氧空位对Co掺杂TiO2稀磁半导体中杂质 分布和磁交换的影响

本文使用基于密度泛函理论的第一性原理方法研究了Co掺杂TiO₂稀磁半导体中氧空位对体系能量和磁性的影响. 通过对总能量的计算发现当引入氧空位后近邻杂质体系能量高于均匀掺杂体系, 同时氧空位易在Co近邻位置富集. 进而发现氧空位的存在及其占位可以影响Co离子间的磁交换, 近邻Co离子体系下氧空位的引入使Co离子间的铁磁耦合减弱; 非近邻Co离子体系下, 底面氧空位使Co离子间呈反铁磁耦合而顶点氧空位使Co离子间呈铁磁耦合. 总之, 氧空位的存在对Co掺杂TiO₂材料的能量及磁性都有较大影响.     

氧空位对钴掺杂氧化锌半导体磁性能的影响

从实验和理论上阐述了氧空位对Co掺杂ZnO半导体磁性能的影响.采用磁控溅射法在不同的氧分压下制备了Zn_095Co_005O薄膜,研究了氧分压对薄膜磁性能的影响.实验结果表明,高真空条件下制备的Zn_095Co_005O薄膜具有室温铁磁性,提高氧分压后制备的薄膜铁磁性逐渐消失.第一性原理计算表明,在Co掺杂ZnO体系中引入氧空位有利于降低铁磁态的能量,铁磁态的稳定性与氧空位和Co之间的距离密切相关. 关键词： Co掺杂ZnO 稀磁半导体 第一性原理计算 氧空位缺陷     

利用第一性原理研究Ni掺杂ZnO铁磁性起源

采用基于密度泛函理论和局域密度近似的第一性原理分析了Ni掺杂ZnO磁性质.文中计算了8个不同几何结构的铁磁(FM)和反铁磁耦合能量,结果表明FM耦合更稳定.态密度结果显示Ni 3d 与O 2p发生杂化,导致费米能级附近电子态自旋极化.文中也分析了O空位对Ni掺杂ZnO铁磁性质的影响,O空位通过诱导电子调节FM耦合,从而稳定Ni掺杂ZnO铁磁性质,其强度足以引发室温铁磁性.通过Ni 3d能级耦合具体分析了Ni 掺杂ZnO铁磁性起源.另外,也分析了晶格应变对Ni掺杂ZnO FM耦合的影响. 关键词： 第一性原理 半导体 铁磁性 缺陷     

TM掺杂的Ⅲ-Ⅴ族稀磁半导体电磁性质的第一原理计算

使用基于自旋局域密度泛函理论的第一性原理方法对3d过渡金属(TM=V，Cr，Mn，Fe，Co和Ni)掺杂的Ⅲ-Ⅴ族半导体(GaAs和GaP)的电磁性质进行了计算.结果发现：用V，Cr和Mn掺杂时体系将出现铁磁状态，而Fe掺杂时将出现反铁磁状态，Co和Ni掺杂时，其磁性则不稳定.其中，Cr掺杂的GaAs和GaP将可能是具有较高居里温度的稀磁半导体(DMS).在这些DMS系统中，V离子的磁矩大于理论期待值，Fe，Co和Ni离子的磁矩小于理论期待值，Cr和Mn离子的磁矩与期待值的差距取决于晶体的对称性以及磁性离子的能带分布.此外，使用Si和Mn共同对Ⅲ-Ⅴ族半导体进行掺杂，将有利于DMS表现为铁磁状态，并可以使体系的T_C进一步提高. 关键词： 稀磁半导体 过渡金属 掺杂 共掺杂     

Co掺杂的ZnO室温铁磁半导体材料制备与磁性和光学特性研究

采用溶胶-凝胶法制备出具有室温铁磁性的Co掺杂的ZnO稀磁半导体材料. 通过对样品的结构、磁性和发光特性的研究发现,样品具有室温铁磁性,并发现其铁磁性源于磁性离子对ZnO中Zn离子的取代. 对不同温度制备的样品的磁性以及其发光特性的变化研究发现,样品的铁磁性与样品中锌间隙位（Zn_i）缺陷的密度有关. 关键词： ZnO 稀磁半导体 铁磁性     

