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 (CoFe 2 O 4 ) 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 CoFe 2 O 4 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‐CoFe 2 O 4 NPs. Molecular‐level modeling further supports the origin of ferromagnetism in aryl‐functionalized‐CoFe 2 O 4 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.