Spin-resolved Photoemission Spectra Research Articles

Nature of the magnetic moment of cobalt in ordered FeCo alloy

The magnets are typically classified into Stoner and Heisenberg type, depending on the itinerant or localized nature of the constituent magnetic moments. In this work, we investigate theoretically the behaviour of the magnetic moments of iron and cobalt in their B2-ordered alloy. The results based on local spin density approximation for the density functional theory (DFT) suggest that the Co magnetic moment strongly depends on the directions of the surrounding magnetic moments, which usually indicates the Stoner-type mechanism of magnetism. This is consistent with the disordered local moment picture of the paramagnetic state, where the magnetic moment of cobalt gets substantially suppressed. We argue that this is due to the lack of strong on-site electron correlations, which we take into account by employing a combination of DFT and dynamical mean-field theory (DMFT). Within LDA + DMFT, we find a substantial quasiparticle mass renormalization and a non Fermi-liquid behaviour of Fe–3d orbitals. The resulting spectral functions are in very good agreement with measured spin-resolved photoemission spectra. Our results suggest that local correlations play an essential role in stabilizing a robust local moment on Co in the absence of magnetic order at high temperatures.

Electronic structure and magnetism of strained bcc phases across the fcc to bcc transition in ultrathin Fe films

We investigated the electronic structure of the bcc metastable phases involved in the fcc to bcc transition of Fe. Ultrathin Fe films were grown on a 2-monolayer (ML) Ni/W(110) substrate, where a fcc lattice is stabilized at low Fe coverages and the transition proceeds through the formation of bcc nuclei showing a specific ``Kurdjumov-Sachs'' orientation with the substrate. A comprehensive description of the electronic structure evolution is achieved by combining spin-resolved UV photoemission spectroscopy and ab initio calculations. According to our results, an exchange-split band structure is observed starting from 2 ML of Fe, concomitant with the formation of ferromagnetic bcc nuclei. Continuous modifications are observed in the spin-resolved photoemission spectra for increasing Fe coverage, especially for what concerns the minority states, possibly indicative of the progressive relaxation of the strained bcc phase starting from the bcc/fcc interface.

Performance of a quadruple reflector circular polarizer in VUV region for angle- and spin-resolved photoemission spectroscopy

A quadruple-reflector circular polarizer has been installed and tested at the U5UA beamline of National Synchrotron Light Source. With an intense and highly linearly polarized vacuum ultraviolet (VUV) undulator light source, the polarizer can produce pure circular polarization in the 20–50 eV photon energy range, enabling angle- and spin-resolved photoemission spectroscopy. The optical conditions for circularly polarized light (CPL) as well as the efficiency of the polarizer have been investigated and compared with calculation. Example valence band magnetic circular dichroism and spin-resolved photoemission spectra taken with CPL demonstrate the advantages of CPL in the VUV energy range.

Evidence for the half-metallic ferromagnetic state ofFe3O4by spin-resolved photoelectron spectroscopy

The spin-dependent electronic structure of thin epitaxial films of magnetite, ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111),$ has been investigated at room temperature by means of spin-, energy-, and angle-resolved photoemission spectroscopy. Near the Fermi energy ${E}_{F}$ a spin polarization of $\ensuremath{-}(80\ifmmode\pm\else\textpm\fi{}5)%$ is found. The spin-resolved photoemission spectra for binding energies between 1.5 eV and ${E}_{F}$ show good agreement with spin-split band energies from density-functional calculations.

Electronic structures of magnetic ultrathin films Co/Au(1 1 1) studied by spin-resolved photoelectron spectroscopy

We have measured the spin-resolved photoemission spectra of the Co thin films grown epitaxially on Au(1 1 1) substrate in order to investigate their valence band structures. It is proved that the electronic structures of Co thin films are pretty different from that of bulk hcp-Co. It is observed that as the films grow thicker, the electronic structures become closer to those of the bulk Co with the magnetic anisotropy turning into in-plane magnetization from out-of-plane magnetization.

Helicity Dependence of the Spin Polarization of Ni 6 eV Satellite

Spin-resolved photoemission spectra of the 6 eV satellite of ferromagnetic Ni valence band have been measured with circularly polarized light at Ni 3 p core level excitation threshold. We have observed that the spin polarization of the 6 eV satellite was enhanced according to the Δ m =+1 excitation of the dipole selection rule for magnetic quantum numbers and suppressed to the Δ m =-1 excitation. The observed helicity dependence of the spin polarization was discussed based on an atomic model including the spin-orbit interaction.

