H-impurity induced high-temperature ferromagnetism in Co-doped ZnO

Through first-principles total-energy calculations, the effect of H-impurity on the magnetic properties of Co-doped ZnO is studied. Instead of an antibonding location, a bond-centered location of Co-O is the most stable location for isolated H in Co-doped ZnO with a strong bond with oxygen which results in the Co neighbor displaced from the host site to form a Co dimer with the other Co. At the most stable position, due to the strong hybridization between the H-impurity states and the Co 3d-t_2g minority spin states at the Fermi level in the gap, H-impurity can mediate a strong short-ranged and long-ranged ferromagnetic spin-spin interaction between neighboring Co atoms. Results based on first-principles total-energy calculations show that H-impurity is a very effective agent that can make Co-doped ZnO process high-temperature ferromagnetism.     

Photoluminescence properties of Co-doped ZnO nanocrystals

We performed photoluminescence experiments on colloidal, Co²⁺-doped ZnO nanocrystals in order to study the electronic properties of Co²⁺ in a ZnO host. Room temperature measurements showed, next to the ZnO exciton and trap emission, an additional emission related to the Co²⁺ dopant. The spectral position and width of this emission does not depend on particle size or Co²⁺ concentration. At 8 K, a series of ZnO bulk phonon replicas appear on the Co²⁺-emission band. We conclude that Co²⁺ ions are strongly localized in the ZnO host, making the formation of a Co²⁺d-band unlikely. Magnetic measurements revealed a paramagnetic behaviour.     

Local fields in Co and Mn Co-doped ZnO

The magnetic properties of ZnO co-doped with 5 at. % Co and 5 at. % Mn(Zn_0.90Co_0.05Mn_0.05O) synthesized by a solid-state reaction were investigated by means of ⁵⁷Co emission Mössbauer spectroscopy. The majority of the probe ions (80 %) residing in defect-free substitutional Zn sites take the oxidation state of ⁵⁷Fe ²⁺, and the others presumably form local defects taking the state of ⁵⁷Fe ³⁺ at room temperature. Both components show doublets, and RT ferromagnetism was thus absent in the sample. For the measurement at 10 K, spectral broadening was observed, implying a possible presence of a weak magnetic component.     

Investigation of n-type donor defects in Co-doped ZnO

We investigate using density functional theory (DFT) the electronic structure of (∼3%) Co-doped ZnO in the presence of native n-type donor defects such as V_O and Zn_I. In particular, we apply a pseudopotential-based self-interaction correction (pseudo-SIC) scheme as an improvement to the local spin-density approximation (LSDA). This overcomes major short comings of the LSDA in describing Co-doped ZnO. Donor+dopant pair complexes such as Co–V_O and Co–Zn_I are studied as relevant magnetic centres for long-range magnetic interactions at low-dopant concentrations. We find that complex formation is energetically favourable but the inter-complex magnetic coupling is too weak to account for room temperature ferromagnetism in ZnO:Co     

ZnCoO稀磁半导体的室温磁性

采用固相反应法，将ZnO和Co₂O₃粉末按不同的成分配比混合，制备了稀磁半导体Zn_1-xCo_xO (x=0.02,0.06,0.10)材料.并使用H₂气氛退火技术对样品进行了处理，得到了具有室温铁磁性的掺Co氧化锌稀磁半导体.利用全自动X射线衍射仪、X射线光电子能谱仪、高分辨透射电子显微镜和超导量子干涉器件磁强计对样品的结构、晶粒的尺寸、微观形貌以及磁性等进行了测量和标度. 关键词： 稀磁半导体 氧化锌 掺杂 固相反应法     

Spin manipulation in semiconductor quantum dots qubit

Thirty years of effort in semiconductor quantum dots has resulted in significant developments in the control of spin quantum bits(qubits). The natural two-energy level of spin states provides a path toward quantum information processing. In particular, the experimental implementation of spin control with high fidelity provides the possibility of realizing quantum computing. In this review, we will discuss the basic elements of spin qubits in semiconductor quantum dots and summarize some important experiments that have demonstrated the direct manipulation of spin states with an applied electric field and/or magnetic field. The results of recent experiments on spin qubits reveal a bright future for quantum information processing.     

