The determination of electron mean free paths in solids by depth and angular dependences of inelastically scattered electron spectra

A technique is presented of electron mean free path determination in solids using the spectra of backscattered electrons. The mean free path without energy loss (λ₀) in Ge, and the mean free paths for excitation of volume (λ_v) and surface (λ_s) plasmons in Al, are determined in the electron energy range E₀ = 0.1–10 kev. The depth d_s of surface plasmon localization is estimated for Al.     

Investigation of intrinsic plasmon energy losses associated with the Fe 1s core level in metallic iron

A recently developed Cu Kα₁ (hν = 8047.8 eV) X-ray source/ESCA300 electron spectrometer combination has been used to investigate the intrinsic plasmon energy losses associated with the Fe 1s core level (binding energy = 7111 eV) in metallic iron. The surface and bulk intrinsic plasmon energy losses were separated and it was found that using the theoretically calculated extrinsic energy loss cross-section to represent the bulk intrinsic energy loss cross-section gave an overall intrinsic loss probability which is approximately the same as if a Lorentzian type cross-section is used. However, this approach does not separate the surface and bulk intrinsic losses properly and is not a good approximation for peak shape analysis in the near peak region. A more realistic approximation is provided by using a Lorentzian type energy loss cross-section to represent the bulk intrinsic energy losses. It has also been shown that for the Fe 1s core level of metallic iron the probability that a photoelectron will suffer an intrinsic energy loss is higher at the surface than in the bulk. Also for this core level the excitation probability for the intrinsic plasmons is greater than that of the extrinsic plasmons. Hence ignoring the intrinsic plasmons would cause considerable error in peak shape analysis in the near peak region.     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

The peculiarities of the low-energy ion scattering by polycrystal targets

In the present work, experimental and computer simulation studies of low-energy (E₀ = 80-500 eV) Cs⁺ ions scattering on Ta, W, Re target surfaces and K⁺ ions scattering on Ti, V, Cr target surfaces have been performed for more accurate definition of mechanism of scattering, with a purpose of evaluation of an opportunity of use of slow ions scattering as a tool of surface layers analysis. The choice of the targets was based on the fact that the ratios of atomic masses of target atoms and ions μ = m₂/m₁ were almost the same for all cases considered and greater than 1 (direct mass ratio) however, the difference of binding energies of target atoms in the cases of Cs⁺ and K⁺ scattering was almost twice as much. It has been noticed that the dependencies of the relative energy retained by scattering ions at the maximum of energy distribution versus the initial energy E_m/E₀ (E₀) have a similar shape in all cases. The relative energy retained by scattering ions increases while the initial energy of incidence ions decreases. The curves are placed above each other relative to the binding energies of target atoms, to show what this says about the influence of binding energy on a process of scattering of low-energy ions. The correlation between value of energy change maintained by an ion for different values of E₀ in the case of scattering by targets with different masses of atoms and its binding energies is experimentally established. The contrary behavior of the E_m/E₀ (E₀) dependencies concerning the target atom binding energy quantity E_b for cases with direct (μ > 1) and inverse (μ < 1) mass ratio of colliding particles is established. The comparison of experimental energy distributions with calculated histograms shows that the binary collision approximation cannot elucidate the abnormally great shift in the maxima of relative energy distributions towards greater energy retained by scattering ions.     

Calculation of the surface energy of bcc transition metals by using the second nearest-neighbor modified embedded atom method

The surface energies for 24 surfaces of all bcc transition metals Fe, Cr, Mo, W, V, Nb and Ta have been calculated by using the second nearest-neighbor modified embedded atom method. The results show that, for all bcc transition metals, the order among three low-index surface energies E_(1 1 0) < E_(1 0 0) < E_(1 1 1) is in agreement with experimental results and E_(1 1 0) is also the lowest surface energy for various surfaces. So that from surface energy minimization, the (1 1 0) texture should be favorable in the bcc films. This is also consistent with experimental results. The surface energy for the other surfaces increases linearly with increasing angle between the surfaces (h k l) and (1 1 0). Therefore, a deviation of a surface orientation from (1 1 0) can be used to estimate the relative values of the surface energy.     

Variation of relative intensities between surface and bulk plasmon losses due to crystal orientations for aluminium in low energy electron reflection loss spectroscopy

The contrast change of secondary electron images due to the crystal orientations is observed by the ultra high vacuum scanning electron microscope (UHV-SEM) for crystal grains of clean surface of polycrystalline Al in the primary energy E_p of 200 eV to 5 keV. The low energy electron loss spectra are measured by the cylindrical mirror analyzer. The relative intensity ratio between surface and bulk plasmon loss spectra was dependent on the crystal orientations. The SEM images taken by the surface and bulk plasmon signals at E_p = 230 eV show the inverse contrast depending on the grains. The inversion of the relative intensities between the surface and bulk plasmon losses is explained qualitatively by taking into account of variation of the penetration depth of the incident beam caused by the electron channeling.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Scattering effects of primary electrons in Co/Cu(1 1 1) measured by Auger; elastic and loss electrons