Spin Dependent Electronic Structure of Doped Manganese Perovskites

The spin-resolved photoemission spectra were successfully obtained from La0.7Sr0.3MnO3 190 nm thick epitaxial film on SrTiO3(001). Well below Tc the results clearly manifest the half-metallic nature, i.e., for the majority spin, the photoemission spectrum clearly shows a metallic Fermi cut-off, whereas for the minority spin, it shows an insulating gap with disappearance of the spectral weight at ∼0.6 eV binding energy. On heating through Tc the spectra show no difference for different spins and the spectra weight at the Fermi level (EF disappears, indicating that the Mn 3d spins become disordered) and the system undergoes the ferromagnetic metal to paramagnetic non-metal transition.

Structure of Cr overlayers on Fe surfaces: a new approach for the interpretation of spin-resolved photoemission and magnetic dichroism spectra

Results of investigations of Cr overlayers on Fe surfaces by magnetic linear dichroism in the angular distribution and spin-resolved core level photoemission are analysed within the framework of a new model approach. It includes a special algorithm for modelling epitaxial growth of overlayers with different roughness together with self-consistent calculations of the magnetic moment distribution within a Periodic Anderson Model. On this basis the value of the magnetic dichroism and the spin polarisation in photoemission as functions of the Cr coverage can be modelled also for rough surfaces. The comparison of experimental spectra and theoretical coverage dependencies obtained for the different surface roughnesses leads to conclusions about the microscopic structure of Cr overlayers.

Ab initio studies of Ni-Cu-Ni trilayers: layer-projected densities of states and spin-resolved photoemission spectra

First-principles calculations have been performed in order to determine local magnetic moments, partial densities of states and spin-resolved photoemission spectra of Ni-Cu-Ni-trilayers. The tight-binding linearized muffin-tin orbitals (LMTO) method within the atomic sphere approximation has been used, and structural models have been built by translational repetitions of a face centred tetragonal-(001) supercell (N = 2,...,6), where ES stands for an empty sphere and the four ES monolayers, separating the trilayers from one another, mimic the vacuum. The main finding of the present paper is that both densities of states as well as a Fermi energy (and thereby a work function) are to some extent sensitive to Cu-spacer thickness. In addition, a crossover from the antiparallel alignment of magnetizations of neighbouring Ni layers to the parallel configuration is computed, and it is confirmed that surface-Ni magnetic moments are enhanced in contrast to the Ni-Cu interface ones which are suppressed.

Magnetic dichroism in valence-band photoemission spectroscopy of disordered bccFexCo1−xalloys

A fully relativistic calculation of spin-resolved valence-band photoemission spectra for bcc ${\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ alloys is presented that is based on the Korringa-Kohn-Rostoker coherent-potential approximation method of band-structure calculation. The result for the spin-integrated spectra for unpolarized radiation was found to be in very satisfying agreement with corresponding experimental data. For the spin-resolved spectra a pronounced magnetic circular dichroism is predicted that should be observable in a corresponding experiment.

Valence band structure of ferromagnetic Ni observed by spin- and angle-resolved photoemission

Abstract We have measured spin-resolved photoemission spectra of ferromagnetic Ni valence bands in normal emission mode and obtained quasi-particle energy band dispersion along the Γ – K – X direction of the Brillouin zone. Comparing the experimental band structure with theoretical calculations, we found that the electron correlation effect appearsmore obvious in the majority spin states of the valence band spectra more than in the minority spin states. We also measured spectra in off-normal emission mode and observed considerable negative spin polarization in the valence band, resulting in spin dependent 3d band dispersion along the Q line between the L and W point in the Brillouin zone

Analysis of temperature-dependent spin-resolved photoemission of Ni(110) by a single-particle model

We have measured temperature-dependent spin-resolved photoemission spectra of Ni(110) valence band along theΓ- K-C direction in the Brillouin zone and interpreted the observed spectra by a single-particle approximation of photoelectron excitation. We have attempted to explain the spectra by a model which describes the temperature dependence of the spinresolved spectra as functions of magnetization, band width and effective intra-atomic Coulomb energy. Although the model does not consider the whole Ni3 d bands nor mutual interactions between them, we have obtained qualitative good agreement between the model and the observed spectra with comparatively small Coulomb energy.

One-electron theory of core-level photoemission from ferromagnets

A one-electron theory is presented for photoemission from a $p$ core level of a ferromagnet. The approach permits the straightforward calculation of angle- and spin-resolved photoemission spectra for an arbitrary geometry and photon polarization. The Hamiltonian employed simultaneously includes both spin-orbit and exchange interactions, and thereby accounts for mixing between the ${p}_{3/2}$ and ${p}_{1/2}$ levels. Photoelectron diffraction effects are neglected. Using this model, good agreement is obtained with experimental spin-resolved and magnetodichroic Fe $2p$ photoemission spectra.

