  The last two decades have witnessed various experiments reporting very
unusual magnetic properties of ensembles of gold nanoparticles surrounded by
organic ligands, including ferromagnetic, paramagnetic, or (large) diamagnetic
responses. These behaviors are at odds with the small diamagnetic response of
macroscopic gold samples. Here we theoretically investigate the possibility
that the observed unusual magnetism in capped gold nanoparticles is of orbital
origin. Employing semiclassical techniques, we calculate the orbital component
to the zero-field susceptibility of individual as well as ensembles of metallic
nanoparticles. While the result for the orbital response of individual
nanoparticles can exceed by orders of magnitude the bulk Landau susceptibility
in absolute value, and can be either diamagnetic or paramagnetic depending on
nanoparticle size, we show that the magnetic susceptibility of a noninteracting
ensemble of nanoparticles with a smooth size distribution is always
paramagnetic at low magnetic fields. In particular, we predict that the
zero-field susceptibility follows a Curie-type law for small nanoparticle sizes
and/or low temperatures. The calculated field-dependent magnetization of an
ensemble of diluted nanoparticles is shown to be in good agreement with
existing experiments that yielded a large paramagnetic response. The width of
the size distribution of the nanoparticles is identified as a key element for
the quantitative determination of the orbital response.
