摘要
基于密度泛函理论的平面波超软赝势方法模拟计算了金红石相TiO_2的四种本征缺陷(氧空位、钛空位、钛间隙缺陷、氧间隙缺陷)和两种复合缺陷(氧空位与氧间隙复合缺陷、钛空位与钛间隙复合缺陷)的铁磁特性.结合态密度、电子分布及晶体结构变化分析可知,四种本征缺陷均会在系统内引入缺陷态.氧空位、钛间隙缺陷使费米面升高,引起自旋极化,引入磁矩分别为1.62μB与3.91μB;钛空位的缺陷态处于价带顶,使费米面进入价带,表现出明显的p型半导体特性,引入磁矩约为2.47μB;氧间隙缺陷引入缺陷态但仍然处于自旋对称状态,费米面略微下降;氧空位与氧间隙复合缺陷使费米面上升的程度比单个氧空位时大,模拟的超晶胞保持了氧空位的铁磁特性,大小为1.63μB;钛空位与钛间隙复合缺陷以反铁磁方式耦合,但超晶胞仍具有一定的铁磁特性.
The TiO_2 based diluted magnetic semiconductors（DMSs） have aroused the considerable interest as one of the promising candidates for the spintronic devices accommodating both charge and spin of electrons in a single substance.Unfortunately, however, throughout most of the published papers, the question how to clearly elucidate the role of defects which may be played in the experimentally observed room temperature ferromagnetism（RTFM） remains open,especially after a new concept of d^0 ferromagnetism. In such a case, to further understand this issue and also to explore the origin of the RTFM in rutile TiO_2, we here first perform a first principles calculation on the magnetic properties of the intrinsic defects, namely oxygen vacancy（V_O）, Ti vacancy（V_（Ti））, Ti interstitial（Ti_（in））, oxygen interstitial（O_（in））and two complex defects of V_O＋ O_（in） and V_（Ti）＋Ti_（in）. Combining the density functional theory and the Perdew-BurkeErnzerhof functional of the generalized gradient approximation, we calculate various model structures of rutile TiO_2 constituted by 48-atom 2 × 2 × 2 supercell. The cutoff energies in these calculations of the planewave basis are all set to be 340 eV and the Monkhorst-Pack scheme k points are set to be 3 × 3 × 4 for an irreducible Brillouin zone. The convergence threshold for self-consistent field iteration is 0.1455 × 10^（-6） eV /atom. Structural relaxation is taken into account in each of all calculations. It is found that each defect we created in the structure leads to a lattice expansion and that the positive value for spin up and the negative value for spin down of the density of states（DOS） of the structure without defect are symmetric, suggesting that the perfect rutile TiO_2 lattice is nonferromagnetic. For the system with one V_O, the total energy of the spin-polarized system is 200 meV lower than that of the non-spin-polarized system,which indicates ferromagnetic behavior in this system. The defect brings in an impurity state near Fermi level located at about 0.7–1.0 eV down below the conduction band, resulting in an excess of spin up over spin down for the presences of the two localized electrons left by the vacancy. At this point the supercell bears a magnetic moment of about 1.62 μB.In contrast, V_（Ti） also brings in an impurity state near Fermi level but above the valence band, which reveals a p-type characteristic semiconductor nature. Since a lower total energy requires more spin-up electrons, the asymmetric DOS induces a magnetic moment of 2.47 μB. When a neutral Ti occupies an interstitial lattice site, the system requires it to be oxidized into a Ti^（3＋） ion to increase the stabilization of the system. The three delocalized electrons tend to occupy the 3d or 4s orbital of the neighbor Ti^（4＋） ions and then have strong exchange interactions with the 2p electrons of the local O atom. This can distort octahedral symmetry and give rise to a ferromagnetic moment of 3.91 μB. O_（in） defect in the supercell is extremely unstable. It can easily be reduced and escape from the host in terms of an oxygen molecule so that the system is in a manner similar to the perfect lattice, showing no ferromagnetism. It is interesting to note that the properties of the system with the complex defect of one V_O and O_（in） are similar to that of the structure with one V_O and the magnetic moment of this system is 1.63 μB. For the Ticom complex defect, our results point out the fact that the magnetic properties of the supercell are related to the distance between V_（Ti） and Ti_（in）. The spin up and spin down states are symmetric when they are close to each other, while, in addition to some ferromagnetic behavior, the system mainly exhibits antiferromagnetism when the distance increases.
作者
林俏露
李公平
许楠楠
刘欢
王苍龙
Lin Qiao-Lu Li Gong-Ping Xu Nan-Nan Liu Huan Wang Cang-Long(School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China)
出处
《物理学报》
SCIE
EI
CAS
CSCD
北大核心
2017年第3期302-315,共14页
Acta Physica Sinica
基金
国家自然科学基金(批准号:11575074
11304324)
山东大学晶体材料国家重点实验室开放基金(批准号:KF1311)资助的课题~~
关键词
第一性原理
本征缺陷
态密度
自旋磁矩
first-principles
intrinsic defect
density of states
magnetization
