Magnetic orientation of soft particles in a jammed solid

Densely packed atoms, molecules, or small particles can get trapped in a jammed state thereby avoiding crystallization. The resulting amorphous structures display complex, spatially heterogeneous, trapping potentials in stark contrast to the uniform case of a crystal. Here we study active and passive rotational motion in a jammed colloidal dispersion of particles consisting of an anti-ferromagnetic core and a thermo-sensitive microgel soft shell. We determine the order parameter as a function of volume fraction from optical birefringence measurements. A simple model allows us to discriminate the orientation and relaxation of different subsets of particles according to their elastic trapping state.