Surface hysteresis curves of Fe(110) and Fe(100) crystals in ultrahigh vacuum — evidence of adsorbate influences

Abstract We have investigated hysteresis curves of silicon-stabilized Fe(100) and Fe(110) crystals using Polarized Secondary Electron Emission (PSEE) for surface magnetization processes and Magneto-Optical Kerr Effect (MOKE) for bulk hysteresis curves. In particular, the influence of adsorbates (oxygen and sulphur) and surface roughness on the magnetization reversal has been studied. For surface hysteresis curves we found adsorbate-induced reductions of coercive force of up to 50%; the bulk hysteresis curves showed significantly smaller deviations. Similar changes were caused by surface disorder on an atomic scale induced by ion bombardment. The results agree qualitatively with a previous e…

Angular-resolved electron spectroscopy from (110) surfaces of ternary Ce-based intermetallics: CePd2Si2 and CeNi2Ge2

Investigations of electronic properties were carried out for the ternary Ce-based intermetallic systems CeT2X2 (T = Ni, Pd; X = Ge, Si). To produce well-ordered and atomically clean surfaces, preparation is carried out in UHV. The polycrystalline substance was evaporated on a W(110) substrate with subsequent annealing. The single-crystalline layers obtained are characterised by MEED (thickness), AES (surface stoichiometry), LEED and SEM (surface structure). For electron-spectroscopic investigations, ARUPS (angle-resolved photoemission spectroscopy) was used. In the photoemission spectra, dispersion effects could be detected by variation of the detection angle.

Circular Dichroism in Photoemission from Nonmagnetic, Low- Z Solids: A Conspicuous Effect of the Photon Spin

A new phenomenon has been observed in solid-state photoemission with circularly polarized light. Its outward appearance is similar to the circular dichroism in photoabsorption. For special observation geometries, large photoelectron intensity asymmetries arise upon reversal of the photon helicity. The effect is not based on spin-orbit interaction, as is demonstrated by the example of photoemission from graphite (Z = 6). Similar asymmetries are expected for all single-crystal solids thus opening up a new class of experiments providing new tests for photoemission theories. Detailed information on the dynamics of photoexcitation of a solid can be gained, as discussed in terms of a simple orbit…

Oxygen on Fe(110): magnetic properties of the adsorbate system

Abstract Investigations concerning the electronic and magnetic properties of oxygen adsorbed on magnetized iron films were carried out by means of angle and spin resolving photoelectron spectroscopy. Iron(1 1 0), epitaxially grown on a W(1 1 0) crystal, served as the ferromagnetic substrate. Exchange splittings of the O 2p x derived level were detected demonstrating a magnetic coupling between the chemisorbate and the iron layer. This observation indicates the presence of an induced magnetic moment within the adsorbate overlayer. Variations of the exchange splitting occurred as a function of the oxygen coverage, energy of the exciting radiation, and detection angle of the emitted photoelect…

Spin resolved photoemission study of Co(0001) films

Abstract Thin ferromagnetic films are of great practical interest as they can exhibit a different magnetic behaviour compared to the bulk crystals due to, e.g., the surface anisotropy. The electronic and magnetic properties of thin cobalt films evaporated on W(110) have been investigated by means of angle and spin resolving photoelectron spectroscopy. The study was focused on electron spin polarization, spin resolved intensities and band structure behaviour in dependence of film thickness, excitation energy, and photoelectron detection angle. In order to interpret the experimental results, we have performed relativistic band structure calculations for the Co(0001) surface. The observed agre…

Chalcogen adsorption and surface magnetism

Abstract Investigations concerning the electronic and magnetic properties of oxygen and sulfur adsorbed on magnetized surfaces were carried out by means of angle and spin resolving photoelectron spectroscopy. Iron(110), a polycrystalline iron alloy, and an amorphous metallic glass (Fe 79 B 16 Si 5 ) served as ferromagnetic substrates. Exchange splittings of the O 2p and S 3p derived levels could be detected, demonstrating a magnetic coupling between the chemisorbate and iron. This observation presents a prerequisite for an induced magnetic moment within the adsorbate overlayer. For sulfur an adsorbate-induced structure only in the minority spin channel near the Fermi level was observed whic…

