Magnetic Anisotropy of Co2MnSn1−xSbx Thin Films Grown on GaAs (001)

Heusler alloy Co2MnSn1−xSbx (x = 0.0, 0.5, and 1.0) thin films were grown on GaAs (001) substrates using pulsed laser deposition techniques. Growth parameters have been determined that result in highly magnetically anisotropic, crystalline, and oriented (001) films. The angular dependences, relative to the GaAs (001) crystallographic directions, of the coercive field Hc(θ) and the remanence Mr(θ) were determined from angle dependent magneto-optic Kerr effect (MOKE) measurements. It was found that Hc(θ) revealed higher order symmetry contributions to the magnetic anisotropy than did Mr(θ). The Fourier analysis of rotational MOKE data was used to determine the symmetry contributions to the total anisotropy

Induced Magnetic Anisotropy and Spin Polarization in Pulsed Laser-Deposited Co2MnSb Thin Films

Co2MnSb thin films were grown on glass and GaAs (001) substrates using pulsed laser deposition. The films were grown in magnetic fields (HG = 500 Oe and 0 Oe) that were applied in the plane of the substrate during the deposition process. Angle-dependent magneto-optic Kerr effect measurements for films grown on glass revealed a uniaxial magnetic anisotropy in the direction of the applied growth field. Films grown on GaAs (001) exhibited more complicated magnetic anisotropy behavior, due to additional contributions from the substrate. Point contact Andreév reflection spectroscopy measurements indicated that the spin polarizations of the films were about P~ 50%, with negligible difference between films grown in zero and non-zero applied fields

Magnetic and Transport Properties of Co2MnSnxSb1−x Heusler Alloys

We present the magnetic, structural, and transport properties of the quaternary Heusler alloys Co2MnSnxSb1−x (x = 0, 0.25, 0.50, 0.75, and 1.0), which have been theoretically predicted to be half-metallic. Magnetization measurements as a function of applied field show that the saturation moment for x = 1 (Co2MnSn) is near the Slater–Pauling value of 5μB; however, the moment for x = 0 (Co2MnSb) falls far short of its predicted value of 6μB. Resistivity as a function of temperature was measured from 5 to 400 K, and a phase transition from a half-metallic ferromagnetic phase to a normal ferromagnetic phase was observed between 50 and 80 K for all of the alloys. At low temperature (10 K\u3cT\u3c40 K), the resistivity ratio was found to vary as R(T)/R(T = 5 K) = A+BT2+CT9/2, where the T2 term results from electron-electron scattering, whereas the T9/2 term is a consequence of double magnon scattering

X-Ray Magnetic Circular Dichroism of Pulsed Laser Deposited Co2MnSn and Co2MnSb Thin Films Grown on GaAs (001)

We present the structural and element specific magnetic properties of Co2MnSn and Co2MnSb thin films grown on GaAs (100) substrates using pulsed laser deposition. X-ray magnetic circular dichroism (XMCD) spectra were measured for 400 A thick films at the L2,3 edges of Co and Mn. Element specific moments for Co and Mn in Co2MnSn were calculated from the x-ray absorption and XMCD spectra using the XMCD sum rules. The ratios of orbital to spin magnetic moments for Co and Mn were calculated for Co2MnSn and Co2MnSb

The UCSD HIRES/KeckI Damped Lya Abundance Database III. An Empirical Study of Photoionization in the Damped Lya System Toward GB1759+7539

We investigate the ionization state of the damped Lya system at z=2.62 toward GB1759+7539 through an analysis of ionic ratios sensitive to photoionization: ArI/SII, FeIII/FeII, NII/NI, AlIII/AlII. Approximately half of the metals arise in a mostly neutral velocity component with HI/H > 0.9, based on FeIII/FeII < 0.013. In contrast, the remaining half exhibits FeIII/FeII~0.3 indicative of a partially ionized medium with HI/H~0.5. These conclusions are supported by the observed NII/NI, AlIII/AlII, and ArI/SII ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible except for ArI. However for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (SiII, SII, NiII, AlII)/FeII ratios and more for (NI, ArI)/FeII. Independent of the shape of the photoionizing spectrum and assumptions on the number of ionization phases, these ionization corrections have minimal impact (<0.1dex) on the total metallicity inferred for this damped Lya system. Measurements on the relative elemental abundances of the partially ionized gas, however, have a greater than ~0.15 dex uncertainty which hides the effects of nucleosynthesis and dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g. a very low ArI/SII ratio) and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics like FeIII/FeII in a larger sample of systems.Comment: 19 pages with 11 b&w figures. Visit the DLA database at http://kingpin.ucsd.edu/~hiresdla for more. Accepted to the Astrophysical Journal, Feb 5 200

Why do models overestimate surface ozone in the Southeast United States

Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx  ≡  NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°  ×  0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC4RS observations of NOx and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NOx emissions from mobile and industrial sources must be reduced by 30–60 %, dependent on the assumption of the contribution by soil NOx emissions. Upper-tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NOx emissions. We find that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 6 ± 14 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer