The Effect of Annealing on Phase Stability and Magnetic Properties of Hexaferrite BaFe₁₂ O ₁₉

The hexaferrite BaFe₁₂O₁₉ phase was synthesized through the mechanical alloying process followed by subsequent annealing. Rietveld refinements of as-milled powder annealed at 700 ^∘C confirm the formation of the BaFe₁₂O₁₉ phase with the presence of an important amount of the α-Fe₂O₃ phase. Thus, prior mechanical milling shows much lower reaction temperature and less reaction time compared to conventional methods. Further annealing up to 900 and 1100 ^∘C could not enable the formation of a single BaFe₁₂O₁₉ phase, reaching an optimum phase composition ratio close to BaFe₁₂O₁₉/ α-Fe₂O₃ 70/30. The crystallite size was found to be in the nanoscale level but increases with increasing temperature (BaFe₁₂O₁₉ = 20–62 nm; α-Fe₂O₃ = 31–74 nm). SEM micrographs show that as the annealing temperature rises, the particles become more regular with sharp edges and hexagonal-like shapes. Magnetic measurements reveal that both M_s and M_r increase with annealing temperature to reach maximum values at 900 ^∘C then remain unchanged, associated with phase composition. The coercivity H_c increases upon annealing up to 700 ^∘C to a much higher value, from 1.7 kOe for as-milled powder to 4.8 kOe. Its value then decreases, attributed to grain (particle) growth (formation of larger particles) due to high annealing temperatures: 900–1100 ^∘C. The obtained composites show very interesting magnetic properties and can be considered for potential applications, such as hyperthermia, heavy metal and dye removal, and hard/soft magnetic composites.