ORIGINAL PAPER Enhancement of Virtual Magnetic Moment Formation in ZnO NPs by Li + Ion Doping Supawika Tanyawong 1 & I-Ming Tang 2 & Tun Seng Herng 3 & Sirikanjana Thongmee 1 Received: 7 March 2020 /Accepted: 18 May 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Lithium-doped ZnO (Zn 1-x Li x O) nanoparticles were fabricated using the hydrothermal process. XRD patterns showed that the Li + ions substituted into different lattice sites depending on the level of the doping of Li + into this semiconductor. The Williamson-Hall plots of the same XRD showed that the strains and sizes of the nanoparticles do not changed monotonically. Using the Tau plot to analyze the effects of the Li + substitution on the optical (UV-Vis absorption) by the nanoparticles, the changes in the observed values of the energy gaps of the different doped semiconductor were seen not to decrease continuously. A VSM (vibrating sample magnetometer) was used to measure the hysteresis loops of our Zn 1-x Li x O NPs (originally, the nonmagnetic semiconductor XnO doped with the nonmagnetic Li + ions). Our VSM measurements shows that the saturation magnetization (emu/g) and coercive force of these nanoparticles increased or decreased, respectively, as the level of Li + doping increased. Photoluminescence emission at ≈ 510 nm showed that the Li + doping led to the creation of more zinc vacancies, which in turn generates more virtual magnetic moments. Our results support the vacancy-induced d 0 electron model of ferromagnetism in nonmagnetic ion-doped ZnO nanoparticles. Keywords Lithium-doped zinc oxide semiconductor . d 0 ferromagnetism . Zinc vacancy . Li + ion doping . 510 nm photoluminescence peaks 1 Introduction The discovery of ferromagnetic behavior in thin films of HfO 2 by Venkatesan et al. [1] caused renewed interest in magne- tism, since magnetic properties in insulators and semiconduc- tor normally require the ions to have unpaired d or f electrons. For paired electrons, the magnetic moments created by each of these electrons would be in the opposite directions and would therefore cancel out. Since neither the Hf 4+ or the O 2- ion has any unpaired d or f electrons, HfO should not exhibit any magnetic behavior. However, Hong et al. [2] observed room temperature in undoped semiconducting and insulating oxides such as TiO 2 and In 2 O 3 . Sundaresan et al. [3] stated that fer- romagnetism is a universal feature of nanoparticles (NPs) of the otherwise nonmagnetic oxides (CeO 2 , Al 2 O 3 , ZnO, In 2 O 3 , and SnO 2 ), Both Hong et al. and Sundaresan et al. pointed to the need of oxygen- or zinc-related defects (vacancies or in- terstitials) to induce ferromagnetism into the nonmagnetic ox- ides (when they are in the bulk form). Researchers therefore began to look for correlations between the onset of the ferro- magnetic behaviors and the presence of these defects, as their presence are believed to be responsible for the observed mag- netic behaviors [4, 5]. Zuo et al. [6] found that both oxygen interstitials and zinc vacancies may induce ferromagnetism into the ZnO nanostructure systems. Based on first-principle density function theory (DFT) cal- culations, Wang et al. [7] showed that the introduction of a zinc vacancy into the wurtzite structure of ZnO created virtual energy levels around the vacancy. These levels can then be occupied by the 2p orbital electrons of the nearby O 2- ion. The configuration where the two electrons are aligned has a lower energy than the configuration having non-aligned electrons, and so a pair of aligned O-2p electrons would localized about * Sirikanjana Thongmee fscisjn@ku.ac.th 1 Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand 2 Computational and Applied Science for Smart Innovation Cluster (ClASSIC), Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand 3 Department of Materials Science, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore Journal of Superconductivity and Novel Magnetism https://doi.org/10.1007/s10948-020-05547-6