Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p_3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.     

Low energy electron attenuation length studies in thin amorphous carbon films

The results of experiments on low energy electrons passing through thin amorphous carbon films are reported. For electron energies between 50 and 3000 eV, electron attenuation lengths were determined by electron transmission measurements through carbon films of thicknesses between 81 and 210 Å. The density of these amorphous carbon films was also obtained by a sink—float method and found to be 1.90 ± 0.05 g cm^?3. The electron attenuation length for inelastic scattering was found to increase monotonically from approximately 10 to 55 Å over the 50 to 3000 eV range. Over the 500–3000 eV energy region, the results are in good agreement with mean free paths calculated using optically measured dielectric functions.     

Crystalline structure of copper-gold alloys investigated by DEPES: Theoretical approach

We present results of calculations concerning the angle integrated electron emission from crystalline samples as a function of the incidence angle of the primary electron beam. Diffraction processes of primary electrons in directional elastic peak electron spectroscopy (DEPES) were described by the single scattering cluster (SSC) approximation applied to clusters composed of sublattices formed by different atoms. Theoretical polar profiles were calculated for Cu(0 0 1), Au(0 0 1), AuCu(0 0 1), and AuCu₃(0 0 1) in terms of phase shifts, inelastic mean free paths and scattering cross-sections. In calculations a real experimental geometry was considered concerning the acceptance angle of the analyser and polar rotation of the sample. Profiles calculated for particular incident electron beam energies depend on the structure and stoichiometry of crystalline samples. Application of above investigations is discussed in the context of superficial alloy formation and order-disorder transitions associated with the change of the concentration and order of alloy components in first few atomic layers.     

Gold nanoparticle induced conformational changes in heme protein

Change of α-helical structure of heme protein (Hb) to a β-sheet and random coil conformation because of the interaction of glycine capped gold nanoparticles (20–60 nm) as observed from attenuation total reflectance, absorption, Fourier transform infra red, and Circular Dichroism spectroscopy has been reported in this article. Upon interaction, protein takes a cylindrical shape of length 12 μm and diameter 0.35 μm as revealed from scanning electron microscopy and transmission electron microscopy. The Selected-Area Electron beam Diffraction pattern shows change of crystalline structure in GNP to amorphous nature with the interaction of Hb.     

Elastic origin of the O/Cu(1 1 0) self-ordering evidenced by GIXD

We have studied by grazing incidence X-ray diffraction the self-ordering of the Cu-CuO stripe pattern. By comparing the experimental results to molecular dynamics simulations and anisotropic linear elastic calculations, we have been able to determine the atomic relaxations within the Cu substrate. The results show the importance of the crystalline anisotropy in the relaxation field. These relaxations are due to the surface stress difference Δσ between oxygen-covered and bare Cu(1 1 0) regions. For the different oxygen coverages studied, we have always found Δσ=1.0±0.1 N m⁻¹. This surface stress difference is shown to be the origin of the self-ordering.     

Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.     

Hot electron spin attenuation lengths of bcc —Room temperature Magnetocurrent of 1200%

We investigate spin-dependent hot electron transport through metallic epitaxial spin valves by ballistic electron magnetic microscopy (BEMM). By variation of the thickness of one of the ferromagnetic layers we determine the spin dependent attenuation lengths which reflect hot electron transport along the vicinity of the [1 0 0]-axis of the bcc Fe₃₄Co₆₆ lattice. The majority spin attenuation length is more than 6 times larger than that of the minority spins within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently a Magnetocurrent effect exceeding 1200% accompanied by a monotonic bias voltage behavior is observed at room temperature.     

Reflection electron energy loss spectroscopy of aluminum

Reflection electron energy loss spectra (REELS) of Al(111) single crystal and of the aluminum polycrystalline (poly Al) film were measured at 200 eV and 1000 eV electron energies for a variety of experimental geometries and were mutually compared. No anisotropy was found for the poly Al, as expected. Polar intensity plots evaluated from the elastic (no loss) and inelastic first surface plasmon- and first bulk plasmon-loss intensities of the Al(111) surface show clearly discernable peaks for both considered electron energies. Their positions on the angular axis are the same for the elastic as well as for the inelastic, surface and bulk plasmon-loss peaks. The polar plots of intensities of the elastically and inelastically reflected electrons were compared to calculated intensities of photoelectrons emitted from the Al 2s core level to the same kinetic energy. Peak positions in the theoretically determined polar plots of electron intensities agree with those obtained experimentally in REELS.     

