  Present work investigates the structural, electronic and magnetic properties
of wurtzite (0001) GaN nanowires (NWs) doped with Gd and point defects by
employing the GGA+U approximation. We find that Ga vacancies (VGa) exhibit
lower formation energy compared to N vacancies (VN). Further stabilization of
point defects occurs due to the presence of Gd and ambient ferromagnetism (FM)
can be stabilized in the NW by the additional positive charge induced by the
VGa. Electronic structure analysis shows that VGa introduces additional levels
in the band gap leading to ferromagnetic coupling due to the hybridization of
the p states of the Ga and N atoms with the Gd d and f states. Ferromagnetic
exchange coupling energy of 76.4meV is obtained in presence of Gd-VGa complex,
and hence the FM is largely determined by the cation vacancy-rare earth complex
defects in GaN NWs. On the other hand, the VN, which introduce additional
electron carriers, does not assist in increasing the ferromagnetic exchange
energy.
