Perpendicular magnetic anisotropy of V/Co(0 0 1)

Based on the full-potential linearized augmented plane wave (FLAPW) calculations, various magnetic properties of ultra thin face centered cubic (fcc) Co(0 0 1) film and V adsorbed systems on Co(0 0 1) surface are explored. It was found that the V film grown on fcc Co(0 0 1) surface has large induced magnetic moment and the direction of magnetization is antiparallel to that of Co atom in the submonolayer coverage. Very interestingly, we found that the surface alloy and 0.5 ML adsorbed V/Co(0 0 1) systems have perpendicular magnetocrystalline anisotropy and the magnitude of anisotropy energy in 0.5 ML V on fcc Co(0 0 1) surface is greatly larger than that of surface alloy, while we observed in-plane magnetization in pure fcc Co(0 0 1) film. It was found that the spin-orbit interaction through spin-flip process cannot be ignored, therefore the simple relation with orbital anisotropy is not applicable in the interpretation of magnetocrystalline anisotropy.     

(2 × 1)-Na surface reconstruction induced by NaCl dissociation on Ag(1 1 0) during LEED analysis

This study first reports the initial growth stages of sodium chloride (NaCl) on Ag(1 1 0) at room temperature. NaCl grows in bi-layer mode along its [1 0 0] axis and gives rise to (4 × 1) and (1 × 2) reconstructed domains for coverages lower than two monolayers (ML), a minimal thickness inducing a bi-dimensional closed film. In addition, a 10 ML NaCl film has been examined by low energy electron diffraction (LEED). LEED analysis leads to the dissociation of the NaCl deposit in a few minutes. The NaCl dissociation implies Cl desorption from the surface and Na remaining on it. The residual Na is arranged in the form of a (2 × 1) surface reconstruction and is found to be strongly bounded to the Ag substrate. These findings have been established by using the X-ray photoelectron spectroscopy technique.     

Growth of Al2O3 thin films on NiAl(1 0 0) by gas-phase oxidation and electro-oxidation

The growth and structures of aluminum oxides on NiAl(1 0 0) have been investigated by RHEED (reflection high energy electron diffraction), complemented by LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and STM (scanning tunneling microscopy). Crystalline θ-Al₂O₃ phase grows through gas-phase oxidation on the NiAl(1 0 0) substrate with its a and b-axes parallel to [0 −1 0] and [0 0 1] direction of the substrate, respectively, forming a (2 × 1) unit cell. Whilst, three-dimensional nano-sized NiAl(1 0 0) protrusions and Al₂O₃, NiAl(0 1 1) clusters were found to co-exit at the surface, evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns. Particularly, the NiAl(0 1 1) clusters develop with their (0 1 1) plane parallel to the NiAl(1 0 0) surface, and [1 0 0] axis parallel to the [0 −1 1] direction of the substrate surface. STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxides during annealing. On the other hand, smoother (2 × 1) oxide islands with thickness close to a complete monolayer of θ-Al₂O₃ can be formed on NiAl(1 0 0) by electro-oxidation, in contrast with the large crystalline films formed by gas-oxidation.     

A density functional theory study of ethylene adsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models and Ni13 nanocluster

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian’03 software. It was found that ethylene has adsorbed molecularly on all clusters with π adsorption mode. Relative energy values were calculated to be −50.86 kcal/mol, −20.48 kcal/mol, −32.44 kcal/mol and −39.27 kcal/mol for Ni₁₃ nanocluster, Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) surface cluster models, respectively. Ethylene adsorption energy is inversely proportional to Ni coordination number when Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) cluster models and Ni₁₃ nanocluster are compared with each other.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Simplified method to prepare atomically-ordered TiO2(1 1 0)-(1 × 1) surfaces with steps and terraces

An effective way to prepare atomically-ordered rutile TiO₂(1 1 0) surfaces that have distinct step and terrace structures suitable for oxide thin film deposition is demonstrated. Only a two-step procedure, consisting of 20% HF etching and UHV-annealing at 1100 °C, was required to yield a clean (1 × 1) structure with step and terrace structures. Investigation of the surface using scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy reveals that carbon contamination is removed at around 800 °C, and straight steps with clear terraces appear at around 1000 °C.     

