Spin-resolved Photoemission Spectroscopy Research Articles

Significant spin polarization of Co2MnGa Heusler thin films on MgO(100) measured by ultraviolet photoemission spectroscopy

The spin dependent valence band structure of the Heusler compound Co2MnGa grown on MgO(100) is investigated by spin resolved ultraviolet photoemission spectroscopy (SRUPS). Using a vacuum suitcase, epitaxial thin film samples are transferred from a deposition chamber into an SRUPS chamber with a highly efficient spin detector. The transport results only in weak oxidation of the moderately reactive compound Co2MnGa. A spin polarization of 34% is obtained at the Fermi energy. The energy dependence of the spin polarization including a sign change at E−EF=−0.6 eV is in good agreement with theoretical predictions of the bulk properties of the Heusler compound.

Effect of sublattice asymmetry and spin-orbit interaction on out-of-plane spin polarization of photoelectrons

We study theoretically the effect of spin-orbit coupling and sublattice asymmetry in graphene on the spin polarization of photoelectrons. We show that sublattice asymmetry in graphene not only opens a gap in the band structure but in case of finite spin-orbit interaction it also gives rise to an out-of-plane spin polarization of electrons close to the Dirac point of the Brillouin zone. This can be detected by measuring the spin polarization of photoelectrons and therefore spin resolved photoemission spectroscopy can reveal the presence of a band gap even if it is too small to be observed directly by angle resolved photoemission spectroscopy because of the finite resolution of measurements or because the sample is $p$-doped. We present analytical and numerical calculations on the energy and linewidth dependence of photoelectron intensity distribution and spin polarization.

Investigation of the spin-dependent properties of electron doped cobalt–CuPc interfaces

We have grown metal–organic interfaces by in-situ deposition of ultrathin copper phthalocyanine (CuPc) films on a thin cobalt film on Cu(0 0 1). Evidence for layer-by-layer growth is found. The spin-dependent electronic properties of the Co–CuPc interface and their modification under caesium doping are investigated by spin-resolved photoemission spectroscopy. We observe a doping-induced shift of the highest occupied molecular orbital (HOMO) of CuPc away from the Fermi level ( E F), accompanied by the formation of an unpolarised gap-state at 0.7 eV below E F in the high doping regime. Such features are reflected in the behaviour of the detected interfacial spin-polarisation.

Direct observation of spin-polarized surface states in the parent compound of a topological insulator using spin- and angle-resolved photoemission spectroscopy in a Mott-polarimetry mode

We report high-resolution spin-resolved photoemission spectroscopy (spin-ARPES) measurements on the parent compound Sb of the recently discovered three-dimensional topological insulator Bi1−xSbx (Hsieh et al 2008 Nature452 970, Hsieh et al 2009 Science 323 919). By modulating the incident photon energy, we are able to map both the bulk and the (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin–orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin-polarized electronic channels. These results complement our previously published works on this class of materials and re-confirm our discovery of topological insulator states in the Bi1−xSbx series.

Three-Dimensional Topological Insulators

Topological insulators in three dimensions are nonmagnetic insulators that possess metallic surface states (SSs) as a consequence of the nontrivial topology of electronic wavefunctions in the bulk of the material. They are the first known examples of topological order in bulk solids. We review the basic phenomena and experimental history, starting with the observation of topological insulator behavior in BixSb1−x by angle and spin-resolved photoemission spectroscopy (spin-ARPES) and continuing through measurements on other materials and by other probes. A self-contained introduction to the single-particle theory is then given, followed by the many-particle definition of a topological insulator as a material with quantized magnetoelectric polarizability. The last section reviews recent work on strongly correlated topological insulators and new effects that arise from the proximity effect between a topological insulator and a superconductor. Although this article is not intended to be a comprehensive review of what is already a rather large field, we hope that it serves as a useful introduction, summary of recent progress, and guideline to future directions.

