Enhancement of hard-magnetic properties in Mn-Al-Cr substituted M-type hexaferrite

This study investigates the magnetic properties of isotropic sintered magnets based on M-type hexaferrite SrFe12O19,enhanced through multi-cation substitution with Al, Cr, and Mn. M-type hexaferrite samples with the general formulaSrFe12 − 2xAlxCrxO19 (x = 0–0.3) were synthesized via a solid-state reaction method to investigate the effects of Al–Cr substitutionon the structural and magnetic properties. X-ray diffraction (XRD) analysis confirmed the formation of a singlephaseM-type hexaferrite with minor traces of Fe2O3 in some samples. Magnetic characterization showed that coercivity(HC) increased while remanent magnetization (4πMr) decreased with increasing x, exhibiting a typical trade-off behavior. Among the compositions, x = 0.2 exhibited the most balanced magnetic properties. Based on this, further substitutionswith Mn, Co, La, and Ce were introduced, and Mn substitution slightly enhanced HC. Optimization of sintering additivesand temperature revealed that the composition SrFe11.5Mn0.1Al0.2Cr0.2O19, sintered with 1 wt% CaCO₃ + 1 wt% SiO₂ at1230 °C, exhibited the best performance with 4πMr = 2207 G and HC = 5304 Oe. The results demonstrate that simultaneousmulti-cation substitution and sintering condition control can significantly enhance the hard-magnetic properties ofM-type hexaferrites.