Effects of Al and Ca ions co-doping on magnetic properties of M-type strontium ferrites

M-type strontium ferrite is a kind of magnetic functional material with wide application prospect. In this work, SrFe_12−xAl_xO₁₉ (0 ≤ x ≤ 1) and Sr_1−yCa_yFe₈Al₄O₁₉ (0 ≤ y ≤ 0.4) have been prepared by a hydrothermal method, and the effects of Al³⁺ doping and Al³⁺ and Ca²⁺ co-doping on the magnetic properties have been investigated. X-ray diffraction analysis indicates all the samples are a hexagonal structure, and the shift of the diffraction peaks suggests that Al³⁺ and Ca²⁺ are successfully doped in SrFe₁₂O₁₉. Field emission scanning electron microscope images show the particles have the hexagonal shape with the size of 1.1–2.9 µm. Vibrating sample magnetometer reveals that the doping of Al³⁺ and the co-doping Al³⁺ and Ca²⁺ can enhance magnetic properties of strontium ferrite. For example, the values of coercivity and the saturation magnetization of SrFe₈Al₄O₁₉ are 914.1 Oe and 44.0 emu/g, which are 3% and 117% enhancement compared with SrFe₁₂O₁₉, respectively. For Sr_1−yCa_yFe₈Al₄O₁₉, coercivity and saturation magnetization increase from 914.1 Oe and 41.8 emu/g to 1047.2 Oe and 50.2 emu/g when the Ca²⁺ doping concentration increases from 0 to 0.4. Moreover, the effects of acid washing also on phase composition, morphology and magnetism are researched. The as-synthesized doping strontium ferrite will be useful for the applications in security switching and recording media.