EXAFS and XRD studies of an amorphous Co57Ti43 alloy produced by mechanical alloying

We have investigated the local atomic order of an amorphous Co₅₇Ti₄₃ alloy produced by mechanical alloying by means of x-ray diffraction and EXAFS analyses on Co and Ti K-edges. Average coordination numbers and average interatomic distances between first neighbors where found from EXAFS and compared with those determined using an additive hard sphere (AHS) model associated with an deconvolution, and also with data from bcc- Co₂Ti compound. EXAFS results obtained indicated a shortening in the Co-Ti and Ti-Ti average interatomic distances when compared to those found by using the AHS-RDF method and an increase in the Co-Co and Ti-Ti average interatomic distances and a shortening in the Co-Ti one when compared to the interatomic distances found in the bcc- Co₂Ti compound. In spite of these differences, average coordination numbers obtained from EXAFS and AHS-RDF are similar to each other and also to those found in bcc- Co₂Ti.     

Optical and Magnetic Properties of Fe-Doped GaN Diluted Magnetic Semiconductors Prepared by MOCVD Method

Fe-doped GaN thin films are grown on c-sapphires by metal organic chemical vapour deposition method （MOCVD） Crystalline quality and phase purity are characterized by x-ray diffraction and Raman scattering measurements. There are no detectable second phases formed during growth and no significant degradation in crystalline quality as Fe ions are doped. Fe-related optical transitions are observed in photoluminescence spectra. Magnetic measurements reveal that the films show room-temperature ferromagnetic behaviour. The ferromagnetism may originate from carrier-mediated Fe-doped GaN diluted magnetic semiconductors or nanoscale iron clusters and FeN compounds which we have not detected.     

Compression Behaviour of Bulk Metallic Glasses and Binary Amorphous Alloy

The compression properties of Zr41Ti14Cu12.aNi10Be22.5, Zr44.4Nb7Cu13.5Ni10.8Be24.3 bulk metallic glasses and Ni77P23 binary amorphous alloy are investigated at room temperature up to 24 GPa, 39 GPa and 30.5 GPa, respectively, using in-situ high pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The pressure-volume relationship of Ni77P23 amorphous alloy is consistent well with the second order BirchMurnaghan （B-M） equation within the experimental pressure range. However, under higher pressure, the experimental data of Zr-based specimens deviate from the B-M equation. Compare to the binary amorphous alloy less excess free volume existing in the bulk metallic glass and multi-component atomic configuration results in a two-stage relationship between compressibility and pressure.     

采用PVA方法制备ZnO基发光稀磁半导体

不同的制备工艺对ZnO薄膜的微结构和性能有很大的影响,为了得到成本较低,样品具有较好特性的实验方法,对于制备手段进行了探索。使用PVA溶胶-凝胶法制备了Zn_0.88Co_0.12O薄膜,研究了不同退火工艺对其微结构的影响。对于Zn_0.88Co_0.12O样品的微结构和室温下的铁磁性和发光特性,具体比较分析了产生原因。对比了Co掺杂和复合Co、Fe掺杂Zn_0.88(Co_0.5Fe_0.5)_0.12O样品的微结构,采用振动样品磁强计(VSM)测量了样品的磁特性,发现单一掺杂的薄膜具有更好的晶体质量和更强的磁性。     

稀磁ZnO纳米颗粒的生长和性能

利用Zn、Fe、Mn、Co的铜铁试剂盐为前驱体,胺为表面包裹剂,在200℃N2保护下生长了2%的过渡金属离子掺杂的ZnO稀磁纳米晶体,研究了纳米晶体的结构、形态、光学和磁学性能。所有ZnO纳米晶体均为近圆形的颗粒,晶体结构为六角纤锌矿结构,无其他氧化物相的析出,但过渡金属离子的掺入使纳米颗粒的尺寸增大。在掺杂纳米颗粒的吸收谱和发射谱中均可以观察到明显的激子吸收和发射峰,所有纳米颗粒在温度高于43K时只有顺磁性。     

Age-induced phase transitions on mechanically alloyed amorphous GaSe

After aging it for four years at room temperature, a mechanically alloyed amorphous GaSe powder was transformed to a multi-phase crystalline alloy, where major phase is the trigonal Se one. The structural, thermal and optical properties of this aged amorphous GaSe were investigated through systematic X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The X-ray diffraction results on the aged GaSe powder suggest the presence of oxides, and X-ray absorption spectroscopy was employed to further investigate it.     

