Binary Alloy Surfaces Research Articles

Compensation Temperatures of Ferrimagnetic Ising Thin Films with Binary Alloy Surface Layers

Department of Natural Science Informatics, School of Informatics and Sciences,Nagoya University, Chikusa-ku, 464-01 Nagoya, Japan(Received December 2, 1996)Ferrimagnetic Ising thin films with binary alloy type surface layers are studied within the frame-work of standard mean-field theory. The particular emphasis is given on the investigation of (mul-ti)compensation points that may appear in the system.

First-principles approaches to surface segregation

Abstract The progress in the first-principles determination of the thermal equilibrium concentration profile at the surface of a random binary alloy is reviewed. After a brief introduction to the (single-site) coherent-potential approximation used to model concentrated alloys, and to the elements of density functional theory necessary for the ab initio treatment of such systems, three self-consistent Green function techniques for calculating ground-state properties of random alloy surfaces, namely the layer Korringa-Kohn-Rostoker (KKR), the screened KKR and the tight-binding linear muffin-tin orbital (LMTO) method are presented, with particular emphasis on the relationship between the latter two approaches. Although each of these methods is capable of yielding the internal energy of the system for any given profile, a ‘brute force’ search for the optimum configuration is unrealistic, in view of the large number of possibilities available. Instead, one uses the results of the above self-consistent calculations to construct an effective Ising model, which can then be treated by the standard methods of classical statistical mechanics. The two schemes currently used to generate the parameters of this effective Ising model are presented. The first, based on the treatment of Connolly and Williams for the study of phase stability in bulk alloys, consists in fitting the model parameters to the calculated internal energies of a sufficiently large number of composition profiles. The second, the generalized perturbation method (GPM), also first proposed in the context of bulk phase diagrams, derives them from the electronic structure of a reference system, relative to which the free energy of the actual configuration is expanded in powers of local concentration fluctuations. A detailed derivation of the GPM with respect to an arbitrary composition profile is given, in both the multiple-scattering and the tight-binding LMTO formalism, and recent developments of the method are discussed. Finally, the results of applications of the two approaches by different groups are presented.

Substitutional adsorption of Li on Al: The structure of the Al(111)-( sqrt 3 x sqrt 3)R30 degrees-Li phase.

The atomic geometry of the Al(111)-$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase formed by adsorption of one-third monolayer Li on Al(111) at room temperature has been determined by analysis of extensive low-energy electron-diffraction measurements. The structure is found to be a binary surface alloy, in which Li atoms occupy sixfold coordinate substitutional sites formed by displacing every third Al atom in the first layer of the substrate, similar to the structures found previously for the corresponding $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phases formed by adsorption of Na, K, and Rb.

PHASE TRANSITIONS ON ALLOY SURFACES

Various kinds of quasi-two-dimensional order-disorder phase transitions on binary alloy surfaces are discussed on the basis of the lattice gas model with appropriate approximations. The importance of surface segregation is pointed out in understanding phase transitions on alloy surfaces. Ordered structures localized on the surfaces are found above the bulk transition temperature, and a possibility of finding surface ordered structures with a different symmetry from the bulk ordered one is discussed, too. As for both ordering and segregating alloys, semi-infinite systems with surfaces are discussed above and below the bulk transition temperatures; in particular, the relation of the surface phase transitions to the bulk ones is focused on.

Angle Resolved XPS Study of Thin Oxide Layers Formed on the Surface of Iron-Chromium Binary Alloys Exposed to Air.

Recent angle resolved X-ray photoelectron spectroscopy (AR-XPS) on the surface of iron-low chromium alloys due to air exposure at room temperature after sputter cleaning has suggested that an oxide layer of nanometer order of magnitude of thickness formed on the alloys, and a contaminated overlayer covered the oxide layer. In the present work, we have subsequently studied the surface of iron-chromium alloys containing chromium more than 50 mass%, in order to confirm the previous remarks for these thin oxide layers on a wider base. Estimation of the effective thickness of these layers by coupling with a model indicates that the thickness of the oxide layer on the surface decreases with increasing chromium concentration up to 50 mass%, and is kept almost unchanged with higher chromium. It is, however, noted that the intensity of metallic peaks in Cr 2p XPS spectra still increases with chromium concentration more than 50 mass%, and the position of O is XPS spectra shifts as the chromium concentration increases. These facts suggest that the characteristic features of the oxide layers are affected, more or less, by the bulk chromium composition.

