等价电子数组元Heusler合金Fe_2RuSi中的原子占位

对等价电子数组元Heusler合金Fe_2RuSi的原子占位、电子结构与磁性进行了理论与实验研究.第一性原理计算表明,虽然Fe_2RuSi中Fe,Ru均有8个价电子,但是Ru仍表现出强烈的占据A,C晶位倾向.基态总能最低的是Fe与Ru分别占据A,C晶位的XA结构,次低的是Fe,Ru在A,C位混乱占位的L2_1B结构,且两者能量差很小.这说明决定Heusler合金中过渡族原子占位的因素除价电子数以外还可能有原子半径和共价杂化作用等.态密度和差分电荷密度计算表明Heusler合金中主族元素与最近邻过渡族元素之间的p-d共价杂化对Heusler合金的占位有明显影响,在XA结构中Ru与Si和Fe(B)之间都存在明显的杂化作用,而在高能的L2_1结构中,Si与最近邻的Fe杂化作用相当弱.XRD测试表明在室温Fe_2RuSi存在A,C位之间的Fe-Ru反占位,形成了能量次高的L21B结构,这主要来自于混合熵对自由能的贡献及其引起的原子自发混乱占位.在5 K下Fe_2RuSi的饱和磁矩为4.87μB/f.u.,与计算值符合得相当好.     

The short range of the electronic promoter effect of potassium

Photoemission of Adsorbed Xenon atoms (PAX) as a local work function probe is used to investigate the range of the electronic promoter effect of potassium submonolayers on a Ru(001) surface. Three Xe states on these bimetallic K/Ru surfaces are clearly distinguishable by their 5p photoemission and are associated with Xe probe atoms at basically unmodified Ru sites, at “mixed” K.Ru sites next to K ions, and on top of potassium, respectively. From the relative intensities of these three states as well as from their 5p electron binding energies as a function of potassium coverage it is concluded that the radius of the “sphere” of modified charge density around one K ion is ～ 6 Å.     

二苯基硫脲的配位取代反应及其红外光谱表征

研究了二苯基硫脲分别与二十羧基三钌和十二羰基二铁的配位取代反应,红外光谱跟踪反应从开始到结束,谱图数据揭示出原始物消失和新物质的形成过程,并对产物Ru3(CO)9CSN2HPh2和Fe3(CO)sS2CNPh作了KBr压片红外光谱表征。     

Magnetism and structure of hexagonal Fe in Fe/Ru superlattices

Mössbauer spectroscopy is applied to study the magnetism and the local structure of Fe/Ru superlattices. Fe Layers show hcp structure, isomorphous with Ru. A well-defined symmetry is observed at Fe sites, withV _zz along thec axis. The Fe/Ru interfaces are non-magnetic over the two first layers, but a large moment (2μ_B/Fe) exists beyond. Magnetic coupling between neighbouring Fe Layers is observed at small spacing, i.e. for Ru thickness lower than 13 Å typically.     

X‐ray absorption spectroscopy analysis of Ru in La2RuO5

The compound La₂RuO₅ was examined by the x‐ray absorption spectroscopy (XAS) methods, x‐ray absorption near edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS). XANES technique was used to probe directly the average valence of Ru atoms in the compound. The energy shift of the Ru K‐edge in the XANES signal gave the average Ru valence state as 4.0 ± 0.1. EXAFS analysis provided, by yielding directly the interatomic distances and coordination numbers, the first information on the Ru atom neighborhood, on which the model for the Rietveld refinement of the unit cell of the new compound was devised. Finally, the local structure around the Ru atoms from the refinement was used in the FEFF6 code for a model EXAFS spectrum. The very good quantitative agreement with the measured spectrum proves that the refined crystal structure contains no systematic defects in the vicinity of Ru atoms. This result, together with the valence obtained from XANES, strongly confirms the proposed La₂RuO₅ stoichiometry. Copyright © 2007 John Wiley & Sons, Ltd.     