Tailoring the Spin Functionality of a Hybrid Metal-Organic Interface by Means of Alkali-Metal Doping

We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration.

TOF electron energy analyzer for spin and angular resolved photoemission spectroscopy

Current pulsed laser and synchrotron x-ray sources provide new opportunities for Time-Of- Flight (TOF) based photoemission spectroscopy to increase photoelectron energy resolution and efficiency compared to current standard techniques. The principals of photoelectron timing front formation, temporal aberration minimization, and optimization of electron beam transmission are presented. We have developed these concepts into a high resolution Electron Optical Scheme (EOS) of a TOF Electron Energy Analyzer (TOF-EEA) for photoemission spectroscopy. The EOS of the analyzer includes an electrostatic objective lens, three columns of transport lenses and a 90 degree energy band pass filter (BPF). The analyzer has two modes of operation: Spectrometer Mode (SM) with straight passage of electrons through the EOS undeflected by the BPF, allowing the entire spectrum to be measured, and Monochromator Mode (MM) in which the BPF defines a certain energy window inside the scope of the electron energy spectrum.

Influence ofMgOoverlayers on the electronic states ofFe(001)thin films grown onGaAs(001)

The valence electronic states at the model tunneling junction interface $\mathrm{MgO}∕\mathrm{Fe}(001)$ system were systematically studied by comparing the spin-resolved photoemission spectroscopy of clean $\mathrm{Fe}(001)$ and $\mathrm{MgO}$ covered $\mathrm{MgO}∕\mathrm{Fe}(001)$ surfaces using linearly $p$-polarized light. For the clean $\mathrm{Fe}(001)$ film on $\mathrm{GaAs}(001)$, five distinct features including bulk and surface-related transitions are found. The bulk and surface-state transitions are well-accounted for by the direct transition model based on the calculated energy band structure of bcc bulk $\mathrm{Fe}(001)$. The previously observed minority feature at a binding energy ${E}_{B}=\ensuremath{-}1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is reinterpreted as a surface roughness associated transition. Upon the $\mathrm{MgO}$ adsorption on $\mathrm{Fe}(001)$, the surface-state transitions at ${E}_{B}=\ensuremath{-}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below Fermi energy ${E}_{F}$ appearing in both the majority and minority spin spectra at low photon energy ($18\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to $35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) were quenched. This is also the first direct experimental evidence of a minority spin surface state located just below the Fermi energy as predicted previously. The bulk states at the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface exhibit a layer-dependent modification, i.e., the bulk states in the deeper Fe layer(s) remain unaffected, while the states of ${\ensuremath{\Delta}}_{5}^{\ensuremath{\downarrow}}$ band symmetry in the Fe layer(s) closest to the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface are strongly modified, in contrast to the states of ${\ensuremath{\Delta}}_{1}^{\ensuremath{\uparrow}}$ symmetry. As a consequence of this interface effect, the ``partial spin polarization'' at the Fermi level changes sign from negative to positive values as seen in the spin asymmetry spectra at photon energies of $40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $60\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The origin of this spin- and symmetry-dependent modification observed at $\mathrm{MgO}∕\mathrm{Fe}(001)$ interfaces is discussed.

Temperature dependent phase transition of EuO on MgO(100)

We investigated the changes of magnetic properties and phase transition in EuO films grown on a MgO(100) substrate as a function of temperature. As the temperature was varied, we observed a clear phase transition and polarization change at 69K using spin resolved photoemission spectroscopy and low energy electron diffraction. We will elucidate the temperature driven changes in the electronic and magnetic properties of EuO films on a MgO(100) substrate.

