Vacancy induced p-orbital ferromagnetism in MgO nanocrystallite

The collective magnetic response in MgO is due to the presence of magnesium vacancy (V-Mg) centers. Herein, this observation is verified under an experimental and density functional theory (DFT) framework. MgO grown at 500 degrees C, 650 degrees C and 800 degrees C attains a saturation magnetisation (M-s) of 0.13 emu/g, 0.15 emu/g and 0.22 emu/g, respectively. M-s drops from 0.22 emu/g to 0.04 emu/g when MgO (at 800 degrees C) is vacuum treated at 200 degrees C for 5 h. Ferromagnetism persists far above room temperature, attaining a Curie temperature (T-c) of 822 K. Electron spin resonance (ESR) data of the samples at 77 K and 300 K confirm the presence of oxygen (Vo) and magnesium (V-Mg) vacancy centers. DFT calculation reveals that only V Mg contributes to a magnetic moment in MgO, with no apparent contribution from Vo or Vmg+o. An increase in the V-Mg concentration results in a concomitant increase in the magnetic moment. Spin polarised charge density distribution analysis shows spin polarised O 2p states in the vicinity of V-Mg. Based on these results, we speculate that a localized polaron is formed by the hole trapped on the oxygen sub-lattice, nearest to V-Mg. Finally, the p-p interactions of several nearest polarons contribute to the long range ferromagnetism. (C) 2019 Elsevier B.V. All rights reserved.