Spin-resolved Electron Spectroscopy Research Articles

Probing the magnetic moments of [MnIII6CrIII]3+ single-molecule magnets—A cross comparison of XMCD and spin-resolved electron spectroscopy

Single-molecule magnets (SMM) of the [MnIII6CrIII]3+ structural type prepared on Si and gold-coated glass substrates have been investigated by spin-resolved electron spectroscopy (SPES) and X-ray magnetic circular dichroism (XMCD) at the Mn L3,2 edge and in addition by XMCD at the Cr L3,2 edge using synchrotron radiation. Differences between the two methods are discussed. Despite its severe limitations for 3d transition metals, a spin sum rule evaluation is nevertheless performed for the MnIII centres in the [MnIII6CrIII]3+ SMM to provide a simple means of comparing XMCD and spin-resolved electron spectroscopy results.

Spin resolved photoelectron spectroscopy of[Mn6IIICrIII]3 + single-molecule magnets and of manganese compounds as reference layers

Properties of the manganese-based single-molecule magnet are studied. It contains six MnIII ions arranged in two bowl-shaped trinuclear triplesalen building blockslinked by a hexacyanochromate and exhibits a large spin ground state ofSt = 21/2. The dominant structures in the electron emission spectra of resonantly excited at the L3-edge are the L3M2, 3M2, 3, L3M2, 3V and L3VV Auger emission groups following the decay of the primaryp3/2 core hole state. Significant differences of the Auger spectra from intact and degraded show up. First measurements of the electron spin polarization in theL3M2, 3V andL3VV Auger emission peaks from the manganese constituents in resonantly excited at the L3-edge near 640 eV by circularly polarized synchrotron radiation are reported. Inaddition spin resolved Auger electron spectra of the reference substances MnO,Mn2O3 andMnII(acetate)2·4H2O are given. The applicability of spin resolved electron spectroscopy forcharacterizing magnetic states of constituent atoms compared to magneticcircular dichroism (MCD) is verified: the spin polarization obtained fromMnII(acetate)2·4H2O at room temperature in the paramagnetic state compares tothe MCD asymmetry revealed for a star-shaped molecule with aMn4IIO6 core at 5 K in an external magnetic field of 5 T.

Magnetic properties of Fe3O4 surface

The magnetic properties of the magnetite Fe3O4(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe2+) and trivalent (Fe3+) ions on the Fe3O4(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.

Band symmetries of GaSe(0001) studied by spin-resolved electron spectroscopy using circularly polarized radiation

We have studied the symmetry properties of the bands of the layered semiconductor GaSe along the $\ensuremath{\Delta}$ direction by means of spin-resolved electron spectroscopy using circularly polarized synchrotron radiation and by means of a relativistic LAPW band-structure calculation. The photoelectron spectra show in the main a pair of weakly dispersing peaks caused by direct transitions from valence-band states with symmetries ${\ensuremath{\Delta}}_{9}^{5},$ ${\ensuremath{\Delta}}_{9}^{6}$ and ${\ensuremath{\Delta}}_{7}^{5},$ ${\ensuremath{\Delta}}_{8}^{6}.$ Our data determine the spin-orbit splitting of these valence bands to be $\ensuremath{\Delta}{E}_{\mathrm{so}}=0.3\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}.$ We find a gap in the unoccupied bands along $\ensuremath{\Delta}$ about 8 eV above the fundamental gap. The analysis of the photoelectron spin polarization reveals that also unoccupied states with low symmetries ${\ensuremath{\Delta}}_{7}^{5},$ ${\ensuremath{\Delta}}_{8}^{6}$ and ${\ensuremath{\Delta}}_{9}^{5},$ ${\ensuremath{\Delta}}_{9}^{6}$ are involved in excitation and emission. Spin resolved spectra measured at Auger electrons following the decay of Ga 3d core holes confirm that p derived unoccupied states exists below the vacuum level.

