Stabilization of high spin FCC Fe in (Fe/Pd)n multilayers

Polycrystalline (Fe/Pd)_n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t_Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d_Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d_Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd)_n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ_B, which is about 20% larger compared to μ=2.2 μ_B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.     

Chemically driven diffusion of Pd in the surface region of Zn(0001): An ultraviolet photoemission investigation

The diffusion of deposited Pd through the (0001) surface region of zinc has been studied with photoemission at hν = 21.2 eV by following the time evolution of the Zn 3d and Pd 4d peaks for a Pd initial coverage of 1, 3, 10 and 15 monolayers. The time decay of the Pd 4d signal is explained with a model where the diffusion coefficient D is not constant; it is (4.6 ± 0.5) × 10^?19 cm² s^?1 for t?7000s, then decreases to (5.5 ± 1) × 10^?20 cm² s^?1 for t ? 15,000 s. The D values correlate well with the spectroscopic results on the valence state evolution during diffusion. At short times (higher D) the spectra show an electron energy gain of Pd atoms during diffusion while at higher time (lower D) this gain is negligible. The initial diffusion is chemically driven while at longer times the diffusion becomes gradually entropic.     

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).     

Final state band gap effects in UV photoemission from Cu(111)

The crystal momentum dependence of the final states involved in bulk photoemission from the sp-bands through a (111) surface of copper was investigated with a photon energy hν = 11.7 eV. Plane wave hybridization was observed and is described in terms of a 2-OPW model Hamiltonian with parameters determined to be V₀=7.1±0.1 eV for the inner potential and E_g = 1.6 ± 0.2 eV for the energy gap. The model is also shown to account for a rapid intensity variation with crystal momentum of the form |a_G(k)|², where a_G(k) is a plane wave amplitude of the final state wave function.     

Spin waves in Fe3−xMnxSi ordered alloys

Spin wave dispersion relations in Fe_3?xMn_xSi ordered alloys have been measured by neutron scattering for alloys with x = 0.04 and x = 0.18. The dispersion relations have been found to be quadratic in the magnon wavevector q? up to about half of the Brillouin zone along [111] direction. The measured values of the spin wave stiffness constant are 169±7 meV Å⁰² and 207±7 meV Å⁰² for samples with x = 0.18 and x = 0.04, respectively. The energy gap found from the rough data turned out to be almost totally due to the triple-axis spectrometer resolution properties. The nearest neighbour exchange integral for FeFe interaction, calculated from the above data, equals to 22.5±1.0 meV.     

Determination of the cation site-occupation parameter in a cobalt ferrite from Synchrotron-Radiation diffraction data

Energy-tuned synchrotron radiation (SR) was used to collect Bragg diffraction intensity data from a single crystal of a cobalt ferrite (nominally CoFe₂O₄). X-Ray energies were chosen just below the Fe and Co K absorption edges where large changes in anomalous dispersion enhance differences in scattering between the two metals. Observations were analyzed by least-squares methods to determine the value of the parameter, X, that describes the distribution of cobalt and iron ions on the octahedral and tetrahedral sites of the cubic spinel structure. Refinements based on over 290 SR data gave X = 0.83 ± 0.01 (83% of the cobalt ions located on octahedral sites); with only 30 specially selected SR data, this value could be fixed at 0.80 ± 0.07. Estimates of dispersion corrections to the X-ray scattering factors for iron near the Fe K edge were obtained through observations of SR diffraction from a magnetite crystal. Results do not differ appreciably from predicted values in the range 6358–7105 eV, through the precision in estimates of the imaginary term is poor at the lower energies.     

γ-Detected NMR of103 mRhFe

Using the method ofγ-detection, the NMR in the metastable 40 keV-state of¹⁰³Rh in Fe (thin foils with diffused¹⁰³Pd activity) was measured in external fields of 0.5 to 14 kG. We find a zero-field resonance frequency ofv ₀=(550.3±0.5) MHz and a slope ofdv/dH= ?(0.933 ±0.017) MHz/kG, yielding g=1.22 ± 0.02. The resulting value for the hyperfine field, H_hf=(590±10) kG, is inconsistent with that of an NMR measurement in the ground state of¹⁰³Rh. Possible reasons for this discrepancy are discussed.     