Influence ofMgOoverlayers on the electronic states of bctCo(001)thin films grown on bccFe(001)∕GaAs(001)

The spin polarization of the valence band electronic states of strained bcc $\mathrm{Co}(001)$ and $\mathrm{MgO}∕\mathrm{Co}(001)$ thin films grown onto a bcc $\mathrm{Fe}(001)$ seed layer on $\mathrm{GaAs}(001)$ are investigated by employing spin-resolved photoemission spectroscopy. The experimental results are compared with the calculated energy band structure of bcc and bct $\mathrm{Co}(001)$, and discussed in the framework of the interband transition model, which allows one to ascribe the observed spectral features to bands of given spin and spatial symmetry. In contrast to the positive spin polarization observed at the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface, a large negative spin polarization of the electronic states at the Fermi level is observed for the $\mathrm{MgO}∕\mathrm{Co}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ system. Such a large negative spin polarization is attributed to a change in the energy band structure at the bct Co/bcc $\mathrm{Fe}(001)$ interface.

Spin Injection and Spin Dynamics at CuPC/GaAs(100) Interface

ABSTRACTSpin injection from GaAs(100) to organic semiconductor copper phthalocyanine (CuPc) has been investigated experimentally with spin-resolved two-photon photoemission (SR-2PPE) spectroscopy. With SR-2PPE, the dynamics of both electron and spin relaxation have been studied with femtosecond time resolution. The spin-polarized electrons are originally generated in GaAs through optical pumping and injected into CuPc. We observed an enhancement in spin polarization at the interface after initial CuPc deposition. This demonstrates that interface spin scattering is insignificant, which is similar to our previous result of spin injection at CuPc/Co interface. The spin polarization dropped when the CuPc film became thick, an effect attributed to bulk attenuation in CuPc. The lifetime of the unoccupied orbits in CuPC was also studied with red-blue excitation of photon energy 1.56 eV and 3.12 eV, respectively. There was a strong asymmetry in the time-resolved spectra, and an unexpected long lifetime when the lower unoccupied orbital was excited. A simple explanation of this phenomenon will be discussed.

Spin-polarized conduction in localized ferromagnetic materials: The case of Fe3O4 on MgO(100)

The surface electronic structure of 1500-Å-thick Fe3O4(100) films has been investigated by spin-resolved photoemission spectroscopy with vacuum ultraviolet synchrotron radiation. The films, epitaxially grown on MgO(100) substrates using plasma-assisted molecular-beam deposition, are briefly exposed to air during transfer to the photoemission chamber. It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere. The presence of the characteristic Verwey transition, as detected in the valence-band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films. The top of the valence band is found to be negatively spin polarized, with a value of ≈−50%; this result strongly points towards the localized nature of the 3d states in this system.

Reinvestigation of Co 2p Satellite Peak on the Co Ultrathin Film: Screening Channel at Interface

Reinvestigation of Co 2p Satellite Peak on the Co Ultrathin Film: Screening Channel at Interface

Epitaxial growth and characterization of layered magnetic nanostructures

We describe the construction and operation of an ultrahigh-vacuum system devoted to the study of layered magnetic nanostructures. The apparatus includes two growth chambers, where specimens nanostructured along the direction of growth (heterostructures, nanometric and subnanometric thin films and multilayers) are deposited either by molecular beam epitaxy or pulsed laser deposition, and a measurement chamber, where they are analyzed in situ by a variety of electron spectroscopies. Magnetic characterization is obtained by spin resolved inverse photoemission spectroscopy and magneto optical Kerr effect technique. Vacuum transfer towards other experimental facilities is also available. As examples of application, results from half metallic magnetic oxides, such as magnetite (Fe 3O 4) and manganite (La 2/3Sr 1/3MnO 3) thin films, and ferromagnet/semiconductor interfaces (Fe/Ge(0 0 1)) are also reported.