Unusual magnetization anisotropy in amorphous Nd–Fe–Al ribbons

Nd₆₀Fe₃₀Al₁₀ ribbons has been prepared by chill-block melt-spinning with different wheel speeds from 5 to 30 m/s. Fully amorphous ribbons were obtained at wheel speeds of 25 and 30 m/s. These ribbons exhibited an unusually large anisotropy in magnetization. The effect of the magnetic anisotropy decreased with decreasing wheel speed, and nearly disappeared at the wheel speed of 5 m/s, at which the ribbon consisted of a mixture of a more stable Fe-rich amorphous phase and a crystalline Nd phase with a strong crystallographic texture.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Chemically prepared well-ordered InP(0 0 1) surfaces

In the present work HCl-isopropanol treated and vacuum annealed InP(0 0 1) surfaces were studied by means of low-energy electron diffraction (LEED), soft X-ray photoemission (SXPS), and reflectance anisotropy (RAS) spectroscopies. The treatment removes the natural oxide and leaves on the surface a physisorbed overlayer containing InCl_x and phosphorus. Annealing at 230 °C induces desorption of InCl_x overlayer and reveals a P-rich (2 × 1) surface. Subsequent annealing at higher temperature induces In-rich (2 × 4) surface. The structural properties of chemically prepared InP(0 0 1) surfaces were found to be similar to those obtained by decapping of As/P-capped epitaxial layers.     

Surface magnetic anisotropy in glass-coated amorphous microwires as determined from ferromagnetic resonance measurements

The ferromagnetic resonance frequency of different Co base glass-coated amorphous magnetic microwires about 3.5 μm in diameter with negative, vanishing and positive magnetostriction has been investigated from power absorption measurements in the microwave frequency range. The experimental technique employed here involves the replacement of the dielectric of a coaxial transmission line by the sample to be measured. From the evolution of the resonance frequency with DC applied magnetic field, the surface magnetic anisotropy field of the microwires has been quantitatively obtained and, as expected, found to depend on the sign and strength of the magnetostriction. Similar values for the surface anisotropy are obtained in comparison with bulk anisotropy as determined from quasi-static hysteresis loops measurements.     

Layer-resolved photoelectron diffraction from Si(0 0 1) and GaAs(0 0 1)

Photoelectron diffraction in the layer-resolved mode brings more detailed information about local atomic arrangement than is obtained in the standard mode. This is demonstrated in crystals with diamond and zinc-blende structures, both for unpolarized photon excitation as well as for circularly polarized excitation. The full angular distributions of photoemission intensities are evaluated for large atomic clusters representing ideally truncated surfaces of Si(0 0 1) and GaAs(0 0 1). Highly structured layer-resolved patterns enable a more detailed understanding of the standard mode outcomes. Photoelectron intensities from atomic layers placed at different depths under the crystal surface provide direct evidence about electron attenuation and its anisotropy in crystals.     

Substitutional adsorption geometry for Pb(1 1 1)-(√3 × √3)R30°-K

Intermixed structures for alkalis (larger than Li) on close-packed substrates have previously been observed only on Al(1 1 1). This study shows that K forms an ordered intermixed structure on Pb(1 1 1). The structures of clean Pb(1 1 1) and Pb(1 1 1)-(√3 × √3)R30°-K were studied using dynamical low-energy electron diffraction (LEED). The clean Pb(1 1 1) surface at 47 K was found to be a relaxed version of the bulk structure, in agreement with an earlier study of the same surface [Y.S. Li, F. Jona, P.M. Marcus, Phys. Rev. B 43 (1991) 6337]. At room temperature, adsorption of K on this surface results in a (√3 × √3)R30° structure, which was shown using dynamical LEED to consist of K atoms substituted in surface vacancies. The K-Pb bond length was found to be 3.62 ± 0.3 Å, with no significant change to the Pb interlayer spacings.     

