“…Therefore, some work has been contributed with additional dopants to enhance or induce ferromagnetism in ZnO:Co systems. In the case of (Co, Al)-codoped ZnO, for example, Liao et al [8] used the water-assistance reactive vapor deposition method and Liu et al [9] used a simple solgel technique, although completely different methods have been used, all findings indicated that introducing the additional Al dopant played an important role in enhancing the FM of Co-doped ZnO owing to the increase of the free electron carrier density, while Alaria et al [10] using the co-precipitation method, confirmed that Al ions increase the free carriers concentration, however, those n-type free carriers are not sufficient to create ferromagnetic interactions. Therefore, the reported findings are controversial and more effort should be made to determine the origin of the RT FM in Co and Al co-doped ZnO.…”