Spin-resolved photoemission of in situ sputtered iron and iron-yttrium alloys

Spin-resolved photoemission spectra of amorphous Fe-Y alloys have been obtained and compared with both those of pure Fe and with theoretical electronic structure calculations. The thin iron and alloy films were prepared by in situ sputtering, and the spectra were obtained using 110-eV photons from the UK Synchrotron Facility. From secondary electron spin hysteresis loops, ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ starts to order ferromagnetically well before the lowest temperature accessible with our equipment (114 K), but ${\mathrm{Fe}}_{41}$${\mathrm{Y}}_{59}$ shows no polarization down to this temperature. Although the spin-integrated photoemission spectrum of the valence band of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ shows little difference from that of pure Fe, the spin-resolved spectra are significantly different. ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ also exhibits a reduced valence band polarization (14%) compared with iron, and contrary to predictions, the majority spin states of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ dominate at the Fermi edge. No evidence is found for strong ferromagnetism. Unexpectedly, the 1-eV secondary electron polarization of ${\mathrm{Fe}}_{60}$${\mathrm{Y}}_{40}$ (7%) is found to be low compared with the valence band polarization, an effect that may be due to a negative moment on the yttrium.

Spin-Resolved Photoemission Spectroscopy

The spin-resolved photoemission spectroscopy is utilized to study the spin dependent electronic structures of solids and solid surfaces. The spinand angle-resolved valence band spectra of ferromagnetic Ni are investigated by comparing with an atomic model and a one-electron band calculation. The electron correlation effects which appear in the spin-resolved photoemission spectra are discussed. The spin dependent electronic structures of S atoms adsorbed on Fe(100) surface are also investigated. PACS numbers: 75.25.+z, 73.20.-r, 79.60.-i

Spin-resolved photoemission of valence-band satellites of Ni

Spin-resolved photoemission spectra of 6-, 9-, and 13-eV satellites of the ferromagnetic Ni valence band are reported. The spin polarization of the valence-band satellites and their photon energy dependence are interpreted by the multiplet configuration of ${3\mathrm{d}}^{8}$ and ${3\mathrm{d}}^{7}$ final states. This implies the localization of a photoexcited final state reflecting the strong electron correlation in Ni.

Resonant enhancement of valence band satellites of Ni: A spin-resolved photoemission study

We have measured spin-resolved resonant photoemission spectra of 6 eV and 13 eV satellites of ferromagnetic Ni(1 1 0) valence band near the 3 p threshold. By the analyses of the spin polarization of the 6 eV satellite, the main part of the resonant enhancement of the satellite is confirmed due to the interference between two photoexcitation processes leading to the same phtoexcited 3 d 8 final state. In 13 eV satellite, resonance enhancement occurs mainly in the majority spin 3 d 7 final state.

Spin-resolved core-level photoemission spectra of ferromagnetic nickel

Spin-resolved core-level photoemission spectra of ferromagnetic nickel

Spin-resolved core-level photoemission spectra of ferromagnetic nickel

We have measured the spin-resolved 3 p and 3 s core-level photoemission as well as M 23 VV Auger electron spectra of ferromagnetic Ni. The observed core level spin polarization spectra can be interpreted in terms of the multiplet structures based on the impurity Anderson model. In the non-resonant normal M 23 VV Auger-electron spectrum, positive spin polarization is clearly found not only for the “3 d 8” final state but also for the state with the kinetic energy beyond 62eV (binding energy of the Ni 3 p core level minus the work function). The positive spin polarization of the “3 d 8” main structure is understood as the final state mixing of the 3 d 7 and 3 d 8 states in contrast to the previous interpretation. The positive spin polarization above 62eV is ascribed for the first time to the secondary electron excitation.

Photon energy dependence of spin-resolved photoemission spectra in normal emission from Pt(110) by linearly polarized light

Spin-resolved photoemission spectroscopy in normal emission has been performed with Pt(110). The photoelectrons were emitted by off-normally incident linearly polarized synchrotron radiation from BESSY with photon energies between 10 and 16 eV. The photoelectron spectra obtained reveal spin polarization for the component perpendicular to the reaction plane which is defined by the momenta of the incoming light and the outgoing electrons. In agreement with previous measurements with unpolarized light [1] and theoretical calculations [2] the spin polarization depends strongly on the azimuthal angle of the crystal reflecting its two-fold symmetry. The spin polarization depends significantly on the photon energy and even changes its sign in the main photoemission peak between 11 and 14 eV. This behaviour is due to the fact that the transitions occur into different final bands (with different band hybridization) for photon energies below and above 13 eV.