Surface structure determinations using X-ray absorption spectroscopy

Download options Please wait... Article information DOI https://doi.org/10.1039/FT9969201653 Article type Paper Download Citation J. Chem. Soc., Faraday Trans., 1996,92, 1653-1667 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Surface structure determinations using X-ray absorption spectroscopy J. Haase, J. Chem. Soc., Faraday Trans., 1996, 92, 1653 DOI: 10.1039/FT9969201653 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author J. Haase

Glow Discharge Optical Emission Spectrometry on the Formation of Thin Oxide Layers on the Iron Binary Alloy Surface.

Glow discharge optical emission spectrometry (GDOS) has been used for characterizing thin oxide layers formed on iron-base binary alloys with aluminum, silicon, phosphorus, chromium, manganese, nickel and molybdenum by heating in air at 773 and 873 K. We focused on the enrichment of alloying elements in the thin oxide layers and substrates as well as the effect of alloying elements on the thickness of the oxide layers. It was found in the quantitative depth profiles that aluminum, silicon, phosphorus, chromium and nickel are enriched at the oxide/metal interface, while manganese is distributed to the oxide layer and molybdenum is observed neither at the oxide/metal interface nor in the oxide layer. It is also suggested that aluminum, silicon, chromium and molybdenum have a significant influence on the reduction in the thickness of the oxide layers, and the present results are in good agreement with structural data obtained by X-ray diffraction.

Localized compositional depth profiles in near surface of Cu-50 wt% Ag alloy

The segregation-induced depth profiles of the constituent in a binary alloy system under irradiation with energetic particles are interesting phenomena and have been extensively studied by many authors in the last two decades [1-3], because of their importance for various technological applications and many areas of material science. In general, segregation-induced or bombardment-induced depth profiles are often studied by use of ion sputtering in combination with surface analytical methods such as secondary ion mass spectrometer (SIMS) and auger electron spectrometer (AES). However, these analytical results for depth profile, i.e. the composition distribution in the near-surface region, are average values since the beam spot of the probe in SIMS or AES apparatus is very large (>/50/lm), one to two orders of magnitude beyond the size of a micro-phase region on the surface of the two-phase binary alloy as in previous studies [4, 5]. Therefore, a rational hypothesis is that research on the depth-profile of near-surface composition must involve a study of the depth profile changes combined with micro-phase region changes. Recently, depth-profile measurements for Ar ÷ ion bombarded A170Sn30 alloy have shown significant variation between two different micro-phase regions [6]. At present, in order to investigate further the composition distribution in the near-surface region of a two-phase binary alloy, we chose a Cu-50 wt % Ag alloy as sample. This is a typical two-phase system with which to study the depth profile of elements in the near surface. We shall see that differences in the composition depth profile for different micro-phase regions are observed. The Cu-50 wt % Ag alloy sample was in the form of a disc of 15 mm diameter and i mm thickness. The sample surface was mechanically polished, and in the final step, 0.05/~m alumina was used to give an optically smooth surface with no protrusions observable by scanning electron microscopy (SEM) (x5000). The sample was bombarded in the electromagnetic isotope separator with normally incident Ar ÷ ions (27 keV). During the bombardmental experiment the vacuum was kept below 1.33 × 10 .4 Pa, and the Ar ÷ ion beam current density was kept below 3 #A/cm 2. In addition, the sample was mounted on a LN2 indirectly cooled copper backer, so the temperature within the beam spot (3 mm diameter) was estimated not to exceed 300 K. After ion bombardment the irradiated surface was exmained in a S-570 model SEM. A typical surface region is shown in Fig. 1.