The Electronic and Magnetic Properties of FCC Iron Clusters in FCC 4D Metals

The electronic and magnetic structures of small FCC iron clusters in FCC Rh, Pd and Ag were calculated using the discrete variational method as a function of cluster size and lattice relaxation. It was found that unrelaxed iron clusters, remain ferromagnetic as the cluster sizes increase, while for relaxed clusters antiferromagnetism develops as the size increases depending on the host metal. For iron in Rh the magnetic structure changes from ferromagnetic to antiferromagnetic for clusters as small as 13 Fe atoms, whereas for Fe in Ag antiferromagnetism is exhibited for clusters of 24 Fe atoms. On the hand, for Fe in Pd the transition from ferromagnetism to antiferromagnetism occurs for clusters as large as 42 Fe atoms. The difference in the magnetic trends of these Fe clusters is related to the electronic properties of the underlying metallic matrix. The local d densities of states, the magnetic moments and hyperfine parameters are calculated in the ferromagnetic and the antiferromagnetic regions. In addition, the average local moment in iron-palladium alloys is calculated and compared to experimental results.     

过渡金属掺杂钛酸纳米管的电子结构和光学性质研究

采用平面波超软赝势方法计算了过渡金属(TM)(Fe,Co或者Ru)掺杂钛酸纳米管的电子结构及光学性质.对TM取代钛酸纳米管层间间隙位H+的几何结构进行优化,发现掺杂对几何结构的影响较大,其中Co或者Ru掺杂的形成能均较低.此外,掺杂的TM与周围的O原子成键,有形成固熔体的趋势.掺杂后的能带结构分析表明:Fe,Co或者Ru掺杂导致钛酸纳米管禁带宽度减小并且于禁带中引入了新的能级,这主要归因于b1g(dx2-y2)及a1g(dz2)态的出现;部分杂质能级处于半填充状态,成为空穴的俘获中心,减少电子和空穴的复合;掺杂后,价带顶向低能方向移动,使价带中形成的空穴氧化性更强.最后,掺杂的钛酸盐纳米管的吸收光谱显示,Ru掺杂的钛酸纳米管导致其在可见光范围内有更强的吸收.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

RuFeSi — A new magnetic system

A ternary system consisting of Ru, Fe and Si is identified in which the hyperfine field at Fe is seen to evolve at a characteristic temperature, T¹, far below the magnetic ordering temperature, T_m. It is speculated that the magnetic ordering at T_m is itinerant and T¹ corresponds to the localization of the majority spin electrons at the Fe atoms.     

Magnetic configurations of hexagonal iron rhenium multilayers

We present ab initio calculation of the electronic structure and magnetic properties of Fe/Re multilayers assuming hcp crystallographic phase. The system is modeled according to recent structural analysis which shows that Fe/Re has bct(0 0 1) ordering for 0⩽t_Re⩽8 Å after which a structural phase transition occurs to an hcp phase. Antiferromagnetic and ferromagnetic orders appear more stable than the nonmagnetic (NM) state, only when the Fe atoms begin to adopt an expanded volume of about 27% accompanied by an axial distortion (c/a=1.42). The sizeable magnetic moments calculated on Fe atoms contrast with the paramagnetic bulk hcp. These results are discussed and compared with recent experimental data on hcp Fe/Re superlattices and also with magnetic properties of Fe/Ru multilayers.     

Clusters and magnetic anchoring points in (Ga,Fe)N condensed magnetic semiconductors

The stability and magnetic properties of Fe clusters in the (Ga,Fe)N magnetic semiconductor is investigated by using first-principles density functional theory and local spin density+Hubbard U theoretical methods. The present results reveal the existence of ferrimagnetic clusters formed by three or four peripheral Fe atoms neighboring a central Fe atom acting as a robust magnetic anchoring point. These clusters have magnetic moments 2 or 3 times that of a single Fe atom and, when connected by sharing peripheral Fe atoms, can form stable, ordered magnetic regions where all of the central atoms are ferromagnetically coupled. The formation of these ferrimagnetic clusters is proposed here to be at the origin of the ferromagnetic behavior observed in (Ga,Fe)N samples showing chemical phase separation.     

