Origin of the defects-induced ferromagnetism in un-doped ZnO single crystals

We clarified, in this Letter, that in un-doped ZnO single crystals after thermal annealing in flowing argon, the defects-induced room-temperature ferromagnetism was originated from the surface defects and specifically, from singly occupied oxygen vacancies denoted as F+, by the optical and electrical properties measurements as well as positron annihilation analysis. In addition, a positive linear relationship was observed between the ferromagnetism and the F+ concentration, which is in support with the above clarification. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793574]