Al³⁺ doped M-type hexagonal Ba–Co ferrites synthesized via ball-milling assisted ceramic process: magnetism and its correlation with structural properties

Ba_0.5Co_0.5Al_xFe_12−xO₁₉ (x = 0, 0.08, 0.16, and 0.24) hexaferrites are synthesized via ball-milling assisted ceramic process, and their crystal structure, microstructure, and magnetic properties were studied. The results show that all samples, calcined between 950 and 1150 °C, consist of the main M-type hexagonal Ba ferrite phase in combination of a small amount of CoFe₂O₄ and Fe₂O₃ phase. The lattice parameters of M-type hexagonal Ba_0.5Co_0.5Al_xFe_12−xO₁₉ decrease after Al³⁺ doping. The addition of Al³⁺ ions results in a reduction of crystallite size, which is attributed that the presence of foreign phase CoFe₂O₄ and Fe₂O₃ restrains the growth of the Ba_0.5Co_0.5Al_xFe_12−xO₁₉ crystallite. Magnetic characterization indicates that all samples exhibit hard magnetic properties. Trend of specific saturation magnetization of Ba_0.5Co_0.5Al_xFe_12−xO₁₉ sample, calcined at 1050 and 1150 °C, decreases with the increase in Al³⁺ content. The varied magnetic properties with substitution content (x) are well explained by the occupancy effects of Al³⁺ ions in magnetoplumbite structure. Ba_0.5Co_0.5Fe₁₂O₁₉, calcined at 1150 °C, has the highest specific saturation magnetization value (56.07 emu/g), remanence (28.66 emu/g), and moment (10.76 µ_B). Besides, with the increase of substitution content (x), magnetic domain type of ferrites, calcined at 1150 °C, changes from a single magnetic domain to a multi-domain type.