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  • 11
    Electronic Resource
    Electronic Resource
    Magnetic properties of epitaxial and polycrystalline Fe/Si multilayers : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 4772-4774 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown via ion-beam sputtering on both glass and single-crystal substrates. High-angle x-ray diffraction measurements show that both sets of films have narrow Fe peaks, implying a large crystallite size and crystalline iron silicide spacer layers. Low-angle x-ray diffraction measurements show that films grown on glass have rougher interfaces than those grown on single-crystal substrates. The multilayers grown on glass have a larger remanent magnetization than the multilayers grown on single-crystal substrates. The observation of magnetocrystalline anisotropy in hysteresis loops and (hkl) peaks in x-ray diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The smaller remanent magnetization in Fe/Si multilayers with better layering suggests that the remanence is not an intrinsic property. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361926
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  • 12
    Electronic Resource
    Electronic Resource
    Aging effect of Co/C soft x-ray multilayer mirrors (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 184-189 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of aging on Co/C multilayer x-ray mirrors is presented. One significant result is the enhancement of the reflectivity at grazing incidence with time for the Co/C multilayers with C-on-top stored in air or argon and Co-on-top stored in argon. This can be interpreted in terms of a Co–C phase-separation at Co–C interfaces due to the positive enthalpy of Co–C mixing. Results also show that oxidation of the surface of Co-on-top Co/C multilayers plays an important role in the decrease of reflectivity. The oxidation can be prevented by storing the multilayers in an oxygen-free atmosphere or by depositing amorphous carbon as the top layer. The reflectivity of tarnished multilayers can be restored by removing the oxides by wet chemical methods. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363839
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  • 13
    Electronic Resource
    Electronic Resource
    Thermal evolution of carbon in annealed Co/C soft x-ray multilayers (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 1428-1436 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The structures of the carbon sublayers in the annealed Co/C soft x-ray multilayers fabricated using a dual-facing-target sputtering system have been characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy (RS). The results suggest that the structural variations in the carbon layers can be roughly divided into three stages, i.e. ordering, crystalline and grain growth stages. In the ordering stage with annealing temperatures below 400 °C, the upward shift of D and G lines in Raman spectra indicates that the amorphous carbon layers are changing from ones with bond-angle disorder and fourfold-bonding only to ones containing threefold-bonding. In the crystalline stage, the amorphous carbon layers in the as-deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500–600 °C. The rapid increase in the intensity ratio of D line to G line and dramatic decrease in linewidth further confirm this substantial structural change. In the grain growth stage, the specimens are annealed at temperatures higher than 700 °C. The decrease in the intensity ratio implies a growth in the graphite crystallite dimensions, which is consistent with the XRD and TEM results. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363010
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  • 14
    Electronic Resource
    Electronic Resource
    Quadratic electro-absorption and electro-optic effects in a guest/host nonlinear optical polymeric system (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 1267-1274 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quadratic electro-absorption (QEA) and quadratic electro-optic measurements were performed with a guest/host system consisting of 4(4'-N,N-Dimethylaminophenyl)-1,1-bis(trifluoromethyl sulfonyl)-1,3-butadiene doped in poly(ethyl)methacrylate host polymer. The imaginary part of the QEA response was measured over a wavelength range of 450–600 nm. The real and imaginary parts of the electric field induced optical Kerr coefficient were measured at wavelengths near and far from resonance. Under different polarizations of the optical field in the QEA measurement, the two contributing coefficients of the third order susceptibility (χ(3)) were deduced. A distinctive temperature variation of the third order response was observed. Temperature dependence measurements from ambient to well above the glass transition of the guest-host system were carried out. The same chromophore in a different polymer host, poly(vinyl)chloride, was also investigated. Correct expressions for the real and imaginary parts of the Kerr coefficient are provided. The present results confirm the importance of the electronic contribution to χ(3) at room temperature where molecular reorientation is insignificant. At high temperatures (above the glass transition temperature of the polymer) the ratio of χ(3)3333/χ(3)1133 deviates from −2, which is predicted by the elastic model assuming molecular reorientation to be the dominant mechanism. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361021
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  • 15
    Electronic Resource
    Electronic Resource
    The ac electrical behavior of polycrystalline yttria (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 1757-1762 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ac electrical behavior of the polycrystalline yttria was evaluated in the temperature range of 800–1300 °C in air as a function of frequency (5 Hz≤f≤13 MHz). Resistance-temperature and resistance-time (aging) characteristics were examined using immittance measurements and electron microscopy to establish microstructure-property relationships. The ac electrical data indicated two distinct relaxations when analyzed in the impedance plane. These relaxations are attributed to the lumped grain and grain-boundary contributions in conjunction with a polarization effect at the electrode/sample interface. The admittance plane analysis revealed a semicircular relaxation in the low-frequency region, indicative of a trapping effect associated with grain-boundaries and the electrode/sample interface. The variation in the total electrical resistance with time is found to be dependent on the starting microstructure of the sample. A sample with a larger grain size shows a smaller degree of aging at elevated temperatures. Immittance measurements suggest that the major contribution to the aging behavior comes from the evolution in the microstructure. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360205
