  Here, we investigate the nature of the moment coupling between 10-nm
DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder
form. The individual iron oxide cores were composed of > 95% maghemite and
agglomerated to clusters. At room temperature the ensemble behaved as a
superparamagnet according to M\"ossbauer and magnetization measurements,
however, with clear signs of dipolar interactions at low temperatures. Analysis
of temperature-dependent AC susceptibility data in the superparamagnetic regime
indicates a tendency for dipolar coupled anticorrelations of the core moments
within the clusters. To resolve the directional correlations between the
particle moments we performed polarized small-angle neutron scattering and
determined the magnetic spin-flip cross-section of the powder in low magnetic
field at 300 K. We extract the underlying pair distance distribution function
of the magnetization vector field by an indirect Fourier transform of the
cross-section, and which suggests positive as well as negative correlations
between nearest neighbor moments, with anticorrelations clearly dominating for
next-nearest moments. These tendencies are confirmed by Monte Carlo simulations
of such core-clusters.
