Zong Yan, Sun Yong, Meng Shiyan, Wang Yajing, Xing Hongna, Li Xinghua, Zheng Xinliang
School of Physics, Northwest University, Xi'an 710069 China
RSC Adv. 2019 Jul 25;9(40):23012-23020. doi: 10.1039/c9ra03620b. eCollection 2019 Jul 23.
Co-doped ZnO nanoparticles with different dosage concentrations were fabricated by a thermal decomposition method. The nanoparticles show a pure wurtzite structure without the formation of a secondary phase or Co clusters, in which Co ions present as Co and occupy Zn tetrahedral sites within the ZnO matrix. All the samples show ferromagnetic properties at room temperature with nonzero coercivity and remanence magnetization. Besides, the magnetic data is also fitted by the model of bound magnetic polarons (BMP). By increasing the Co doping concentration, the saturation magnetization values of Co-doped ZnO nanoparticles increase first and then decreases, which is related to the variation tendency of oxygen defects on the surface and the number of BMPs. This phenomenon can be ascribed to the formation of defect-induced BMPs, in which ferromagnetic coupling occurs at lower Co concentration and Co-O-Co antiferromagnetic coupling arises at higher Co concentration. Air annealing experiments further demonstrate this result, in which the saturation magnetization of Co-doped ZnO nanoparticles is reduced after annealing in Air. The doping effect and oxygen defects on the magnetic ordering of Co-doped ZnO were calculated using density functional theory. The calculation results reveal that stable long-range magnetic ordering in Co-doped ZnO nanoparticles is mainly attributed to the localized spin moments from 3d electrons of Co ions. Both the experimental and theoretical studies demonstrate that the ferromagnetism in Co-doped ZnO nanoparticles is originated from the combined effects of Co doping and oxygen vacancies. These results provide an experimental and theoretical view to understand the magnetic origination and tune the magnetic properties of diluted magnetic semiconductors, which is of great significance for spintronics.
采用热分解法制备了不同掺杂浓度的共掺杂ZnO纳米颗粒。这些纳米颗粒呈现出纯纤锌矿结构，未形成第二相或Co团簇，其中Co离子以Co的形式存在并占据ZnO基体中的Zn四面体位置。所有样品在室温下均表现出铁磁特性，具有非零矫顽力和剩余磁化强度。此外，磁数据也通过束缚磁极化子（BMP）模型进行拟合。随着Co掺杂浓度的增加，Co掺杂ZnO纳米颗粒的饱和磁化强度值先增大后减小，这与表面氧缺陷的变化趋势和BMP的数量有关。这种现象可归因于缺陷诱导BMP的形成，其中在较低Co浓度下发生铁磁耦合，在较高Co浓度下出现Co-O-Co反铁磁耦合。空气退火实验进一步证明了这一结果，其中Co掺杂ZnO纳米颗粒在空气中退火后饱和磁化强度降低。利用密度泛函理论计算了Co掺杂ZnO的掺杂效应和氧缺陷对磁有序的影响。计算结果表明，Co掺杂ZnO纳米颗粒中稳定的长程磁有序主要归因于Co离子3d电子的局域自旋矩。实验和理论研究均表明，Co掺杂ZnO纳米颗粒中的铁磁性源于Co掺杂和氧空位的综合作用。这些结果为理解稀磁半导体的磁起源和调控磁性能提供了实验和理论视角，对自旋电子学具有重要意义。
