  The low-energy properties of two-dimensional ensembles of dipole-coupled
magnetic nanoparticles are studied as function of structural disorder and
particle coverage. Already small deviations from a square particle arrangement
lift the degeneracies of the microvortex magnetic configuration, and result in
a strongly noncollinear magnetic order of the particle ensemble. The energy
distribution of metastable states is determined. For a low degree of disorder a
strongly asymmetric shape with a pronounced peak of the ground state energy
results. In contrast, for a strong disorder a Gaussian-like distribution is
obtained. The average dipole coupling energy $\bar E_\mathrm{dip}$ decreases
with increasing structural disorder. The role of vacancies has been studied for
a square particle array by determining the angular distribution of the
preferred microvortex angle as function of the vacancy concentration.
Indications for a preferred angular direction along the axial as well as along
the diagonal directions of the square array are revealed. A corresponding
investigation for disturbed square arrays results in a different angular
distribution. The effect of dipole-quadrupole corrections resulting from the
finite size of the particles is quantified.