Ion neutralization near a disordered binary alloy surface

Ion neutralization near the surface of a disordered binary alloy is investigated within the framework of the approximate many-level method. The disordered binary system is described by the one-dimensional tight-binding model and the coherent potential approximation. The results obtained show that the substrate's electronic structure and the concentration percentages of the two components play important roles in the resonance charge transfer.

Charge transfer during reflection of ions from a disordered binary alloy surface

The resonance charge transfer during the scattering of a positive ion from a disordered binary alloy surface is investigated within the framework of the approximate many-level method. The results obtained show that the substrate's electronic structure and the concentration percentages of the two components play important roles in the charge transfer process.

Determination of surface stoichiometry in polycrystalline alloys by a crystallographic electron attenuation model: Application to the Ce/Rh system

AbstractIn order to obtain quantitative surface compositional information, an earlier crystallographic electron attenuation model has been extended to treat polycrystalline alloys. Simple correction factors are obtained that yield large corrections to elemental Auger intensity ratios. The model has been applied to a binary surface alloy formed by heating cerium overlayers on a polycrystalline rhodium substrate. It is shown that an alloy film is formed whose thickness is proportional to the number of Ce layers initially deposited. The surface alloy is identified as Ce3Rh2, which corresponds to a known bulk phase.

Interaction properties of molecules with binary alloy surfaces

The adsorption properties of binary alloy surfaces may significantly deviate from those of the pure component metals. The physical origin of this deviation arises in a rather complex way from the distribution of both kinds of metal species and their interaction in the surface. In contrast to most other adsorption work on binary alloy surfaces we demonstrate and discuss here a clear-cut distinction between a pure geometrical ensemble effect and an electronic ligand effect on the adsorption of gas molecules by using an ordered alloy surface, namely Cu3Pt(111). Furthermore, we show the extent to which the chemical heterogeneity of the surface playing a role depends also on the nature of the adsorbate.

Geometrical and chemical restructuring of clean metal surfaces as retrieved by LEED

Quantitative structure determination of clean metal surfaces by LEED during the last three decades is reviewed. First, the development of experimental techniques necessary for the fast and reliable measurement of electron diffraction intensities is described. Emphasis is on the structure of geometrically reconstructed clean metals as well as on the monitoring of the corresponding structural phase transitions and their kinetics. The appearance of chemical reconstruction (segregation) in binary alloy surfaces is described as well.

Calculation of the chemisorption energy of H on a multilayer-segregated CuNi alloy

The chemisorption theory of Einstein and Schrieffer is used to calculate the chemisorption energy of hydrogen on a multilayer-segregated CuNi disordered binary alloy surface, in which the Green-function technique, based on the coherent-potential approximation, and the method of complex-energy integration are adopted. When the effect of chemisorption on the surface component is neglected, multilayer surface segregation of Cu leads to an increasein chemisorption energy ΔE as compared to the case of no surface segregation as well as monolayer segregation. In general cases, chemisorption-induced surface segregation can change the surface component and the chemisorption energy to varying degrees. The results show that the chemisorption energy depends sensitively on the adatom coverage θ, and decreases with increasing θ. The calculated result for θ = 0.41 agrees with the experiment of Takeuchi et al.

Novel surface segregation phenomena at the plasma/copper-silver alloy interface

Low-temperature glow discharges are an attractive means for engineering solid surfaces; this can occur via either chemical or physical processes. Applications are widespread and can be as diverse as the molecular rearrangement of polymers,l plasma polymerization,2 deposition of thin inorganic ~ o a t i n g , ~ and the hardening of metals by nitriding.4 A whole variety of plasma/solid interactions are believed to be present in these systems, including the interaction of radicals, atoms, and electronically excited molecules with the surface, neutralization, secondary electron emission, sputtering, ion-induced chemistry, electron-induced chemical reactions, and phot~chemistry.~ The major attribute of nonisothermal plasmas is that they are capable of generating a very reactive medium at room temperature, whereas high temperatures (typically 10 OOO's K) are necessary to achieve the same types of species via thermal ionization. Investigations related to the glow discharge treatment of metals are widespread in the l i terat~re.~?' However, there have been very few studies dedicated to nonisothermal plasma modification of binary alloy surfaces, in fact the first ever report concerning the plasma oxidation of a binary metal alloy system (Cu-A1) has only recently been published.8 In this article, a detailed account of the influence of various glow discharges (02, H2, and N2) on Cu-Ag eutectic alloy substrates is presented. Cu-Ag alloys are potential candidates for electrical switching applications, and therefore it is desirable to devise protective finis he^.^ The microstructure and chemical nature of the resulting surfaces have been evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, Ar+ ion depth profiling, and plasma emission spectroscopy.

