Room-temperature ferromagnetism of the amorphous Cu-doped ZnO thin films

Amorphous copper-doped ZnO thin films (ZnO:Cu) prepared on glass substrates by the radio-frequency magnetron co-sputtering have been investigated. Magnetic measurements indicated that the amorphous ZnO:Cu thin films were ferromagnetic at room temperature and the saturation magnetization was much higher than that of the polycrystalline films. X-ray diffraction results showed there was no Cu₂O phase in amorphous ZnO:Cu films, which might be the reason for the high magnetic moment of the films. On the other hand, the high saturation magnetization of the amorphous ZnO:Cu films could also attribute to that there was no limit of solid solubility of Cu in amorphous ZnO solvent. The X-ray photoelectron spectroscopy study of the amorphous ZnO:Cu thin films reveal that copper was in Cu²⁺ chemical state.     

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.     

磁控溅射法制备钴掺杂氧化锌薄膜室温磁学性质

钴掺杂氧化锌是室温稀磁半导体的重要候选材料,其磁学特性和钴掺杂浓度、显微结构及光学性质密切相关。磁控溅射具有成本低、易于大面积沉积高质量薄膜等特点,是广受关注的稀磁半导体薄膜制备方法。利用磁控溅射方法制备了不同浓度的钴掺杂氧化锌薄膜,并对其显微结构、光学性质和磁学特性进行了系统分析。结果表明：当掺杂原子分数在8%以内时,钴掺杂氧化锌薄膜保持单一的铅锌矿晶体结构,钴元素完全溶解在氧化锌晶格之中;薄膜在可见光区域有很高的透射率,但在567, 615和659 nm处有明显吸收峰,这些吸收峰源于Co2+处于O2-形成的四面体晶体场中的特征d-d跃迁。磁学特性测试结果表明钴掺杂氧化锌薄膜具有室温铁磁性,且钴的掺杂浓度对薄膜的磁学特性有重要影响。结合薄膜结构、光学和电学性质分析,实验中观察到的室温铁磁性应源于钴掺杂氧化锌薄膜的本征属性,其铁磁耦合机理可由束缚磁极化子模型进行解释。     

Influence of reducing anneal on the ferromagnetism in single crystalline Co-doped ZnO thin films

This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on $a$-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become good conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16~$\mu _{\rm B}$/Co$^{2 + })$ in the Zn6110M, 7550P, 7280E, 7870Dhttp://cpb.iphy.ac.cn/CN/10.1088/1674-1056/19/5/056101https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=111756Co-doped ZnO, diluted magnetic semiconductors, x-ray absorption fine structure, single crystalline thin filmsProject partially supported by National Science Foundation of China (Grant No.~10804017), National Science Foundation of Jiangsu Province of China (Grant No.~BK2007118), Research Fund for the Doctoral Program of Higher Education of China (Grant No.~20070286037), Cyanine-Project Foundation of Jiangsu Province of China (Grant No.~1107020060), Foundation for Climax Talents Plan in Six-Big Fields of Jiangsu Province of China (Grant No.~1107020070) and New Century Excellent Talents in University (NCET-05-0452).This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on $a$-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become good conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16~$\mu _{\rm B}$/Co$^{2 + })$ in the Zn$_{0.95}$Co$_{0.05}$O single crystalline film with reducing annealing in the absence of Zn vapour, the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism (0.65~$\mu _{\rm B}$/Co$^{2 + })$ at room temperature. This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films, and the corresponding ferromagnetic mechanism is discussed.Co-doped;ZnO;diluted;magnetic;semiconductors;x-ray;absorption;fine;structure;single;crystalline;thin;filmsThis paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on a-plane sapphire substrates by using molecular-beam epitaxy.The as-grown films show high resistivity and non-ferromagnetism at room temperature,while they become more conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour.The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase.Compared with weak ferromagnetism(0.16 μB/Co2+) in the Zn0.95Co0.05O single crystalline film with reducing annealing in the absence of Zn vapour,the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism(0.65 μB/Co2+) at room temperature.This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films,and the corresponding ferromagnetic mechanism is discussed.     

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.     

