The Origin Of Ferromagnetism Of Doping ZnO: First-principles Study

ZnO-based diluted magnetic semiconductors （DMSs） have attrached muchattention for their broad application prospects in spintronic devices. The research aimof people is to prepare DMSs which have good quality and room temperature （RT）ferromagnetism. Many groups have observed RT ferromagnetism in ZnO-basedDMSs through experiments. However, the origin of ferromagnetism of ZnO-basedDMSs still remains a controversial issue. To improve the physical properties of thesematerials, it is crucial to understand magnetic mechanism, and design new approachesto enhance the magnetic interations. In this work, we researched the ZnO-basedDMSs systemly based on the first-principle calculations. The electronic structures ofZnO-based DMSs were carried out, the results were used to analyze and explain theorigin of ferromagnetism. These research results provide new approaches forimproving the properties of ZnO-based DMSs.The main results are given as follows:1. The origin of ferromagnetism in Mn-doped ZnO was researched systemly frommany aspects. We first researched the magnetic properties of Mn adsorption on ZnO（1010） surfaces, it is found that the Mn atom prefers to reside at the bridge positionbetween the Zn and O atoms. Mn atoms tend to assemble together and form Mnclusters, the system displays stable anti-ferromagnetic （AFM）. With increasing thedistance between two Mn atoms, the AFM coupling diminishes rapidly. We secondresearched the role of oxygen vacancies in the origin of ferromagnetism in Mn-dopedZnO. Results demonstrated that single neutral oxygen vacancy can not induceferromagnetism, while the charged oxygen vacancy can induce ferromagnetism. Theneutral oxygen vacancy in hydrogenated environment also can introduceferromagnetism.2. Co-doped method is used to mediate the Ferromagntism in Mn-doped ZnO andCo doped ZnO sytems.（Mn, N）-codoped ZnO was first researched by GGA andGGA+U schemes of density functional theory. It is found that the introduction of Nelement changes the charge state of Mn ions and makes the system to change fromAFM to FM state. We secondly investigated the magnetic properties of （Co,Al）-codoped ZnO. It is found that the Al substitution for Zn is energetically favorablethan Al substitution of O or Al at the interstitial point and low concentration of Aldopant can’t induce RT ferromagnetism. At low Al dopant concentration, the introduction of oxygen vacancy can induce RT ferromagnetism and the origin offerromagnetism can be explained by an improved BMP model. When the Al dopantconcentration exceeds the critical concentration, the system will also display RTferromagnetism even though without oxygen vacancy in （Co, Al）-codoped ZnO inwhich RT ferromagnetism is mediated by the carriers.3. The origin of ferromagnetism of nonmagnetic metal element doped ZnO areresearched, respectively. In Cu-doped ZnO, it is found that the ferromagnetism arisesfrom the strong hybridization between Cu3d and O2p orbitals. In nonmagneticelements X-doped ZnO （X=Li, Mg and Al, which without3d electrons）, theferromagnetism is attributed to the coupling interaction between two Zn vacancies.The introduction of X reduces the formation energy of Zn vacancy as well asstabilizes the ferromagnetism.4. The magnetic properties of two kinds of nonmagnetic elements doped in （ZnO）₁₂cluster are researched, respectively. In Cu-doped （ZnO）₁₂cluster, it is found that theground state of the system is antiferromagntism, which is opposite to Cu-doped bulkZnO. It demonstrated that the ground state of the system is associated with the size forthe doping system. In C-doped （ZnO）₁₂cluster, the ground state of the system issensitive to the distance between the two doped C atoms. At appropriate C Cdistance, the ferromagnetism will occur and the ferromagnetism is mediated by holesthrough the p-d hybridization-like p-p coupling interaction between the two C dopantsand their mutual neighboring O atoms.