Surface order in body-centered cubic alloys

Free (100)-surfaces of body-centered cubic binary alloys are studied in a parameter range where the bulk turns from the ordered B2-phase to the disordered A2-phase. A model is chosen that describes iron-aluminium alloys in a fairly realistic way. Mean field treatments and Monte Carlo investigations both show that under certain circumstances the surface remains ordered far above the bulk disordering temperatureTc, though the surface order parameter and the surface susceptibility exhibit a singularity atTc with critical exponents characteristic for the ordinary transition. One finds, that if the surface is nonstoechiometric and different layers are not equivalent with respect to perfect bulk ordering, this induces an effective ordering field at the surface. Finally surface potentials are introduced into the model. Realistic values are determined by mean field considerations from experimental data and their effect on the surface is discussed.

Self-consistent ordering energies and segregation profiles at binary-alloy surfaces.

We discuss the application of the direct configurational averaging method to the calculation of the thermodynamic properties of alloy surfaces. We analyze critically a number of approaches previously proposed to determine the parameters in different Ising models. We investigate the physical meaning of various interaction energies occurring in those models and compare our technique to phenomenological approaches. The formalism is applied in electronically self-consistent mean-field calculations of the segregation profile at the surface of Ni-Cu and Rh-Ti alloys at temperatures above the bulk disordering temperature.

The chemisorption energy of CO on a disordered binary alloy

The one-dimensional tight-binding model and the one-electron chemisorption theory are used to calculate the chemisorption energy of CO on a NiCu disordered binary alloy surface, in which the coherent-potential approximation and the technique of complex-energy-plane integration are adopted. The change of surface segregation due to chemisorption is also investigated. The chemisorption of the CO/NiCu system can change the surface component concentration drastically, thereby influencing the chemisorption energy. The result shows that the chemisorption of the system strongly constrains surface segregation, resulting in chemisorption properties more like those of CO/Ni rather than those of CO/Cu.

Determination of the growth mechanism of overlayers on solid surfaces: a method based on combined XPS and LEIS measurements

The conventional method for the determination of the growth mechanism of overlayers on solid surfaces is based on a plot of the AES signal versus the deposition time (“AS- t” method). However, the applicability of this method is not general and in some cases its results may be difficult to interpret. In this paper an alternative method to the AS- t one is discussed. This method is based on plotting XPS (or AES) data versus LEIS data for variable overlayer coverages. By means of numerical simulations it can be shown that the sensitivity of this method in the initial stages of overlayer growth is better than that of the conventional AS- t plots for the same signal-to-noise ratio. Furthermore, the method is applicable in cases where the AS- t plot cannot be used (e.g. when the overlayer segregates from the bulk). Experimental data for the growth of oxide layers on the surface of binary alloys are reported to show the practical applicability of the method.

(√3 × √3)R30° substitutionally ordered structure on fcc (111) surfaces of binary alloys

Order-disorder transitions in semi-infinite systems of binary alloys with the fcc (111) surface are investigated by use of the Bragg-Williams approximation above the bulk ordering temperature. An order-disorder transition is found from the disordered phase to the ordered phase with (√3 × √3)R30° symmetry in the absence of bulk long-range order. This structure is consistent with the surface ordered structure found on the fee (111) surfaces of α-CuAl alloys. For a large surface segregation energy parameter, a re-entrant surface order-disorder transition is found, as well as in cases with bcc (110) and fcc (100) surfaces.