Abstract
By controlling the time to apply magnetic field to a Fe3O4 magnetic fluid with polymer acrylic resin and by evaporating the solvent, linear chain-like structures were obtained and fixed. Optical microscopy and transmission electronic microscopy (TEM) were used to observe the assembly structures of the fixed magnetic nanoparticles. We found that the chain-like structures with size even down to hundreds nanometers had smaller substructures. A picture of aggregation between linear chain-like substructures is proposed to understand the formation of the macrostructure in magnetic fluid induced by a magnetic field. Two basic aggregations, the coarsening and the connection of the substructures, were observed.