  Non-polar magnetic nanoparticles agglomerate upon cooling. This process is
followed by in-situ small angle X-ray scattering to assess structural
properties of the emerging agglomerates. On the length scale of a few particle
diameters, no differences are found between the agglomerates of small (d =
12nm) and large (d = 22nm) nanoparticles. Hard-sphere like random packing with
a local packing fraction of $\eta$ = 0.4 is seen. On larger length scales,
small particle form compact superstructures, while large particles arrange into
agglomerates that resemble chain-like structure in SAXS. This can be explained
by directed magnetic dipole interactions that dominate larger particles, while
isotropic van der Waals interaction governs the agglomeration of smaller
particles.
