Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Determination of the atomic arrangement of the unreconstructed Ir (110) surface by low-energy electron diffraction a

An Ir(110)-(1 × 1) surface structure has been prepared by adsorbing $\text{1}\text{4}$ monolayer of oxygen at 850 K on a clean, reconstructed (1 × 2) surface. Results of the low-energy electron diffraction structure analysis reveal that the oxygen is probably distributed randomly over the crystal surface, and the (1 × 1) structure is the same as a clean unreconstructed (1 × 1) structure, with a topmost interlayer Ir spacing of 1.26 ± 0.05 Å. This is equivalent to a contraction of approximately 7.5% of the bulk interlayer spacing of 1.36 Å.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

LEED crystallographic investigations of two surface structures designated Zr(0001)-(1 × 1)-C

Multiple scattering analyses of LEED intensities have been made for two Zr(0001)-(1 × 1)-C surfaces corresponding to a low and a high coverage of C. For the first, a moderate level of correspondence between experimental and calculated I(E) curves is reached when the C atoms occupy octahedral holes between the first and second metal layers to give a ZrC bond distance of 2.29 Å. In this case the absorbed C causes only a 3.5% increase in the neighbouring ZrZr interlayer spacing. However, the normal incidence study for the high-coverage surface suggests that this spacing is then increased by about 25% over the bulk value; such an expansion is consistent with C incorporation into tetrahedral hole underlayer sites.     

Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being $\text{1.09 ± 0.05}\text{A}\text{?}$ and corresponding to an effective radius of 1.06 Å for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.     

The termination of the titanium (100) surface

We find that a large quantity of normal incidence LEED data from the “non-unique” Ti(001) surface indicates that it is probably uniquely terminated in one of the two possible bulk interlayer spacings. The favored structure has a small first interlayer spacing of about 0.80 Å, similar to that of one bulk distance (0.85 Å) and quite distinct from the second possibility (1.70 Å).     

Multilayer relaxations of Ni(110): New medium energy ion scattering results

Recent ion scattering and LEED measurements of the clean Ni(110) surface structure show a significant difference in the magnitude of the first layer relaxation. Ion scattering has measured the first layer contraction to be 4.5% while LEED I–V analyses have resulted in measurement of an 8.5% contraction. In an attempt to resolve this apparent discrepancy we have undertaken a careful reexamination of the clean Ni(110) surface with medium energy ion scattering using the technique of channeling and blocking. The experiment was conducted in a UHV chamber with a toroidal electrostatic analyzer using 110 keV protons. Our analysis was based on comparisons of measured surface blocking curves in two scattering geometries with full crystal Monte Carlo simulations. These simulations include variations of the interlayer spacing of the outer three surface layers and of the surface Debye temperature. Our measurement shows a first layer contraction, D₁₂ = −9.0% ± 1.0% and a second layer expansion, D₂₃ = +3.5% ± 1.5% (measured as percent of the bulk interlayer spacing) with a surface Debye temperature of 395 K. The present result is in excellent agreement with two recent LEED measurements.     

Limitations in specular beam leed analysis due to measurement of the scattering angle: Examples from a study of Cu(110)

The I–V profiles for specular LEED beams from a clean Cu(110) surface have been measured for three selected angles of incidence of the primary electron beam with respect to the crystal orientation. A comparison of experimental and calculated profiles indicates that the first interlayer spacing is contracted by 8 ± 3%, a value in reasonable agreement with the result obtained by a previous study of non-specular beams. However, this study demonstrates some inherent limitations of LEED analyses due to difficulties in the precise measurement of the alignment of the primary electron beam.     

Multilayer relaxation of the Mg(0001) surface

We have Investigated the interplanar relaxation of the clean (0001)-(1 × 1) surface of magnesium at 100 K using a dynamical LEED I-V analysis. In contrast to almost all other metal surfaces, an expansion has been observed for the first interlayer spacing of this clean surface. Using an extended database, the results indicate that the first three interlayer spacings are relaxed from the bulk value by Δd₁₂ = +1.9 ± 0.3%, Δd₂₃ = +0.8 ± 0.4%, and Δd₃₄ = −0.4 ± 0.5%. A comparison of this observed multi relaxation with experimental and theoretical results for similar free-electron closepacked metal surfaces, e.g. Al(111), suggests that a surface expansion is a normal property of high electron density simple metals.     

An analysis of the structure of the iridium (111) surface by low-energy electron diffraction

Elastic low-energy electron diffraction (LEED) intensity versus voltage (I-V) measurements for the clean Ir(111) surface have been obtained. Seven specular I-V spectra were measured from 15 to 975 eV at incident angles from 7° to 62.5° relative to the surface normal. The outermost atomic layer spacing of the unreconstructed Ir(111) surface was determined both by the convolution-transform method we have presented previously (including certain convenient modifications) and by dynamical calculations. Results from the analysis of the I-V spectra by the convolution-transform method indicate that the outermost Ir(111) layer spacing is either unrelaxed or contracts by 4% of its bulk value depending upon whether the θ=7° data or the θ = 25° data are used. In agreement with this, the dynamical calculations show that the outermost Ir(111) layer spacing contracts by 2.5± 5% and, in addition, that the registry of the first layer of the crystal surface is not shifted, maintaining the fcc structure.     

