1,721,050 research outputs found
Magnetic dichroisms in absorption and photoemission for magnetic characterization in x-ray photoelectron emission microscopy
No full textMagnetic contrast for the operation of a photoelectron emission microscope (PEEM) with synchrotron radiation is provided by magnetic dichroisms. Besides the most frequently employed magnetic dichroism, magnetic circular dichroism in x-ray absorption spectroscopy, energy filtering of photoemitted electrons allows one to also use magnetic dichroisms in photoelectron emission as complementary contrast mechanisms. We demonstrate that it is possible to obtain magnetic contrast in photoemission using PEEM equipped with a simple retarding field electron energy analyzer. Magnetic domain images of an ultrathin film of 10 atomic monolayers of Fe on W(001), obtained by three, different contrast mechanisms (circular magnetic dichroism in x-ray absorption, circular magnetic dichroism in Fe valence band photoemission, and linear magnetic dichroism in Fe 3p photoemission) are presented and compared
Space-charge effects in high-energy photoemission
No full textPump-and-probe photoelectron spectroscopy (PES) with femtosecond pulsed sources opens new perspectives in the investigation of the ultrafast dynamics of physical and chemical processes at the surfaces and interfaces of solids. Nevertheless, for very intense photon pulses a large number of photoelectrons are simultaneously emitted and their mutual Coulomb repulsion is sufficiently strong to significantly modify their trajectory and kinetic energy. This phenomenon, referred as space-charge effect, determines a broadening and shift in energy for the typical PES structures and a dramatic loss of energy resolution. In this article we examine the effects of space charge in PES with a particular focus on time-resolved hard X-ray (∼10 keV) experiments. The trajectory of the electrons photoemitted from pure Cu in a hard X-ray PES experiment has been reproduced through N-body simulations and the broadening of the photoemission core-level peaks has been monitored as a function of various parameters (photons per pulse, linear dimension of the photon spot, photon energy). The energy broadening results directly proportional to the number N of electrons emitted per pulse (mainly represented by secondary electrons) and inversely proportional to the linear dimension a of the photon spot on the sample surface, in agreement with the literature data about ultraviolet and soft X-ray experiments. The evolution in time of the energy broadening during the flight of the photoelectrons is also studied. Despite its detrimental consequences on the energy spectra, we found that space charge has negligible effects on the momentum distribution of photoelectrons and a momentum broadening is not expected to affect angle-resolved experiments. Strategy to reduce the energy broadening and the feasibility of hard X-ray PES experiments at the new free-electron laser facilities are discussed
Influence of exchange bias coupling on the single-crystalline FeMn ultrathin film
No full textPolarization dependent x-ray photoemission electron microscopy was used to investigate the influence of the exchange bias coupling on the disordered ultrathin single-crystalline fcc Fe(50)Mn(50). We find that the critical thickness of the FeMn film, where the antiferromagnetic (AF) order is formed, varies with changing the magnetization direction of the ferromagnetic (FM) layer from out-of-plane to in-plane. Surface magneto-optical Kerr effect measurements (SMOKE) further manifest the shift of the critical thickness with alternating the exchange bias coupling. It indicates that the spin structure of the FeMn layer near the FM layer is modified by the presence of exchange bias coupling and the properties of the coupling. Our results provide direct experimental evidence that the AF spin structure at the interface between the FM and AF layers is strongly influenced by the exchange bias coupling
Microspectroscopic two-dimensional Fermi surface mapping using a photoelectron emission microscope
No full textWe demonstrate the use of a photoelectron emission microscope in connection with a retarding field electron energy analyzer for the fast acquisition of two-dimensional momentum resolved photoelectron angular distribution patterns. This opens the possibility to combine spatial, momentum, and energy resolution of photoelectrons within the same instrument. We have applied this to observe the Cu(001) Fermi surface from a selected region of the sample. A well defined bulk Fermi surface is quickly mapped in this way
Local exchange bias observed by photoemission microscopy
