Hard Magnetic Properties of M-Type Sr-Hexaferrites via Ce–Mg Substitution and Ca-Assisted Solubility Enhancement

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A cobalt-free compositional strategy was pursued to promote Ce incorporation in M-type SrFe12O19 and thereby enhance its hard magnetic performance. In Sr1−xCexFe12−xMgxO19 (x = 0–0.15), the highest coercivity and finest microstructure were obtained at x = 0.1; however, a residual CeO2 phase persisted at all substitution levels, suggesting limited Ce incorporation. Partial replacement of Sr2+ by Ca2+ suppressed detectable CeO2 reflections once y ≥ 0.2 in Sr0.9−yCayCe0.1Fe11.9Mg0.1O19, providing indirect XRD evidence for enhanced Ce incorporation, although an α-Fe2O3 secondary phase remained. Reducing the nominal Fe content in Sr0.7Ca0.2Ce0.1Fe11.9−zMg0.1O19 (z = 0–0.75) further suppressed the α-Fe2O3 reflections, which were no longer detected at z = 0.75 (Sr0.7Ca0.2Ce0.1Fe11.15Mg0.1O19). Because the Ca-containing compositions exhibited reduced coercivity, potentially associated with grain coarsening, high-energy milling combined with sintering-temperature and additive optimization (SiO2, CaO, SrCO3, Cr2O3, ZnO, Mn2O3) was applied to restore a fine growth-controlled microstructure, lifting coercivity from 2714 Oe to 3552–3911 Oe. Additive sets sharing a common SiO2–CaO–SrCO3 base with ZnO, Cr2O3, or Mn2O3 addition yielded a closely matched, favorable remanence–coercivity balance. This work demonstrates a cobalt-free composition–processing route to hard magnetic M-type hexaferrites through Ca-assisted Ce incorporation, nominal Fe-content adjustment, and microstructural control.