Anisotropic aggregates as the origin of magnetically induced dichroism in ferrofluids

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Abstract
Static and dynamic polarized and depolarized light scattering, static, and time‐resolved dichroic anisotropy, as well as conventional magnetization versus applied magnetic field determinations have been carried out on aqueous commercial ferrofluids and on surfactant aggregate stabilized Fe3O4 in aqueous solution. Over a dilution range of more than three orders of magnitude there is no evidence for field‐induced cooperative effects. The shape of the dichroic anisotropy versus applied field curve superimposes virtually exactly onto the magnetization curve. Rotational and translational diffusion coefficients indicate ellipsoidal magnetic aggregates with average minor to major axial ratios around 0.33 and major axis of 285 nm, which are insensitive to dilution, and far above the expected value of around 10 nm. Electron micrographs have revealed polydisperse clusters of around 150 nm composed of particles with sizes on the order of 10 nm. The scattered intensity autocorrelation curve shows no appreciable change upon application of a magnetic field to the ferrofluids. Evidence for the shape anisotropy of the presumed 140‐nm clusters is apparent in the depolarized light scattering autocorrelation decay curves. In the absence of field‐induced particle chaining, aggregation, or shape deformation, the origin of the field‐induced dichroism was attributed to the permanent shape anisotropy of the clusters. Subtle field‐induced alteration of the spatial arrangement of particles within the stable clusters or an unexpected anisotropic polarizability of the magnetite crystals do not seem to be likely origins of the dichroic effect.