Enhanced magnetic properties in Mn Co1−Fe2O4 nanoparticles: Unraveling the composition–annealing synergy

This study systematically explores the interplay between manganese substitution and annealing temperature in the magnetic and structural properties of Mn x Co 1− x Fe 2 O 4 ( x = 0, 0.15, 0.25, 0.35) nanoparticles prepared via sol–gel auto-combustion method. A combination of 57 Fe Mössbauer spectroscopy, XRD, and TEM reveals that Mn substitution progressively decreases the inversion degree ( i ≈ 0.92 → 0.78) and promotes α-Fe 2 O 3 formation above 500 °C. These structural transformations directly correlate with variations in magnetization, coercivity, and energy product (| BH| max ). Notably, the Mn 0.15 Co 0.85 Fe 2 O 4 sample annealed at 600°C in air exhibited a | BH| max ≈ 0.91 MGOe. These findings underscore the critical role of cation redistribution and secondary phase suppression in tailoring the intrinsic magnetic anisotropy of cobalt ferrites. The enhanced magnetic performance achieved here, using a scalable synthesis and air-compatible annealing, supports their use as hard magnetic components in exchange-coupled composite systems aimed at rare-earth-free permanent magnet technologies. • Mn 0.15 Co 0.85 Fe 2 O 4 MNPs annealed at 600°C exhibited the |BH| max of ∼0.91 MGOe. • Annealing in the air of Mn-substituted CoFe 2 O 4 MNPs above 500 °C triggers α-Fe 2 O 3 formation. • The inversion degree decreases from 0.92 for pure CoFe 2 O 4 to 0.79 for the sample with 35% Mn content.