Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co-ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100 K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.