Mn-Induced Stabilization of a β-Alumina-Type Defect Structure in Barium Hexaferrite Nanoplatelets

Hexaferrite nanoplatelets exhibit size-dependent structural variations influencing their magnetic properties. Here, we synthesized Mn-substituted barium ferrite nanoplatelets via hydrothermal methods, achieving up to ∼27% Fe substitution. Advanced STEM and Raman analyses revealed depletion of Fe(2b) trigonal lattice sites and associated oxygen vacancies, forming a β-alumina-type ferrite structure─representing the first pure Ba2+ β-ferrite analogue. First-principles modeling confirmed the thermodynamic stabilization of this defected structure at higher Mn/Fe ratios. Mn substitution reduced nanoplatelet size and suppressed magnetic properties, which were restored upon annealing at 800 °C, reverting to the M-type hexaferrite structure with expected magnetic behavior. These findings elucidate nanoscale structural adaptations induced by chemical substitution and offer insights into tailoring the magnetic properties of barium ferrite nanoplatelets through controlled synthesis and post-treatment.