Douglas M. Pease

Site selectivity in Fe doped β phase NiAl

We have investigated site selectivity in iron doped β phase ternary alloys of the composition NixFeyAl(100−x−y), where the Ni concentration is within a few at. % of stoichiometry. Experimental techniques used include extended x‐ray absorption edge fine structure and Mossbauer measurements. The transition metal atoms in excess of 50 at. %, which occupy sites with transition metal nearest neighbors, are not chosen randomly among Ni and Fe, but are exclusively Fe, to within the accuracy of the measurements. This result requires a re‐analysis of previous magnetic susceptibility measurements on this system, which as re‐analyzed here imply more reasonable moment values for the Fe dopants. Our findings regarding Fe site selectivity are of significance in regards to recent studies showing a ductilization effect of Fe dopants microalloyed into Ni50Al50.

Resistivity of V2O3 thin films deposited on a-plane (110) and c-plane (001) sapphire by pulsed laser deposition

Thin films of V2O3 with thickness of 215nm were grown on a- and c-plane sapphire by pulsed laser deposition with (001)V2O3∥(001)Al2O3 and (110)V2O3∥(110)Al2O3 epitaxy. The effects of the growth direction on the electrical resistivity of the films were examined. Films on c-plane sapphire displayed a metal-to-insulator transition at T=180K compared to T=160K in single-crystal V2O3. The films on a-plane sapphire, however, showed an insulator-to-insulator transition at T=186K. The variation in the phase transformation characteristics and the resistivity can be attributed to different levels of strain and commensurate changes in the film morphology.

Study of the crystalline surface of Metglas 2605 CO

X‐ray studies using a diffractometer and Read camera are reported for Metglas 2605 CO. A crystalline surface layer, which extends into the sample ≲3 μm occurs on only the substrate wheel side of the as‐quenched ribbon. Most of the crystallites consist of an α‐iron phase and are preferentially oriented in a fiber texture configuration; the fiber axis being a mixture of the [100] and [110] directions, and normal to the ribbon surface. A small amount of non α‐iron crystallinity is also present. The crystalline surface layer does not appear to influence the high value of the magnetomechanical coupling factor obtained through magnetic annealing.

Accurate X-ray absorption spectra obtained from concentrated bulk samples by fluorescence detection

Abstract We have obtained accurate X-ray absorption spectra from concentrated bulk samples by using a normal incidence low angle fluorescence detection method. Our calculated sampling depth with negligible spectrum distortion in copper samples is ≲8000 A. Our method is a truly bulk probe and is therefore complementary to more surface sensitive methods such as the electron yield technique or the technique utilizing the evanescent wave effect used in a recent study by Suzuki.

The Thickness Effect in X-ray Absorption Edges of Metals and Alloys

The thickness effect is treated using a simplified form for the window function and the absorption edge. It is shown that for the low absorbing portion of the K edge the relative error caused by the tail of the window function becomes larger, rather than vanishing, in the limit of zero thickness foils. A means is described and experimentally demonstrated for separating changes in K absorption edge fine structure due to alloying from those due to the thickness effect.

Spin‐echo NMR study of the atomic site environments in the Fe67Co18B14Si1 metallic glass

In order to investigate the near neighbor atomic environment and, consequently, the relationship of such environments to the observed magnetoelastic properties, a spin‐echo NMR study of the hyperfine field distributions in Metglas 2605 CO (Fe67Co18B14Si1) for various magnetic annealing treatments has been undertaken. Here, initial spectra for the 11B metalloid and 59Co transition metal nuclei, obtained at 4.2 °K and zero magnetic field, are presented along with supplementary magnetization and x‐ray diffraction measurements. Upon annealing, a bcc α‐Fe phase precipitates first, leaving a residual amorphous phase of lower Fe content. The appearance of such crystalline regions is accompanied by a strong reduction in the magnetomechanical coupling coefficient.

Growth of V 2 O 3 thin films on a-plane (110) and c-plane (001) sapphire via pulsed-laser deposition

We report the direct deposition of epitaxial 215-nm-thick vanadium sesquioxide (V2O3) films on a- and c-plane sapphire substrates from powder-pressed V2O3 targets via pulsed-laser deposition (PLD) in an evacuated deposition chamber devoid of O2. The films were characterized using x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS), x-ray absorption fine structure (XAFS) spectroscopy, and atomic force microscopy (AFM). XPS measurements confirmed that the stoichiometry of the powder was conserved in the films. XRD patterns together with XAFS measurements proved that V2O3 was epitaxial on the a-sapphire substrate with epitaxial relation (110)film//(110)substrate, and the results are consistent with the epitaxy on the c-plane substrate as well. The room-temperature resistivities of V2O3 films on a- and c-plane substrates were 1.49 x 10-4 and 3.00 x 10-5 O m, respectively. The higher resistivities of the films compared to bulk V2O3 might be attributed to thermal stresses resulting from difference in thermal expansion coefficients (TECs) of the films and the substrates.

Log spiral of revolution highly oriented pyrolytic graphite monochromator for fluorescence x-ray absorption edge fine structure

We have constructed an x-ray monochromator based on a log spiral of revolution covered with highly oriented pyrolytic graphite. Such a monochromator is used for obtaining x-ray absorption edge fine structure by the fluorescence method, and is particularly useful for measuring the fine structure of dilute element A in a concentrated matrix of element B, where B is to the left of A in the Periodic Table. Using the log spiral monochromator, we measure good Cr x-ray fine structure in an alloy of 1% Cr in a V matrix, whereas the corresponding spectrum is severely distorted by the V background if nonmonochromatized fluorescence is used. We also obtain excellent rejection of Mn fluorescence relative to Cr fluorescence in a Cr80Mn20 alloy, and can tune the monochromator such that the entire Mn step height is significantly smaller than the Cr x-ray absorption edge fine structure oscillations for this system.

Silicon photodiode detector for a glancing-emergence-angle EXAFS technique

We have constructed a silicon photodiode detector for use with a glancing‐emergence‐angle (GEA) geometry useful for obtaining fluorescence EXAFS spectra from thick specimens with concentrated absorbing species. We present a description of the detector and the results of tests, including dark‐noise tests, EXAFS spectra from a standard sample, and a comparison to an ion chamber also in the GEA configuration. Data obtained from the two detectors are comparable in quality, making the diode detector a preferable choice for this application due to factors such as simplicity of construction and compact size. The diodes also have potential for significant further improvement in the quality of the signal due to their high quantum efficiency if the dark noise can be reduced. We present suggestions for achieving this in future generations of the detector.

On the density of unoccupied d states in transition metal-metalloid metallic glasses☆

Abstract New data on the X-ray absorption edge near-in structure of the (Ni0.50Pt0.50)75P25 system is compared with previous results on random solid solution alloys and other TM-M metallic glasses. Whereas experimental probes of the density of d states at the Fermi energy, ϱd (Ef), often indicate a sharp reduction in ϱd (Ef) due to the presence of metalloids or nontransition metals, absorption edge results usually indicate a negligible decrease in the total number of d holes per transition metal atom. In the (Ni0.50Pt0.50)75P25 system the number of 5d holes per Pt site is actually found to be greater than for the case of pure Pt. A model is proposed which may reconcile the results of absorption edge measurements and ϱd(Ef) determinations.