Spin-polarized Fermi surface mapping

The magnetic and electronic properties of itinerant ferromagnets and their interplay have been studied in the last few years by spin resolved electron spectroscopy on one hand and by high-resolution angle-resolved photoemission experiments on the other. We discuss how the two approaches can be combined in a high resolution electron spectrometer with spin resolution for angle-scanned Fermi surface mapping experiments. We have built this new instrument, which allows an advance into a deeper understanding of magnetic thin film or multilayer systems, where band structures become intricately dense in momentum space and where the magnetization direction can change from layer to layer. Spin-resolution is thus required to arrive at a correct assignment of spectral features. A fully three-dimensional polarimeter makes the instrument ‘complete’ in the sense that all properties of the photoelectron are measured. First experiments on Ni(111) conclusively confirm previous band and spin assignments at the Fermi level and demonstrate the correct functioning of the apparatus.

A new analyzer for spin resolved electron spectroscopies

We have developed a new instrument which performs simultaneously energy and spin-polarization analysis of an electron beam, to be used in spin resolved electron spectroscopies from magnetic surfaces and thin films. The device consists of a novel polarization detector with high efficiency and analyzing power coupled to a large commercial hemispherical energy analyzer (HEA). The spin polarimeter is based on the spin dependence of the low energy (4–6 eV kinetic energy) reflectivity from a magnetic target, namely a Fe(0 0 1)- p(1×1)O surface. We describe reliable target preparation procedures and the design of the electron-optical system transferring the electrons from the HEA exit slit to the polarimeter target.

Study of xenon4d,5p,and5sphotoionization in the shape-resonance region using spin-resolved electron spectroscopy

Photoionization of Xe atoms is investigated close to the maximum of the $4d$ shape resonance by means of spin-resolved photoelectron spectroscopy. Using circularly and linearly polarized synchrotron radiation of 93.8-eV energy, we investigate the photoionization processes of the $5p,$ $5s,$ and $4d$ shells. By combination of earlier data for the partial and differential cross sections with our measurements, we are able to perform a complete characterization of the Xe $4d$ photoionization process, i.e., we can determine all the relativistic dipole matrix elements. Comparison with different theoretical calculations shows the importance of relaxation effects at this photon energy.

Magnetic order and electronic structure in thin films

The magnetic order in ultrathin films depends critically on a variety of film parameters. Therefore, to understand the magnetic properties of thin films on the basis of the spin-dependent electronic structure is a challenging task for both experimentalists and theoreticians. Experimentally, spin-resolved electron spectroscopies probe directly either the spin-dependent density of states or specific energy states, often at a defined wave vector. In this paper, two surface-sensitive techniques, that provide complementary information about the unoccupied electronic states, are described. Spin-resolved appearance potential spectroscopy gives element-specific access to the spin-dependent local density of states. Spin-resolved inverse photoemission permits detailed investigations of electron states characteristic of the surface and the layers underneath as a function of the wave vector. Both techniques are sensitive also to the film structure. A variety of magnetic film properties is discussed in the light of the electronic structure. The examples described in this paper include Fe films on W(110) and Cu(001) as well as Gd films on W(110).

Novel compact 60-kV Mott detector for spin-polarization electron spectroscopy

A compact 60-kV Mott polarimeter designed specially for the local analysis of surface and two-dimensional magnetism by spin-resolved electron spectroscopy is developed and tested. The use of a design which combines a spherical accelerating field and the absence of a retarding potential after scattering of the electron beam ensures high stability of the measured polarization even when the potential and diameter of the beam being investigated vary. As a result of optimization of the scattering angle (118°) and the use of surface-barrier detectors with a large collection angle (∼48°), the efficiency or figure of merit of the polarimeter, which determines the signal-to-noise ratio ɛ=(I/I0)·(Seff)2, equals 2.5×10−4. Specially developed electronic circuits and optimum positioning of the detectors provide a maximum electron counting rate as high as 5×106 counts/s. Consequently, it is possible to calibrate the polarimeter (to find the effective Sherman function Seff) by extrapolating the measured asymmetry to a high discrimination level. This instrument can also be used in other areas of solid-state physics, atomic physics, and high-energy physics.

Spin-resolved resonant Auger spectroscopy of condensed iodine on Pt

In molecular iodine condensed onto Pt the N 4,5 VV Auger electron intensity and the valence band photoemission intensity are resonantly enhanced if the photon energy is just the energy necessary for exciting 4d 5/2 and 4d 3/2 core electrons to unoccupied valence states derived from the molecular 5p σ u states. These resonant emission processes are studied by spin resolved electron spectroscopy using circularly polarized radiation for the primary excitation. In the spectra measured on-resonance, spectator and participator emission processes can clearly be distinguished by the energy and by the preferential spin direction of the emitted electrons. In the 4d 3/2 →5p σ u resonance spectrum an emission process following a 4d 3/2 →4d 5/2 Coster–Kronig decay of the primary excited hole-state can be identified by its spin polarization. For the spectator emission processes the two-hole states excited in the decay of the primary holes are predominantly coupled to a singlet.