Circular dichroism in x-ray photoemission from core levels of nonmagnetic species

Circular dichroism in the angular distribution of photoelectrons (CDAD) has been observed in core-level photoemission from spherically symmetric initial states of nonmagnetic species utilizing circularly polarized soft-x-ray radiation from BESSY. Up to now, CDAD was predicted and observed only for aligned initial states. The data for oriented CO molecules prove that circular x-ray dichroism in photoemission from core levels is a general phenomenon that is not restricted to ferromagnets. High asymmetries of 50% suggest future applications as an effective circular x-ray analyzer.

Valence band photoemission from thin iron films - A comparison of experimental and theoretical results

Abstract In this contribution, we report on photoemission data from thin iron films on W(110) that have been carried out by means of spin-resolved photoelectron spectroscopy as well as Magnetic Circular Dichroism in valence band photoemission at the storage ring BESSY. These techniques give complementary information on the electronic and magnetic properties, such as exchange and spin–orbit splitting. We compare our experimental data to new photoemission calculations, which have been obtained in the framework of a fully relativistic one-step theory for ferromagnetic materials.

Magnetic circular and linear dichroism in VUV-photoemission from thin iron films on W(110)

Abstract We report on two different photoemission techniques for ferromagnetic surfaces: the phenomena of magnetic circular and linear dichroism in the angular distribution of photoelectrons MCDAD and MLDAD. MCDAD had already been studied in detail for different systems. Now we are able to compare our MCDAD results from thin iron films on W(110) to new data from magnetic linear dichroism MLDAD. Both effects (MCDAD and MLDAD) have been studied in the identical photon energy region (up to hv = 36 eV). The results will be discussed with respect to a bandstructure calculation.

Oxygen on Fe(100) and Fe(110)

Investigations of the electronic and magnetic properties of oxygen adsorbed on magnetized iron films have been carried out by means of angle and spin resolving photoelectron spectroscopy. Iron, epitaxially grown on W(100) and W(110) crystals, served as the ferromagnetic substrate. Exchange splittings of the O 2px derived level have been detected, demonstrating a magnetic coupling between the chemisorbate and the iron layer. Variations of the exchange splitting have occurred as a function of the oxygen coverage, photon energy, and emission angle. High oxygen exposures have lead to a FeO overlayer at the surface.

Clean and ordered surfaces of CeNi 2 Ge 2 layers on W(110)

Investigations of the geometric and electronic properties of ternary Ce-based heavy fermion systems CeT2X2 (T : Ni,Pd,Rh; X : Ge,Si) were carried out by means of electron spectroscopic methods. The main problem for these surface-sensitive techniques is the preparation of well-ordered and atomically clean surfaces. The ternary substance CeNi2Ge2 was grown on a W(110) substrate by MBE with subsequent annealing. A nearly layer-by-layer growth mode was detected using MEED. The annealed layers are ordered, but show small Ni2Ge crystalline islands. The composition was characterised by means of AES in dependence of the substrate as well as the annealing temperature. Electronic properties are inves…

Electronic structure of terpene derivatives

Abstract He I photoelectron spectra of terpene derivatives and their preparative intermediates have been measured. The changes of the ionization potentials are discussed in relation to the corresponding variations of the electronic structure. Especially, the effect of methyl and oxygen substitution has been studied.

Orientation and substrate interaction of adsorbed CO and NO molecules probed by circular dichroism in the angular distribution of photoelectrons.

