Self-assembly of magnetic nanoparticles into complex superstructures: Spokes and spirals

Dipole-dipole interactions in the most commonly used ferrofluids are relatively weak and there are only a few reports of chain formation in zero magnetic field. Here we report on the pronounced aggregation of a ferrofluid formed by maghemite nanoparticles (10 nm in diameter), investigated by atomic force microscopy (AFM), small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS). The ferrofluid forms chain-like filaments independently of particle concentration. Furthermore, the ferrofluid forms self-assembled patterns, namely ordered superstructures, such as spirals, concentric rings and spokes (radially directed lines), when deposited on a substrate under a magnetic field. Our results reveal an unprecedented level of complexity in the self-assembly of magnetic nanoparticles. Further optimization of the working conditions could eventually enable the reproducible creation of three-dimensional magnetic structures with chosen architecture on submicron length scales.