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

Ion substitution has significantly improved the performance of ferrite magnets, with cobalt remaining a key area of research. Studies on the mechanism of Co2+ in strontium ferrite, especially SrFe2n-x Co x O19-δ (n=6.1-5.4, x=0.05-0.20) synthesized using the ceramic method, showed that Co2+ preferentially enters the lattice as Fe/Sr ratio decreases. This results in a decrease in 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 Co content increases, 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 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.