Measurement of the magnetic moment in a cold worked 304 stainless steel using HTS SQUID

The magnetic properties of stainless steel have been investigated using a radio frequency (RF) high-temperature superconductivity (HTS) SQUID (Superconducting QUantum Interference Device)-based susceptometer. The nuclear grade 304 stainless steel is nonmagnetic at a normal condition but it changes to a partially ferromagnetic state associated with martensitic transformation under a plastic deformation. The magnetic moment of the 304 stainless steels was increased with an increasing cold work rate, and decreased with an increasing annealing temperature. The change of mechanical properties such as yield strength and ultimate tensile strength (UTS) are also analyzed in terms of deformation-induced martensitic transformation.     

新型稀磁半导体Mn掺杂LiZnP的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对纯LiZnP、Mn掺杂LiZnP、Li过量和不足时Mn掺杂LiZnP体系进行了几何结构优化,计算并分析了体系的电子结构、半金属性、态密度及光学性质。结果表明：LiZnP新型稀磁半导体可以实现自旋和电荷注入机制的分离。Mn的掺入使体系产生自旋极化杂质带,自旋极化率达到100%,表现出半金属铁磁性,且体系性质受Li计量数的影响。当Li不足时杂质带宽度增大,半金属性增强,居里温度提高,形成能最低。进一步比较光学性质发现：Mn掺入后体系光学性质没有明显变化,但随Li的化学计量数的改变,介电函数虚部会在低能区中出现新的介电峰,同时复折射率函数对低频电磁波吸收明显加强,且能量损失在Li过量时最小。     

新型稀磁半导体Mn掺杂LiZnP的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对纯LiZnP、Mn掺杂LiZnP、Li过量和不足时Mn掺杂LiZnP体系进行了几何结构优化,计算并分析了体系的电子结构、半金属性、态密度及光学性质.结果表明:LiZnP新型稀磁半导体可以实现自旋和电荷注入机制的分离.Mn的掺入使体系产生自旋极化杂质带,自旋极化率达到100%,表现出半金属铁磁性,且体系性质受Li计量数的影响.当Li不足时杂质带宽度增大,半金属性增强,居里温度提高,形成能最低.进一步比较光学性质发现:Mn掺入后体系光学性质没有明显变化,但随Li的化学计量数的改变,介电函数虚部会在低能区中出现新的介电峰,同时复折射率函数对低频电磁波吸收明显加强,且能量损失在Li过量时最小.     

Influence of Randomness and Localization on RKKY Interactions in Diluted Magnetic Semiconductors

We show that the spatially random distribution of magnetic moments of dopants in diluted magnetic semiconductors can partially localize the itinerant carriers and change the carrier-mediated indirect RKKY interaction. From numerical calculations of the electron states taking into account the interaction with magnetic impurities which are random both in spatial positions and in orientations of magnetic moments, we obtain the electron states and the RKKY interaction as a function of the distance between magnetic dopants L and of the sp - d exchange integral J. With the increase of disorder, the localization of itinerant electrons become stronger and the long-range regular oscillatory behaviour of the RKKY interaction gradually disappears and is replaced by severe fluctuations. The randomness and localization may enhance the RKKY interaction between dopants with short and middle distances and in favour of the ferromagnetism.     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Structural characterization of pumice-supported silver-palladium metal clusters by means of XAFS and AWAXS

Silver-palladium bimetallic clusters were synthesized on pumice by the reduction of aqueous solution of metal salts with different Pd:Ag ratios. Used as bimetallic catalysts, in hydrogenation reactions, in situations where molecules can undergo several different reactions, they eliminate undesired reactions and maximize the desired ones. To characterize the structure of these bimetallic catalysts and control if the two metallic elements do mix or not to each other to form an alloy, the structure of the Ag-Pd bimetallic clusters was investigated by means of X-ray-absorption fine-structure spectroscopy (XAFS) and anomalous wide angle X-ray scattering (AWAXS) experiments performed at the European Synchrotron Radiation Facility (ESRF) using the GILDA and the BM16 beamlines. A correlation between Ag and Pd was found but not a clear evidence of alloying. Received: 21 December 1998 / Received in final form: 19 March 1999     

Mn掺杂GaN稀磁半导体的电磁特性研究

采用基于密度泛函理论的第一性原理平面波超软赝势法计算了不同浓度Mn掺杂GaN（Ga1-xMnxN, x=0.0625和0.1250）的晶格常数、能带结构和态密度,分析比较了掺杂前后GaN的电子结构和磁性。结果表明：Mn掺入后体系仍为直接带隙半导体,带隙宽度随Mn含量的增加逐步增大。Mn掺杂GaN均使得N 2p与Mn 3d轨道杂化,产生自旋极化杂质带,自旋向上的能带占据费米面,掺杂后的Ga1-xMnxN表现为半金属铁磁性,适合自旋注入;随着Mn掺杂浓度的增加,体系的半金属性有所增强。     

