Ferromagnetism from non-magnetic ions: Ag-doped ZnO

To develop suitable ferromagnetic oxides with Curie temperature (T_C) at or above room temperature for spintronic applications, a great deal of research in doping ZnO with magnetic ions is being carried out over last decade. As the experimental results on magnetic ions doped ZnO are highly confused and controversial, we have investigated ferromagnetism in non-magnetic ion, Ag, doped ZnO. When Ag replaces Zn in ZnO, it adopts 4d⁹ configuration for Ag²⁺ which has single unpaired spin and suitable exchange interaction among these spins gives rise to ferromagnetism in ZnO with above room temperature T_C. Experimentally, we have observed room temperature ferromagnetism (RTFM) in Ag-doped ZnO with Ag concentration varied from 0.03% to 10.0%. It is shown that zinc vacancy (V_Zn) enhances the ferromagnetic ordering (FMO) while oxygen vacancy (V_O) retards the ferromagnetism in Ag-doped ZnO. Furthermore, the theoretical investigation revealed that V_Zn along with Ag²⁺ ions play a pivotal role for RTFM in Ag-doped ZnO. The Ag²⁺-Ag²⁺ interaction is ferromagnetic in the same Zn plane whereas anti-ferromagnetic in different Zn planes. The presence of V_Zn changes the anti-ferromagnetic to ferromagnetic state with a magnetic coupling energy of 37 meV. Finally, it has been established that the overlapping of bound magnetic polarons is responsible for RTFM in low doping concentration. However, anti-ferromagnetic coupling sets in at higher doping concentrations and hence weakens the FMO to a large extent.