Impact of Co²⁺ substitution on structure and magnetic properties of M-type strontium ferrite with different Fe/Sr ratios

Ion substitution has significantly improved the performance of ferrite magnets, and cobalt remains a key area of research. Studies on the mechanism of Co²⁺ in strontium ferrite, especially SrFe_2n–xCo_xO_19–δ (n = 6.1–5.4; x = 0.05–0.20) synthesized using the ceramic method, showed that Co²⁺ preferentially enters the lattice as the Fe/Sr ratio decreases. This results in a decrease in the lattice constants a and c due to oxygen vacancies and iron ion deficiency. The impact of Co substitution on morphology is minor compared to the effect of the Fe/Sr ratio. As the Fe/Sr ratio decreases and the Co content increases, the saturation magnetization decreases. The magnetic anisotropy field exhibits a nonlinear change, generally increasing with higher Fe/Sr ratios and Co content. These changes in the performance of permanent magnets are attributed to the absence of Fe³⁺ ions at the 12k + 2a and 2b sites and the substitution of Co²⁺ at the 2b site. This suggests that by adjusting the Fe/Sr ratio and appropriate Co substitution, the magnetic anisotropy field of M-type strontium ferrite can be effectively optimized.