No full textBy using a photoemission electron microscope in combination with X-ray magnetic circular dichroism, the domain configuration in a single crystalline Co film exchange coupled to an Fe50Mn50 film is obtained. The effect of the ferromagnetic/antiferromagnetic interaction is observed to be different in Co domains with different magnetization direction, establishing the fact that the exchange bias' is locally set by the magnetization of the ferromagnetic layer, without external field
A teaser made simple: a didactic measurement of the spectral answer of a human eye calibrated luxmeter
No full textA simple didactic experiment has been designed and realized, in order to
illustrate to undergraduate students in scientific faculties some basic concepts
lying behind the fundamentals of geometrical optics. The spectral response of a
human-eye-calibrated lux meterwas measured using a very trivial experimental
arrangement. The white light of a halogen lamp was decomposed into its
spectral components through a diffraction grating, so that collecting the
radiation at different dispersion angles allowed one to measure the intensity as a
function of wavelength. The experiment can be used to effectively illustrate the
concepts of spectral distribution, the radiometry versus photometry conversion
and photopic response, and the famous historical experience by Herschel on
the ‘temperature of colours’
Structural and magnetic properties of FeχMn1-χ thin films on Cu(001) and on Co/Cu(001)
No full textThe structural and magnetic properties of FexMn1-x alloy thin films deposited on Cu(001) and on Co/Cu(001) are investigated. A layer-by-layer growth mode of the alloy films in a large range of concentration is deduced from the presence of regular oscillations in medium-energy electron-diffraction intensity. Low-energy electron-diffraction experiments reveal that Fe50Mn50 films keep the fcc structure when growing on Cu(001) and on Co/Cu(001). Magneto-optic Kerr effect is used to study the magnetic properties of Co/FeMn bilayers. A temperature-dependent increase in the coercive field of the Co film, when the thickness of the adjacent FeMn film exceeds a certain value, is related to the antiferromagnetic state of the FexMn1-x alloy
Space-charge effect in electron time-of-flight analyzer for high-energy photoemission spectroscopy
No full textThe space-charge effect, due to the instantaneous emission of many electrons after the absorption of a single photons pulse, causes distortion in the, photoelectron energy spectrum. Two calculation methods have been applied to simulate the expansion during a free flight of clouds of mono- and bi-energetic electrons generated by a high energy pulse of light and their results have been compared. The accuracy of a widely used tool, such as SIMION (R), in predicting, the energy distortion caused by the space-charge has been tested and the reliability of its results is verified. Finally we used SIMION (R) to take into account the space-charge effects in the simulation of simple photoemission experiments with a time-of-flight analyzer. (C) 2016 Elsevier B.V. All rights reserved
Hard X-ray PhotoEmission Spectroscopy of strongly correlated systems
Get PDFHard X-ray PhotoEmission Spectroscopy (HAXPES) is a new tool for the study of bulk electronic properties of solids using synchrotron radiation. We review recent achievements of HAXPES, with particular reference to the VOLPE project, showing that high energy resolution and bulk sensitivity can be obtained at kinetic energies of 6-8 keV. We present also the results of recent studies on strongly correlated materials, such as vanadium sesquioxide and bilayered manganites, revealing the presence of different screening properties in the bulk with respect to the surface. We discuss the relevant experimental features of the metal-insulator transition in these materials
Evidence for the collapse of short-range magnetic order in CoO at the Neel temperature RID G-2497-2011
No full text""Auger-photoelectron coincidence spectroscopy is used for investigating the electronic properties of a CoO thin film above and below the magnetic transition temperature (T(N)). By using the dichroic effect in angle-resolved measurements, we identify and assign well-defined high-spin and low-spin structures in spite of the otherwise featureless Auger singles spectra, typically found for open-band systems. The disappearance of the dichroism for temperatures just above T(N) indicates a collapse of the surface short-range magnetic order, presumably due to a strongly reduced exchange field in the surface compared to that in the bulk. Copyright (C) EPLA, 2011"
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