Aimo Winkelmann

High efficiency electron spin polarization analyzer based on exchange scattering at Fe∕W(001)

We report on a compact electron spin analyzer based on exchange scattering from a magnetic surface. The heart of the detector is an Fe(001) thin film grown on W(001) with chemisorbed oxygen in the p(1 x 1) structure. The device is mounted at the exit of an energy dispersive analyzer and works at a scattering energy of about 13.5 eV. Its figure of merit is 2 x 10(-3), combined with an excellent stability of more than 2 weeks in UHV.

Spin resolved photoelectron microscopy using a two-dimensional spin-polarizing electron mirror

We report on an imaging spin-filter for electrons. The specular reflection of low-energy electrons at the surface of a tungsten single crystal is used to project a spin-filtered two-dimensional image onto a position sensitive detector. Spin-filtering is based on the spin-dependent reflectivity of electrons due to spin-orbit coupling in the scattering target, while a two-dimensional field of view, encoded in the angle of incidence, is conserved in the outgoing beam. We characterize the efficiency of the spin-filter by recording photoelectron emission microscopy images of the magnetic domain structure of 8 monolayers cobalt grown on copper (100).

Dynamical effects of anisotropic inelastic scattering in electron backscatter diffraction.

We present a model which describes the appearance of excess and deficiency features in electron backscatter diffraction (EBSD) patterns and we show how to include this effect in many-beam dynamical simulations of EBSD. The excess and deficiency features appear naturally if we take into account the anisotropy of the internal source of inelastically scattered electrons which are subsequently scattered elastically to produce the EBSD pattern. The results of simulations applying this model show very good agreement with experimental patterns. The amount of the excess-deficiency asymmetry of the Kikuchi bands depends on their relative orientation with respect to the incident beam direction. In addition, higher order Laue zone rings are also influenced by the same effect.

Principles of depth-resolved Kikuchi pattern simulation for electron backscatter diffraction.

This paper presents a tutorial discussion of the principles underlying the depth‐dependent Kikuchi pattern formation of backscattered electrons in the scanning electron microscope. To illustrate the connections between various electron diffraction methods, the formation of Kikuchi bands in electron backscatter diffraction in the scanning electron microscope and in transmission electron microscopy are compared with the help of simulations employing the dynamical theory of electron diffraction. The close relationship between backscattered electron diffraction and convergent beam electron diffraction is illuminated by showing how both effects can be calculated within the same theoretical framework. The influence of the depth‐dependence of diffuse electron scattering on the formation of the experimentally observed electron backscatter diffraction contrast and intensity is visualized by calculations of depth‐resolved Kikuchi patterns. Comparison of an experimental electron backscatter diffraction pattern with simulations assuming several different depth distributions shows that the depth‐distribution of backscattered electrons needs to be taken into account in quantitative descriptions. This should make it possible to obtain more quantitative depth‐dependent information from experimental electron backscatter diffraction patterns via correlation with dynamical diffraction simulations and Monte Carlo models of electron scattering.

Tuning the perpendicular magnetic anisotropy in tetragonally distorted FexCo1−x alloy films on Rh (001) by varying the alloy composition

Tetragonally distorted FexCo1−x alloy films are grown on Rh (001) showing a strong perpendicular magnetic anisotropy in a wide thickness and composition range. This large perpendicular magnetic anisotropy is chemical composition dependent and reaches a maximum at x=0.4. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature with film thickness up to 15 ML. Our experiments show that the proper adjustment of the Fermi level (EF) by the variation of the FexCo1−x alloy composition and the corresponding tetragonal distortion results in a large perpendicular magnetic anisotropy.

High-energy photoelectron diffraction: model calculations and future possibilities

We discuss the theoretical modeling of x-ray photoelectron diffraction (XPD) with hard x-ray excitation at up to 20?keV, using the dynamical theory of electron diffraction to illustrate the characteristic aspects of the diffraction patterns resulting from such localized emission sources in a multilayer crystal. We show via dynamical calculations for diamond, Si and Fe that the dynamical theory predicts well the available current data for lower energies around 1?keV, and that the patterns for energies above about 1?keV are dominated by Kikuchi bands, which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations with results from a cluster model that is more often used to describe lower energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.

Dynamical Simulation of Electron Backscatter Diffraction Patterns

To extract the maximum amount of information from experimental electron backscatter diffraction (EBSD) patterns, it is necessary to realistically model the physical processes that lead to the formation of the characteristic diffraction features in the form of Kikuchi bands and lines. Whereas the purely geometrical relations in the observed networks of bands and lines can be explained by mapping out Bragg’s law for the relevant reflecting lattice planes, the dynamical theory of electron diffraction is needed to explain the observed intensities. This theory takes into account the fact that electrons interact strongly with matter, which leads to multiple elastic and inelastic scattering of the electron waves in a crystal. To simulate a realistic EBSD pattern, we will need to model the very general situation of an incident electron beam which hits a sample and which subsequently undergoes elastic and inelastic interactions to result in the intensity pattern on the observation screen. The incident primary beam contains electron waves within a relatively narrow range of energies and directions (defined by the properties of the electron gun), whereas the backscattered electrons have a broad spectrum of energies (due to inelastic scattering) and are distributed over all possible directions (due to momentum changes by inelastic as well as elastic scattering). Because the exact solution of the combined elastic and inelastic

Analysis of Kikuchi band contrast reversal in electron backscatter diffraction patterns of silicon

We analyze the contrast reversal of Kikuchi bands that can be seen in electron backscatter diffraction (EBSD) patterns under specific experimental conditions. The observed effect can be reproduced using dynamical electron diffraction calculations. Two crucial contributions are identified to be at work: First, the incident beam creates a depth distribution of incoherently backscattered electrons which depends on the incidence angle of the beam. Second, the localized inelastic scattering in the outgoing path leads to pronounced anomalous absorption effects for electrons at grazing emission angles, as these electrons have to go through the largest amount of material. We use simple model depth distributions to account for the incident beam effect, and we assume an exit angle dependent effective crystal thickness in the dynamical electron diffraction calculations. Very good agreement is obtained with experimental observations for silicon at 20keV primary beam energy.

Point-group sensitive orientation mapping of non-centrosymmetric crystals

We demonstrate polarity-sensitive orientation mapping of non-centrosymmetric phases by Electron Backscatter Diffraction (EBSD). The method overcomes the restrictions of kinematic orientation determination by EBSD, which is limited to the centro-symmetric Laue-groups according to Friedels rule. Using polycrystalline GaP as an example, we apply a quantitative pattern matching approach based on simulations using the dynamical theory of electron diffraction. This procedure results in a distinct assignment of the local orientation according to the non-centrosymmetric point group of the crystal structure under investigation.

Quantitative spin polarization analysis in photoelectron emission microscopy with an imaging spin filter

Using a photoelectron emission microscope (PEEM), we demonstrate spin-resolved electron spectroscopic imaging of ultrathin magnetic Co films grown on Cu(100). The spin-filter, based on the spin-dependent reflection of low energy electrons from a W(100) crystal, is attached to an aberration corrected electrostatic energy analyzer coupled to an electrostatic PEEM column. We present a method for the quantitative measurement of the electron spin polarization at 4 × 10³ points of the PEEM image, simultaneously. This approach uses the subsequent acquisition of two images with different scattering energies of the electrons at the W(100) target to directly derive the spin polarization without the need of magnetization reversal of the sample.