  We report here enhancement of ferromagnetism in pure ZnO upon thermal
annealing with the ferromagnetic transition temperature Tc above room
temperature. We observe a finite coercive field upto 300K and a finite
thermoremanent magnetization upto 340K for the annealed sample. We propose that
magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur
due to either superexchange between vacancy clusters via isolated F+ centers,
or through a limited electron delocalization between vacancy clusters. Isolated
vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like
behaviour below 10K.
