Rotational dynamics of magnetic silica spheres studied by measuring the complex magnetic susceptibility

The weak permanent magnetic dipole moment of cobalt ferrite-doped colloidal silica spheres was increased by exposure to a saturating magnetic field. The resulting change of the rotational dynamics of the magnetic microspheres in a weak alternating field was measured from low to high volume fraction in ethanol, using a frequency-dependent complex magnetic susceptibility setup. At low volume fractions, the increased dipolar attraction slows down Brownian rotation. At higher volume fractions, however, rotation is no longer slowed down as rapidly with increasing concentration, likely due to dipolar coupling between the particles which accelerates their partial alignment with the alternating field. This explanation is supported by the unexpected finding that salt addition accelerates particle rotation rather than slowing it down. At the highest volume fraction, colloidal crystals and glasses were formed in which only a small fraction of the spheres exhibit rotational mobility