Magnetic states at the oxygen surfaces of ZnO and Co-doped ZnO

First principles calculations of the O surfaces of Co-ZnO show that substitutional Co ions develop large magnetic moments which long-range order depends on their mutual distance. The local spin polarization induced at the O atoms is 3 times larger at the surface than in the bulk, and the surface stability is considerably reinforced by Co. Moreover, a robust ferromagnetic state is predicted at the O (0001) surface even in the absence of magnetic atoms, correlated with the number of p holes in the valence band of the oxide, and with a highly anisotropic distribution of the magnetic charge even in the absence of spin-orbit coupling.     

共沉淀法制备Co掺杂ZnO的室温铁磁性的研究

利用共沉淀法并在5vol.%H₂/Ar气流中于300 ℃退火3 h,制备了Zn_1-xCo_xO稀磁半导体. 扫描电子微探针分析表明,对Co的名义组分分别为0.05,0.10,0.15的样品,其实际组分分别为x＝0.054, 0.100和0.159. X射线衍射表明, 主相为纤锌矿结构, x=0.100和 0.159的样品中含有CoO杂相. X射线光电子谱显示出Co有３种状态: 替代进入Z 关键词： 稀磁半导体 ZnO 共沉淀法 磁性来源     

Hydrogen shallow donors in Co-doped ZnO showing room-temperature ferromagnetism

Room-temperature ferromagnetism has been identified in sol–gel-prepared Co-doped ZnO (ZnCoO), hydrogen being incorporated by a linear polymer polyvinyl pyrrolidone. The magnetic order was investigated by the X-ray photoemission and magnetization measurements in view of the interstitial hydrogen being coupled with Co²⁺. The electron paramagnetic resonance measurements revealed that hydrogens not contributing to the magnetic order occupy the shallow donor levels in the hydrogenated ZnCoO system.     

Influence of defects on magnetism of Co-doped ZnO

Two kinds of Zn_0.97Co_0.03O powders were prepared by precursor thermal decomposition under different conditions. One grown at low temperature has a positive Curie-Weiss temperature Θ, while the other grown at high temperature has a negative Θ. Both of them contain oxygen vacancies. There are more shallow donors in the former than those in the latter. It is proposed that coexistence of oxygen vacancies and shallow donors is necessary to induce ferromagnetic coupling between Co ions.     

Structure and ferromagnetic properties of Co-doped ZnO powders

Single-phase Zn_1−xCo_xO (x=0.02, 0.04) powders were synthesized by a simple co-precipitation technique. X-ray diffraction analysis reveals that the Co-doped ZnO crystallizes in a wurtzite structure. The lattice constants of Co-doped ZnO powders decrease slightly when Co is doped into ZnO. Optical absorption spectra show a decrease in the bandgap with increasing Co content and also give an evidence of the presence of Co²⁺ ions in tetrahedral sites. Raman spectra indicate that Co doping increased the lattice defects and induced another Raman vibration mode around at 538 cm⁻¹, which is an indicator for the incorporation of Co²⁺ ions into the ZnO host matrix. Magnetic measurement reveals that the Zn_1−xCo_xO (x=0.02, 0.04) powders clearly exhibit room-temperature ferromagnetic behavior, which makes them potentially useful as building components for spintronics.     

Growth and characterizations of ZnO and Co-doped ZnO films for their use in spintronic

Diluted Magnetic Semiconductors offer potential applications for spintronics. In this respect, Co-doped ZnO films are particularly interesting due to their Curie temperature. However, the origin of ferromagnetism is controversial. High quality Co-doped ZnO thin films have thusly been grown using the pulsed laser deposition technique on (001) Al₂O₃ substrates. Two series were made. In the first one, the films are grown using metallic targets whereas in the second one, the films are synthesized from ceramic targets. Detailed characterizations have been performed and a comparison have been made, in light of the literature.     

