Vladimir A. Chernov

High gain beam compression in new-generation thin-film x-ray waveguides

X-ray waveguides can compress an incident beam for microscopy applications above 8 keV photon energy to sizes smaller than 100 nm in one dimension, a range which is not routinely accessed with other x-ray optics (e.g., Fresnel zone plates). Beryllium, because of its low absorption, is expected to provide the highest intensity gain. Measured gains for a beryllium waveguide of 74 nm thickness exceed values of 100 at 13 and 20 keV photon energy, which is an improvement by an order of magnitude compared to previously reported performances. The same object works also at 8 keV with gain 43.

Evidence for diffuse interfaces and tensile in-plane strains in evaporated CoPd (1 1 1) multilayers and their role in perpendicular magnetic anisotropy

Abstract The local structure surrounding Co atoms in Co Pd (1 1 1) multilayers, [ Co χ A + Pd 24 A ] × 13 is investigated using polarized extended X-ray absorption fine structure (PEXAFS). The PEXAFS results show ther is a considerable amount of Co-Pd alloy-like phase near interfaces, e.g., 31 at.% Co for χ = 3.2 in a range extending 2.5 A on their side of the nominal Co layer. The in-plane tensile strains mainly lead to out-of-plane magnetization, combined with a much more negative value of the magnetostriction coefficient λ1 1 1 of the alloy phase. The alloy-like region should be considered in experiments and theories.

Anisotropic short-range structure of Co0.16Pd0.84 alloy films having perpendicular magnetic anisotropy

Polarized Co K edge extended x-ray absorption fine structure measurements obtained with electric vector parallel and perpendicular to the film plane indicate differences in Co bonding along these two directions in room-temperature evaporated Co0.16Pd0.84 alloy films. A local modulation of the Co fraction whose amplitudes are 0.05–0.09 exists along the growth direction in the alloy films. Pd underlayer and Pd spacer layers alternated with the alloy layer induce coherency strains resulting in an anisotropic effective strain state. Both anisotropic strain and local compositional modulation are likely to contribute to the perpendicular magnetic anisotropy observed in these alloys.

X-ray absorption spectroscopic analyses and fluorescence emission characteristics of PbO-Bi203-Ga203 glasses doped with rare-earth ions

A representative of heavy metal oxide glasses, i.e., a PbO- Bi2O3-Ga2O3 glass, was investigated to identify the network structure of the glass and the electronic transition properties of rare-earth ions doped. X-ray absorption spectroscopic analyses showed that gallium forms GaO4 tetrahedral units with an average Ga-O bond length of approximately 1.87 A. Lead forms both PbO3 and PbO4 polyhedra, but the fraction of PbO4 decreases with decreasing PbO content. Bismuth in glasses constructs BiO5 and BiO6 polyhedra, which have a similar coordination scheme of the (alpha) -Bi2O3 crystal. Formation of three-coordinated oxygens is necessary to compensate shortage of oxygens to be two-fold coordinated. These glasses exhibit a relatively good thermal stability as well as the lowest phonon energy among oxide glasses, and thereby enhance numerous fluorescence emissions that are quenched in the conventional oxide glasses. Magnitudes of multiphonon relaxation are the lowest among oxide glasses and comparable to those of fluoride glasses. Fluorescence emission characteristics of Pr3+: 1.3 micrometer and Er3+: 2.7 micrometer were discussed in detail. In addition, influence of OH- on the Nd3+: 1.3 micrometer emission was analyzed. Further research efforts on impurity minimization and fiberization may realize a new oxide-based fiber-optic host.

Cryogenic EXAFS investigations of the short-range structural environment of gallium in PbO–Ga2O3 glasses

Gallium K-edge extended X-ray absorption fine structure (EXAFS) spectra of PbO-Ga 2 O 3 glasses were recorded at liquid-nitrogen temperature (77 K) to understand the coordination scheme of gallium in these glasses. Four oxygens were found to be bonded to each Ga, forming GaO 4 tetrahedra with an average bond distance of 1.855-1.862 A. Assuming a non-Gaussian distribution of Ga-O bond distances provided better results with a smaller R-factor compared to the Gaussian counterpart. Two-subshell fitting indicated that oxygens with Ga-O bond distance of ∼1.87 A are bonded to three cations, while the shorter Ga-O bonded oxygens are two-coordinated.

Anisotropic Local Structure of Co-Pd Alloy and Pd/Co Multilayer Films Having Perpendicular Magnetic Anisotropy

Anisotropie short-range structure in Co-Pd alloy and Pd/Co multilayer films is determined using polarized Co K-edge extended x-ray absorption fine structure. A local modulation of Co fraction exists along the growth direction in the alloy films. Pd underlayer and Pd spacers alternated with the alloy layer influence an effective strain anisotropy. Pd/Co multilayers have an alloy-like region near nominal interfaces by diffuse interface. Pd spacers in the multilayer play a role in coherency strain rather than on broken symmetry of Co atoms. Local structural anisotropy of both strain and chemical environment contribute to the perpendicular magnetic anisotropy observed in these films.