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.     

Periodic lattice distortions in epitaxial films of Fe(110) on W(110)

The epitaxial growth of Fe(110) on W(110) at 500 K is analyzed using LEED and AES. Frank-van der Merwe growth is established by AES. According to LEED, pseudomorphism occurs up to θ = 1.64, where every W atom of two topmost W layers is just covered by exactly one Fe atom. For 2?θ?9, characteristic reflection-multiplets are observed, symmetric about the basic Fe(110) reflections, which are interpreted in terms of periodic lattice distortions. The latter are caused by interaction with the misfitting W substrate.     

SPIN-DEPENDENT TUNNELING EFFECTS ON MAGNETIC NANOSTRUCTURES

The contrast mechanism of spin-polarized scanning tunneling spectroscopy (SP-STS) is demontrated on in-plane magnetized Gd islands grown on W (110) using Fe-coated tips. The use of Gd-coated tips enables the imaging of the antiferromagnetic domain structure of Fe nanowires being perpendicularly magnetized grown on a stepped W(110) substrate. The influence of an external magnetic field on the nanowire domain structure is demonstrated. It is shown that the antiferromagnetic domain structure of the Fe nanowires strongly depends on the miscut of the W(110) substrate. While at high miscut the magnetization direction alternates between adjacent DL stripes it was found to alternate within single Fe DL stripes at low miscut. Nanoscale Fe islands with a height of two atomic layers were found to be single domain particles.     

Noncollinear spin reorientation transition in S?=?1 ferromagnetic thin films

An in-plane spin reorientation transition in thin ferromagnetic films is discussed in terms of the thermodynamics of inhomogeneous low-dimensional systems based on a Néel sublattices concept while using a spin 1 Heisenberg Hamiltonian. The model allows us to investigate in a straightforward manner the layer-dependent phenomena. In this context, we propose a model of noncollinear magnetization structure based on the appropriate distribution of the anisotropy parameters inside the Fe films on W(110). The spin reorientation transition originates at the Fe/W(110) interface and proceeds via noncollinear magnetization structure toward the surface with increasing film thickness in accordance with the experimental findings. The temperature-driven spin reorientation transition in freestanding Fe films and in Fe/W(110) systems is also discussed in detail.     

In-situ conversion electron Mössbauer spectroscopy on Fe(110)-surfaces and thin films

Using in-situ conversion electron Mössbauer spectroscopy the layer-by-layer analysis of epitaxial 21-monolayer Fe(110)-films on W(110) was carried out. The hyperfine interaction parameters near the Ag-coated and free Fe(110) surfaces could be measured with the monolayer resolution. The quadrupole interaction deviates strongly from zero in the topmost iron monolayer. The reduction of hf-magnetic field for the free surface is demonstrated. The temperature dependence of the hf-magnetic field across the Ag-coated films is discussed.This work was supported by the Stiftung Volkswagenwerk.     

Large wave vector spin waves and dispersion in two monolayer fe on w(110)

We present the first surface spin-wave (SW) dispersion measurements up to the surface Brillouin zone boundary of a two monolayer Fe film on W(110) by using spin-polarized electron energy loss spectroscopy. Pronounced features of SW peaks are observed in the spectra at room temperature. We found that the SW energies in the Fe film are strongly reduced compared to spin waves in bulk Fe and to theoretical predictions. Our results suggest that this reduction is caused by the reduction of exchange interaction within the 2 ML Fe on W(110) as compared to bulk Fe.     