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     

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。     

Comparison of ferromagnetism in n- and p-type magnetic semiconductor thin films of ZnCoO

Both n- and p-type diluted magnetic semiconductor ZnCoO are made by magnetron co-sputtering with, respectively, dopants of Al and dual dopants of Al and N. The two sputtering targets are compound ZnCoO with 5% weight of Co and pure metal Al. Sputtering gases for n- and p-type films are pure Ar and N₂, respectively. These films are magnetic at room temperature and possess free electron- and hole-concentration of 5.34×10²⁰ and 5.27×10¹³ cm⁻³. Only the n-type film exhibits anomalous Hall-effect signals. Magnetic properties of these two types of films are compared and discussed based on measurements of microstructure and magneto-transport properties.     

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.     

Study of ferromagnetism in Co-doped rutile powders and float-zone grown single crystals

Co-doped rutile samples in the form of both powders and bulk single crystals have been studied with particular emphasis on the dependence of their magnetic, compositional and structural properties upon the type of atmosphere used during their preparation. Both powders and single crystals were characterized using X-ray diffractometry and vibrating sample magnetometry, while the crystals were also studied using the X-ray Laue technique, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that an oxygen deficient environment during the preparation of Co-doped TiO₂ powders is crucial for the observation of room temperature ferromagnetism, while preparation in oxygen rich conditions destroys the ferromagnetism due to the formation of the paramagnetic second phase CoTiO₃. Floating zone growth of crystals under oxygen also led to the formation of material containing second phase CoTiO₃ that was paramagnetic at room temperature, while crystals grown under argon were ferromagnetic and contained Co-rich inclusions.     

Study of the magnetic and structural properties of Mn-, Fe-, and Co-doped ZnO powder

A study of the magnetic and structural properties of Zn_1−xM_xO powder (where x=0 or 0.01, and M=Mn, Fe or Co) produced by the proteic sol–gel process was undertaken. The sample crystal structure was analyzed by XRD and magnetic measurements were carried out in a SQUID magnetometer. Of the XRD analysis, all samples had hexagonal wurtzite crystal structure with P63mc space group, and no secondary phase was observed. It is observed of the M(H) measures at 2 K, that the Co- and Mn-doped ZnO displayed saturation magnetizations (M_s) of approximately 2 and 3.2 emu/g, respectively, and no remanence (M_r) was observed, indicating a superparamagnetic behavior in these samples. However, the Fe-doped sample showed a ferromagnetic behavior with M_s∼0.34 emu/g, M_r∼0.05 emu/g, and coercivity (H_c)∼1090 Oe. Already at room temperature, the M(H) measurements reveal a purely paramagnetic behavior for Mn- and Fe-doped ZnO, indicating that the Curie temperature (T_c) is below 300 K. However, a weak superparamagnetic behavior was observed in the Co-doped sample, indicating that T_c>300 K.     

Room temperature ferromagnetic pure ZnO

Pure ZnO films were prepared by pulsed laser deposition on oxidized Si substrates under different oxygen pressure and substrate temperature. Clear room temperature ferromagnetism has been observed in the ZnO film prepared under high vacuum and room temperature. The observation of anomalous Hall effect confirms the intrinsic nature of the ferromagnetism. The photoluminescence and X-ray photoelectron spectroscopy spectra show the high concentration of oxygen vacancies in the ferromagnetic ZnO film. Our results clearly demonstrate the ferromagnetic contribution of the oxygen vacancies mediated by the spin polarized electrons hopping between discrete states in pure ZnO.     

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     

Co掺杂ZnO薄膜的局域结构和电荷转移特性研究

采用磁束缚电感耦合等离子体溅射沉积法在不同的氧气分压下制备了Zn_0.95Co_0.05O和Zn_0.94Co_0.05Al_0.01O薄膜.利用X射线吸收精细结构技术对薄膜O-K,Co-K和Co-L边进行了局域结构研究,结果表明：Co²⁺取代了四配位晶体场中的Zn²⁺而未改变ZnO的六方纤锌矿结构,高真空条件下制备的薄膜 关键词： Co掺杂ZnO 稀磁半导体 X射线吸收精细结构 共振非弹性X射线散射     

Studies of the physical properties of Co, Cu codoped ZnO powders

Zn_0.95−xCo_0.05Cu_xO powders have been synthesized by the sol-gel method and the structural, magnetic and electrical properties of the powders have been investigated. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicate that the Co ions do not change the ZnO wurtzite structure. Magnetic measurements indicate that Co doping can induce room temperature (RT) ferromagnetism and the addition of Cu to the powders further increases the magnetic moment per Co ion. The effects of the introduction of Cu as an acceptor dopant in the host matrix are further studied using resistance measurements. It is demonstrated experimentally that acceptor doping plays an important role in realizing dominant ferromagnetic ordering in Co doped ZnO powders.     