Environment effect on the hyperfine field distributions in ordered Ni3Fe

It was found that the Mössbauer pattern of Fe in a partially ordered Ni₃Fe consists of three component spectra, each corresponding to a local configuration assigned by the number of Fe atoms on the first neighboring shell. It was concluded from the results that the hyperfine field at an Fe nucleus increases slowly with increasing number of Fe atoms on the first neighboring shell but the magnetic moment of an Fe is not influenced appreciably by the surrounding conditions.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Magnetic properties of stable structures of small binary clusters

We have studied the optimum geometries and the magnetic behavior of small binary Fe_nGe_m (n+m≤4) clusters usingab initio spin-polarized density functional calculations. Our results reveal that the optimized clusters present high values in the HOMO–LUMO gap and generally prefer structures with high geometries, the local magnetic moments at Fe atoms present an enhancement with respect to Fe bulk magnetization, whereas the Ge atoms present local magnetic moments whose modulus take significative values. The magnetic coupling between Fe and Ge atoms is mainly antiferromagnetic-like. All the clusters studied here present a charge transference from Fe atoms to Ge atoms.     

Detecting positron-atom bound states through resonant annihilation

A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation due to electronic Feshbach resonances at electron-volt energies. The method is applicable to a range of open-shell transition-metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb, Ta, W, Os, Ir, and Pt. Estimates of their binding energies are provided.     

Local structure of core/shell nanocrystalline particles produced by surfactant-assisted ball milling of Fe powder in paraffin

Surfactant-assisted ball milling of the Fe powder in paraffin has been used for fabrication of core–shell nanocrystalline particles. The local atomic structure of the bulk and surface layers of the mechanically milled particles has been studied using X-ray absorption spectroscopic techniques with synchrotron radiation from the DORIS storage ring at DESY, Hamburg. Regardless of milling environment composition, the as-prepared powders were shown to be characterized by a significant drop in the EXAFS signal intensity and coordination numbers of the Fe–Fe pairs due to the formation of nanocrystalline state in the particles. It has been shown that an addition of perfluorononanoic acid as a surfactant has a more prominent effect on the structure of the shell layers. The effect is revealed as an appearance of light element atoms (O, F, C) in the local atomic environment of the Fe atoms due to formation of oxide, carbide and adsorbed structures of different types in the particle shell.     

合金化元素Ru和Re在Ni3Al中的相互作用及其对力学性质的影响

采用基于密度泛函理论和广义梯度近似的第一原理方法,探究了Ru元素和Re元素在Ni₃Al中的相互作用及其对力学性质的影响.计算表明：在绝大多数化学计量比范围内,Ru原子优先占据Ni₃Al中的Ni位,Re原子优先占据Ni₃Al中的Al位,Ru和Re易于聚集分布,占据近邻的Ni-Al原子对.通过差分电荷密度图和态密度图分析得到Ru和Re掺杂会与近邻Ni原子产生轨道相互作用,Ru和Re之间也会产生轨道相互作用.通过计算弹性模量,应用经验判据得到Ru的加入使得Ni₃Al材料抗压缩能力和韧性明显增强,但硬度有所降低.而Ru、Re的同时加入使其抗压缩能力、韧性和硬度都有所增强.     

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

An in-situ study of structure and magnetic properties of Fe films on the sulphur passivated Ge(100) surface at 150°C

Ultra-thin Fe films have been grown on the sulphur passivated Ge(100) surface at 150°C. The growth, structure and the magnetic properties of the thin films were studied with LEED, AES, angle resolved AES and in-situ magneto-optical Kerr effect measurements. For the first five monolayers of Fe, no long-range order was observed . However, angle resolved AES data suggest that local order occurs with a small fraction of the Fe atoms adsorbed on bcc sites. For thicker Fe films, the growth becomes ordered. A comparison of the present study and with a previous study of the growth of Fe on sputter cleaned Ge(100)(2×1) surface, shows that sulphur passivation effectively prevents Fe–Ge intermixing. During the Fe deposition process, most of the sulphur atoms migrate to the growing surface, thus acting as surfactants. The presence of sulphur at the surface also affects the growth and magnetic properties of the thin films.