Ferromagnetic order and critical behavior at surfaces of ultrathin epitaxial films

The critical behavior, ferromagnetic order and magnetic anisotropies of ultrathin, epitaxial, magnetic films is studied using electron capture spectroscopy (ECS), which is capable of probing the long-ranged and short-ranged electron spin polarization (ESP) at the topmost surface layer of uncoated and coated magnetic structures. For all systems [Ni(100)/Cu(100), Ni(100)/NaCl(100), fcc Fe(111)/Cu(111), Fe(100)/Ag(100), Tb/Fe(100)/Ag(100), Fe(100)/Au(100), hcp Tb(0001)/W(110), Fe(110)/W(110), V(100)/Ag(100), Pd(100)/Ag(100), Pd/W(110)] investigated so far, ferromagnetic order is detected. It is found that the surface Curie temperatureT _Cs depends on film thicknessd. ECS data obtained at the surface of various systems reveal the existence ofT- andd-dependent magnetic anisotropies. Although for V(100)/Ag(100) the measured critical exponent=0.128 agrees very well with=1/8 predicted for the two-dimensional Ising model, for other systems, such as Fe(100)/Au(100), the measured value (0.25) is in disagreement with theoretical predictions. The experimental results are discussed within the framework of presently available experimental and theoretical data.     

Spin-polarized scanning tunneling spectroscopy on Fe nanowires

The contrast mechanism in spin-polarized scanning tunneling spectroscopy (SP-STS) is demonstrated on perpendicularly magnetized Fe nanowires grown on stepped W (110). By using Gd-coated tips, the antiferromagnetic domain structure of the nanowires can be imaged. The influence of an external magnetic field on the nanowire domain structure is demonstrated. It is shown that the antiferromagnetic domain structure of the Fe nanowires depends strongly on the miscut of the W (110) substrate. At high miscut the magnetization alternates between adjacent stripes, whereas at low miscut it alternates within the individual stripes.     

Onset of magnetization in monolayer films

Correlations between structural and magnetic properties for ultra-thin iron films are discussed. Some selected methods of structural and magnetic analysis are reviewed. The onset of magnetism for submonolayer Fe(110) film on W(110) is explained based on STM (scanning tunneling microscopy) imaging. Hyperfine magnetic fields in iron films sandwiched between Cr show unusual temperature behavior due to the influence of magnetic ordering in Cr. Magnetic properties of metastable iron phases on Cu and Ru are discussed, based on CEMS measurements.     

CHx(x=2~4)在Fe(110)表面吸附的DFT研究

采用密度泛函理论与周期平板模型相结合的方法,对物种CHx(x=2~4)在Fe(110)表面的top,hcp,SB和LB位的吸附模型进行了结构优化、能量计算,得到了各物种较有利的吸附位;并对最佳吸附位进行密立根电荷和总态密度分析。结果表明：CH4在Fe(110)表面的最稳定吸附位都是SB位,吸附能别是-38.14 kJ•mol-1,CH3在Fe(110)表面的最稳定吸附位都是top位,吸附能别是-171.78 kJ•mol-1,而CH2在Fe(110)表面的最稳定吸附位hcp的吸附能是-342.43 kJ•mol-1;CH3 和CH2两物种与金属表面成键,属于化学吸附。     

CEMS analysis of Fe thin films

The application of the Conversion Electron Mössbauer Spectroscopy (CEMS) to the structural and magnetic analysis of ultrathin films and their interfaces is reported. Fe(110) films were deposited on W(110) under UHV conditions and analyzed in situ using CEMS. The changes ofB _hf from layer to layer across the film are discussed with respect to the modifications of magnetic properties caused both by the finite film thickness and by the specific electronic structure of the interfaces.     

CHx(x=2~4)在Fe(110)表面吸附的DFT研究

采用密度泛函理论与周期平板模型相结合的方法,对物种CHx(x=2~4)在Fe(110)表面的top,hcp,SB和LB位的吸附模型进行了结构优化、能量计算,得到了各物种较有利的吸附位;并对最佳吸附位进行密立根电荷和总态密度分析。结果表明：CH4在Fe(110)表面的最稳定吸附位都是SB位,吸附能别是-38.14 kJ•mol-1,CH3在Fe(110)表面的最稳定吸附位都是top位,吸附能别是-171.78 kJ•mol-1,而CH2在Fe(110)表面的最稳定吸附位hcp的吸附能是-342.43 kJ•mol-1;CH3 和CH2两物种与金属表面成键,属于化学吸附。     

Spin-polarized electron scattering at single oxygen adsorbates on a magnetic surface

