La–Mg-Substituted M-Type Sr-Hexaferrites for Enhanced Hard Magnetic Properties

This study investigates the structural and magnetic properties of La–Mg co-substituted M-type strontium hexaferrites with the compositions Sr1−xLaxFe12−xMgxO19 (x = 0, 0.05, 0.1, 0.15), synthesized via a conventional solid-state reaction route. X-ray diffraction confirmed the formation of single-phase M-type hexaferrites up to x = 0.15. The Sr0.9La0.1Fe11.9Mg0.1O19 composition (x = 0.1) composition exhibited the best magnetic performance, achieving a saturation magnetization, MS = 71.9 emu/g, remanent magnetization, 4πMr = 2280 G, and coercivity, HC = 3549 Oe. These improvements were attributed to the synergistic effect of La3+ and Mg2+ substitution, which enhanced magnetocrystalline anisotropy and refined grain morphology. Further enhancement was obtained by adding sintering additives (0.8 wt% SiO2, 0.5 wt% CaO, and 0.7 wt% SrCO3), resulting in an HC of 4489 Oe and a 4πMr of 2285 G. The results demonstrate that La–Mg co-substitution, combined with optimized sintering conditions, provides a cobalt-free strategy for developing high-performance, environmentally benign M-type ferrite magnets for advanced motor and electronic applications.