Surface structure magnetic

Bohr, J., 1991, Surface Structures, Magnetic Structures, and Small Inclusions, as Studied by X-ray Diffiaction, Report Riso-R-585, 69pp. 

The many papers in this proceedings are partitioned into very abstruse theoretical analyses of structure and stability of quasicrystals on the one hand, and practical studies of surface structures, mechanical properties and potential applications. The subject shows signs of becoming as deeply divided between theorists and practical investigators, out of touch with each other, as magnetism became in the preceding century. 

Most microscopic theories of adsorption and desorption are based on the lattice gas model. One assumes that the surface of a sohd can be divided into two-dimensional cells, labelled i, for which one introduces microscopic variables Hi = 1 or 0, depending on whether cell i is occupied by an adsorbed gas particle or not. (The connection with magnetic systems is made by a transformation to spin variables cr, = 2n, — 1.) In its simplest form a lattice gas model is restricted to the submonolayer regime and to gas-solid systems in which the surface structure and the adsorption sites do not change as a function of coverage. To introduce the dynamics of the system one writes down a model Hamiltonian which, for the simplest system of a one-component adsorbate with one adsorption site per unit cell, is... 

Girard, C., Weeber, J.-C., Dereux, A., Martin, O. J. F. and Goudormet, J.-P. (1997) Optical magnetic near-field intensities around nanometer-scale surface structures. Phys. Rev. B, 55, 16487—16497. 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

In electron microscopy, generally intense electron beams are used which can severely damage the surface (however, at high energies the electron-atom cross sections become smaller). Often, large magnetic fields are also present that could affect the surface structure. 

The quadrupole splitting (as shown in Section I, C, 3) and the relaxation of the magnetic hyperfine interaction (as will be seen presently) are also related closely to the symmetry, and thus the structure of the surface. Determination of the surface structure using these parameters then follows the pattern outlined above for the recoil-free fraction. For clarification, this pattern will be illustrated in the following section for the magnetic relaxation. 

To realize second-order anisotropy, the atomic environment of the transition-metal atoms must have a sufficiently low symmetry [49, 62-65]. Figure 6 illustrates that this is often, but not always, the case for surface atoms. Magnetic surface anisotropy, first analyzed by Neel [62], is important in complicated structures and morphologies such as ultrathin transition-metal films [66], multilayers [67], rough surfaces [65], small... 

Reflection at a surface of a beam of linearly polarized photons alters the direction and amplitude of the electric and magnetic vectors. It is these differences between incident and reflected beams that give information concerning surface structure, as they depend on the interaction of the beam with the electronic distribution and with the associated local electric and magnetic fields on the surface. The phase and amplitude change for the vectors is different for the component parallel to the plane of incidence than for the component perpendicular to it. The result is a vector that follows a spiral during its propagation, and is referred to as elliptically polarized, Fig. 12.2. A deeper treatment of these optical properties can be found in Ref. 9. Such measurements are referred to as specular reflectance. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

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