Communication: Colloidal particles,vith magnetic properties are attractive due to their tunable anisotropic interactions. Here is reported a new class of polystyrene-core magnetite-shell particles produced by the sequential adsorption of nanoparticles and polyelectrolyte, a process that allows the shell thickness and composition to be controlled with nanometer precision, The Figure shows the alignment of the nanocomposite particles in the presence of a magnetic field.