Proc. Natl Acad. Sci. USA http://doi.org/885 (2015)

Sara Mehdizadeh Taheri and colleagues have uncovered a surprising geometry-dependent ordering principle in assemblies of magnetic nanocubes. The team showed that they could use a magnetic field to wrangle small nanocubes in solution into one-dimensional chains, two-dimensional sheets and three-dimensional cuboids — all with surprisingly regular internal order. But the same was not true of spherically shaped nanoparticles, which showed no such tendencies to order.

The group posited that the difference lay with the fact that spheres have much lower contact area than their cuboid counterparts. The cubes assembled for particle sizes so small that the particle–particle and particle–field interactions were on the order of the thermal energy and van der Waals forces. The external field induced dipole moments in the particles, bringing them close enough that the short-range interactions could facilitate highly regular ordering. The local interactions between spherical particles were therefore even weaker, and incapable of stabilizing the assemblies. Despite the fact that the cube assemblies showed remarkable regularity, they had unusually small magnetization given their size.