X-Ray Scattering Studies of the SiO2_2/Si(001) Interfacial Structure

X‐ray scattering has been utilized in a study of the SiO$_2$/Si(001) interfacial structure. Scattering data provide evidence for a low coverage 2×1 epitaxial structure at the SiO$_2$/Si interface for dry oxides grown on highly ordered Si surfaces at room temperature. The observed scattering is consistent with distorted dimer models of the interfacial structure. Thermal annealing substantially reduces the order of the 2×1 structure while prolonged exposure to humid air almost eliminates the 2×1 symmetry scattering. These findings suggest that the observed 2×1 order is associated with a metastable, intermediate state of the dry oxidation process.Engineering and Applied Science

X‐Ray Reflectivity Studies of SiO2_2/Si(001)

X‐ray reflectivity has been utilized in a study of the SiO$_2$/Si interfacial structure for dry oxides grown at room temperature on highly ordered Si(001) surfaces. Scattering near ($\pm$110) demonstrates the Si lattice termination of the wafers studied is characterized by a highly ordered array of terraces separated by monoatomic steps. Specular reflectivity data indicate the ‘‘native’’ dry oxide thickness is approximately 5 Å with a 1‐Å vacuum interface width. Residual laminar order in the oxide electron density along the oxide/Si interfacial normal decays exponentially from the oxide/Si interface with a $\sim$2.7‐Å decay length.Engineering and Applied Science

X-Ray Grazing Incidence Diffraction from Alkylsiloxane Monolayers on Silicon Wafers

X‐ray reflection (both specular and off‐specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers ($n‐C_{18}H_{37}SiO_{1.5}$) formed by self‐assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 Å. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n‐alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal‐expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational correlation lengths and integrated peak areas are significantly reduced relative to complete monolayers, the peak positions of the incomplete monolayers are comparable to those of complete monolayers. Given the lower average areal density of incomplete monolayers, this finding implies that incomplete monolayers are inhomogeneous.Engineering and Applied Science

Liquid-Vapor Density Profile of Helium: An X-Ray Study

The average liquid-vapor density profiles 〈ρ(z)〉 of thick $^4$He films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2$\pm$1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6$\pm^{1}_{2}$ Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.Engineering and Applied Science

X-Ray Specular-Reflectivity Study of the Liquid-Vapor Density Profile of 4^4He

The helium liquid-vapor interfacial density profile has been measured with x-ray specular reflectivity. Measurements were performed on thick films of helium adsorbed onto atomically flat silicon substrates. Both the amplitude and the phase of the complex scattering amplitude of the helium-vapor interface were obtained from measured interference between reflections from the helium liquid-vapor interface and the silicon-helium interface. Films whose thickness varied from 15 Å to 220 Å over a range of temperatures from 1.1 K to 3.0 K were studied. At T=1.13 K the film thickness is 215 Å and the interfacial width is 9.2 $\pm$ 1 Å. No significant variation was seen in the interfacial widths measured at temperatures between 1.1 K and 1.8 K. Analysis of these measurements indicates that the interface is asymmetric, with the decay of the density into the vapor having the sharper falloff. The zero-K interfacial width extrapolated from the finite-temperature measurements with a quantized capillary-wave theory is 7.6$\pm^{1}_{2}$ Å.Engineering and Applied Science

Observation of isotropic giant magnetoresistance in paramagnetic Au80 Fe20

Magnetization and magnetoresistance were measured at room temperature and above on Au80Fe20 platelets and ribbons obtained by solid-state quenching and melt spinning. The as-quenched samples contain a solid solution of Fe in Au and exhibit a paramagnetic (Curie-Weiss) behavior in the considered temperature range; magnetic data indicate very short-ranged magnetic correlation among adjacent spins, enhanced by local composition fluctuations. The solid solution is very stable. Only a very limited fraction (never exceeding 1%) of nanometer-sized, bcc Fe particles appears after long-time isothermal anneals at suitable temperatures. A negative magnetoresistance was observed at room temperature in all examined samples. The observed effect is anhysteretic, isotropic, and quadratically dependent on magnetic field H and magnetization M. The signal scales with M rather than with H, indicating that it depends on the field-induced magnetic order of the Fe moments, as it does for conventional giant magnetoresistance in granular magnetic systems. This effect derives from spin-dependent scattering of conduction electrons from single Fe spins or very small Fe clusters. The scattering centers are almost uncorrelated at a distance of the order of the electronic mean free path (of the order of 1.5 nm, or a few atomic spacings, at RT

Dipolar interactions and anisotropic magnetoresistance in metallic granular systems

We revisit the theory of magnetoresistance for a system of nanoscopic magnetic granules in metallic matrix. Using a simple model for the spin dependent perturbation potential of the granules, we solve Boltzmann equation for the spin dependent components of the non equilibrium electronic distribution function. For typical values of the geometric parameters in granular systems, we find a peculiar structure of the distribution function of conduction electrons, which is at variance with the two-current model of conduction in inhomogeneous systems. Our treatment explicitly includes the effects of dipolar correlations yielding a magnetoresistance ratio which contains, in addition to the term proportional to the square of uniform magnetization (), a weak anisotropic contribution depending on the angle between electric and magnetic fields, and arising from the anisotropic character of dipolar interactions.Comment: 9 pages, 2 figures, accepted in PR

Complete Wetting of a Rough Surface: An X-Ray Study

The evolution of the surface structure of a wetting film on a rough surface as a function of the film thickness has been studied by x-ray specular reflection and surface diffusion scattering. For thin films ($\lesssim$60 Å) the liquid surface is characterized by static undulations induced by the roughness of the substrate; however, with increasing film thickness the structure is dominated by thermally induced capillary waves. The data are quantitatively described by a model with exclusively van der Waals liquid-substrate interactions.Engineering and Applied Science

X-Ray Specular Reflectivity of the 4He^4He Liquid-Vapor Interface

X-ray specular reflectivity measurements of a saturated film of helium absorbed onto an atomically flat silicon substrate have been made at several temperatures. The thickness of the film has been determined and some preliminary data on the structure of the $^4He$ liquid-vapor interface are reported.Engineering and Applied Science

Wetting Films on Chemically Modified Surfaces: An X-Ray Study

The wetting of silicon wafers and silicon wafers coated with alkylsiloxane monolayers by saturated vapors of cyclohexane and methanol were studied using x-ray specular reflection. Differentially heating the substrate surface relative to the temperature of a liquid reservoir was used to probe the disjoining pressure as a function of the film thickness and surface chemistry. Uncoated silicon wafers wet completely. The variations in film thickness with $\Delta$T are explained exclusively in terms of the nonretarded van der Waals forces for films 10–120 Å thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film 1–3 Å thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -CH$_3$ to -CH$_2$OH converts the surface from incompletely to completely wet. Surfaces coated with partial monolayers of methyl terminated alkylsiloxane of greater than 50% coverage are incompletely wet by cyclohexane, with the monolayers ‘‘swelling’’ to a thickness close to that of fully extended alkane chains through incorporation of cyclohexane into the film structure. The data are consistent with a first-order transition to complete wetting upon reduction of the alkylsiloxane coverage below approximately 50%. The importance of the surface atomic layer in the promotion or suppression of complete wetting is explained in the context of van der Waals forces.Engineering and Applied Science