Massively ordered microstructures composed of magnetic nanoparticles

Magnetic γ-Fe₂O₃ nanoparticle toluene solution was placed in a restricted, axially symmetric geometry composed of a spherical lens on a Si substrate (i.e., sphere-on-Si geometry), forming a capillary-held solution. The solvent evaporation led to the self-assembly of γ- Fe ₂O₃ nanoparticles into hundreds of spatially well-ordered, gradient concentric 'coffee rings'. The rings were two nanoparticle layers high with a monolayer of nanoparticle thin film formed in between. The ring width ranged from submicron to a few microns. The center-to-center distance between adjacent rings was on the order of a few microns. The size and packing of γ- Fe₂O₃ nanoparticles, both within individual microscopic rings and in between, were dependent upon the preparation of the γ-Fe₂O₃ nanoparticle toluene solution.