Plasmon dispersion in Beryllium

We have measured the plasmon dispersion in polycrystalline Beryllium at wave vectors up tok=1.29 Å^?1 with electron energy loss spectroscopy. The plasmon energy is 18.8±0.1 eV, the fitted quadratic dispersion coefficient is α=0.36±0.03 for 0<k<1.29 Å^?1 and α _L =0.52+0.04 fork<0.55 Å^?1. We compare our results with the plasmon dispersion in other simple metals and point out discrepancies with the theories of the homogeneous electron gas.     

The surface ionization of gadolinium,terbium and dysprosium on the tungsten (110) surface

A study of the surface ionization of gadolinium, terbium, and dysprosium (Gd, Tb, and Dy) is described and the results compared with the predictions of the Saha-Langmuir equation. A tentative measurement of holmium is also included. The experimental index parameter, E, which should be equal to the difference between the first ionization potential of the element and the apparent work function of the surface, was found to exceed the predicted value by the following amounts: Gd, 0.66 ± 0.14 eV; Tb, 0.27 ± 0.13 eV; and Dy, 0.24 ± 0.12 eV. The absolute ionization efficiencies were estimated by comparison of our gadolinium results with those reported by Dresser and Hudson. The preexponential coefficients are found to exceed the theoretically predicted values by an order of magnitude. Surface ionization efficiency of these rare earths is larger than predicted by the Saha-Langmuir equation by factors ranging from 5 to 25 at 2500 K. The (110) face of a tungsten single crystal was used as the ionizing surface, and its work function was determined to be 5.46 ± 0.11 eV in an auxiliary study of the emission of tungsten ions.     

Exafs study of the sructure of boron in palladium

The local lattice structure around B atoms in Pd was determined from the EXAFS at the K-edge of' Pd in to $\text{Pd}\text{B}$ alloys with B/Pd = 16 and 11.4 %. It was confirmed that B occupies octahedral interstices in the fcc lattice of Pd. The lattice expansion of the Pd matrix by the dissolved B is da/a = (0.169±0.003) for B/Pd = 1. Boron displaces its 6 nearest Pd neighbors outwards by (0.106 ± 0.005)Å relative to pure Pd and by (0.061 ± 0.005)Å relative to the average $\text{Pd}\text{B}$ lattice. The local lattice distortion has practically vanished beyond the second Pd neighbors.     

Investigation of linear momentum transfer on the 35 MeV/u Ar+U system

All significant experimental results on the ³ H-³ He mass difference are reviewed. The weighted average of mass doublet measurements is 18596.7±3.6 eV, of tritium β measurements in magnetic spectrometers is 18610±7 eV and in implanted Si(Li) detectors is 18578±6 eV. The data within each group are consistent but there are discrepancies among the groups; possible explanations are proposed. Our best estimate for the ³ H-³ He atomic mass difference is 18599.4 ± 3.0 eV.     

First direct observation of strong interaction spin-orbit effects in antiprotonic atoms

The strong ¯p-nucleus spin-orbit interaction was investigated in a measurement of the strong-interaction effects of the 9→8 transition in ¯p ¹⁷⁴Yb at the Low-Energy Antiproton Ring (LEAR) at CERN. This measurement was part of an experimental programme where, for the first time, the fine-structure components of the last observable X-ray transition in a ¯p atom, which carries information on the strong ¯p-nucleus interaction, were resolved and studied individually. The observed splitting ΔE _exp=2408±26 eV consists of the electromagnetic fine-structure splitting ΔE _FS=2350 eV and an additional splitting Δ?=58±26 eV. In addition, one finds a significant difference in the level widths of Δ=195±59 eV with the larger value_?=1216±41 eV for the lower fine-structure level. This experiment follows an earlier measurement on ¯p ¹³⁸Ba, where the transition 8→7 is influenced by the strong interaction. In this case, however, the fine-structure components could not be resolved. The results for¹⁷⁴Yb may be attributed to a spin-orbit (LS) term in the complex strong-interaction potential.     

