Leed measurements of Fe epitaxially grown on Cu(100)

Iron was epitaxially grown on a Cu(100) surface. Low energy electron diffraction (LEED) intensity versus energy curves were recorded for 1 and 10 layers of iron on Cu(100) at room temperature. A full dynamical analysis was performed using the renormalized forward scattering perturbation method. The surface Debye temperatures were determined to be 233 K for 1 ML Fe and 380 K for 10 layers of Fe. The value obtained for fcc iron was 550 K. A multiple relaxation approach was employed in analyzing the experimental data. The estimated interlayer spacings for the first and second layers were 1.78±0.02 Å (first) and 1.81±0.02 Å (second) for 1 ML Fe, and 1.81±0.02 Å (first) and 1.78±0.02 Å (second) for 10 layers of Fe on Cu(100). Auger electron spectroscopy was used to determine the thickness of the Fe films, and the LEED measurements indicate approximately a layer-by-layer growth until about 17 layers at room temperature. At higher temperatures there is evidence of iron diffusion or copper surface segregation.     

The superstructures of the clean Pt(100) and Ir(100) surfaces

The clean and reconstructed surfaces of Pt(100) and Ir(100) were investigated by low energy electron diffraction (LEED). It is shown that two superstructures can be observed in the case of platinum. The structure Pt(100)-hex, which is commonly called Pt(100)-(5 × 20), transforms to Pt(100)-hex-R0.7° above 1100 K. It is shown that this stable phase differs from the first one by a slight rotation of the hexagonal surface layer by 0.7°. For Ir(100) only the well known (1 × 5) superstructure is observed without any rotation of the outer layer. The rotation angle of 0.7° for platinum and the stability of the unrelated structure for iridium can be interpreted by simple calculations of the coordination of surface atoms with those of the second layer. The method assumes that the surface layer is of ideal hexagonal structure in the case of platinum and nearly hexagonal in the case of iridium. The results are in good agreement with the experiment.     

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3（001） and polar（111） surfaces

The influence of surface polarity on the structural properties of BiFeO3 （BFO） thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 （STO） （100） and polar （111） surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar （001） or polar （111） surfaces perform very differently. BFO/STO （001）is a fully strained tetragonal phase with orientation relationship （001）[100]BFOII（001）[100]STO, while BFO/STO （111） is a rhombohedral phase. BFO/STO （111） has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO （001） and STO （111） are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111)

We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moiré cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.     

Local atomic structure of ultra-thin Fe films grown on Cu(100)

Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering. PACS 68.55.-a; 64.70.-p; 81.30.-t     

Variable-temperature micromagnetic study of epitaxially grown MnAs films on GaAs(001)

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs layers on GaAs(001). In MnAs, a structural and magnetic phase transition occurs at T_c40 °C from the hexagonal, ferromagnetic -phase below T_c to the orthorhombic, paramagnetic -phase above T_c. In the investigated MnAs-GaAs system, both phases coexist over a temperature range of 30 °C below T_c due to the involved strain. Using MFM we are able to distinguish between the ferromagnetic and the paramagnetic phases by measuring topographic and magnetic contrast of the same sample area. For VT-MFM studies, we have employed a temperature stage that allows heating and cooling in a controlled atmosphere with small thermal drift (for this system, the temperature was varied from below 0 °C to above 45 °C). The ratio of the ferromagnetic to the paramagnetic phases shows a temperature hysteresis, i.e. the ratio is dependent on whether the sample was heated or cooled to reach the measurement temperature. Detailed studies of the domains and their arrangements over the hysteretic temperature cycle are shown and compared. Mainly three different domain types are found that are dominant in their respective temperature ranges. PACS 68.37.Rt; 68.35.Rh; 75.70.-i     

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O₂ was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O₂, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.     

The critical layer number of epitaxially grown Cu and Ni films with strained structure

The thickness dependent structural transition of epitaxially grown thin films from a tetragonal structure to the corresponding bulk structure is thermodynamically considered. It is found that there exists a competition between elastic energy of the tetragonal structure and film–substrate interface energy. Equilibrium between these energies is present at a critical layer number n_c. The predictions for n_c are in agreement with the experimental results of some different metallic films deposited on fcc metallic substrates.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

The condensation of gold onto tantalum (100) single crystal surfaces: LEED,AES analysis

The condensation of gold onto clean and contaminated, single crystal, tantalum (100) surfaces has been followed by using LEED and AES. On a contaminated surface gold condenses as crystallites in a (211) surface orientation with some degree of preferred, azimuthal orientation. On a clean surface gold condenses in an ordered overlayer. Up to approximately $\text{3}\text{4}$ monolayer the structure conforms to the (1 × 1) tantalum surface. Beyond this, the observed LEED structure may be interpreted initially in terms of a TaAu overlayer made up of 90° rotated domains with (001)TaAu//(100)Ta and 〈 10 〉 TaAu// 〈 11 〉 Ta, and then in terms of a gold overlayer in a “distorted (111)” orientation. Annealing of these gold films always results in the formation of a (1 × 1) TaAu overlayer of small crystallite size.     