CoPt alloy grown on the WSe2(0 0 0 1) van der Waals surface

The structural and magnetic properties of 3-nm-thick CoPt alloys grown on WSe₂(0 0 0 1) at various temperature are investigated. Deposition at room temperature leads to the formation of a chemically disordered fcc CoPt alloy with [1 1 1] orientation. Growth at elevated temperatures induces L1₀ chemical order starting at 470 K accompanied with an increase in grain size and a change in grain morphology. As a consequence of the [1 1 1] growth direction, the CoPt grains can adopt one of the three possible variants of the L1₀ phase with tetragonal c-axis tilted from the normal to the film plane direction at 54°. The average long-range order parameter is found to be 0.35(±0.05) and does not change with the increase in the deposition temperature from 570 to 730 K. This behavior might be related to Se segregation towards the growing facets and surface disorder effects promoted by a high surface-to-volume ratio. Magnetic studies reveal a superparamagnetic behavior for the films grown at 570 and 730 K in agreement with the film morphology and degree of chemical order. The measurements at 10 K reveal the orientation of the easy axis of the magnetization lying basically in the film plane.     

Density functional theory study of selenium adsorption on Fe(1 1 0)

The adsorption of atomic Se on a Fe(1 1 0) surface is examined using the density functional theory (DFT). Selenium is adsorbed in high-symmetry adsorption sites: the -short and long-bridge, and atop sites at 1/2, 1/4, and 1 monolayer (ML) coverages. The long bridge (LB) site is found to be the most stable, followed by the short bridge (SB) and top sites (T). The following overlayer structures were examined, p(2 × 2), c(2 × 2), and p(1 × 1), which correspond to 1/4 ML, 1/2 ML, and 1 ML respectively. Adsorption energy is −5.23 eV at 1/4 ML. Se adsorption results in surface reconstruction, being more extensive for adsorption in the long bridge site at 1/2 ML, with vertical displacements between +8.63 and −6.69% -with regard to the original Fe position-, affecting the 1st and 2nd neighbours. The largest displacement in x or y-directions was determined to be 0.011, 0.030, and 0.021 Å for atop and bridge sites. Comparisons between Se-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the Se. At the long bridge site, the presence of Se causes a decrease in the surface Fe d-orbital density of states between 4 and 5 eV below Fermi level. The density of states present a contribution of Se states at −3.1 eV and −12.9 eV. stabilized after adsorption. The Fe-Fe overlap population decrease and a Fe-Se bond are formed at the expense of the metallic bond.     

Infrared reflection absorption spectroscopic study of the adsorption structures of dimethyl ether and methyl ethyl ether on Cu(1 1 1) and Ag(1 1 1)

Infrared reflection absorption (IRA) spectra measured for dimethyl ether (DME) adsorbed at 80 K on Cu(1 1 1) and Ag(1 1 1) give IR bands belonging only to the A₁ and B₂ species, indicating that the adsorbate takes on an orientation in which the C₂ axis bisecting the COC bond angle tilts away from the surface normal within the plane perpendicular to the substrates. The DFT method was applied to simulate the IRA spectra, indicating that the tilt angles of DME on Cu(1 1 1) and Ag(1 1 1) are about 50° and 55°, respectively, at submonolayer coverages. The results are in contrast to the case of DME on Cu(1 1 0) and Ag(1 1 0), where the C₂ axis is perpendicular to the substrates [T. Kiyohara et al., J. Phys. Chem. A 106 (2002) 3469]. Methyl ethyl ether (MEE) adsorbed at 80 K on Cu(1 1 1) gives IRA bands mainly ascribable to the gauche (G) form, whereas the IRA spectra measured for MEE on Ag(1 1 1) are characterized by the trans (T) form. The rotational isomers are identical with those on Cu(1 1 0) and Ag(1 1 0); i.e., MEE on Cu(1 1 0) takes the G form and the adsorbate on Ag(1 1 0) the T form [T. Kiyohara et al., J. Phys. Chem. B 107 (2003) 5008]. The simulation of the IRA spectra indicated that (i) the G form adsorbate on Cu(1 1 1) takes an orientation, in which the axis bisecting the COC bond angle tilts away from the surface normal by ca. 30° within the plane perpendicular to the surface to make the CH₃-CH₂ bond almost parallel to the surface, and (ii) the T form adsorbate on Ag(1 1 1) takes an orientation, in which the bisecting axis tilts away by ca. 60° from the surface normal within the perpendicular plane. Comparison of these adsorption structures of MEE on the (1 1 1) substrates with those of MEE on Cu(1 1 0) and Ag(1 1 0) indicates that the structures are mainly determined by a coordination interaction of the oxygen atom to the surface metals and an attractive van der Waals interaction between the ethyl group of MEE and the substrate surfaces. The coordination interaction plays an important role on Cu(1 1 1) and Cu(1 1 0), which makes the adsorbate on the Cu substrates to take the orientations with the bisecting axis near parallel to the surface normal and to assume the G form in order to make the ethyl group parallel to the surface, which is favorable for the van der Waals interaction. In the case of MEE on the Ag substrates the attractive van der Waals interaction plays a dominant role, resulting in the T form which is more favorable for the interaction than the G form.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Fe monolayers on InAs(0 0 1): An in situ study of surface, interface and volume magnetic anisotropy

