Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.     

Electronic structure of gadolinium calcium oxoborate

Gadolinium calcium oxoborate (GdCOB) is a nonlinear optical material that belongs to the calcium-rare-earth (R) oxoborate family, with general composition Ca₄RO(BO₃)₃ (R³⁺ = La, Sm, Gd, Lu, Y). X-ray photoemission was applied to study the valence band electronic structure and surface chemistry of this material. High resolution photoemission measurements on the valence band electronic structure and Gd 3d and 4d, Ca 2p, B 1s and O 1s core lines were used to evaluate the surface and near surface chemistry. These results provide measurements of the valence band electronic structure and surface chemistry of this rare-earth oxoborate.     

Comparative study of the electronic structure of the ordered and disordered Cu3Au (100) and Cu3Au (110) surfaces

The influence of structural changes on the electronic structure has been investigated by a comparison of the angle-integrated valence and core level photoelectron spectra of ordered and disordered Cu₃Au(100) and (110) surfaces. The total width of the Au 5d and Cu 3d bands does not change with the ordering state or surface orientation. The spectra for the (100) surface are compared with selfconsistent calculations and good agreement is found, for the ordered state, however with a 0.6 eV correction of the calculated Fermi level position. We observe three Au 5d derived bands at 5.1, 6.0 and 6.9 eV, in contrast to previous experimental findings. Our results indicate the existence of shortrange order above the critical temperature.     

A theoretical study of the electronic structure of Pd(111) clean surface

A detailed investigation of the electronic structure of Pd(l 11) clean surface is presented in terms of a LCAO band model. Results including surface bands and local densities of states are given and a comparison with recently performed photoemission experiments is presented. For hω = 21.2 eV we find that the angle resolved distribution curves can be explained in terms of initial state density and k_∥ conserving transitions. For lower photon energies a mixture of bulk transition calculations and surface density of states seems more appropriate. Comparison with previous theoretical work is also presented.     

Spin-orbit hybridization points in the face-centered-cubic cobalt band structure

Linear magnetic dichroism is observed in spin-, time-, and energy-resolved two-photon photoemission from valence bands of epitaxial fcc cobalt on Cu(001). With image-potential states as spectator states we identify initial bulk and surface states with minority spin character as the source for dichroic intensities and apparent dichroic lifetimes. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. These spin hot spots enhance spin-flip scattering by several orders of magnitude and are therefore assumed to be crucial in ultrafast demagnetization.     

Pure and Sb-doped SnO 2 nanoparticles studied by photoelectron spectroscopy

We describe photoemission results from pure and Sb-doped SnO₂ nanoparticles deposited on gold substrates. Photoelectron spectra with synchrotron radiation were recorded for Sn 3d, Sb 3d and O 1s core levels and valence bands in the 500-1200 eV energy range. For pure SnO₂ nanoparticles the surface is terminated by an oxygen rich layer with no obvious surface environment for Sn. When doped n-type with 9.1% or 16.7% Sb, dopant atoms are concentrated near the surface of the nanoparticles. The valence state of the dopant atoms is predominantly Sb^V. Plasmon satellite features are also observed in core level photoemission spectra and their intensity relative to the main peak increases with increasing photon energy. Received 30 November 2000     

Magnetic circular dichroism in Co 2p photoemission of Co/Cu(1 1 13): Separation of the fundamental spectra

We measured high-quality Co 2p magnetic circular dichroism (MCD) spectra in photoemission for > 5 ML Co films grown on Cu(1 1 13) using a “complete” experiment, where the sample magnetization and the light helicity vector were reversed separately. We show how the four measured spectra, M^±P^±, can be used to make new linear combinations, which correspond to the circular dichroism in the angular dependence (CDAD), magnetic linear dichroism in the angular dependence (MLDAD) and MCD spectra. The integrated signals of the MLDAD and CDAD can be used to estimate the error caused by the difference in the degrees of magnetization and light polarization, respectively, in the opposite alignments. The MCD signal integrated over the entire 2p region does not average to zero, as one would have expected from the sum rule for photoemission to a non-interacting continuum state. There is a strong MCD signal in the entire region between the 2p _3/2 and 2p _1/2 main lines with pronounced satellite structure. The differences between the measured and calculated results for an independent-particle and an atomic model indicate the presence of interatomic electron correlation effects and configurational mixing. Received 26 September 2000     

Surface magnetism of iron and cobalt on W (110) probed with polarized synchrotron radiation

I report investigations on the magnetic properties of epitaxially ordered ultrathin metal films and nanoscaled islands on a tungsten surface. The systems are fabricated by thermal evaporation under ultrahigh vacuum conditions and characterized using standard surface science techniques, the 3D iron islands are created by a thermally stimulated self-organization process on the surface. The magnetic properties of these systems are investigated using polarized synchrotron radiation ranging from the VUV region in valence band photoemission to the X-ray regime (14.4 keV) for synchrotron-based M?ssbauer spectroscopy. In angle resolved photoemission the effect of magnetic dichroism gives rise to different photoelectron intensities when the magnetization states are reversed. This phenomenon is used to analyze the electronic and magnetic properties. On the other hand, synchrotron-based M?ssbauer spectroscopy is applied to an oriented iron island structure in order to obtain information on the strength and direction of the magnetic fields from the nuclear energy levels. Received: 28 April 2000 / Accepted: 4 September 2000 / Published online: 7 March 2001     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Photoemission study of the growth of the NdF3/Si(111) interface

