<p>The growing demand for rare-earth-free magnetic materials with high coercivity (<i>H</i>_<i>c</i>) and thermal stability has motivated extensive research on M-type hexaferrites as alternatives for high-density magnetic storage and microwave applications. In this work, M-type hexaferrites A_0.54Ca_0.46Fe_6.5Al_5.5O₁₉ (A = Ba and Sr) were synthesized via the sol-gel auto-combustion method to investigate the effect of the ionic radius of Ba²⁺ and Sr²⁺ on the structural, microstructural and magnetic properties. X-ray diffraction confirmed the formation of the M-type structure with minor traces of α-Fe₂O₃ in the Ba_0.54Ca_0.46Fe_6.5Al_5.5O₁₉ sample. Rietveld refinement revealed that replacing Ba²⁺ with the smaller Sr²⁺ ion induces lattice contraction and shortens the Fe-O bond length at the (2b) bipyramidal site, thereby enhancing magnetocrystalline anisotropy (<i>K</i>_<i>eff</i>). This structural modification leads to a higher <i>H</i>_<i>c</i> (26.33 kOe) for the Sr_0.54Ca_0.46Fe_6.5Al_5.5O compound compared to 23.43 kOe for the Ba_0.54Ca_0.46Fe_6.5Al_5.5O₁₉. Temperature-dependent magnetic measurements showed that at 10&#xa0;K, reduced thermal disorder exposes the intrinsically higher magnetic moment of the Ba_0.54Ca_0.46Fe_6.5Al_5.5O₁₉ sample, whereas at 300&#xa0;K the Sr_0.54Ca_0.46Fe_6.5Al_5.5O₁₉ sample exhibits superior magnetic hardness due to stronger Fe–O superexchange interactions. These results demonstrate that precise cation tuning in Ca–Al co-substituted M-type hexaferrites offers an effective route to achieving high <i>H</i>_<i>c</i> without relying on scarce or costly rare-earth elements. The optimized Sr_0.54Ca_0.46Fe_6.5Al_5.5O₁₉ composition combines structural stability, high anisotropy, and strong room-temperature <i>H</i>_<i>c</i>, making it a promising candidate for next-generation high-density magnetic recording and high-frequency electromagnetic applications.</p>