Origin of room temperature ferromagnetism in optically transparent 2D graphene/Co-doped ZnO/graphene

Despite extensive efforts to find room temperature ferromagnetic (FM) two-dimensional (2D) materials, only a few 2D FM materials have been experimentally synthesized. Particularly, it is a still challenging task to find optically transparent room temperature 2D FM systems. Recently, room temperature ferromagnetism was confirmed in 2D graphene/Co doped ZnO/graphene multilayer structure. However, in our calculations, the Co doped ZnO (ZCO) monolayer showed an antiferromagnetic (AFM) ground state. Moreover, both ZCO/graphene bilayer and graphene/ZCO/graphene trilayer systems also exhibited an AFM state. Thus, we investigated the possibility of hydrogen-induced ferromagnetism. After the introduction of the H atom (H-ZCO), we found a ferromagnetic (FM) ground state in both H-ZCO monolayer and graphene/H-ZCO/graphene trilayer systems. We obtained that the FM state appeared only when the H atom stayed on the same ZCO layer. Thus, we propose that the ferromagnetism in the graphene/ZCO/graphene originates from the H effect. Also, we obtained a Curie temperature (TC) of 500 K in H-ZCO and graphene/H-ZCO/graphene trilayer systems. Moreover, we find that the graphene/H-ZCO/graphene shows optically transparent behavior in the visible frequencies.