Spin-resolved electron spectroscopy with highly polarized sources: Inverse photoemission from ferromagnets

We report on the use of recently developed spin-polarized electron sources with very high polarization for electron spectroscopy. In particular we present data of spin-resolved inverse photoemission from Fe(100) films excited by polarized electrons produced by a strained GaAsP negative electron affinity photocathode. This highly polarized source (beam polarization P=65%) allows a direct and almost complete decoupling of the majority and minority empty states in Fe(100), much better than the standard GaAs sources (P=20%–30%). The preparation and characterization of the strained photocathode is also discussed.

Magnetic properties of ultrathin epitaxial γ-Fe films on Cu 3Au(001)

We have studied by LEED, electron energy loss and spin-resolved secondary electron spectroscopy structural and magnetic properties of ultra thin γ-Fe films grown epitaxially on Cu 3Au(001). We find that inplane ferromagnetic order sets in step-like at 6 Å film thickness. A strong film thickness dependence of the Curie temperature is observed. The films are inherently unstable as evidenced from the non-reversiblity of the spin polarization vs temperature curves even after annealing by only a few degrees which is accompanied also by a strong enhancement of the substrate signal in electron energy loss spectra.

Symmetry characterization of unoccupied states in thick alkaline layers by spin-resolved Auger electron spectroscopy using primary excitation by circularly polarized light

CVV Auger electrons emitted from K, Rb and Cs layers are studied using spin-resolved spectroscopy. Oriented 3p, 4p and 5p hole states are excited by circularly polarized radiation in normal incidence. The photon energies range from 12 to 24 eV. With all three materials, the degree and sign of the Auger electron spin polarization vary with the photon energy. As an atomic model of the Auger process predicts, and as a comparison of measurements with the calculated densities of states shows, the spin polarization is essentially determined by the symmetry of the final states reached in the primary (photo)excitation. Just above the excitation threshold, the preferential spin direction of the Auger electrons is measured to be parallel to the spin of the exciting photons corresponding to a predominantly s-like symmetry of the unoccupied final states reached by the excitation. At higher photon energies the preferential spin direction changes to be antiparallel to the photon spin, corresponding to the mainly d-like symmetry of unoccupied states reached by the excitation.

Magnetic reconstruction of the Gd(0001) surface.

The surface magnetism of Gd(0001) films is studied using spin resolved electron spectroscopies. Both a canted surface magnetization vector out of the plane of the film and an enhanced surface magnetic ordering temperature 60[plus minus]2 K higher than the bulk Curie temperature are observed. Our results show that the surface magnetic layer behaves as a separate entity from the bulk. The in-plane component of surface magnetization aligns ferromagnetically to the bulk. Spin resolved photoemission from the Gd 4[ital f] core levels shows [approx]65% spin polarization at 150 K, which extrapolates to [approx]91% at 0 K.

Auger electron spectroscopy of molecules: Theory for spin polarization following photoabsorption in rotating linear molecules

This paper develops theoretical expressions to study angular distribution and spin polarization of those Auger electrons which are emitted in the decay of a vacancy created by the absorption of a photon in a rotating linear molecule. Identical expressions except, of course, for different decay amplitudes, in both the Hund’s coupling schemes (a) and (b), are obtained for the differential Auger current emitted in the transition J→Jf measured by an electron spectrometer sensitive to spin detection. The structure of these angular distributions is exactly the same as that of the spin-resolved photoelectrons from unoriented atoms and molecules. The present paper thus puts the angle- and spin-resolved Auger and photoelectron spectroscopies on the same footing wherein identical geometrical and kinematical analysis is applicable. The four parameters needed to completely characterize such distributions depend, in the present case, on rotational orientation and/or alignment of the photoexcited molecule, in addition to its Auger decay amplitudes. The use of parity-adapted molecular states separates the Auger spectra into even and odd partial wave components of the ejected electron continuum in both of the coupling schemes. Our analysis shows that the integrated Auger current is spin resolved provided it is produced in the decay of oriented vacancies. We further find that Auger electrons which leave the molecular ion in Jf=0 state may have nonzero degree of spin polarization if they follow absorption of only circularly polarized light. In this case, both the angular distribution and spin polarization of emitted electrons become totally independent of Auger dynamics. Thus, angle- and spin-resolved Auger electron spectroscopy can be used to produce polarized electrons, to determine rotational orientation and alignment of linear molecules, to study their structure and dynamics, and to prepare ions of such molecules in selective ro-vibronic states.

