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.     

Magnetic anisotropy and X-ray magnetic circular dichroism of MnNi surface alloy: Ab initio studies

The magnetic properties of MnNi surface alloy grown on fcc Co(0 0 1) surface have been explored through the full potential linearized augmented plane wave (FLAPW) method. It has been found that one monolayer (ML) of Mn_0.5Ni_0.5 alloy on Co(0 0 1) surface has a ferromagnetic (FM) ground state. We suggest that the direct exchange coupling between Mn and Ni plays an essential role for the ferromagnetism. The vertical position of Mn is higher by 0.26 Å than that of Ni, i.e., the surface has a buckling geometry. In contrast to pure fcc Co(0 0 1) film which has in-plane magnetization, we have obtained that the magnetocrystalline anisotropy of MnNi/Co(0 0 1) due to spin-orbit interaction is perpendicular to the film surface and the magnetic anisotropy energy is 146 μeV/cell. In addition, the theoretically calculated X-ray magnetic circular dichroism (XMCD) is presented.     

Spin-engineering in the Co75Fe25/Cu(1 1 0) system

We present results on the growth and magnetic anisotropies of Co₇₅Fe₂₅ films grown on a Cu(1 1 0) single crystal. Angular dependent MOKE measurements show a thickness dependent, in-plane rotation of the easy axis of magnetisation of up to 60° from the [0 0 1] direction (towards [−1 1 0]). For a film thickness of 5 ML, just greater than that required for the onset of ferromagnetism, uniaxial anisotropy is observed with the easy axis along the [0 0 1] direction. As the film thickness increases this is seen to rotate in-plane towards the [−1 1 0] direction as the contribution from the cubic anisotropy constant grows. At a film thickness of 9 ML there is predominantly cubic anisotropy and at 10 ML the easy axis is rotated to 150° with respect to the [1 −1 0] axis, where it is stabilised.     

Surface structures of atomic hydrogen adsorbed on Cu(1 1 1) surface studied by density-functional-theory calculations

The surface structures of atomic hydrogen adsorbed on Cu(1 1 1) surface have been studied theoretically by using density-functional-theory calculations. The results show that 0.67 ML hydrogen adsorbed on threefold hollow sites forming (3 × 1) superstructure and 0.5 ML hydrogen adsorbed on threefold hollow sites forming (2 × 2)-2H superstructure with central H at trigonal sites induce most significant substrate reconstructions and that fits best the observed (3 × 3) and (2 × 2) LEED patterns, respectively. The potential energies for the hydrogen in these two models are also lower than those in other competing models. Accordingly, these two models are the most preferable structures for 0.5-0.67 ML and 0.3-0.5 ML hydrogen adsorbed on the Cu(1 1 1) surface. In addition, the calculations also suggest that the lateral H-H interaction is not of simple repulsion and how the adsorbed hydrogen is arrayed is important in modifying the adsorption energy.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

Density-functional study of the CO adsorption on ferromagnetic Co(0 0 0 1) and Co(1 1 1) surfaces

The regular CO overlayers at coverage θ = 1/3 adsorbed on the (0 0 0 1) surface of hcp Co and (1 1 1) surface of fcc Co are studied by first-principles density-functional theory with the exchange-correlation component in the PBE form. Adsorption in atop, bridge, and three-fold hcp or fcc position are considered. The adsorption energies, CO stretching frequencies, geometry, work function, and local magnetic moments are studied, and, when possible, compared with experimental or theoretical data. Particularly, we show that the recently proposed correction to adsorption energy of CO prefers correctly the atop adsorption site, whereas the remaining sites are almost degenerate in energy. The CO molecule lowers magnetization on neighbouring Co atoms, and the effect decreases with the adsorption site coordination. We show, however, that this trend is not the result of the different C-Co separation at different adsorption sites. A very small magnetic moment appears on CO that couples antiferromagnetically to Co. Most results are very similar for the Co(0 0 0 1) and Co(1 1 1) surfaces.     

A first-principles study of C + O reaction on NiCo(1 1 1) surface

A density-functional theory method has been conducted to investigate the association of C + O on (1 1 1) facets of ordered NiCo alloy and the results have been compared with those obtained on pure Ni(1 1 1) surface. In reaction of C + O, the favorable reaction path is that C adsorbed on HCP-1 site moves to the nearest Ni-Co bridge site, and associates with O migrating from FCC-1 site to result in CO adsorbed on the bridge site of Ni-Co. However, the reaction barrier is higher by 0.35 eV than that on pure Ni(1 1 1), which indicates that the incorporation of Co into the Ni crystal is not in favor of the reaction of carbon delimination.     