Theoretical prediction of a surface-induced spin-reorientation phase transition in BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanocrystals

The equation of the magnetization of a hexagonal crystal is derived for the first time for an arbitrary orientation of the external magnetic field relative to the crystallographic c axis. In order to clarify the magnetization mechanism for a real ensemble of small particles in the framework of the given problem, surface anisotropy (which is significant for nanosize objects) was taken into account along with crystalline magnetic anisotropy and anisotropy in the particle shape. Model computer experiments prove that the magnetization curves for nanocrystals oriented in a polar angle range of 65–90° exhibit an anomaly in the form of a jump, indicating a first-order spin-reorientation phase transition. This explains a larger steepness of the experimental curve reconstructed taking into account the interaction between particles as compared to the theoretical dependence obtained by Stoner and Wohlfarth [IEEE Trans. Magn. MAG 27 (4), 3469 (1991)]. An analysis of variation of the characteristic anisotropy surface and its cross section with increasing ratio |K₂|/K₁ of the crystalline magnetic anisotropy constants upon a transition from a macroscopic to a nanoscopic crystal shows that surface anisotropy leads to a change in the magnetic structure. As a result, an additional easy magnetization direction emerges in the basal plane apart from the easiest magnetization direction (along the c axis). The direction of hard magnetization emerges from the basal plane, the angle of its orientation relative to the c axis being a function of the ratio | K₂|/K₁.     

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

Reflection and scattering of spin waves by surface roughness of a ferromagnet

Abstract

A detailed theory of volume spin wave reflection from the randomly rough surface of a ferromagnet is presented. The contribution to damping of the reflected wave is calculated. This contribution is due to the scattering of the initial volume wave into secondary surface and volume spin waves. The value of damping is proportional to the correlation length and the square of the roughness amplitude. Numerical calculations of the attenuation rate as a function of the angle of incidence and the ratio between the surface anisotropy and the wavenumber are provided. They yield the angle of incidence where the attenuation has a maximum. In analogy to optics, this angle is similar to the Brewster angle. Numerical estimations of damping and a comparison of its value with the ferromagnetic resonance linewidth are also made. Finally, the results of the calculation of the scattering of surface exchange spin waves by surface roughness are presented and discussed.     

Scattering effects and damping of electrons in Pt(1 1 1) and Cu(1 1 1)

In this work an analysis of experimental and theoretical data associated with the scattering and attenuation of electrons in the Pt(1 1 1) and Cu(1 1 1) crystalline samples is presented. The information about the crystalline structure of the first few atomic layers was obtained by the directional elastic peak electron spectroscopy (DEPES) at the primary electron beam energies E_p from 1.5 keV to 2.0 keV. The comparison of the experimental and theoretical DEPES distributions indicates a qualitative agreement between experiment and theory. The relative signal values associated with the intensity maxima were found to be different. The latter effect suggest that the electron attenuation in the crystalline samples can have an anisotropic character. A qualitative analysis of the characteristic pattern around the [1 1 1] direction concerning the calculation of the scattering factors was performed. The collective scattering of electrons by atoms located around the threefold symmetry axis resulting in the so called ring focusing effect is discussed.     

Characterization of adsorbed arsenate on amorphous and nano crystalline MgFe-layered double hydroxides

This article investigated sorption of toxic and carcinogenic arsenate (AsO₄ ³⁻) ions on positively charged surface of amorphous and nano crystalline MgFe-layered double hydroxides (LDHs). Based on Brunauer–Emmett–Teller (BET) and Transmission Electron Microscopy (TEM), average size, and specific surface area of nano crystalline MgFe-LDHs, which was about range of about 50–200 nm and 90.2 m²/g, was lower and higher when compared to them of amorphous MgFe-LDHs, respectively. In addition, X-ray diffraction (XRD) peak and point of zero charge (PZC) of crystalline MgFe-LDHs was higher intensity and same, respectively, when compared to that of nano crystalline FeMg-LDHs. Adsorption rate of arsenate on amorphous MgFe-LDHs was a little faster when compared to that of nano crystalline MgFe-LDHs. In addition, as pH decreased, adsorption amount of arsenate on amorphous MgFe-LDHs increased significantly when compared to that of nano crystalline MgFe-LDHs. These results indicate that mechanism of arsenate in two materials was significantly different. We investigate sorption characteristic at pH 5, based on XRD and Fourier-transformed infrared (FTIR). In amorphous MgFe-LDHs, ferric arsenate precipitate was formed on surface of amorphous MgFe-LDHs and constituted the predominant surface arsenate. However, in nano crystalline MgFe-LDHs, arsenate was dominantly sorbed as a “non-surface-complexed” As–O bond on surface and anion exchange in interlayer.