Measurement Of Spin Polarization Research Articles

Experimental study of longitudinal and transverse spin polarization in LEED from platinum (111)

In LEED from the (111) surface of platinum the dependence of the spin polarization on the polar angle ϑ and the azimuthal angle φ in the energy range fromE=60 eV toE =70 eV was investigated. Our results further confirm earlier findings concerning the symmetry properties of the polarization vectorP. An appropriate rotation diagram clearly demonstrates that by means of momentum averaging the strong influence of multiple scattering on spin polarization cannot be eliminated. By systematic analysis ofP(E, ϑ, φ) data it is shown that there are strong polarization features which are coupled to a fixed direction of the crystal. Spin up/spin down partial intensities evaluated from simulataneously measured spin polarization and total intensity data show quite different behaviour. Therefore, if spin dependent scattering is strong, the total intensity cannot be calculated correctly within a theory, which handles the multiple scattering spin-independent.

Magnetic-field-free vapor furnace for photoelectron spectroscopy. I. Temperatures up to 900 K

A vapor furnace producing stable atomic and molecular beams for photoionization experiments is described. During operation the furnace causes no detectable magnetic stray field because it is heated by a stream of hot air. Thus angle- and energy-resolved measurements of photoelectron spin polarization could be done at a kinetic energy as low as 0.5 eV. The high temporal stability of the target beam is demonstrated by means of an angular distribution measurement with mercury atoms.

Spin polarization in low energy electron diffraction from Ni(001) and Ni(001) c(2×2)Te

Combined measurements of both electron spin polarization and intensity are described for several LEED beams from Ni(001) and Ni(001)c(2 × 2)Te surfaces. Polarizations and intensities were determined in the energy ranges 20–110 eV and 25–200 eV, respectively. Measurements on the specular 00 beam were undertaken at several angles of incidence in the range 10.2–18° for azimuthal angles φ = 0° and φ = 45°. The 01 beam, and the 1 2 , 1 2 beam from the Ni-Te surface, were studied at normal incidence. Sizable polarization features, ¦ P¦ ≲ 25%, were observed, even for Ni(001), indicating that polarized LEED measurements may be used to study surfaces of low atomic number. Pronounced changes in the polarization features were evident following deposition of a c(2 × 2) tellurium adlayer. The experimental data are in general agreement with the results of initial calculations undertaken using a relativistic multiple-scattering theory.

Surface barrier determination using spin-polarized LEED: W(001)

Spin-polarized LEED intensities have been calculated for the W(001) surface for a variety of surface barriers and compared with both spin-polarized measurements (15° ≤ angle of incidence Θ≤45°) and high resolution unpolarized data ( Θ=48°). Observed features, in particular the striking differences between spin-up and spin-down intensities, are reproduced satisfactorily by a barrier with an image plane 3.3 bohr from the outermost atomic layer and with a value of 70% of the bulk inner potential at that layer.

Spin polarization measurements of secondary electrons as a tool to study scattering mechanisms in a solid

Energy, angle, and spin resolved measurements of secondary electrons from Au(110) excited with an unpolarized beam of primaries indicate that the secondary electrons are mostly produced in a two-step scattering event. A calculation reproducing the experimental results is presented.

Electron spectrometer for spin-polarized angle- and energy-resolved photoemission from ferromagnets

An electron spectrometer allowing energy- and angle-resolved spin-polarized photoemission from ferromagnets is described. The system is designed for use with samples magnetized perpendicular to the photoelectron extraction direction (transverse magnetization geometry) and a Mott detector (100 keV) for the spin-polarization measurements. It transforms the emitting spot at the magnetic photocathode to a parallel beam which matches the size of the apertures of the Mott detector, resulting in a high efficiency. Angle selection is achieved despite initial acceleration of the photoelectrons. Energy resolution is obtained with a 90° spherical deflector connected to the Mott detector whose apertures act as the spectrometer exit slits. The performance of the setup is demonstrated by measurements with an evaporated iron film and a neon resonance lamp as light source. Sample geometries for use with the transverse magnetization configuration are discussed.

