Tunable Formation of Ferromagnetic Nanoparticle Rings: Experiments and Monte Carlo Simulations

Anisotropic interactions in self-assembly of nanostructures always result in novel patterns. We demonstrate that, by the aid of high-power sonication, 16 nm ε-cobalt ferromagnetic nanoparticles (FMNPs) dispersed in dilute suspensions at room temperature would self-assemble into rings as small as ∼50 nm in diameter. The well-defined size and shape and the uniform surfactant coating layer of the cobalt nanoparticles enable quantitative calculations of particle–particle and particle–interface interactions. The experiments, in conjunction with cluster-moving Monte Carlo simulations mimicking the self-assembly in solution and dynamics during solvent evaporation, have revealed three key factors that influence the ring formation most, i.e., FMNP density, dipolar strength, and surfactant layers. Two very different mechanisms of the FMNP-ring formation are found by changing these factors. The results provide a guide to the fabrication of nanorings as well as diverse patterns assembled by FMNPs