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.     

A study of Ga layers on Si(1 0 0)-(2 × 1) by SR-PES: Influence of adsorbed water

Results for deposition and thermal annealing of gallium on the Si(1 0 0)-(2 × 1) surface achieved by synchrotron radiation photoelectron spectroscopy (SR-PES) and low energy electron diffraction (LEED) are presented. In addition to deposition of Ga on a clean surface, the influence of water adsorption on the arrangement of gallium atoms was also studied. The results on Ga deposition at a higher temperature (490 °C) are consistent with a Ga ad-dimer model showing equivalent bond arrangement of all Ga atoms for coverages up to 0.5 ML. The deposition onto a surface with adsorbed water at room temperature led to a disordered gallium growth. In this case gallium atoms bind to silicon dimers already binding fragments of adsorbed water. A subsequent annealing of these layers leads to a surface structure similar to the Ga-(2 × 2), however, it is less ordered, probably due to the presence of silicon oxides formed from water fragments.     

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).     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Ge(1 1 1) films

Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced.     

(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.     

Energetics of oxygen embedment into unreconstructed and reconstructed Cu(1 0 0) surfaces: Density functional theory calculations

Atomic oxygen embedment into a Cu(1 0 0) surface is studied by density functional theory calculation and the nudged elastic band method. As the oxygen coverage increases on the unreconstructed surface from 0.25 monolayer (ML) to 0.75 ML, the energy barrier for oxygen embedment decreases and an energetically favorable sub-surface site is found at 0.75 ML coverage. At a fixed oxygen coverage of 0.5 ML, the oxygen embedment energetics vary with the surface morphology and the embedment is found to be more probable for reconstructed structures compared to the bare surface. On the missing-row reconstructed surface, we find that the energy barrier for atomic oxygen embedment is smaller through the missing-row compared to other paths, suggesting a mechanism for the formation of sub-surface oxygen structures that are consistent with a recent experiment. The energy barrier for sub-surface oxygen diffusion is predicted to be less than that for on-surface diffusion.     

Coverage-dependent absorption of atomic hydrogen into the sub-surface of Cu(1 1 1) studied by density-functional-theory calculations

With density-functional-theory calculations, we have studied coverage-dependent absorption of H atoms into the sub-surface below a face-centered-cubic (fcc) hollow site of Cu(1 1 1). Both frozen and relaxed surface lattices were considered when the atomic H migrated from the surface to the sub-surface. The potential energy curve for the absorbing H shows that the surface site is in general favored over the sub-surface site, and this trend varies little with the H coverage (0.11-0.67 ML). If the hexagonal-close-packed (hcp) hollow sites immediately vicinal to the absorbing H are pre-adsorbed with other H atoms, the surface adsorption potential is greatly increased, because of the repulsive H-H interaction, to a value near, or even greater than, the sub-surface absorption potential; when two or three H atoms (on the hcp sites) are beside the absorbing H, the energy barrier for the sub-surface absorption is decreased, whereas that for diffusion from the sub-surface to the surface is enhanced. These results indicate that, on an H-saturated Cu(1 1 1) surface (0.67 ML), the sub-surface sites below the fcc sites with two or three neighboring H atoms can trap the sub-surface H.     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films have been studied. In ultrahigh vacuum environment, Auger electron spectroscopy (AES) analysis shows that no oxygen adsorption occurs on Si(1 1 1)-7×7 surface and Co-Si compound interfaces. As the thickness of Co films increases above 5 monolayers (ML), pure cobalt islands form on the surface and the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. From the results of slight chemical shift and depth profiling measurements, the oxygen is weakly adsorbed on the topmost layer of 15 ML Co/Si(1 1 1) films. The adsorbed oxygen influences the electronic density of states of Co and leads to the changes of the magnetic properties. The appearance of the O/Co interface could modify the stress anisotropy, as a result, the coercivity of ultrathin Co/Si(1 1 1) films are enhanced. As an example for 15 ML Co/Si(1 1 1), the coercivity increases from 140 to 360 Oe with 5000 Langmuir of oxygen exposure.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

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.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.