Microstructure and effective properties of nanocomposites: ferrofluids as tunable model systems

We have studied the correlation between microstructure and effective material properties using colloidal dispersions of magnetic nanoparticles in a carrier liquid (ferrofluids). Their microstructure can be altered in a continuous and reversible way via an external magnetic field. Two-dimensional small angle x-ray scattering and Monte Carlo simulations show that field-induced structural anisotropy develops due to the formation of anisometric particle clusters having a preferred orientation parallel to the field. In this polydisperse system particles of all sizes take part in cluster formation. The structural data are compared with results of dielectric measurements in the frequency range from 5 Hz to 1 GHz. We show that dielectric anisotropy is correlated with the shape anisometry of oriented clusters.