Tuning of magnetic properties of aluminium-doped strontium hexaferrite powders

M-type Al-doped strontium ferrite powders (SrAl_xFe_2n-xO₁₉, n=5.9) with nominal Al content of x=0-2.0 are prepared by traditional ceramic technology. The phase identification of the powders, performed using x-ray diffraction, shows the presence of purity hexaferrite structure and absence of any secondary phase. The lattice parameters decrease with increasing x. The average grain size of the powders is about 300 nm-400 nm at Al³⁺ ion content x=0-2.0. The room-temperature hysteresis loops of the powders, measured by using vibrating sample magnetometer, show that the specific saturation magnetization (σ_s) value continuously decreases while the coercivity (H_c) value increases with increasing x, and H_c reaches to 9759 Oe (1 Oe=79.5775 A/m) at x=2.0. According to the law of approach saturation, H_c value increases with increasing Al³⁺ ion content, which is attributed to the saturation magnetization (M_s) decreasing more rapidly than the magnetic anisotropy constant (K₁) obtained by numerical fitting of the hysteresis loops. The distribution of Al³⁺ ions in the hexaferrite structure of SrAl_xFe_2n-xO₁₉ is investigated by using ⁵⁷Co Mössbauer spectroscopy. The effect of Al³⁺ doping on static magnetic properties contributes to the improvement of magnetic anisotropy field.