  : Understanding the correlations between magnetic nanoparticles is important
for nanotechnologies, such as high-density magnetic recording and biomedical
applications, where functionalized magnetic particles are used as contrast
agents and for drug delivery. The ability to control the magnetic state of
individual particles depends on the good knowledge of magnetic correlations
between particles when assembled. Inaccessible via standard magnetometry
techniques, nanoscale magnetic ordering in self-assemblies of Fe3O4
nanoparticles is here unveiled via x-ray resonant magnetic scattering (XRMS).
Measured throughout the magnetization process, the XRMS signal reveals
size-dependent inter-particle magnetic correlations. Smaller (5 nm) particles
show little magnetic correlation, even when tightly close-packed, yielding to
mostly magnetic disorder in the absence of external field, which is
characteristic of superparamagnetic behavior. In contrast, larger (11 nm)
particles tend to be strongly correlated, yielding a mix of magnetic orders
including ferromagnetic and anti-ferromagnetic orders. These strong magnetic
correlations are present even when the particles are sparsely distributed.