Electron energy losses were measured as a function of the incidence angle of the primary electron beam for the Co/Cu(1 1 1) adsorption system. The measurements performed for the clean and covered substrate reveal characteristic intensity maxima associated with the close packed rows of atoms, as it was observed in the so called directional Auger and directional elastic peak electron spectroscopy profiles. The incidence angle dependent signal of electron energy losses measured for the clean (Cu 3p_3/2) and covered (Co 3p_3/2) substrate gives the so called directional electron energy loss spectroscopy (DEELS) profiles which contain structural as well as chemical information. The scattering of primaries and different emission processes associated with electron energy losses, Auger, and elastically backscattered electrons are discussed. A change in the h_Cu (Cu M_2,3VV transition) Auger signal recorded during the continuous cobalt deposition shows that the growth mode is not a pure layer by layer type. The complete covering of the substrate by Co at higher coverages is confirmed by the comparison between experimental and theoretical ratios of the Auger peak heights.     

Adsorption of titanium,chromium, and copper atoms on thin aluminum and magnesium oxide film surfaces

Methods of Auger electron spectroscopy (AES), spectroscopy of characteristic electron energy losses (SCEEL), slow electron diffraction (SED), and contact potential difference (CPD) in ultrahigh vacuum are used to investigate the adsorption-emission properties and stability of two-component film systems formed by putting of Ti, Cr, and Cu atoms on MgO–Mo(011) and Al₂O₃–Mo(011) surfaces. All atoms have the properties of electronegative adsorbates. Continuous adatom monolayers are formed on the Al₂O₃–Mo(011) system surface, and three-dimensional islands are formed on the MgO–Mo(011) surface. The properties of monoatomic films on the oxide layer surface are close to those observed for bulk materials. No radical changes of the system properties are detected with increasing dielectric layer thickness. The thermal stability of the newly formed structures decreases in the order Ti, Cr, Cu, Al₂O₃(MgO), and Mo(011).     

Photoemission loss spectra of metals with emergence parallel to the surface

A theory is presented for the energy loss spectra of photoelectrons emitted from atoms located outside a metal sample at a distance z₀ from the surface. These electrons travelling along trajectories parallel to the surface, loss energy mostly by surface plasmon excitation, the losses by bulk plasmon or pair excitations being negligible. The intensity of losses by surface plasmon excitations is found to be proportional to the logarithm of the length l of the trip of the photoelectrons along the surface, a length which extends from the emitting atom to the surface edge. But this intensity does not depend on the distance z₀ as long as z₀/l small. Considering only the high energy photoemission where the initial energy ?₀ of the electrons is much larger than the surface plasmon energy ω_s, we find that these l-dependent losses only occur when the angle of emergence is smaller than $\text{(ω}{}_{\mathrm{s}}\text{/?}{}_0\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and peak strongly when this angle is smaller than ω_s/2?₀. An experiment with appropriate parameters is proposed to verify the principal results of our theory. Further investigation will yield interesting information on the surface plasmon structure, e.g. the dispersion rule.     

Site occupancy behaviours of ternary elements (Zr,Mo, Cr) in the Laves phase of C15 NbCo2: a first-principles study

Abstract

First-principles method based upon the density functional theory has been carried out to investigate the site occupancy behaviours of typical ternary elements Zr, Cr and Mo in the C15 NbCo₂ Laves phase. The calculated equilibrium lattice constant of the Nb₈Co₁₆ unit cell is in good agreement with the previous experimental results. The calculated results indicate that all the considered ternary elements show the preference to occupy the Co sites, and the site preference of these elements in the Laves phase is in the order of Cr, Mo and Zr. Mo and Cr atoms tend to participate in the formation of the Laves phase, generating Nb₇MoCo₁₆, Nb₈Co₁₅Mo and Nb₈Co₁₅Cr, respectively. Zr atoms are not prone to concentrate in the Laves phase. The partial density of states was also calculated to investigate how atoms interact, and the charge density difference is analysed to reflect the bonding characteristics.     

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

The internal nitriding response of a ternary Fe–1 at.%Cr–1 at.%Mo alloy, which serves as a model alloy for many CrMo-based steels, was investigated. The nitrides developing upon nitriding were characterised by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and atom probe tomography. The developed nitrides were shown to be (metastable) ternary mixed nitrides, which exhibit complex morphological, compositional and structural transformations as a function of nitriding time. Analogous to nitrided binary Fe–Cr and Fe–Mo alloys, in ternary Fe–Cr–Mo alloys initially continuous precipitation of fine, coherent, cubic, NaCl-type nitride platelets, here with the composition (Cr_½,Mo_½)N_¾, occurs, with the broad faces of the platelets parallel to the {1?0?0}_α-Fe lattice planes. These nitrides undergo a discontinuous precipitation reaction upon prolonged nitriding leading to the development of lamellae of a novel, hexagonal CrMoN₂ nitride along {1?1?0}_α-Fe lattice planes, and of spherical cubic, NaCl-type (Cr,Mo)N _x nitride particles within the ferrite lamellae. The observed structural and compositional changes of the ternary nitrides have been attributed to the thermodynamic and kinetic constraints for the internal precipitation of (misfitting) nitrides in the ferrite matrix.     