Polarization of

We show, for the first time, that the density of states of a magnetic thin film covered by an insulating barrier may be measured by using spin-resolved X-ray photoemission spectroscopy (SR-XPS). The basic idea is the lack of density of states of an insulator from the top of the valence band to the Fermi level. In this range of energy, the density of states of the underneath magnetic layer can be identified. This simple principle is demonstrated here for the [001] Fe/MgO system. From SR-XPS measurements, a polarization fully integrated in the /spl kappa/ space and corrected from remanence near 42% is obtained. Furthermore, we evidence a weak Fe-O hybridization and an enhancement of the Fe magnetic moment at the interface with MgO. These results are in agreement with previous ab initio calculations and certainly explain the fact that the Fe polarization is not destroyed in contact with MgO.

Polarization of Fe(001) covered by MgO analyzed by spin-resolved x-ray photoemission spectroscopy

Single-crystalline (001) Fe thin films were prepared by molecular beam epitaxy and covered by epitaxial MgO barriers. The MgO growth was confirmed to be layer by layer as shown by reflection high-energy electron diffraction, which allowed us to prepare Fe films covered by only two atomic planes of MgO for spin polarization measurements. On the one hand, regular x-ray photoemission spectroscopy measurements evidenced a weak hybridization between Fe and O. Moreover, a large Fe magnetic moment at the interface with MgO was shown by x-ray magnetic circular dichroism measurements. These two observations are in agreement with theory. On the other hand, we show that the Fe density of states can be measured by photoemission through the oxide barrier, since the oxide valence band only starts at 4 eV below the Fermi level. Spin-resolved x-ray photoemission measurements performed at ESRF allowed us to demonstrate that the electrons emitted from the (001) Fe layer through the barrier are polarized. A polarization fully integrated in k space and corrected from remanence near 42% was measured.

Growth and spin-resolved photoemission spectroscopy of the epitaxial α-Al2O3/Fe(110) system

Electronic and structural properties of epitaxial ultrathin aluminum oxide layers grown on epitaxial Fe(110) films were investigated at room temperature by means of spin-, angle- and energy-resolved photoemission spectroscopy. A spin polarization of −(15±5)% near the Fermi energy EF is found for 1 (ML) of α-Al2O3 on epitaxial Fe(110). This value results from the attenuation of −80% of Fe(110) by the oxide layer thickness and the spin-independent part of the scattering cross section.

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.

The use of NiMnSb Heusler alloy in MBE Grown Tunnel Junction

ABSTRACTIn this paper, the structural, magnetic and electronic properties of single-crystalline NiMnSb thin films grown by molecular beam epitaxy are presented. The growth process and surface morphology were studied by RHEED and STM. The structural quality of the films was investigated by EXAFS and HRTEM. For the optimized growth conditions, the expected NiMnSb C1b structure was obtained. Moreover, the most stable surface was found to be 4x1 reconstructed. The magnetic and electronic properties were studied by X-ray magnetic circular dichroïsm and spin-resolved x-ray photoemission spectroscopy experiments. The magnetization was observed to be essentially due to Mn, in agreement with theory. The surface was not found to be fully polarized but a polarization near 50% at room temperature was observed. Finally, the preparation of fully epitaxial NiMnSb/MgO/NiMnSb(001) trilayers is presented.

Recent progress in high-resolution magnetic imaging using scanning probe techniques

We have applied magnetic force microscopy (MFM) and spin-polarized scanning tunnelling spectroscopy (SP-STS) in ultrahigh vacuum (UHV) to study the magnetic domain structure of Co/Au(111) and Gd/W(110), respectively. A new measurement mode will be presented which minimizes possible accidental interactions between the probe tip and the sample in UHV-MFM. This allows the investigation of the well known spin reorientation transition of Co films grown on Au(111) at nanometre-scale spatial resolution. Details of domain walls and the domain structure as a function of CO coverage are studied. Spin-polarized tunnelling is realized by using an Fe-coated W tip and a sample which exhibits an exchange-split surface state. Magnetic domain imaging with a spatial resolution below 20 nm is demonstrated for Gd/W(110). The observed bias dependent spin polarization of Gd(0001) is in good agreement with spin-resolved inverse photoemission spectroscopy results.