Nanostructures and ordering phenomena in ferrofluids investigated using polarized small angle neutron scattering

Polarized small angle neutron scattering (SANSPOL) was used to investigate the microstructure of various ferrofluids (FF) where magnetic materials (Co, Fe magnetite), stabilization mechanisms (electrostatic, monolayers and bilayers of surfactants) and carrier liquids (water, organic solvents) have been systematically varied. Magnetic core–shell particles, non-magnetic micelles and magnetic aggregates were identified and size distributions and density, composition, and magnetization profiles were determined. Partial penetrations of solvent molecules inside the surfactant layer and formation of non-magnetic oxide coatings were established. The magnetic nanostructure in diluted samples consists of non-interacting ferromagnetic single domain particles. In concentrated Co FF a pseudo-crystalline ordering was found to be induced by an external magnetic field where cobalt core–shell particles are arranged in hexagonal planes. The particle ordering and magnetic moment direction followed the direction of the applied field. In addition, segments of uncorrelated dipolar chains were found to be present. The dynamics of the field induced ordering was studied by means of time-resolved SANS. Individual particle moments are stuck by field induced dipolar interactions in domains of local hexagonal ordering which relax by rotational diffusion when the field is switched off, with a characteristic time of a few seconds.