  The interplay between structural and magnetic properties of nanostructured
magnetic materials allows to realize unconventional magnetic effects, which
results in a demand for experimental techniques to determine the magnetization
profile with nanoscale resolution. Magnetic small-angle neutron scattering
(SANS) probes both the chemical and magnetic nanostructure and is thus a
powerful technique e.g. for the characterization of magnetic nanoparticles.
Here, we show that the conventionally used particle-matrix approach to describe
SANS of magnetic particle assemblies, however, leads to a flawed
interpretation. As remedy, we provide general expressions for the
field-dependent 2D magnetic SANS cross-section of correlated moments. It is
shown that for structurally disordered ensembles the magnetic structure factor
is in general, and contrary to common assumptions, (i) anisotropic also in zero
field, and (ii) that even in saturation the magnetic structure factor deviates
from the nuclear one. These theoretical predictions explain qualitatively the
intriguing experimental, polarized SANS data of an ensemble of dipolar-coupled
iron oxide nanoparticles.