Angular and energy dependences of the surface excitation parameter for electrons crossing a solid surface

When fast electrons cross a solid surface, surface plasmons may be generated. Surface plasmon excitations induced by electrons moving in the vacuum are generally characterized by the surface excitation parameter. This parameter was calculated for 200-1000 eV electrons crossing the surfaces of Au, Cu, Ag, Fe, Si, Ni, Pd, MgO and SiO₂ with various crossing angles. Such calculations were performed based on the dielectric response theory for both incident (from vacuum to solid) and escaping (from solid to vacuum) electrons. Calculated results showed that the surface excitation parameter increased with crossing angle but decreased with electron energy. This was due to the longer time for electron-surface interaction by glancing incident or escaping electrons and by slow moving electrons. The results were fitted very well to a simple formula, i.e. , where P_s is the surface excitation parameter, E is the electron energy, α is the angle between the electron trajectory and the surface normal, and a, b and c are material dependent constants.     

Temperature-dependent plasmon excitations in the surface layer of ordered Cu-22.5 at % Mn alloy

Temperature and energy dependences of the characteristic electron energy losses on plasmon excitations are studied in the surface layer of ordered polycrystalline Cu-22.5 at % Mn alloy. Features in the near-surface distribution of Mn are found from an analysis of plasmon excitations and the data of Auger spectroscopy. The observed temperature dependences of the electron energy loss spectra have features in the range of 650–750 K that include the temperature of atomic disordering (T _k = 675 K) in the bulk of the alloy.     

Energy loss spectroscopy of sulphide films on Mo (100)

Energy loss spectroscopy has revealed features on clean Mo(100) due to bulk and surface plasmons, surface state emission, core-conduction band transitions, and multiple losses. On random adsorption of H₂S at room temperature new peaks appear which are associated with chemisorption bonds. Spectra from epitaxially grown MoS₂ on Mo(100) are in broad agreement with those from bulk MoS₂. The results complement other studies of sulphide film growth using low energy electron diffraction and Auger spectroscopy.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

Direct experimental study of the equilibrium diffusion of carbon atoms between the (100)Mo surface and the bulk

A direct experimental study of the diffusion of carbon atoms between the (100)Mo surface and the bulk has been carried out at process temperatures in the range 1400–2000 K, and the total balance of carbon atoms in the system has been determined. The difference in the activation energies of carbon dissolution and precipitation ΔE=E _S 1?E _{1 S} has been found under conditions of a dynamic equilibrium between both processes. This difference determines the temperature dependence of the degree of surface enrichment with carbon in reference to the bulk. The activation energy of the dissolution of carbon atoms has been determined in special experiments (E _S 1=3.9 eV), and the activation energy of the precipitation of carbon atoms E _{1 S} has been calculated (E _{1 S} =1.9 eV), which turns out to be close to the energy of carbon bulk diffusion in molybdenum.     

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers

Magnetic polarization of Mo atoms in Co₉₆Mo₄ alloy film and Co/Mo multilayered structures has been studied by X‐ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of ?0.21μ_B in the alloy. In the multilayer with the thinner Mo spacer (d_Mo = 0.9 nm) the magnetic moment is much smaller (?0.03μ_B). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.     

Magnetic properties of Zr(Cr1−xCox)2 alloys and their hydrides

Magnetic properties of Zr(Cr_1?xCo_x)₂ alloys, with 0 ? x ? 0.75, and their hydrides were determined over the temperature range 4 to 275 K. All systems were paramagnetic. The susceptibility (?) is significantly enhanced both by increase of the Co content and by hydrogenation in all cases except cubic ZrCr₂, for which ? is virtually unchanged by hydrogenation. The rise in ? is ascribed to electron transfer from Co to Cr and to H. This transfer increases the local density of states for Co at the Fermi energy. The temperature dependence of the susceptibility can be represented by an equation of the form ? = ?₀ + C/(T ? θ), indicating Pauli and Curie-Weiss contributions to ?. The contributions are attributed to the ZrCr₂ matrix and the solute Co atoms, respectively. The Co atoms on Zr sites are weakly coupled antiferromagnetically. Pressure-composition isotherms for the ZrCrCo-H₂ system are given at 80, 120 and 165°C. At 80°C this alloy hydrogenates to the composition ZrCoCrH_3.25 at a pressure of 50 atm.