The Effect of Annealing on Phase Stability and Magnetic Properties of Hexaferrite BaFe12O19

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 Ms and Mr increase with annealing temperature to reach maximum values at 900 ∘C then remain unchanged, associated with phase composition. The coercivity Hc 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.