAl_xFe_12–xO₁₉, SrAl_xFe_12–xO₁₉, BaGa_xFe_12–xO₁₉, SrGa_xFe_12–xO₁₉, BaSc_xFe_12–xO₁₉ and SrSc_xFe_12–xO₁₉ have been studied. Isomorphic substitutions of Al₂O₃, Ga₂O₃, and Sc₂O₃ for Fe₂O₃ in barium and strontium ferrites are found to increase coercivity due to increasing crystallographic anisotropy constant and to reduce the saturation magnetization value. Processes controlling microstructure formation, specifically recrystallization processes, are shown to have a noticeable effect on the level of properties of the ferrites under study with the use of dopants. The most efficient dopants are boron, calcium, and silicon oxides, which provide the formation of relatively fine-grained structures. The increased coercivity upon doping with these dopants is also due to the formation of grain-boundary interlayers of a nonmagnetic glassy phase and the associated efficient retardation of moving domain walls.