Structural and Magnetic Properties of High Coercive Al-Substituted Strontium Hexaferrite Nanoparticles

The aim of this work is to investigate the correlation between replacement of Fe³⁺ ions by Al³⁺ and structural and magnetic properties of Sr-hexaferrites. To this regard, a simple sol–gel auto-combustion method followed by subsequent heat treatment in air was employed to synthesize nanocrystalline SrFe_12−xAl_xO₁₉ (0 ≤ x ≤ 4). X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared (FT-IR), and vibrating sample magnetometer (VSM) methods were used in order to characterize the produced samples. The results show that the formation of the M-type hexaferrite phase is associated with some α-Fe₂O₃ as a secondary phase at low calcination temperature. By increasing the calcination temperature, high-purity hexaferrite phase without any unwanted phases can be formed. The average particle size of the substituted strontium ferrite gradually became smaller by addition of aluminum. The room temperature saturation magnetization values continuously reduced by increasing Al³⁺ that is contributed to substitution of Fe³⁺ by nonmagnetic ions in the lattice. However, the coercivity initially increases and then decreases with increasing the Al³⁺ content. A high coercivity up to 9.1 kOe was obtained for SrFe₁₀Al₂O₁₉ which is much higher than the maximum theoretical value of H_C for pure SrFe₁₂O₁₉ (7.5 kOe).