Annealing temperature dependence of the structures and properties of Co-implanted ZnO films

- To avoid the forming of Co clusters and explore the origin of the magnetism, detailed investigation on the properties of the Co-implanted ZnO films with a rather low dose of 8 × 1015 cm−2 and high implantation energy of 1 MeV were carried out.
- The crystalline structure of the damaged region caused by ion-implantation has been recovered via the thermal annealing treatment at the temperature of 900 °C and above.
- The low temperature magnetic hysteresis loops have indicated paramagnetism for the annealed films with weak ferromagnetic characteristics.
- The zero-field cooling (ZFC) magnetization curves of the Co-implanted ZnO samples have varied from concave shape to convex one as the annealing temperature increased from 750 °C to 1000 °C.

The effects of thermal annealing treatment on the structural, electrical, optical and magnetic properties of Co-implanted ZnO (0 0 0 1) films have been investigated in detail. The crystalline structure of the damaged region caused by ion implantation has been recovered via the thermal annealing at the temperature of 900 °C and above, and no Co clusters or its related oxide phases have been observed. The electrical and optical properties of the annealed films have shown strong dependence on the annealing temperature. The zero field cooling magnetization curves of the annealed films have varied from concave shape to convex one as the annealing temperature increased from 750 °C to 1000 °C, which are possibly tuned by the changes of the ratio of the itinerant carriers over the localized spin density. The low temperature magnetic hysteresis loops have indicated paramagnetic behavior for the annealed films with weak ferromagnetic characteristics. The ferromagnetism is attributed to the substituted Co2+ ions and vacancy defects, while the paramagnetism could be induced by ionized interstitial Zn defects.