Mn掺杂GaN稀磁半导体的电磁特性研究

采用基于密度泛函理论的第一性原理平面波超软赝势法计算了不同浓度Mn掺杂GaN(Ga1-xMnxN,x=0.0625和0.1250)的晶格常数、能带结构和态密度,分析比较了掺杂前后GaN的电子结构和磁性.结果表明:Mn掺入后体系仍为直接带隙半导体,带隙宽度随Mn含量的增加逐步增大.Mn掺杂GaN均使得N2p与Mn3d轨道杂化,产生自旋极化杂质带,自旋向上的能带占据费米面,掺杂后的Ga1-xMnxN表现为半金属铁磁性,适合自旋注入;随着Mn掺杂浓度的增加,体系的半金属性有所增强.     

Electronic States and Spatial Charge Distribution of Single Mn Impurity in Diluted Magnetic Semiconductors

The electronic and magnetic properties as well as the spatial charge distribution of single Mn impurity in III--V diluted magnetic semiconductors are obtained when the degeneracy of the p orbits contributed from the four nearest-neighbouring As(N) atoms is taken into account. We show that in the ground state, the Mn spin is strongly antiferromagnetically coupled to the surrounding As(N) atoms when the p-d hybridization V_pd is large and both the hole level E_v and the impurity level E_d are close to the Fermi energy. The spatial charge distribution of the Mn acceptor in the (110) plane is non-spherically symmetric, in good agreement with the recent STM images.     

Origin of Room-Temperature Ferromagnetism for Cobalt-Doped ZnO Diluted Magnetic Semiconductor

The pure single phase of Zn0.95 Co0.05 O bulks is successfully prepared by solid-state reaction method. The effects of annealing atmosphere on room-temperature ferromagnetic behaviour for the Zn0.95 Co0.05 O bulks are investigated. The results show that the air-annealed samples has similar weak ferromagnetic behaviour with the as-sintered samples, but the obvious ferromagnetic behaviour is observed for the samples annealed in vacuum or Ar/H2 gas, indicating that the strong ferromagnetism is associated with high oxygen vacancies density. High saturation magnetization Ms = 0.73μg/Co and coercivity Hc = 233.8 Oe are obtained for the Ar/H2 annealed samples with pure single phase structure. The enhanced room-temperature ferromagnetic behaviour is also found in the samples with high carrier concentration controlled by doping interstitials Zn （Zni）.     

Magnetic Impurity Band and Photoinduced Magnetic Phase Transition in Diluted Magnetic Semiconductors

Applying the dynamical coherent potential approximation (dynamical CPA) to a model of diluted magnetic semiconductors (DMSs), in which both random impurity distribution and thermal fluctuation of localized spins are taken into account, the spin-polarized band and the carrier spin polarization are calculated for various magnetizations. In order to clarify the role of impurity depth on the occurrence of ferromagnetism, three typical cases are investigated: (a) II-VI DMS, (b) deep impurity level, and (c) strong exchange interaction. The present study reveals that the impurity depth of magnetic ions strongly enhances the carrier spin polarization (CSP) and accordingly, leads to a high Curie temperature. This means that photoinduced ferromagnetism with high Curie temperature can be expected in a DMS with a deep impurity depth and strong exchange interaction.     

Magnetic calorimeter for registration of small energy release

A scheme of magnetic calorimeter for registration of rare events characterized by small energy release (cosmic rays, WIMPs, solitary X-ray quanta) is proposed. The calorimeter is brought to operation by adiabatic demagnetization, and its magnetic response is measured by a quantum interferometer (SQUID, A. Barone and G. Paterno, Physics and applications of Josephson Effect). Special consideration is given to the specific features of calorimeter operation in the ferromagnetic transition region. The trigger registration of ultrasmall energy release by a ferromagnetic system in the metastable state is described.     

第一性原理计算Cu-Cr共掺AlN稀磁半导体

采用基于密度泛函理论的第一性原理平面波超软赝势方法,结合广义梯度（GGA）近似对Cr单掺AlN和Cu-Cr共掺AlN的32原子超原胞体系进行几何结构优化,计算它们的晶格常数,能带结构,电子态密度以及光学性质.结果表明Cr单掺AlN和Cu-Cr共掺AlN均表现为半金属性质,带隙变窄,且Cu-Cr共掺体系自旋极化作用较Cr单掺强,材料表现出良好的铁磁性.共掺杂后,光吸收的范围增宽,体系对长波吸收加强,能量损失明显减小.