Experimental and first-principles study of defect-induced electronic and magnetic properties of ZnO nanocrystals

We investigate defect-induced electronic and magnetic properties of zinc oxide (ZnO) nanocrystalline powders prepared by mechanical milling. The average crystallite size decreases as the milling time is increased but the wurtzite structure is stable even after 60 hours of milling. The structural properties are characterized by X-ray diffraction and transmission electron microscopy. The chemical composition of pure unmilled and as-milled powders is determined by X-ray photoelectron spectroscopy. Raman scattering spectra were analyzed to find evidence of crystallinity and defect structures in the milled powders. The results obtained from electronic paramagnetic resonance studies demonstrate that the defects are due to both oxygen and zinc vacancies. Since defect-induced magnetism has potential application in spintronic devices, we measured the M-H loops of ZnO powders at different temperatures. We observed that there is a transition from paramagnetic to ferromagnetic behavior due to the vacancies caused by the crystallite size reduction. We also calculated the band structures and density of states within the framework of density functional theory with Hubbard U correction to understand the electronic properties of ZnO for both pure crystals and crystals with defects. A simple mechanical milling process can produce a crystallite size of approximately 10 nm after 40 hours of milling, and the resulting significant changes in electronic properties favor superior application in devices as compared with the bulk. This change of intrinsic properties in ZnO is largely due to the size effect and the increase of defect density.