Angular distributions of electrons elastically scattered from argon and krypton at energies between 10 and 50 eV

This article describes an experiment on elastic scattering of electrons by argon and krypton in the energy range of 10–50 eV. Angular distributions over a range of 40–110° were obtained as a byproduct of spin polarization measurements. There is good agreement with previous experiments in the range of 50-25 eV, except for differences in the depth of the first diffraction minimum. However, at 20 and 10 eV large discrepancies exist between various experiments. Theoretical results are presented, obtained with a code by Walker including both exchange and polarization terms in the scattering potential. This calculation gives a reliable prediction in the energy range of 50-25 eV. Below 25 eV clear discrepancies occur, whereas the theory of Weiss appears to give better results. The calculation of McCarthy et al. shows significant discrepancies with experiment over the whole energy range.

Spin polarization in directional photoemission from non-magnetic crystals by unpolarized light

A spin-dependent three-step photoemission formalism for non-magnetic crystals is outlined, in which unpolarized Bloch electrons are diffracted at the surface. Assuming no spin dependence of the excitation process in the bulk, appreciable spin polarization features are obtained for W(001) at off-normal emission, in fairly good agreement with the first angle- and energy-resolved measurements of photo-electron spin polarization (using unpolarized light).

Erratum: Implications of nucleon-nucleon spin-polarization measurements

Erratum: Implications of nucleon-nucleon spin-polarization measurements

Spin-polarized low-energy electron diffraction: Theory, experiment and analysis of results from W(001)(1 × 1)

The theory of spin-polarized low-energy electron diffraction (SPLEED) is outlined with an emphasis on symmetry-induced properties and models for the effective single-sphere potential. Details are given of the experimental set-up involving a new type of spin polarization detector based on SPLEED. Numerical calculations for the W(001)(1× 1) surface proceeded in close coordination with measurements of spin polarization (normal to the scattering plane) and intensity. A variety of calculated spin polarization and intensity profiles exhibits features, which are very sensitive to changes in the top interlayer spacing, as well as features sensitive to various ingredients of the potential model. The overall agreement between theory and experiment is very good not only for intensities, but also for spin polarization. It implies a contraction of the topmost interlayer spacing of W(001) with respect to the bulk spacing by 7% ± 1.5%, and supports the use of an energy-dependent exchange part and a nonuniform absorption part in the effective ion-core potential.

Symmetry of the Au (110) surface reconstruction studied by spin-polarized low-energy electron diffraction

Spin-polarized low-energy electron diffraction measurements of the analyzing power of two specular beams and the (0, 1/2 ) beam are presented. A theoretial argument relates the analyzing power S and polarizing power P of these beams for an unreconstructed Au (110) surface. This is used to deduce the symmetry of the reconstructed surface. In particualr, these results are inconsistent with the missing- and pairing-row models, suggesting rotation within the reconstructed unit cell.

Optical pumping of cesium atoms with second resonance light

We report the first successful optical pumping of cesium atoms to a state of high spin polarization with a CW dye laser tuned to the D1 second resonance line (4593 Å) of the cesium atom. Results of spin polarization versus laser power measurements for cells containing high-density cesium vapors (1013–1015 cm-3) plus mixtures of nitrogen (10–200 torr) and helium (≈1 atmosphere) gases are presented. These experiments have revealed unexpected limits to the spin polarization of dense alkali vapors.

Crossover from Negative to Positive Spin Polarization in the Photoyield from Ni(111) near Threshold

The measured spin polarization of the photoelectrons emitted from Ni(111) shows a crossover from negative to positive values at 270\ifmmode\pm\else\textpm\fi{}90 meV above photothreshold. This is in constrast to the case of Ni(100), where a crossover at about 75 meV occurs. A single-particle photoemission calculation with an exchange splitting of 0.33 eV, and which does not invoke emission from surface states, is in good agreement with the experimental results for both faces. These new results corroborate the empirical band structure obtained by Eastman and co-workers from angle-resolved photoemission data.

