It was demonstrated that the bound magnetic polaron model was responsible for ferromagnetism in Co–ZnO semiconductors, where the carriers were provided by the interstitial zinc (Zni). The experiment was unique since, by changing the temperature, it was possible to cross the carrier concentration threshold above which a long-range ferromagnetic order was established. Consequently, the ferromagnetic order was observed at room temperature but was weakened at temperatures below 100K. To support this conclusion, a systematic investigation was made of the structural, magnetic and transport properties. This gave consistent results in the context of the proposed two-region model, i.e.: a Zni layer where carriers were sufficient to couple Co ions ferromagnetically and a region with little carriers that remained in a paramagnetic state.

Effect of Zn Interstitials on the Magnetic and Transport Properties of Bulk Co-Doped ZnO. L.R.Shah, H.Zhu, W.G.Wang, B.Ali, T.Zhu, X.Fan, Y.Q.Song, Q.Y.Wen, H.W.Zhang, S.I.Shah, J.Q.Xiao: Journal of Physics D, 2010, 43[3], 035002