Carriers induced ferromagnetism in Co(II)-doped ZnO monolayers and their optical properties: First principles calculations

Diluted magnetic semiconductors are advantageous for magneto-electronic devices because they create ferromagnetism at high Curie temperatures and exhibit spin-dependent interactions. The complex microstructural and compositional aspects of optical emission dynamics and ferromagnetism are still unknown. Here, using first-principles calculations, we investigate the magnetic and opto-electronic characteristics of Co(II) mono-doped and (Co, Na) co-doped ZnO monolayers. The results show that Na co-doping can change the ground state of a Co(II)-doped monolayer from AFM to FM due to the exchange coupling between the Co-d spins and hole produced by Na. The observed room-temperature (RT) ferromagnetism in the co-doped monolayer below with 5% Na doping is due to exchange coupling via electron trapping oxygen vacancies with the magnetic Co ions. The optical investigation reveals a red shift in the d-band to d-band transition of Co(II) and a blue shift in the fundamental energy gap with increasing Co concentration. Na co-doping results in mid-IR and near-infrared absorption peaks. Furthermore, the correlation between spin-spin coupling and optical absorption shows that the FM(AFM) systems exhibit a red-shift (blue-shift) in the fundamental band gap energy and spin-allowed d-band to d-band transition of Co ions. The enhanced optical and magnetic findings suggest that the (Co, Na);ZnO monolayer might be a suitable option for spin-photonic and spintronic devices in the future.