Defect induced ferromagnetism in NiO nanocrystals: Insight from experimental and DFT plus U study

We investigate the defects induced magnetic properties of NiO nanocrystals prepared by mechanical alloying ball milling process. The average crystallite size reduces to similar to 12 nm after 40 hours of milling but the face centered cubic structure is stable both in pure and milled NiO powders. The unmilled pure NiO is anti -ferromagnetic in nature, whereas the as-milled nanoparticles are ferromagnetic at room temperature with saturation magnetization reaching a maximum of 0.85 emu/g. X-ray photoelectrons spectroscopy (XPS) shows two distinct peaks at 854.2 and 856.1 eV specifically related to the typical Ni-O bonds from Ni2+ and Ni3+ respectively. This is clearly supports the presence of non-stoichiometry in the milled samples due to decrease in Ni2+-O2--Ni2+ spin correlation length caused by defects. These defects and crystallinity are characterized by Raman spectra which shows that the two-magnon band disappears, one-phonon (1P) longitudinal optical (LO) band broadens, two-phonon (2P) LO band exhibits red-shift and disappears eventually with decreasing of particle size. Magnetic hysteresis (M-H) loops reveals the ferro-magnetic nature of NiO nanocrystals. Considering these defects, we have also investigated the electronic and magnetic properties systematically within the framework of density functional theory. Our results suggest that the vacancies created by only oxygen as well as both nickel and oxygen are largely responsible for ferro-magnetism in nanoscale NiO powders.