Exploring the magnetic properties of W -type SrFe18O27 hexaferrite: Insights from a first-principles study

$W$-type ${\mathrm{SrFe}}₁₈{\mathrm{O}}₂₇$ hexaferrite is emerging as a potential material for permanent magnet applications. Despite this, theoretical modeling on $W$-type hexaferrites is still lacking, leaving only experimental findings to date. Employing density functional theory, we conduct a detailed analysis of pure $W$-type ${\mathrm{SrFe}}₁₈{\mathrm{O}}₂₇$ hexaferrite and its compositions with Ni/Zn substitutions, to explore their intrinsic magnetic properties and assess their viability as potential permanent magnets. We found no significant effect of Ni/Zn substitution on the magnetocrystalline anisotropy energy (MAE); however, Zn substitution is particularly helpful for improving the magnetization ${\ensuremath{\mu}}₀{M}_s$. The calculated MAE constant ${K}_u$ values indicate that the compounds are uniaxial with easy axis along the (001) direction. The origin of the predicted MAE is investigated using second-order perturbation theory analysis and the electronic structure. We found that different Fe sublattices contribute differently to the MAE, providing a unique way to enhance the MAE with small site-specific substitutions. The results highlight the challenge of simultaneously enhancing ${K}_u$ and ${\ensuremath{\mu}}₀{M}_s$ in $W$-type hexaferrite compounds. However, the compounds show intriguing properties with moderate ${K}_u$ and high ${\ensuremath{\mu}}₀{M}_s$, which may outperform the conventional $M$-type ferrite magnets in some applications.