Positron annihilation studies of vacancy-type defects and room temperature ferromagnetism in chemically synthesized Li-doped ZnO nanocrystals

Abstract

In this article, we have investigated the effects of Li incorporation on the lattice defects and room-temperature d0 ferromagnetic behaviour in ZnO nanocrystals by correlating X-ray photoelectron, photoluminescence and positron annihilation spectroscopic study in details. It is found that at low doping level (<7at.%), Li1+ is an effective substituent of Zn site, but it prefers to occupy the interstitial positions when Li-doping exceeds 7at.% resulting in lattice expansion and increase of particle sizes. The pristine ZnO nanocrystals exhibit ferromagnetic behaviour which is further enhanced significantly after few percentage of Li-doping in ZnO. The magnitude of both saturation magnetizations (MS) as well as the Curie temperature (TC) are found to increase considerably up to Li concentration of 10at.% and then started to decrease on further Li-doping. The gradual enhancement of Zn vacancy (VZn) defects in ZnO nanocrystals due to Li substitution as confirmed from photoluminescence and positron annihilation spectroscopy measurements might be responsible to induce paramagnetic moments within ZnO host. The ferromagnetic exchange interaction between the localised moments of VZn defects can be mediated though the holes arising due to Li-substitutional (LiZn) acceptor defects within ZnO. Hence, Li doping in ZnO favours in stabilizing considerable VZn defects and thus helps to sustain long-range high-TC ferromagnetism in ZnO which can be a promising material in future spintronics.