Resonance effects in photoemission from Si- and P-covered Fe(100) surfaces at the 3p- and 3s-thresholds

The photon energy dependences of the valence-band photoemission spectra of Fe(100)c(2×2)-Si and Fe(100)c(2×2)-P surfaces have been measured in the vicinity of the Fe 3p photothreshold to investigate the existence of valence-band satellites. For Fe(100)-P, the spectra near the 3s threshold were also investigated. The results demonstrate that the main d-band emission exhibits an interference variation characteristic of a Fano resonance near both thresholds. For Fe(100)-Si and Fe(100)-P, resonant satellites are observed at 6.0 eV and 6.3 eV, respectively, below the Fermi level at the 3p threshold. We also find a very weak satellite at 7.3 eV for Fe(100)-P near the 3s threshold. The possibility that this may be identified as a shake-up excitation with two 3d holes plus one 4p electron is considered.     

Anomalous enhancement of DD reaction in Pd and Au/Pd/PdO heterostructure targets under low-energy deuteron bombardment

Yields of protons emitted in the D+D reaction in Pd, Au/Pd/PdO, Ti, and Au foils are measured by a d E-E counter telescope for bombarding energies between 2.5 and 10 keV. The experimental yields are compared with those predicted from a parametrization of the cross section and stopping power at higher energies. It is found that for Ti and Au target the enhancement of the D(d,p)T reaction is similar to that observed with a deuterium gas target (several tens of eV). The dependence of the yields on the bombarding energy corresponds well to the screening potential parameters U _s=250±15 eV for Pd and 601 ±23 eV for Au/Pd/PdO. Possible models of the enhancement obtained are discussed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 785–790 (10 December 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.     

Diffusion of gallium into cadmium sulphide

Measurements of the Ga diffusion into CdS, in the presence of exces Ga metal, using optical and mixroprobe analyser techniques are reported. A reaction layer of CdGa₂S₄ forms on the CdS crystals below 1240±20 K. Above this temperature the reaction layer is liquid. The Ga diffusion is concentration dependent and also orientation dependent with the faster diffusion perpendicular to the hexagonal c-axis of CdS. The anisotropy of the activation energy was calculated to be 0.20±0.06 eV. In the temperature range 940–1240K the linearly concentration dependent diffusion yielded activation energy values for defect motion of 2.39±0.13 eV perpendicular to the c-direction and 2.21 ±0.13 eV parallel to the c-direction.     

Measurement of the lattice parameter of sodium by neutron-back-scattering

We report high precision results (δa/a ≈ 3 x 10^-6) of the lattice parameter of high purity sodium (99.99%) in the temperature region from 32°C up to the melting point. Data of the crystal quality are also given: the half width of the angular mosaic spread amounts to about 4'. Preliminary results on vacancy formation energy and entropy, using literature data of volume expansion, are 0.35 ± 0.02 eV and (3.7 ± 0.5) k_B resp. for a single vacancy.     

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.     

Measurement of partial radiation widths of high energy transitions from keV capture resonances in56Fe and58,60Ni

High energyγ-ray transitions to low-lying states following neutron capture in⁵⁶Fe and^58,60Ni have been investigated for individual resonances in the neutron energy range 7–70keV using a 50cc Ge(Li)-detector. Partial radiation widths fors- and 1>0 wave resonances were determined. For the 1>0 wave resonance in⁵⁸Ni at 47.8 keV a spin of 3/2 was derived. Theγ-ray strength forM1 transitions was found to be (21±17)×10^?9 and (16±7)×10^?9MeV^?3 for⁵⁶Fe and⁵⁸Ni, respectively. The partial radiation widths of the studieds-wave resonances were compared with valence model calculations.     

Dispersion relation for the 3.8 eV volume plasmon of silver

The dispersion curve of the 3.8 eV volume plasmon of Ag has been obtained by electron energy loss measurements for wave vectors q in the range $\text{0.1 ?q? 0.35}\text{A}\text{?}{}^{-1}$. Results from the low-q end of this range match well with the optical results of Lindau and Nilsson. The curve can be described by a dispersion constant a = 0.8 ± 0.1 and the intensity of the loss goes like I≈q^-2.9±0.4. A simple model for the longitudinal dielectric function yields results in good agreement with the experimental conclust