The sensitivity and utility of circular dichroism in the angular distribution of photoelectrons (CDAD) as a probe of molecular orientation is demonstrated for adsorbed CO and NO molecules. A comparison between measured CDAD spectra and calculated values for spatially oriented CO and NiCO clearly confirms the well-known perpendicular adsorption for CO on Ni(100), whereas for CO adsorbed on Fe(100) a tilted adsorption geometry was found. For NO/Ni(100) and for NO on the oxygen-preadsorbed Ni(100) surface, an average tilt angle of α=40±10° was observed. In the case of the oxygen-preadsorbed Ni(100) surface, a higher fraction of NO molecules was found to be in a tilted orientation than on the c…

Iodine on a magnetized iron film evidence for a magnetic coupling

Abstract Spin-resolved photoelectron spectra of iodine chemisorbed on a magnetized iron (110) surface have been taken using unpolarized VUV light. The partial spin spectra reveal the I 5p x and 5p z signals being split by up to 0.35 eV. This exchange splitting occurs due to hybridization of adsorbate and substrate bands as a consequence of the chemical bond of the halogen atoms to the iron surface.

k-resolved electronic properties of ternary heavy fermion systems

Magnetic dichroism in photoemission with unpolarized light.

A magnetic dichroism in photoemission from ferromagnets induced by unpolarized light is observed for the first time in the valence band region as well as for surface states. The study was carried out with magnetized Fe(110), Co(0001), and Gd(0001) films grown epitaxially on W(110). This phenomenon leads to an intensity asymmetry for opposite directions of the sample magnetization. It can be easily used for a routine laboratory diagnostic of magnetic behavior. A simple theoretical interpretation of the phenomenon based on the atomic model is given.

Spin-polarization effects for electrons passing through thin iron and cobalt films

Abstract Spin-dependent effects of the inelastic mean free path (IMFP) are evident for low-energy electrons passing through magnetized ferromagnetic films caused by a different attenuation within the layer. Values of IMFP for both spin components were determined for ultrathin iron and cobalt layers on W(1 1 0) by means of spin-resolving photoelectron spectroscopy.

Electron-spectroscopic investigations on ternary HFS: CeT2X2

Investigations of the electronic properties were carried out for ternary Ce-based heavy fermion systems. The well-ordered surfaces of HFS were prepared by MBE on W (110) with subsequent annealing. The layers are characterised by MEED, LEED, AES and XPS. For the electron-spectroscopic investigations, ARUPS and SPEELS were used. In the photoemission spectra, dispersion effects could be detected. By means of SPEELS, the dipole-forbidden Ce f-f transitions could be observed. The comparison of the energy loss spectra above and below the characteristic temperature T∗ reveals differences in the energy losses as well as in the asymmetries.

Adsorbates on thin iron(100) films

The electronic and magnetic properties of different kinds of adsorbates on thin magnetized Fe(100) films have been investigated by means of spin resolving photoelectron spectroscopy. The study has been carried out with atomically physisorbed xenon, the molecule carbon monoxide, and metallic gold. The spin splitting of the Xe 5p signal can be explained by magnetic interactions with the final ionic hole state. By spin analysis, it could be shown that at room temperature for low exposures the adsorption of CO is dissociative and with increasing exposure additionally molecular. The Au related features for the monolayer show different line shapes in both spin channels but no splittings due to an…

Electronic structure of adsorbed organic molecules

We report on investigations of chiral organic molecules in the gas phase and physisorbed on graphite. The measurements were carried out by means of angle-resolving VUV photoelectron spectroscopy. The emission from specific orbitals in oriented molecules can be forbidden because of dipole selection rules. Due to the lack of certain features in the spectra, it was possible to determine the adsorption geometry of cyclohexanone and camphor as being with their ring system parallel to the surface. By comparison of gas-phase and adsorbate spectra of ethyl benzene derivatives, adsorption could be proved to be molecular without decomposition. For amino acids on graphite, the occurrence of characteri…

Circular dichroism in X-ray photoemission from Pd(111) and CO/Pd(111)

It is shown experimentally that in the soft X-ray region a large circular dichroism in the photoelectron angular distribution (CDAD) exists for both valence orbitals and core levels of CO molecules adsorbed on Pd(111). From theoretical consideration it follows that in the case of a spherically symmetric ground state wave function, like the 1σ and 2σ orbitals of CO, CDAD appears due to the lack of spherical symmetry in the final state. For carbonK-shell experimental results are compared to model calculations. Investigations at the Pd core levels proved that CDAD does also arise in X-ray photo-emission from non-magnetic crystals.