Simple band theory interpretation of the spin polarization of tunnel currents in nickel and some nickel alloys

Measurements of spin polarization of tunnel currents from nickel films are shown to be compatible with a simple band theory interpretation.

Electron field emission from ferromagnetic europium sulfide on tungsten

The emission process from W-EuS junction field emitters was studied by combination of several experimental methods: measurement of emission current, spin polarization, energy distribution, and energy-selective spin polarization as function of emitter temperature for different annealing conditions. The results show that with critical annealing a stoichiometric crystalline EuS layer can be obtained; the interface of such a layer on tungsten acts as a spin filter below the EuS Curie temperature. With emitters annealed at higher temperatures, evidence for electron trapping and the formation of spin clusters has been obtained.

Precision regulator for annealing of polarised-electron field emitters and multiple multichannel analyser based on a microprocessor board

A precision regulator for annealing of polarised-electron field emitters without thermal overshoot is described. It is implemented by means of a microprocessor board which is used in the same experiment as a multichannel scaler for measurement and differentiation of retarding-field electron energy distributions. The same microprocessor board can be equipped with several counter inputs for setting up a multiple-input multichannel scaler with which energy-selective electron spin polarisation measurements can be performed.

ProposedE1−M1Interference Experiment in Hydrogenic Ions to Measure Spin Polarization

It is shown that in the Stark quenching of a spin-polarized beam of metastable hydrogenic ions, a measurable right-left asymmetry can be produced in the emitted radiation which is proportional to the degree of spin polarization. The effect, which has previously been observed in absorption in neutral cesium and thallium, is due to interference between spontaneous magnetic dipole and induced electric dipole decay modes. Asymmetry measurements offer a simple means of determining the degree of spin polarization in the heavier hydrogenic ions.

Implications of nucleon-nucleon spin-polarization measurements

We discuss available data for spin observables in $\mathrm{pp}$ and $\mathrm{np}$ elastic scattering above 3 GeV/c. We identify those measurements which isolate exchange-amplitude components of specific interest---those which provide firm tests of Regge-exchange models and, on the other hand, those which models are unable to predict reliably. Using a simple Regge-pole-exchange model whose trajectory parameters and coupling properties are determined from data on other reactions, we compute expectations for all available $\mathrm{pp}$ spin observables and find good agreement with the data. We find that a hadronic axial-vector ${A}_{1}$-like exchange term is present in the data with strength compatible with theoretical estimates. The low-lying isoscalar exchange observed in $\mathrm{pp}$ and $\mathrm{np}$ polarization is consistent with estimates based on $\ensuremath{\epsilon}({0}^{++})$ exchange. We make a careful study of possible nonasymptotic Regge contributions which could obscure the interpretation of amplitude structure at momenta up to 6 GeV/c. We provide predictions for spin-correlation observables in $\mathrm{np}$ elastic and in $\mathrm{np}$ charge-exchange scattering.

New vacuum-ultraviolet absorption data for lead vapour obtained by spin-polarisation measurements

The photoelectrons emitted by lead atoms exposed to circularly-polarised VUV radiation are partially spin polarised. Unlike the photoionisation cross section, the measured polarisation does not show autoionisation resonances as a function of wavelength. The polarisation and the known cross section results together yield new absorption data, which can be compared with the oscillator strengths of the Rydberg series in the discrete spectrum. Full information about the first three autoionisation resonances is also given. Some selected parts of the apparatus used for the spin-polarisation measurements are discussed in detail.

Electron spin polarization in field emission

The field emission microscope can be extended to determine the directionally weighted spin-dependent surface density of states of magnetic metals or to obtain information on the electronic structure of metal to magnetic-semiconductor interfaces. We describe the techniques of spin polarization measurements in field emission. The measurement of Kisker et al. on ferromagnetic EuS evaporated on W are discussed along with recent investigations on Ni.