Enhancement of ferromagnetism upon thermal annealing in pure ZnO

We report here the enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature T-c above room temperature. We observe a finite coercive field up to 300 K and a finite thermoremanent magnetization up to 340 K for the annealed sample. We propose that magnetic moments can be formed 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 paramagneticlike behavior below 10 K. (C) 2007 American Institute of Physics.