Giant temperature coefficient of resistance in Co-doped ZnO thin films

A novel high-performance thermistor material based on Co-doped ZnO thin films is presented. The films were deposited by the pulsed laser deposition technique on Si (111) single-crystal substrates. The structural and electronic transport properties were correlated as a function of parameters such as substrate temperature and Co-doped content for Zn_1?x Co _x O (x=0.005,0.05,0.10 and 0.15) to prepare these films. The Zn_1?x Co _x O films were deposited at various substrate temperatures between 20 and 280 °C. A value of 20 %/K for the negative temperature coefficient of resistance (TCR) with a wide range near room temperature was obtained. It was found that both TCR vs. temperature behavior and TCR value were strongly affected by cobalt doping level and substrate temperature. In addition, a maximal TCR value of over 20 %?K^?1 having a resistivity value of 3.6 Ω?cm was observed in a Zn_0.9Co_0.1O film near 260 °C, which was deposited at 120 °C and shown to be amorphous by X-ray diffraction. The result proved that the optimal Co concentration could help us to achieve giant TCR in Co-doped ZnO films. Meanwhile, the resistivities of the films ranged from 0.4 to 270 Ω?cm. A Co-doped ZnO/Si film is a strong candidate of thermometric materials for non-cooling and high-performance bolometric applications.     

Co掺杂闪锌矿ZnO的磁性和光学性质

采用自旋极化密度泛函理论方法对Co掺杂闪锌矿ZnO的能带结构、态密度、磁学和光学属性进行了研究.计算结果显示:Co掺杂闪锌矿ZnO的基态是反铁磁态,具有金属性特征;而铁磁态具有半金属性特征.铁磁耦合在费米能级附近出现了明显的自旋劈裂现象,表现出明显的不对称性和强烈的Co 3d和O 2p杂化效应.磁矩主要来源于Co 3d轨道电子以及部分近邻耦合的O 2p轨道电子,大小与Co原子的掺杂位置有关.光学性质计算结果显示,Co掺杂闪锌矿ZnO在可见光范围内都有较强的光吸收能力,吸收峰在高能区发生了红移现象.理论计算结果表明,Co掺杂闪锌矿ZnO或许是一种优异的磁光材料.     

Co掺杂ZnO薄膜的结构和磁学性能

研究了用单束脉冲激光沉积法制备的Co掺杂ZnO薄膜的结构和磁学性能。XRD表征结果表明制备的薄膜是具有沿c轴择优取向的纤锌矿点阵结构。然而,进一步的高分辨电子显微镜结果显示整个样品上的晶体取向并不完全相同。很难说明形成了单晶。结果分析表明Co占据了部分Zn的格点,并对电子结构产生了影响。室温下观察到了磁滞回线,显示掺杂Co可以实现ZnO的磁性翻转,但磁性比较小。该薄膜与我们以前用双束脉冲激光沉积法制备的Co掺杂ZnO薄膜具有相似的性能,提示我们其内部的机制可能相似。     

Structure and room-temperature ferromagnetism of Co-doped ZnO DMS films

Co-doped ZnO diluted magnetic semiconductor films were prepared on Si(100) substrates by magnetron sputtering system and the Co content varies from 0.01 to 0.15. The X-ray diffraction results showed ZnO of the wurtzite structure. The ferromagnetism was observed at room temperature. The X-ray near-edge absorption spectroscopy revealed that Co substitutes for Zn²⁺ ions in the valence of +2 state in the Co-doped ZnO films.     

Na-Mg共掺杂ZnO薄膜的结构和光学性质

利用溶胶-凝胶法,在普通载玻片上使用旋转涂膜技术制备了具有c轴择优取向生长的Na-Mg共掺杂的ZnO薄膜。用XRD、SEM、光致发光(PL)及透射光谱对薄膜样品进行了表征。结果表明:Na-Mg共掺杂有利于ZnO薄膜的c轴择优取向生长,并且随着Na+掺杂浓度的增加,晶粒尺寸先增大后减小;通过比较不同掺杂浓度ZnO薄膜的PL谱,推测发光峰值位于380nm的紫外发射与ZnO的自由激子复合有关;发现掺入Mg的确能使ZnO禁带宽度增大,掺杂组分为Na0.04Mg0.2Zn0.76O时,其PL谱只有一个很强的紫光发射峰,其近带边紫外光发射强度较未掺杂的ZnO增强了近10倍,极大地提高了薄膜紫外发光性能;并且随Na+浓度增加薄膜透光性减弱。