Photoemission spectroscopy with synchrotron radiation was used to study the NdF₃/Si(111) interface as a function of annealing temperature for NdF₃ films. These films range in thickness from 1–20 monolayers and were deposited at room temperature. Without annealing, both F-Si and Nd-Si bonding is observed, indicating that the planar triangular NdF₃ molecules lie flat on the Si(111) substrate. At annealing temperatures between 400 and 500° C, the NdF₃/Si(111) interface is dominated by Nd-Si bonding as evidenced from a line-shape analysis of the Si 2 p and Nd 4 f core levels. By resonant excitation of the giant 4 d-4 f absorption resonance, the photoemission signal from the partially occupied 4 f orbitals is enhanced and can be distinguished from the photoemission signal of the overlapping F 2 p valence band. At higher temperatures F is completely lost due to the decomposition of NdF₃.     

Reactivity and magnetism of Fe/InAs(100) interfaces

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L_{2, 3} edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk α-Fe (2.2 μ _B) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices. Received 4 April 2002 / Received in final form 13 May 2002 Published online 31 July 2002     

Experimental correlation between photoemission matrix elements and LEED intensities in superperiodic structures

A direct comparison between photoemission measurements and band structure calculations is sometimes tricky. Matrix element effects may affect considerably the spectral weight of the electronic states and prevent the expected translational symmetry of the band structure from being observed. We show how matrix element effects can be qualitatively described to a certain extent by making an analogy between photoemission and low energy electron diffraction. We have tested this approach in two superperiodic systems. We have first explained the intensity distribution in different Brillouin zones of a surface state in Si(1 1 1)-(7 × 7), where the surface state spectral intensity does not exhibit the (7 × 7) symmetry. We have also compared the LEED intensity of superperiodic LEED spots with the energy dependence of bulk bands on a facetted Si surface as measured by photoemission.     

Influence of local environment on electronic properties of Co atoms in the Tm3Co11B4 compound

The electronic structure of the Tm₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. This compound crystallizes in the hexagonal Ce₃Co₁₁B₄-type structure (P6/mmm). We have found a good agreement between the experimental XPS valence band spectra and theoretical ab initio calculations. The calculated total magnetic moment is equal to 13.635 μ_B/f.u. The magnetic moments on the Co atoms are antiparallel to the moments of the Tm atoms. Their values are depended on the local environment, especially on the number of the Co neighbors. The theoretical results are compared with other calculations, saturation magnetization measurements as well as neutron diffraction data for R₃Co₁₁B₄ (R=Y, Nd, Gd, Tb).     

Core level spectroscopy and RHEED analysis of KGd0.95 Nd0.05(WO4)2 surface

A study of the surface structure and electronic properties of (010) KGd_0.95Nd_0.05(WO₄)₂ (Nd:KGW) using RHEED analysis and XPS is presented. It is shown that Nd doping has a negligible effect on the core levels of the basic elements. A bombardment of the Nd:KGW crystal with 3-keV Ar ions results in surface amorphization accompanied by the generation of tungsten ions in lower valence states.     

The electronic structure of cobalt phthalocyanine

The electronic structure and morphology of organic semiconducting cobalt-phtalocyanine (CoPc) films in situ prepared on the Au(001)-5×20 superstructure have been studied by a combination of experimental and theoretical work. The CoPc molecular film was characterized by photoemission spectroscopy (PES, valence band and core-level). The experimental results were simulated and have been explained in the framework of density functional theory (DFT) calculations. The C 1s and N 1s core level spectra were analyzed by taking into account the fact that both types of atoms have different nonequivalent positions in the molecule. And finally, the experimentally obtained electronic valence band structure of CoPc is in very good agreement with ab initio density of state results, allowing a detailed site-specific insight into the system.     

X-ray magnetic circular dichroism and resonant photomission of V(TCNE)x hybrid magnets

Thin films of V(TCNE)x were deposited in ultrahigh vacuum using a film growth technique based on in situ chemical vapor deposition of tetracyanoethylene, TCNE, and bis-benzene vanadium, V(C6H6)2. The in situ preparation method enabled, for the first time, experimental analysis of oxygen-free films. X-ray magnetic circular dichroism measurements recorded at the V L(2,3) edge confirmed room temperature magnetic ordering. A combination of conventional photoelectron spectroscopy (PES) and resonant photoemission (RPE) measured at the V L3 edge shows that the highest occupied electronic state is V(3d) derived. The rearrangements of the TCNE- related valence electronic states observed in PES and the evidence of V(3d) and TCNE- pi(pi*) orbital overlap contained in RPE spectra, indicate that strong, covalent type bonding occurs between the vanadium and the TCNE molecules.