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  • 16
    Electronic Resource
    Electronic Resource
    Free-volume distribution and positronium formation in amorphous polymers: Temperature and positron-irradiation-time dependence (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4654-4661 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84–414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84–454 K and as a function of time (〈200 h) at four temperatures (T=84, 149, 224, and 249 K). The glass transition temperature Tg and secondary transition temperature were determined from the average o-Ps lifetime τ3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation σ(τ3) of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature Tg; (ii) the σ(τ3) in PMMA has no observable variation as a function of positron irradiation time, which suggests that the measured free-volume distribution is not influenced by the positron irradiation. The o-Ps formation probability shows large variations with temperature, thermal history, and measuring time. These effects are discussed in the framework of the spur model. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475876
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  • 17
    Electronic Resource
    Electronic Resource
    Effects of viscoelasticity of bulk polymer solution on the surface modes as probed by laser light scattering (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6103-6109 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of surface tension, surface dilational modulus, stress relaxation time, and shear stress modulus on the surface laser light scattering (SLLS) spectrum of polymer solutions are investigated. The capillary wave component is significantly affected by both the stress relaxation time and the amplitude of the shear stress modulus. At moderate concentration, before crossing over to the elastic regime, a strong quasielastic component dominates the SLLS spectrum in polymer solution. In the viscoelastic regime, the capillary wave component of the SLLS spectrum is suppressed by the increase of the surface dilational modulus. The decrease of the surface tension gives rise to an effect similar to that caused by increasing the dilational modulus. However, at high polymer concentration in the elastic regime, the dilational modulus has only a negligible effect on the SLLS spectrum, in contrast to the effect of surface tension. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477237
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  • 18
    Electronic Resource
    Electronic Resource
    Clear-cut experimental observation of the capillary wave to elastic wave crossover in concentrated polymer solutions (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 3827-3834 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A detailed experimental study of the surface laser light scattering (SLLS) spectrum of the polyisobutylene (PIB)/decane solution over a wide range of polymer concentration is carried out. In the dilute solution, the SLLS spectrum is found to be described adequately by a scattering theory from a capillary wave. As the polymer concentration increases, the solution viscoelasticity is found to play an important role in affecting the dynamics of surface waves. By comparing experiment with theory, a clear-cut capillary wave to elastic wave evolution in the PIB/decane solution is found. The nature of the quasielastic peak that appears in the SLLS spectrum of the viscoelastic polymer solution is delineated. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475771
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  • 19
    Electronic Resource
    Electronic Resource
    Hydrodynamic surface waves in concentrated polymer solutions in the presence of surface adsorption (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5898-5906 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory of light scattering from the polymer solution/air interface that includes the effect of surface adsorption is developed. The polymer solution is assumed to exhibit a linear viscoelastic response. The surface laser light scattering (SLLS) spectrum is calculated from surface hydrodynamic equations appropriate for linear viscoelastic fluids using the method of initial value problems. Numerical calculations are made to investigate the effects of surface tension and dilational modulus on the SLLS spectrum. The results show that the SLLS spectrum from polymer solutions having a high surface tension solvent displays a two-peak structure. The high-frequency peak exhibits more capillary wave character, and diminishes rapidly with decreasing surface tension or increasing dilational modulus as a result of surface adsorption. The two-peak structure is, however, found to be much less pronounced in a solution with a low surface tension solvent. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474315
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  • 20
    Electronic Resource
    Electronic Resource
    Poling dynamics and investigation into the behavior of trapped charge in poled polymer films for nonlinear optical applications (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6834-6842 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The behavior of surface and trapped charge that originate during contact electric-field poling of a model guest–host nonlinearly active polymer has been investigated. Charge effects in thin films of para-nitroaniline doped polymethylmethacrylate were studied during and after the poling process by simultaneous measurement of the current through the poling circuit and the second harmonic generation (SHG) signal from the polymer system. The poling current present in a fresh sample was found to differ in both magnitude and temporal dependence to that observed in the sample during subsequent polings. The magnitude of the steady-state current eventually reached during the poling process was found to depend on temperature and is related to the mobility of charge. A sharp drop-off in SHG signal intensity that occurred when the poling electrodes were grounded is shown to result from the removal of surface charge that orients chromophore dipoles near the surface of the film; the decay in the macroscopic polarization that then occurs is prolonged by charges trapped in the polymer matrix. The build-up of trapped charge explains a memory effect in which the SHG signal relaxation time gradually increased as the film was subjected to multiple polings until a steady-state value was reached. Finally, it is demonstrated that the charges trapped in the polymer matrix are released only after the polymer is heated to high above its temperature of glass transition and the SHG signal has completely decayed away. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471350
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