Structural transition in Fe ultrathin epitaxial films grown on Ni(111)

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni(111), has been performed in the 1.5-18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc(110) phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann (NW) in-plane orientation of the bcc(110) cell ((bcc)parallel to(fcc)) is favored over the Kurdjumov-Sachs ((bcc)parallel to(fcc)) orientation. The best-fit vertical interlayer distance between bcc(110) planes is d(NW)=2.11 Angstrom (+3.9% expansion) at 6 ML and relaxes to d(NW)=2.05 Angstrom (+1.0%) at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc(110) surface basis vectors changes from 67.7degrees to 69.0degrees, corresponding to -1.7% and -1.0% contractions of the surface cell area, respectively

Structural analysis of Fe/Ni(001) films by photoelectron diffraction

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0-14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fee to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the bcc[111]parallel to fcc[110] in-plane orientation. Interlayer spacings of 2.05 +/- 0.068 Angstrom, 2.01 +/- 0.03 Angstrom, and 1.85 +/- 0.03 Angstrom were found in the bcc region, between bcc and fee layers and in the fee region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was 2.49 +/- 0.02 Angstrom, in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of 2.04 +/- 0.04 Angstrom and in plane n-n distance of 2.47 +/- 0.01 Angstrom. At 14 ML the PD pattern collected over a 94 degrees azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the bcc[111]parallel to fcc[110] alignment

Young's double-slit interference experiment with electrons

In this short Note we report a method for producing samples containing two nano-sized slits suitable for demonstrating to undergraduate and graduate students the double-slit electron interference experiment in a conventional transmission electron microscope. (C) 2007 American Association of Physics Teachers

Element-specific, surface and subsurface structural analysis by scattering-interference of primary electrons

We investigated the effects of scattering-interference of primary electrons on the secondary electron emission intensity fron ordered surfaces and interfaces. Because of the focusing-defocusing of the primary wave along atomic chains, maxima in the electron yield occur when the exciting beam is aligned with low index axes. Therefore the electron intensity distributions as a function of the angle of incidence of the primary beam can be interpreted as projected images of real space, local atomic arrangement. The process can be modelled in a single scattering cluster (SSC) approximation. Potential of primary-beam diffraction modulated electron emission (PDMEE) technique for surface and subsurface structural characterization is shown, for Co epitaxy on Fe(001)

Fabrication by electron beam induced deposition and transmission electron microscopic characterization of sub-10-nm freestanding Pt nanowires

We present a method to reduce the size and improve the crystal quality of freestanding nanowires grown by electron beam induced deposition from a platinum metal organic precursor in a dual beam system. By freestanding horizontal growth and subsequent electron irradiation in a transmission electron microscope, sub-10-nm polycrystalline platinum nanowires have been obtained. A combined transmission electron microscopy-electron energy loss analysis has shown that the amorphous carbon, mixed to nanocrystalline platinum in the as-deposited material, is removed from the wires during irradiation. The same treatment progressively transforms nanocrystals dispersed in the amorphous matrix in a continuous polycrystalline platinum wire

Four slits interference and diffraction experiments

By means of a focused ion beam apparatus slits about 90 nm in width, 1500 nm in length and with a pitch of 430 nm were prepared on a gold film of 100 nm in thickness. The diffraction and interference experiments carried out in a transmission electron microscope equipped with a field emission gun are an amazing and impressive demonstration of the wave behaviour of the electrons

Modulated electron emission: The effect of elastic and inelastic electrons on core-level ionization

We investigated the elastic and inelastic contributions to the focusing and defocusing properties of low-index Fe atomic chains on keV electron beams. To this end, we measured the intensity of the 2p-ionization loss signal from a thin Co film buried in a Fe matrix as a function of the incidence angle of the electron beam for different values of the beam energy. The angular anisotropy of the measured signal, A(z), directly measures the focusing effect at the depth z where the Co marker is located. The maximum (102%) in A(z) occurs at z = 5 scatterers; at about 12 scatterers below the surface the anisotropy is still 86%. The intensity anisotropy of the 2p-ionization loss we measured is compared to that of the Co LMM Auger signal and to single-scattering cluster calculations, It has been found that a finite spatial extension of the ionization region has to be taken into account to reproduce the experimental results. The relative weight of elastic and inelastic contributions to the 2p core-level ionization cross section has been evaluated as a function of depth and electron energy

Ab initio calculation of electron energy loss spectra of clean and 1ML Fe-covered Ni(111)

The dielectric and the loss functions of Ni(111) are calculated by the Full Potential Linear Muffin Tin Orbitals (FPLMTO) ab initio method whithin the three-layers model (vacuum/surface/bulk). Particular attention is devoted to determine surface and bulk state contributions to the spectra. Good agreement is found with recent experimental EELS data on the Fe-covered Ni(111) surface

X-ray magneto-optics and surface science (X-MOSS) beamline at ELETTRA

The elements of the design of the X-MOSS (X-ray magneto-optics and surface science) beamline, to be installed at a bending magnet of the ELETTRA synchrotron radiation facility, are presented. The beamline is intended for optical study (reflectivity and absorption) of low-dimensional systems, typically surfaces and interfaces, with emphasis on magnetic systems. The optical range will extend from visible up to similar to 1400 eV with full control of linear and circular polarisation

Growth of Fe ultrathin films on Ni(111): structure and electronic properties

We investigated the evolution during film growth of the electronic properties of Fe/Ni(111) ultrathin films with electron energy loss spectroscopy from the valence band and from the Fe L-2,L-3 ionization edges, relating them with structural modifications monitored with low energy electron diffraction. At ca. 4 monolayers of Fe a fee to bce phase transition was observed with the major changes in the electronic structure occurring in the 3d band region. (C) 2000 Elsevier Science B.V. All rights reserved