表面活性剂和n型载流子对钴掺杂氧化锌薄膜铁磁性的双重调控

本文提出了一种利用表面活性剂钝化和电子载流子掺杂来调制钴掺杂氧化锌稀磁半导体薄膜固有铁磁性的"双重操纵效应"新方法.第一性原理计算表明,氢表面钝化作为薄膜表面Co-O-Co磁耦合的磁开关,可以控制掺杂钴原子的自旋极化.同时,电子载流子掺杂可以进一步用作类层状铁磁性媒介来调节薄膜内部原子层的铁磁性.该双重操纵效应揭示了n型钴掺杂氧化锌稀磁半导体薄膜的本质铁磁性来源,并且还有可能用作其他n型稀磁半导体氧化物薄膜增强铁磁性的替代策略.     

表面活性剂和n型载流子对钴掺杂氧化锌薄膜铁磁性的双重调控（英文）

本文提出了一种利用表面活性剂钝化和电子载流子掺杂来调制钴掺杂氧化锌稀磁半导体薄膜固有铁磁性的"双重操纵效应"新方法.第一性原理计算表明,氢表面钝化作为薄膜表面Co-O-Co磁耦合的磁开关,可以控制掺杂钴原子的自旋极化.同时,电子载流子掺杂可以进一步用作类层状铁磁性媒介来调节薄膜内部原子层的铁磁性.该双重操纵效应揭示了n型钴掺杂氧化锌稀磁半导体薄膜的本质铁磁性来源,并且还有可能用作其他n型稀磁半导体氧化物薄膜增强铁磁性的替代策略.     

Zn1-xCoxO稀磁半导体薄膜的结构及其磁性研究

利用X射线吸收精细结构、X射线衍射和磁性测量等技术研究脉冲激光气相沉积法制备的Zn1-zCoxO(x=0.01,0.02)稀磁半导体薄膜的结构和磁性.磁性测量结果表明Zn1-xCoxO样品都具有室温铁磁性.X射线衍射结果显示其薄膜样品具有结晶良好的纤锌矿结构.荧光X射线吸收精细结构测试结果表明,脉冲激光气相沉积法制备的样品中的Co离子全部进入ZnO晶格中替代了部分Zn的格点位置,生成单一相的Zn1-xCoxO稀磁半导体.通过对X射线吸收近边结构谱的分析,确定Zn1-xCoxO薄膜中存在O空位,表明Co离子与O空位的相互作用是诱导Zn1-xCoxO产生室温铁磁性的主要原因.     

氮掺杂的金刚石磁性研究

采用第一性原理计算方法研究金刚石中由空位或者N掺杂引起的磁特性. 发现-1价和-2 价的碳空位能分别产生3 μ_B和2μ_B的磁矩; -2价的碳空位能够引发长程有序的铁磁耦合状态, 而-1价的碳空位更倾向于反铁磁耦合.掺杂N元素能有效地控制空位的荷电状态及相应的磁相互作用, 这一结果为在金刚石中实现非过渡族金属掺杂的铁磁性提供了一条新的路径. 关键词： 第一性原理计算 氮掺杂 金刚石 磁性     

Cu掺杂ZnO磁性能的实验与理论研究

采用固相反应法制备了Cu掺杂ZnO样品.在室温下Cu含量3%的样品在室温下表现为铁磁性.样品为n型半导体,载流子的浓度为1015cm-3.利用密度泛函理论(DFT+U)计算了CuZnO体系的Cu2+—O2-—Cu2+,Cu2+—Vo—Cu2+,Cu2+—Vo+—Cu2+,Cu2+—V++o—Cu2+磁交换耦合作用,给出了不同束缚电荷的氧空穴Vo与Cu2+离子之间的超交换机理,提出了CuZnO体系中铁磁性机理为Cu2+—V++o—Cu2+束缚磁极化子模型.     