Magnetic-closure domains and interpretation of spin-resolved electron spectroscopies.

In spin-resolved electron spectroscopies at surfaces that do not contain an easy axis of bulk magnetization, the detection of spin effects can be hindered by closure domains. This is demonstrated by combining the magneto-optical Kerr effect and spin-resolved inverse photoemission (IPE) using Ni(001) as an example. On a Ni(001) surface with high remanent magnetization, spin-resolved IPE reveals a spin splitting of (80\ifmmode\pm\else\textpm\fi{}20) meV for transitions between bulk sp bands. The discrepancy in recent spectroscopic data regarding the splitting of Ni sp bands is explained by closure domains.

Growth mode and magnetic behavior of thin Fe films on Cu3Au(001) studied by low-energy electron diffraction and spin-resolved electron spectroscopies.

The structure and magnetic behavior of ultrathin Fe films on ${\mathrm{Cu}}_{3}$Au(001) have been examined. By low-energy electron diffraction (LEED) and Auger measurements it is shown that films up to a thickness of 7 monolayers (ML) deposited at room temperature grow pseudomorphically as face-centered-cubic Fe(001) with the ${\mathrm{Cu}}_{3}$Au lattice parameter of 3.75 A\r{} parallel to the surface. The films grow layer by layer. The magnetism of the fcc Fe films has been investigated by spin-polarized secondary-electron spectroscopy as a function of film thickness. The spin-polarization component parallel to the film plane is zero for films thinner than 3.6 ML. The nonzero spin polarization for films thicker than 3.6 ML indicates the existence of remanent magnetization parallel to the surface at room temperature. Spin-resolved photoelectron spectroscopy with synchrotron radiation has been used to study the electronic and magnetic structure of the films. The high spin polarization P>50% of the photoelectrons clearly demonstrates the ferromagnetism of \ensuremath{\gamma}-Fe with a lattice constant of 3.75 A\r{}. The spin-resolved energy distribution curves (SREDC's) may be understood according to the spin-resolved band-structure calculations of Bagayoko and Callaway [Phys. Rev. B 28, 5419 (1983)]. The results indicate that in contrast to \ensuremath{\alpha}-Fe, \ensuremath{\gamma}-Fe with a lattice parameter of 3.75 A\r{} is a strong ferromagnet.

Electronic Resonances in Rare-Gas Adsorbates Observed by Spin-Resolved Electron Spectroscopy

Recent experimental results of spin-polarized electron emission following photoexcitation of the valence orbitals of adsorbed rare-gas atoms are presented. As light source we employed the circularly polarized synchrotron radiation from the storage ring BESSY (Berlin). As detection technique served angle- and spin-resolved electron spectroscopy. The resolution of the experimental method is demonstrated by the example of photoemission from Ar on Pt(111), where a doublet feature with a splitting of only 90 meV could be resolved, which is hardly discernible in intensity spectroscopy. Below the threshold for direct photoemission narrow electronic resonances have been observed, resulting from discrete np → n's excitations in the adsorbate. For the system Xe on Ir(111) the dependence of the resonance features on the coverage from the sub-monolayer regime up to the 3D-crystal is discussed.

Assignment of excited states of adsorbed Xe and Kr

We tentatively propose an assignment of the excited states of adsorbed Xe and Kr by means of electron energy loss and spin-resolved electron spectroscopies on the basis of a molecular orbital description.

Assignment of excited states of adsorbed Xe and Kr

We tentatively propose an assignment of the excited states of adsorbed Xe and Kr by means of electron energy loss and spin-resolved electron spectroscopies on the basis of a molecular orbital description.