Co doped ZnO films have been deposited by a laser-molecular beam epitaxy system. X-ray diffraction and UV spectra analysis show that Co effectively substitutes the Zn site. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy analysis indicate that there are no clusters. Co dopants are uniformly distributed in ZnO film. Ferromagnetic ordering is observed in all samples deposited under an oxygen partial pressure, ${\mathrm{PO}}_2={10}^{-3}, {10}^{-5}$, and ${10}^{-7}$ torr, respectively. However, the magnetization of ${\mathrm{PO}}_2={10}^{-3}$ and ${10}^{-5}$ is very small at room temperature. At low temperature, the ferromagnetic ordering is enhanced. Muon spin relaxation ($\mu \mathrm{SR}$) measurements confirm the ferromagnetism in all samples, and the results are consistent with magnetization measurements. From $\mu \mathrm{SR}$ and TEM analysis, the film deposited under ${\mathrm{PO}}_2={10}^{-7}$ torr shows intrinsic ferromagnetism. However, the volume fraction of the ferromagnetism phase is approximately 70%, suggesting that the ferromagnetism is not carrier mediated. Resistivity versus temperature measurements indicate Efros variable range hopping dominates the conductivity. From the above results, we can confirm that a bound magnetic polaron is the origin of the ferromagnetism.

• Received 13 March 2017

DOI:https://doi.org/10.1103/PhysRevB.96.104423