Orientational epitaxy of an incommensurate neon monolayer adsorbed on graphite

The structure of a monlayer of neon adsorbed on the (0001) surface of a graphite single crystal has been studied at T? 10 K by using high resolution Low Energy Electron Diffraction. The incommensurate monlayer is rotated by ?17° with respect to the (√3 × √3) 30° commensurate structure for a mean overlayer interatomic spacing d? 3.24 Å. Thus result agrees with the prediction of Novaco—McTague theory.     

Leed,auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I(E) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R|ΔE| factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 Å, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 Å. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.     

Surface structure determination of W(001)p(1 × 1)-2H: Chemisorption induced substrate relaxation

Surface structural parameters for the full coverage W(001)p(1 × 1)-2H system have been determined using new LEED measurements and model calculations for bridge-bonded hydrogen. The technique is shown to be clearly sensitive to hydrogen position at the surface with the H-W layer spacing determined to be $\text{1.17 ± 0.04}\text{A}\text{?}$ resulting in a H-W bond length of 1.97 Å. The distance between the top two tungsten layers has been determined to be less than 2% contracted relative to bulk termination which is less contraction than measured the for clean W(001) surface by other studies.     

Structural models of the reconstructed W{001} surface

A LEED intensity analysis of 5 beams from the low-temperature W{001}c(2×2) structure indicates that the surface reconstruction involves shifts of the surface atoms along 〈110〉 directions within the plane of the surface, as suggested by Debe and King. At temperatures 100–140K the shifts are in the range 0.15–0.3 Å, with the first interlayer spacing 1.48–1.58 Å (bulk value 1.58 Å). Similar analysis of the room-temperature W{001}c(2×2)-H phase indicates: (i) none of the models proposed, which ascribe the c(2×2) structure directly to ordered hydrogen adsorption, can explain the experimental data; (ii) the W{001}c(2×2)-H structure is probably impurity stabilized by H at room temperature in the same W lattice as the low-temperature reconstructed phase.     

Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films

The spectrum of standing spin waves has been detected by the ferromagnetic resonance method in NiFe(740 Å)/Cu/NiFe(740 Å) three-layered film structure in the perpendicular configuration for the copper thickness d _Cu ≤ 30 Å. At thicknesses d _Cu > 30 Å, the resonance absorption curve is a superposition of two spinwave resonance spectra from individual ferromagnetic NiFe layers. For Co/Pd multilayer films, united spinwave responance spectra have also been observed at thicknesses of the paramagnetic palladium layer up to d _Pd < 30 Å. The partial exchange stiffness has been calculated for a spin wave propagating across the Pd layer (A _Pd = 0.1 × 10^?6 erg/cm). This value is always positive (up to the critical thickness of the palladium interlayer d _Pd < d _c) or equal to zero (d _Pd > d _c).     

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.     

Edge atom depression on stepped Cu(410)

The position of the edge atoms of a stepped Cu(410) surface has been measured by Ion Scattering Spectroscopy using 21 keV H⁺. The edge atoms are depressed 5.0±1.5% of the copper lattice spacing, corresponding to 0.18±0.05 Å.     

Chlorine chemisorption on Cu(0 0 1) by surface X-ray diffraction: Geometry and substrate relaxation

We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 × 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu-Cl interlayer spacing of 1.60 Å and a slightly expanded Cu-Cu first interlayer spacing of 1.85 Å (1.807 Å for bulk Cu) was determined by LEED. The resulting Cu-Cl bond length, 2.41 Å, is very similar to the SEXAFS value of 2.37 Å. Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu-Cl interlayer spacing of 1.60 Å, no first Cu-Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)-c(2 × 2)-Cl system using surface X-ray diffraction technique with synchrotron radiation. We find a Cu-Cl interlayer spacing of 1.584(5) Å and confirm the expansion of the first Cu-Cu interlayer, with an average spacing of 1.840(5) Å. In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu-Cu interlayer expansion.     

Influence of the scattering potential model on low energy electron diffraction from Cu(001)−c(2 × 2)-Pb

A dynamical LEED intensity analysis is reported for Cu(001)−c(2 × 2)-Pb. The adsorbate layer distance from the substrate is determined as 2.29 Å, and the topmost interlayer spacing for the substrate is the same as in bulk Cu, in contrast to a contraction for clean Cu(001). This structural result is, within the accuracy reached, insensitive to changes in the assumed scattering potential models. The r-factors suggest a weak preference for an energy-dependent exchange correlation and a moderate one for adding a localized adsorption part inside the muffin-tin spheres. The sensitivity of spectra and r-factors to changes in the assumed isotropic Debye temperature for Pb suggests that vibrational anisotropy should be taken into account in order to improve the accuracy of the analysis. Calculated spin polarization spectra are very sensitive to the exchange approximation, the localized absorption and the Debye temperature. Together with experimental data, they should be useful in particular for determining the vibrational anisotropy.     

Long range ferromagnetism in tunable cobalt(II) layered compounds up to 25 Å apart

We discuss the structure-magnetic property correlations for a series of layered cobalt(II) compounds, derived from the parent host Co(OH)₂. Intercalation takes place by exchange of OH⁻ by organic or inorganic anions resulting in interlayer spacing between 4.65 and 25.4 Å. For all the compounds, ferromagnetic in-plane interaction dominate the magnetic behavior at high temperature, but long range order occurs at low temperature; 3d antiferromagnetic order is observed for small basal spacing and ferromagnetic for large spacing. The nature of the magnetic interaction is discussed briefly.