Rotational dynamics and aging in a magnetic colloidal glass

We follow here the freezing of the orientational degrees of freedom of strongly interacting magnetic and charged nanoparticles, as the colloidal glass transition is approached. Using a magnetoinduced birefringence technique, we show that the rotational dynamics drastically slows down following a Vogel-Fulcher law. More precisely, this slowing down occurs above a volume fraction threshold phi(*), the value of which depends on the range of electrostatic repulsion between nanoparticles. An interpretation in terms of effective spheres, slightly anisotropic, is proposed. The aging of the rotational dynamics of the more concentrated samples is reported on long time scales, with an exponential growth of the rotational characteristic time with the age t(w) of the sample. An attempt of age rescaling at different volume fractions leads us to introduce a phi-dependent "birth age" t(w)(0)(phi), which diverges analytically at the Vogel-Fulcher volume fraction.