  Ensembles of gold nanoparticles present a magnetic behavior which is at odds
with the weakly diamagnetic response of bulk gold. In particular, an unusual
ferromagnetic order has been unveiled by several experiments. Here we
investigate if the combined effect of orbital magnetism of conduction electrons
and interparticle dipolar interaction can lead to magnetic ordering. Using
different model systems of interacting mesoscopic magnetic dipoles, together
with a microscopic description of the electron dynamics within the
nanoparticles, we find that a spontaneous magnetic moment may arise in dimers
of metallic nanoparticles when the latter are characterized by a large orbital
paramagnetic susceptibility.
