Insights into magnetism generation in pristine O-polar ZnO thin films through Kr ion implantation: experimental and theoretical study

Current research is in constant evolution, regularly revealing surprises that challenge our current understanding. Introducing magnetism into non-magnetic materials like ZnO is motivated by the desire to create new properties and develop innovative technological applications. In this work, magnetization is generated in O-polar ZnO by implanting krypton ions. Krypton is chemically unreactive and does not engage in chemical bonding within the compound; magnetization results from the radiation damage caused by the implantation process. The Kr implantation increases the lattice parameters and defect density. Large magnetization values of approximately 186 emu cm⁻³ were observed upon implantation. To discern the mechanism leading to the induced magnetism, we have performed ab initio calculations. We modeled the effects caused by beam damage by simulating mechanical constraints and introducing Zn and O vacancies. The calculations clearly show that the magnetism in pristine ZnO was attributed to Zn vacancies, where a magnetic moment of around 2µ_B was observed. Oxygen defects lead to negligible magnetization within the cell (10⁻⁵ µ_B). The effects of strain on the observed magnetism were also negligible. The introduction of Zn vacancies in the ZnO matrix leads to localized unpaired electrons, creating large local magnetic moments. This disruption of the electronic structure results in an asymmetry between spin-up and spin-down states, promoting magnetic alignment. Interaction between these magnetic moments near the vacancies establishes a pronounced magnetic region with a reduced charge density, while the regions far away from the vacancy were found to be non-magnetic with a higher charge density, shedding light on the induced magnetism in the material. Our research is of crucial importance in shedding light on the fundamental mechanisms of magnetism induced in ZnO.