  In this paper we study the rheological behaviour (in the absence of magnetic
field and upon its application) of multi-component magnetic suspensions that
consist of a mixture of magnetic (iron) and non-magnetic (PMMA) particles
dispersed in a liquid carrier. These suspensions exhibit considerably higher
viscosity and yield stress in the absence of magnetic field than
single-component suspensions of the same solid fraction, as a consequence of
the adsorption of the iron particles on the PMMA ones. The adsorbed layer of
iron particles on the PMMA ones is observed through optical microscopy of
dilute samples and confirmed by attenuated total reflectance. Microscopic
observations also show that the resulting non-magnetic-core-magnetic-shell
composites move upon magnetic field application and aggregate into particle
structures aligned with the applied field. These structures, which consist of
both types of particles, give rise to high values of the static and dynamic
yield stresses upon field application. Actually, both quantities are much
higher than those of a suspension with the same volume fraction of magnetic
particles, and increase when the amount of non-magnetic ones increases. These
trends are adequately predicted by a theoretical model that considers that the
main contribution to the yield stress is the change of the suspension magnetic
permeability when particle chains are deformed by the shear.
