Synthesis and Characterization of Magnesium Co-Substituted M-Type Ferrites BaFe12−x−yMgxMyO19 with M = Zr, Hf

M-type hexaferrites are widely used in magnetic applications, and tailoring their properties via aliovalent substitution requires a detailed understanding of charge compensation and cation distribution. In this work, Mg2+/M4+ (M = Zr, Hf) co-substituted BaFe12O19 was synthesized via Na2CO3 flux and comprehensively characterized by wavelength-dispersive X-ray spectroscopy, powder and single-crystal X-ray diffraction, Rietveld refinement, X-ray absorption near-edge structure, and magnetic measurements. Increasing substitution levels x, y in BaFe12−x−yMgxMyO19 result in increasing lattice parameters and decreasing the room-temperature magnetic parameters saturation magnetization, remanence, and coercivity, while remanence and coercivity increase at low temperatures. Secondary phases form for nominal substitution ≥ 1. Zr4+ and Hf4+ preferentially occupy the 4f2 site, whereas Mg2+ is distributed over multiple sites, as indicated by polyhedral volume analysis. Wavelength-dispersive X-ray spectroscopy confirms homogeneous elemental distribution within individual crystals but reveals significant variation in substitution levels within batches. The maximum degree of substitution for the tetravalent metals was y ≈ 1.2–1.7, with lower Mg incorporation of x ≈ 0.9–1.1. Charge compensation was found to be partially achieved via vacancy formation, while minor Fe2+ contributions cannot be excluded.