Physicochemical Necessary and Sufficient Conditions for Superferromagnetism in High‐Resolution Magnetic Particle Imaging

Magnetic Particle Imaging (MPI) is a novel tracer modality that images the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs) by exploiting their nonlinear response to magnetic fields. MPI image resolution is primarily influenced by the strength of the MPI gradient field and the physical and magnetic properties of the nanoparticle tracers. Although MPI offers exceptional sensitivity, contrast, and penetration depth, its spatial resolution remains limited, particularly in clinical systems due to safety limits on gradient field strength. Consequently, efforts to improve resolution focus on developing MPI-tailored tracers with enhanced magnetic resolution. This work explores the effects of strong dipole-dipole coupling between SPIOs in linear chain-like structures. This leads to the observation of superferromagnetism, a phenomenon in which assemblies of superparamagnetic nanoparticles exhibit transient magnetic behavior similar to ferromagnetism. This transition is associated with order-of-magnitude improvements in resolution and sensitivity. The influlence of these interactions on the magnetization curve is analyzed using a positive feedback model, and experimental observations such as coercivity are successfully predicted. The necessary conditions for the observation of superferromagnetism suggested by the theory are also experimentally validated. The positive feedback model is found to be an indispensable tool for the synthesis of high-resolution MPI tracers, optimizing core size, oxidation state, and coating.