  We present a comprehensive study of the energetics and magnetic properties of
ZnO clusters doped with 3d transition metals (TM) using ab initio density
functional calculations in the framework of generalized gradient approximation
+ Hubbard U (GGA+U) method. Our results within GGA+U for all 3d dopants except
Ti indicate that antiferromagnetic interaction dominates in a neutral,
defect-free cluster. Formation energies are calculated to identify the stable
defects in the ZnO cluster. We have analyzed in details the role of these
defects to stabilize ferromagnetism when the cluster is doped with Mn, Fe, and
Co. Our calculations reveal that in the presence of charged defects the
transition metal atoms residing at the surface of the cluster may have an
unusual oxidation state, that plays an important role to render the cluster
ferromagnetic. Defect induced magnetism in ZnO clusters without any TM dopants
is also analyzed. These results on ZnO clusters may have significant
contributions in the nanoengineering of defects to achieve desired
ferromagnetic properties for spintronic applications.