Scanning tunneling spectroscopy (STS) on the system of isolated oxygen atoms adsorbed on the double layer of Fe on W(110) shows highly anisotropic spatial oscillations in the local density of states in the vicinity of the adsorbates. We explain this in terms of a single-particle model as electron waves being scattered by the potential induced by the presence of the oxygen atoms. Analysis of the wavelength of the standing electron waves and comparison with ab initio spin-resolved electronic structure calculations reveal that minority-spin bands of d-like symmetry are involved in the scattering process. By applying spin-polarized STS, we observe this standing wave pattern on one particular type of magnetic domain of Fe on W(110) only, thereby proving that the standing electron waves are highly spin polarized.     

Quantum-well states and resonances in thin single-crystal layers of noble metals on W(110) substrates

This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.     

Effects of bimetallic modification on the decomposition of CH3OH and H2O on Pt/W(110) bimetallic surfaces

The decomposition pathways of methanol and water on Pt-modified W(110) bimetallic surfaces have been investigated using density functional theory (DFT), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The reaction of methanol on submonolayer and monolayer Pt-modified W(110) surfaces is compared to that on clean Pt(111) and W(110). Similar to clean W(110), the Pt/W(110) bimetallic surfaces remain active toward the dissociation of methanol, although the reaction pathway leading to the production of CH₄ is reduced on the bimetallic surfaces. The Pt/W(110) surfaces are also active toward the decomposition of water. These results are compared with previous studies of the reactions of H₂ and ethylene on Pt/W(110) bimetallic surfaces to reveal the different Pt-modification effects for the dissociation of oxygen-containing molecules.     

过渡元素掺杂对Mo力学性能的第一性原理研究

基于密度泛函理论,采用第一性原理方法计算了在Mo中掺杂摩尔百分比分别为2.08%和4.17%的过渡金属元素W,Ti,Cu和Fe后,体系在[111](110)滑移系统上的广义层错能以及解理能,并研究了掺杂元素对Mo的剪切形变以及脆性一韧性的影响,研究发现,掺杂W和Ti原子会使体系剪切形变的发生变得困难,并使Mo材料变脆;而掺杂Cu和Fe原子则会使体系剪切形变的发生变得相对容易,并使Mo材料的韧性增强,此外,随着掺杂浓度的增加,掺杂W会使体系剪切形变的发生变得更为困难,并使Mo材料脆性更强;而掺杂Fe则会使体系剪切形变的发生变得更为容易,并使Mo材料的韧性更强。     

Magnetism of ultrathin Fe(110) films

Application of conversion electron Mössbauer spectroscopy (CEMS) to structural and magnetic analysis of ultrathin films and their interfaces is reviewed. Fe(110) films were prepared on W(110) under UHV conditions and analyzed in situ. CEMS provides detailed information on the mode of growth and film structure and on magnetic hyperfine fields, B _hf. Local structure of B _hf across the film is discussed in relation to modifications of magnetic order caused by the finite (including monolayer) film thickness and by the electronic structure of the interface.     

硫在Fe(111)面吸附的密度泛函研究

采用基于密度泛函理论的第一性原理方法,系统研究了不同覆盖度下硫在Fe（111）表面的吸附构型和吸附特性,计算并分析了硫在Fe（111）表面的吸附能、电荷密度、分波态密度、电荷布局、电子局域化函数等数据. 研究结果表明：S在Fe（111）面的H位吸附最稳定,并且吸附能随着覆盖度的增加而增加. 另外,电子态密度、电子局域化函数和布局分析表明Fe、S之间呈较弱的共价键,这种作用力主要是Fe的3d轨道和S的2p轨道杂化所贡献,而随着覆盖度的增加,Fe、S之间的作用力逐渐减弱,这可能是由于S原子之间的排斥力减弱了Fe、S之间的作用. S在Fe（111）、Fe（110）和Fe（100）这三个晶面上吸附情况的对比分析发现,S与Fe（111）表面的相互作用最强,Fe（100）面次之,而Fe（110）面最弱.