The structure of the Pt(100) surface

The atomic arrangement in the first two layers of the clean Pt(100) surface is described from a Fourier analysis of low energy electron diffraction (LEED) intensities. The Fourier transform and sensible platinum-platinum bond lengths require that the top layer is corrugated, the corrugation “step” being approximately 0.4 Å. The structure of the first layer is partly described by a hcp rearrangement from the bulk fcc structure. The rearrangement is related to geometrical and electronic properties of the metals. Some qualitative and quantitative comments are made on the general value of Fourier methods in surface crystallography.     

Dichroism in angular resolved VUV-photoemission from the (0001) surfaces of thin Gd and Nd films epitaxially grown on W(110)

We present investigations of the electronic and magnetic structure of the Rare Earth valence states. In particular, we have examined ultra thin films of the rare earth metals gadolinium and neodymium epitaxially grown on tungsten (110). Various experiments on dichroism in angular resolved photoemission have been performed using circularly as well as linearly polarised light in the VUV-range with photon energies below 40 eV. A special emphasis was placed on the investigation of the surface state, which was observed for both Gd and Nd. A very small magnetic splitting of about 25 meV was observed for the surface state of ferromagnetic Gd. A magnetic ordering of a Nd-monolayer on a remanently magnetised Fe-film is observed. Large dichroism effects are found for the surface state as well as the valence bands of paramagnetic Nd. In the latter case, these are used to determine the dispersion of the valence bands. Different numerical approaches are presented, one based on atomic photoionisation theory, another is based on a one-step model of solid state photoemission. Atomic photoionisation theory is used together with three-step calculations to explain the non-magnetic circular dichroism observed in the Gd 4f emission. The capability of dichroism experiments for resolving details of the electronic structure and for sensitive tests of photoemission calculations is demonstrated. Received 21 September 1998     

High coverage states of oxygen adsorbed on Pt(100) and Pt(111) surfaces

Oxygen adsorption on the Pt(100) and Pt(111) surfaces was investigated using X-ray photo-emission and thermal desorption spectroscopies. Low pressure (ca. 10^?5 Pa) oxygen dosing at near ambient crystal temperature resulted in the formation of dissociated adsorbed species at saturation coverages of nominally 0.2–0.25 monolayer on both surfaces. The combination of higher pressure (ca. 10^?3 Pa) and higher surface temperature (570 K) dosing produced a three to five times higher saturation coverage than the low pressure dosing. The effect of dosing condition on the saturation coverage appears to reconcile apparent discrepancies for the Pt(100) surface in the literature. Characterization by XPS of the higher coverage state for oxygen showed that it is in the same chemical state as the oxygen adsorbed at very low coverage. Angle-resolved XPS has shown that in all cases the oxygen appears to reside on the surface with no significant penetration of oxygen into the bulk, as would be characteristic of oxidation. However, some penetration on the surface by oxygen, such as by a place-exchange type restructuring of the first two atomic layers, cannot be entirely ruled out.     

Copper monolayer formation on platinum single crystal surfaces: Optical and electrochemical studies

The formation of a Cu monolayer on Pt(100), (110) and (111) was investigated by optical and electrochemical techniques. The adsorption isotherms as obtained by cyclic current-potential curves clearly show that the monolayer is deposited in various steps at underpotentials. Differential reflectance spectroscopy at normal incidence was used to detect structural changes of the adsorbate as the coverage increases. For Cu on Pt(110) a pronounced anisotropy in $\text{ΔR}\text{R}$was observed as the electric field vector of the linearly polarized light was rotated. From these measurements it was deduced that Cu in the submonolayer range is deposited onto Pt(110) in rows along the [11̄0] direction of the substrate. No such anisotropy was found for Cu on Pt(100) and Pt(111) and for surface oxide formation on all three low index faces of Pt. The spectral dependence of the normalized reflectance change,$\text{ΔR}\text{R}$, for the Cu monolayer on Pt(hkl) is shown and discussed.