Synthesis of M-type SrFe₁₂O₁₉ by mechanosynthesis assisted by spark plasma sintering

We present a comparative study of synthesis of M-type strontium hexaferrite from strontium and iron single oxides mechanically activated by high-energy ball milling for 5 h, and assisted by two different methods: (a) conventional heat treatment and (b) Spark Plasma Sintering (SPS), both at relative low temperatures (≤900 °C). Although it was found that both methods promoted the complete structural transformation of precursors to Sr-hexaferrite phase (S.G. Pmc21) for temperatures above 700 °C, higher saturation magnetization was found for SPS samples. X-ray diffraction analysis revealed that the structural transformation undergoes formation of an intermediate metastable structure (Fe₂Sr₂O₅) in both methods, but with slight different kinetics. Maximum specific magnetization of 67 emu/g at 18 kOe and coercivity of 3.7 kOe were recorded from powder mixtures milled for 5 h, which were subsequently SPS-ed at 700 °C. By contrast, although magnetization values for the same milled samples after being annealed at 700 °C slightly decreased, it leads to a significant increase in the coercive field reaching 5.4 kOe. These results are explained on the basis of factors such as the complete formation of hexaferrite and the attained density of consolidated powders, in addition to particle and grain sizes also reported.