Effect of (Cd, Al) Co-doping and hydrogenation on the long-range ferromagnetic ordering of ZnO: Experimental and DFT studies

Nanopowders of ZnO codoped with Al and Cd ions (ZnO:Al:Cd) were synthesised by thermal co-decomposition of a mixture of Zn and metals complexes. X-ray diffraction analysis using the Rietveld method confirmed the formation of single nanocrystalline structure, thus, both Al and Cd ions are incorporated into ZnO host lattice forming solid solutions (SSs). Optical properties measured by diffuse reflectance spectroscopy (DRS) revealed a redshift associated with the creation of point defects and that the energy band gap is slightly affected by doping; i.e. 3.23-3.12 eV. Annealing of the as-synthesised SSs in H2-atmosphere (hydrogenation) was conducted in order to clarify the reinforcement and/or the creation of ferromagnetic order. Magnetic measurements showed that ZnO doped with 1.2 at% Al and 0.6 at%Cd became ferromagnetic with a coercivity ∼100 Oe, remanence ∼1.4 memu/g and saturation magnetization of ∼6.5 memu/g. Moreover, hydrogenation of undoped diamagnetic (DM) ZnO transformed into ferromagnetic. To elucidate the changes on the optical properties due to presence of dopants (Al, Cd) and the vacancies, we have employed first principle calculations based on density functional theory to calculate the optical properties and the change operated in the band gap due to structural changes. The observed RT-FM was associated to the doping and vacancies created by the hydrogenation.