Abstract
The weak permanent magnetic dipole moment of cobalt ferrite-doped colloidal silicaspheres was increased by exposure to a saturating magnetic field. The resulting change ofthe rotational dynamics of the magnetic microspheres in a weak alternating field wasmeasured from low to high volume fraction in ethanol, using a frequency-dependentcomplex magnetic susceptibility setup. At low volume fractions, the increased dipolarattraction slows down Brownian rotation. At higher volume fractions, however, rotation isno longer slowed down as rapidly with increasing concentration, likely due to dipolarcoupling between the particles which accelerates their partial alignment with thealternating field. This explanation is supported by the unexpected finding that saltaddition accelerates particle rotation rather than slowing it down. At the highest volumefraction, colloidal crystals and glasses were formed in which only a small fraction of thespheres exhibit rotational mobility.
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