Turning Superparamagnetic Nanoparticles Into Ferromagnetic at Room Temperature via Molecular Functionalization for Magnetic Memory Applications

Molecular functionalization of magnetic oxide nanoparticles presents a powerful strategy for interfacial engineering, enabling the emergence of exotic magnetic phenomena. While non-covalent approaches have been widely explored, they often fail to provide robust interfaces for real applications. We report here covalent surface modification of cobalt ferrite (CoFe₂O₄) nanoparticles (NPs) using a series of judiciously selected aryl diazonium salts to achieve a higher degree of surface functionalization and strong interfacial electronic coupling. This modification establishes robust carbon-metal (C-M) covalent bonds at the nanoparticle-molecule interface, conferring chemical stability and tunable magnetic properties. Notably, π-electron-rich aryl compounds transform pristine superparamagnetic CoFe₂O₄ NPs into room-temperature ferromagnets, with a significant increase in coercivity, remanent magnetization, and saturation magnetization, attributed to enhanced surface spin ordering on different sublattices and elevated effective anisotropy. Zero-field-cooled and field-cooled magnetization measurements reveal elevated blocking (T_b) and irreversibility (T_irr) temperatures, further confirmed by thermoremanent magnetization, with T_b rising from 245.9 K in pristine NPs to 336.8 K in aryl-functionalized-CoFe₂O₄ NPs. Molecular-level modeling further supports the origin of ferromagnetism in aryl-functionalized-CoFe₂O₄ NPs. Chemical functionalization of magnetic nanoparticles offers a versatile strategy to tune the magnetic properties of pristine nanoparticles for applications in spintronics, spin-based memory, and spin logic.