Vacancy induced p-orbital ferromagnetism in MgO nanocrystallite

Abstract

The collective magnetic response in MgO is due to the presence of magnesium vacancy (VMg) centers. Herein, this observation is verified under an experimental and density functional theory (DFT) framework. MgO grown at 500 °C, 650 °C and 800 °C attains a saturation magnetisation (Ms) of 0.13 emu/g, 0.15 emu/g and 0.22 emu/g, respectively. Ms drops from 0.22 emu/g to 0.04 emu/g when MgO (at 800 °C) is vacuum treated at 200 °C for 5 h. Ferromagnetism persists far above room temperature, attaining a Curie temperature (TC) 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 (VMg) vacancy centers. DFT calculation reveals that only VMg contributes to a magnetic moment in MgO, with no apparent contribution from VO or VMg+O. An increase in the VMg 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 VMg. Based on these results, we speculate that a localized polaron is formed by the hole trapped on the oxygen sub-lattice, nearest to VMg. Finally, the p-p interactions of several nearest polarons contribute to the long-range ferromagnetism.