Effects of Surface Polarity on the Structure and Magnetic Properties of Co Implanted and Co-Sm Co-Implanted Polar ZnO Films

We present a comprehensive experimental and theoretical study of the effects of surface polarity on the structure and ferromagnetic properties of Co implanted and Co-Sm co-implanted polar ZnO films deposited on sapphire substrates by molecular beam epitaxy. Substantial intrinsic ferromagnetism (FM) is found for all the implanted polar ZnO films. The magnetization of O-polar ZnO is observed to be higher than that of Zn-polar ZnO under the same implantation conditions, and the magnetization is enhanced for Co-Sm co-implanted ZnO in contrast with unimplanted and Co implanted films. First-principles calculations reveal that the Sm 4f and Co 3d states have strong hybridization with the O 2p state in O-polar ZnO, leading to larger magnetic moments for Co and Co-Sm substituting Zn atoms on the O-polar surface. Meanwhile, X-ray photoelectron spectroscopy results confirm that more oxygen vacancies are introduced into O-polar films by implantation and annealing. We consider that the stronger ferromagnetism in O-polar ZnO is associated with the combined influence of more oxygen vacancies and larger local moments related to Co and Sm doping. These results not only contribute to understanding the origin of FM in diluted magnetic semiconductors but also highlight the feasibility of developing polar spintronic devices for future polar thin film systems.