Half-metallic ferromagnetism in Cu-doped zinc-blende ZnO from first principles study

Electronic structures and magnetism of Cu-doped zinc-blende ZnO have been investigated by the first-principle method based on density functional theory (DFT). The results show that Cu can induce stable ferromagnetic ground state. The magnetic moment of supercell including single Cu atom is 1.0 μ_B. Electronic structure shows that Cu-doped zinc-blende ZnO is a p-type half-metallic ferromagnet. The half-metal property is mainly attribute to the crystal field splitting of Cu 3d orbital, and the ferromagnetism is dominated by the hole-mediated double exchange mechanism. Therefore, Cu-doped zinc-blende ZnO should be useful in semiconductor spintronics and other applications.     

Electrical and magnetic properties of RE-doped ZnO thin films (RE = Gd, Nd)

Due to the small magnetic moments observed for 3d transition metals in ZnO [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117], there is still space for optimizing ZnO-based diluted magnetic semiconductors for spintronics applications. Motivated by the observation of magnetic moments as high as 4000μ_B/Gd atom in GaN:Gd [S. Dhar, O. Brandt, M. Ramsteiner, V.F. Sapega, K.H. Ploog, Phys. Rev. Lett. 96 (2005) 037205], we investigated ZnO films doped with 0.01, 0.1 or 1 at.% rare earth (RE) metals. The films, with thicknesses between 20 nm and 1 μm, have been grown by pulsed laser deposition on a-plane sapphire or fused silica substrates.The homogenous incorporation of the RE ions in ZnO was investigated by combined Rutherford backscattering and particle induced X-ray emission measurements. Hall measurements revealed an unexpected dependence of the electron concentration on film thickness, proving a non-uniform distribution of electrically active defects. Magnetotransport measurements at different temperatures were performed to study the magnetoresistance and the presence of the anomalous Hall effect. Large negative magnetoresistance was obtained at 5 K, while no anomalous Hall effect was observed. These results indicate that there are no exchange interactions between the RE ions.     

单晶和孪晶的Zn0.96Co0.04O稀磁半导体薄膜的制备与研究

采用分子束外延技术分别在不同晶面的蓝宝石（sapphire Al₂O₃）基片上制备了沿c轴生长的Zn_0.96Co_0.04O稀磁半导体薄膜.发现在Al₂O₃（1120）晶面（a面）上薄膜是二维层状外延生长的高质量单晶薄膜,而在Al₂O₃（0001）晶面（c面）上薄膜却具有有趣的孪晶结构,部分区域相互之间有一个30°的面内转动来减少和基片之间的失配度.在孪晶薄膜中存在的这些相互旋转形成的区域界面上会引起载流子强烈的散射作用,导致载流子迁移率的下降和平均自由程的缩短.利用X射线吸收精细结构技术证明了无论单晶还是孪晶的Zn_0.96Co_0.04O薄膜中所有的Co都以+2价替代进入了ZnO的晶格,而没有形成任何杂相.而对其磁性研究发现,孪晶的薄膜样品比高质量的单晶薄膜样品具有大得多的饱和磁矩.这充分说明孪晶薄膜中的铁磁性来源与缺陷有关.我们还对铁磁性耦合机制进行了探讨. 关键词： Co掺杂ZnO 稀磁半导体 X射线吸收精细结构 单晶和孪晶薄膜     

Origins of ferromagnetism in transition metal doped diamond

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.     

The structure and magnetic properties of Cu-doped ZnO prepared by sol-gel method

The Cu-doped ZnO and pure ZnO powders were synthesized by sol-gel method. The structural properties of the samples were investigated by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. All the results confirmed that copper ions were well incorporated into the ZnO lattices by substituting Zn sites without changing the wurtzite structure and no secondary phase existed in Cu-doped ZnO nanoparticles. The Zn_0.97Cu_0.03O nanoparticles exhibited ferromagnetism at room temperature, as established by the vibrating sample magnetometer analysis.