Unraveling the Mn2+ Substitution Effect on the Anisotropy Control and Magnetic Hyperthermia of MnxFe3-xO4 Nanoparticles

Abstract

Composition is a key parameter to effectively tune the magnetic anisotropy of magnetic nanoparticles, which in turn can modulate their structural-magnetic properties and final applications. The Mn2+ content of manganese ferrite nanoparticles (MnxFe3-xO4) deeply impact their structure, anisotropy, magnetism, and their heating capacity. However, a direct correlation between the Mn2+ content, magnetic properties and heating efficiency is not yet clear. Herein we report the synthesis of a wide range of MnxFe3-xO4 with x = 0.14 to 1.40, with similar polyhedral morphologies and sizes (13 to 15 nm). By varying the Mn2+ content (in the range of x = 0.0 up to 0.70), we successfully tuned the effective anisotropy while maintaining saturation magnetization nearly constant. Highest Mn2+ levels (x= 1.40) lead to structural changes and strain defects reflected in their poor saturation magnetization. Mn2+ substitution is not uniform, instead promotes a compositional gradient across the MNPs, with the surface layers having a higher concentration of Mn²⁺ than the core. The Mn²⁺-rich surface likely exhibits superparamagnetic (SPM) relaxation, while the core remains predominantly ferrimagnetic (FiM). Water transference results in cations leaching, promoting vacancies and changes in the local ferrite structure but with minor impact on magnetic properties compared with initial MNPs. We obtained the optimal Mn2+ content that maximizes anisotropy for improved SLP values. Néel relaxation mechanism is warranted regarding variable composition when sizes and shapes are maintained. Our detailed analysis provides a better understanding of the effect of Mn2+ substitution on the heating efficiency through anisotropy modulation and straightforward guidance for the optimized MNPs design for magnetic hyperthermia