Low-temperature dynamics of magnetic colloids studied by time-resolved small-angle neutron scattering

Abstract

The dynamics of ordering and relaxation processes in magnetic colloids has been studied by means of stroboscopic small angle neutron scattering techniques in an oscillating magnetic field. Surfactant stabilized ferrofluids (FFs) of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ and Co nanoparticles have been investigated as a function of temperature and frequency and compared to a solid Cu alloy with $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{vol}\phantom{\rule{0.2em}{0ex}}%$ Co precipitates. This technique allowed elucidating the dynamical nature of the locally ordered domains in both ferrofluids as ``living objects'' becoming arrested below the freezing temperature of the solvent. The time-dependent intensities have been analyzed in terms of Langevin statistics including dynamical interparticle structure factors, which scale with the square of the Langevin function. The local ordering is mainly determined by the effective dipole-dipole interaction, which is enhanced by the partial alignment of the particle moments in an external magnetic field. Starting from the frozen state, the amount of freely rotating particle moments increases continuously with increasing temperature. The dynamical structure factors describing the hexagonal $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{FF})$ or chainlike (Co-FF) ordering reach a maximum around the melting temperature. The alignment of particle moments along the applied field is governed by the fast Brownian rotation of individual nanoparticles and small aggregates, while the magnetic relaxation of longer dipolar chains and local hexagonal domains is much slower. In contrast, no field-induced interparticle correlations occur in the diluted solid CuCo alloy where the moment relaxation is purely of fast N\'eel type, which---due to a low anisotropy constant---follows the oscillating field at all temperatures.