  Fe-doped ZnO nanocrystals are successfully synthesized and structurally
characterized by using x-ray diffraction and transmission electron microscopy.
Magnetization measurements on the same system reveal a ferromagnetic to
paramagnetic transition temperature > 450 K with a low-temperature transition
from ferromagnetic to spin-glass state due to canting of the disordered surface
spins in the nanoparticle system. Local magnetic probes like EPR and Mossbauer
indicate the presence of Fe in both valence states Fe2+ and Fe3+. We argue that
the presence of Fe3+ is due to the possible hole doping in the system by cation
(Zn) vacancies. In a successive ab-initio electronic structure calculation, the
effects of defects (e.g. O- and Zn-vacancy) on the nature and origin of
ferromagnetism are investigated for Fe-doped ZnO system. Electronic structure
calculations suggest hole doping (Zn-vacancy) to be more effective to stabilize
ferromagnetism in Fe doped ZnO and our results are consistent with the
experimental signature of hole doping in the ferromagnetic Fe doped ZnO
samples.
