  Several experimental studies have referred to the grain boundary (GB) defect
as the origin of the ferromagnetism in ZnO. However, the mechanism of this
hypothesis has never been confirmed. Present study investigates the atomic
structure and the effect of point defects in a ZnO GB using the GGA+U
approximation. The relaxed GB possesses large periodicity and channels with 8
and 10 numbered atoms having 4 and 3 fold coordination. Unlike O vacancy (VO),
Zn vacancy (VZn) is preferentially aggregated at the GB, relative to the
bulk-like region, indicating the possibility of obtaining p-type conductivity
in polycrystalline ZnO. Although no magnetization is obtained from point
defect-free GB, VZn induces spin polarization as large as 0.68 {\mu}B/atom to
the O sites at the GB. Ferromagnetic exchange energy > 150 eV is obtained by
increasing the concentration of VZn and by the injection of holes into the
system. Electronic structure analysis indicates that the spin polarization
without external dopants originates from the O 2p orbitals, a common feature of
d0 semiconductors.
