  In the majority of cases nanostructures prepared by focused electron beam
induced deposition (FEBID) employing an organometallic precursor contain
predominantly carbon-based ligand dissociation products. This is unfortunate
with regard to using this high-resolution direct-write approach for the
preparation of nanostructures for various fields, such as mesoscopic physics,
micromagnetism, electronic correlations, spin-dependent transport and numerous
applications. Here we present an in-situ cleaning approach to obtain pure
Co-FEBID nanostructures. The purification procedure lies in the exposure of
heated samples to a H$_2$ atmosphere in conjunction with the irradiation by
low-energy electrons. The key finding is that the combination of annealing at
$300^\circ$C, H$_2$ exposure and electron irradiation leads to compact, carbon-
and oxygen free Co layers down to a thickness of about 20\,nm starting from
as-deposited Co-FEBID structures. In addition to this, in temperature-dependent
electrical resistance measurements on post-processed samples we find a typical
metallic behavior. In low-temperature magneto-resistance and Hall effect
measurements we observe ferromagnetic behavior.