The magnetic anisotropy of epitaxial Fe films with thicknesses in the range of 2-142 monolayers (ML) grown on {4×2} reconstructed InAs(0 0 1) was investigated by in situ ferromagnetic resonance. The easy magnetization direction was found to be parallel to the -direction for Fe films below 4 ML, while it rotates by 45^° toward the -direction. It is observed that both surface-interface and volume contribution to the perpendicular anisotropy favor an easy axis perpendicular to the film plane. The cubic surface-interface anisotropy is relatively large with easy axes along -directions in contrast to the volume contribution which favors easy axes along the -directions. The volume contribution is found to be larger than the Fe bulk cubic anisotropy. A thickness independent uniaxial anisotropy has been found in films with a thickness of 2 up to 142 ML.     

Fabrication of nanostructures by selective growth of C60 and Si on Si(1 1 1) substrate

Using two types of selective growth, selective C₆₀ growth and selective Si growth, on a common Si(1 1 1) substrate, an array of C₆₀ nanoribbons with controlled values of width and thickness is fabricated. On a surface that has Si(1 1 1)√3 × √3R30°-Ag (referred to as Si(1 1 1)√3-Ag hereafter) and bare Si(1 1 1) regions at the same time, the preferential growth of C₆₀ multilayered film is recognized on the Si(1 1 1)√3-Ag region. The growth of Si selectively occurs on a bare Si(1 1 1) region if the substrate surface has C₆₀-adsorbed and bare Si(1 1 1) regions at the same time. As a demonstration of the use of these selective growths, we fabricate an array of well-isolated C₆₀ nanoribbons, which show a well-ordered molecular arrangement and have sizes of about 40 nm in widths and 3-4 nm in thicknesses.     

Li ion neutralization on Ag layers grown on Cu(1 1 1)

Electron transfer processes in the neutralization of Li⁺ ions on Ag layers grown on Cu(1 1 1) are investigated in quest of quantum confinement effects. Neutralization probabilities in the scattering of Li⁺ for incident ion energies in the 300 eV to 2 keV range are reported for Ag coverages ranging from 0.15 ML to 5 ML. Results are compared to those for Ag(1 1 1) and Cu(1 1 1) surfaces of bulk crystals. Although existing studies of the characteristics of Ag layers on Cu(1 1 1) indicate significant differences in electronic structure as a function of film thickness, the electron transfer probabilities we measure are found to be very close to those for bulk Ag(1 1 1). These results are commented on the basis of existing models and earlier studies of Li ion neutralization on various metals.     

Epitaxial growth of Bi thin films on Ge(1 1 1)

We investigated Bi thin film growth on Ge(1 1 1) by using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). In the submonolayer regime, adsorbed Bi atoms form patches of the (2×1) structure. However, the structure does not grow to a long-range order. Following the formation of a (1×1) monolayer (ML) film, two-dimensional (1 1 0)-orientated Bi islands grow. The film orientation changes from (1 1 0) to (1 1 1) at 6-10 ML. The (1 1 0)-oriented Bi film shows a six-domain LEED pattern with missing spots, associated with a glide-line symmetry. The hexagonal (1 1 1) film at 14 ML has a lattice constant 2% smaller than bulk Bi(1 1 1).     

Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.     

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO3 substrates

The (1 0 0) SrTiO₃ substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO₃. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

c-C5H5 on a Ni(1 1 1) surface: Theoretical study of the adsorption, electronic structure and bonding

In the present work the ASED-MO method is applied to study the adsorption of cyclopentadienyl anion on a Ni(1 1 1) surface. The adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically the most favourable. The aromatic ring remains almost flat, the H atoms are tilted 17° away from the metal surface. We modelled the metal surface by a two-dimensional slab of finite thickness, with an overlayer of c-C₅H₅⁻, one c-C₅H₅⁻ per nine surface Ni atoms. The c-C₅H₅⁻ molecule is attached to the surface with its five C atoms bonding mainly with three Ni atoms. The NiNi bond in the underlying surface and the CC bonds of c-C₅H₅⁻ are weakened upon adsorption. We found that the band of Ni 5dz² orbitals plays an important role in the bonding between c-C₅H₅⁻ and the surface, as do the Ni 6s and 6p_z bands.