Effect of defects on room-temperature ferromagnetism in Co and Na co-doped ZnO

The role of defects in the room temperature ferromagnetism of the Co–ZnO based diluted magnetic semiconductor (DMS) was investigated by co-doping the DMS with Na. The structure characterizations indicate that both Na and Co ions enter into the ZnO lattice without the formation of secondary phase. The oxygen vacancy of ZnCoNaO increased while the carrier concentration decreased compared with that of ZnCoO, leading to the enhancement of the ferromagnetic property in the ZnCoNaO. The observed ferromagnetism introduced by Na ions is attributed to the exchange interaction via the electron trapped oxygen vacancies coupled with the magnetic Co ions.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Room temperature ferromagnetism in magic-sized Cr-doped CdS diluted magnetic semiconducting quantum dots

Manipulation of carrier spins in semiconductors for spintronics applications has received great attention driven by improved functionalities and higher speed operation. Doping of semiconductor nanocrystals by transition-metal ions pronounced as diluted magnetic semiconductors (DMS) has attracted tremendous attention. Such doping is, however, difficult to achieve in low-dimensional strongly quantum-confined nanostructures by conventional growth procedures. In the present case, magic-sized, pure, and Cr-doped CdS DM-QDs have been synthesized by solution phase chemistry (lyothermal method). Structural, optical, and magnetic investigation suggest an intrinsic nature of ferromagnetism with highly quantum-confined system. Optical and magnetic results of pure and doped QDs reveal major physical consequences of dopant localization within the capacity to engineer dopant-carrier exchange interactions introducing magnetic functionalities within the host semiconductor lattice. Unpaired Cr ions in Cd substitutional sites could create spin ordering and ferromagnetic coupling. The results presented herein illustrate some of the remarkable and unexpected complexities that can arise in doped QDs.     

Impurity clustering and ferromagnetic interactions that are not carrier induced in dilute magnetic semiconductors: the case of Cu2O:Co

Current models for ferromagnetism in diluted magnetic semiconductors, such as "p-d exchange" or "double-exchange", rely on the presence of partially filled gap states. We point out a new mechanism, not requiring partially filled states, in which ferromagnetic coupling arises from the occupation of previously unoccupied levels when two transition metal impurities form a close pair. We find from first-principles calculations that this mechanism explains strong ferromagnetic coupling between Co impurities in Cu2O, and at the same time gives rise to Co clustering.     

First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations.It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO.The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom.The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.     

Effects of temperature and atmosphere on the magnetic properties of Co-doped ZnO rods

Co-doped ZnO (Zn_0.95Co_0.05O) rods are fabricated by co-precipitation method at different temperatures and atmospheres. X-ray diffraction, Energy dispersive X-ray spectroscopy and Raman results indicate that the samples were crystalline with wurtzite structure and no metallic Co or other secondary phases were found. Raman results indicate that the Co-doped ZnO powders annealed at different temperatures have different oxygen vacancy concentrations. The oxygen vacancies play an important role in the magnetic origin for diluted magnetic semiconductors. At low oxygen vacancy concentration, room temperature ferromagnetism is presented in Co-doped ZnO rods, and the ferromagnetism increases with the increment of oxygen vacancy concentration. But at very high oxygen vacancy concentration, large paramagnetic or antiferromagnetic effects are observed in Co-doped ZnO rods due to the ferromagnetic-antiferromagnetic competition. In addition, the sample annealed in Ar gas has better magnetic properties than that annealed in air, which indicates that O₂ plays an important role. Therefore, the ferromagnetism is affected by the amounts of structural defects, which depend sensitively on atmosphere and annealing temperature.     

Magnetic semiconducting anatase TiO2−δ grown on (1 0 0) LaAlO3 having magnetic order up to 880 K

We demonstrate a semiconducting material, TiO_2−δ, with magnetism up to 880 K, without the introduction of magnetic ions. The magnetism in these films stems from the controlled introduction of anion defects from both the film–substrate interface as well as processing under a deficient oxygen atmosphere. First-principle band structure calculations indicate that the exchange between Ti cations mediated by an oxygen anion is positive, i.e., ferromagnetic, whereas the exchange between cations via a vacancy is negative, i.e., ferrimagnetic. It is likely that both the mechanisms are active in this system. This represents a new and promising approach in the search for room-temperature magnetic semiconductors.