  Above a certain density a granular material jams. This property can be
controlled by either tuning a global property, such as the packing fraction or
by applying shear strain, or at the micro-scale by tuning grain shape,
inter-particle friction or externally controlled organization. Here, we
introduce a novel way to change a local granular property by adding a weak
anisotropic magnetic interaction between particles. We measure the evolution of
the pressure, $P$, and coordination number, $Z$, for a packing of 2D
photo-elastic disks, subject to uniaxial compression. Some of the particles
have embedded cuboidal magnets. The strength of the magnetic interactions
between particles are too weak to have a strong direct effect on $P$ or $Z$
when the system is jammed. However, the magnetic interactions play an important
role in the evolution of latent force networks when systems containing a large
enough fraction of the particles with magnets are driven through unjammed
states. In this case, a statistically stable network of magnetic chains
self-organizes and overlaps with force chains, strengthening the granular
medium. We believe this property can be used to reversibly control mechanical
properties of granular materials.
