High-quality epitaxial iron nitride films grown by gas-assisted molecular-beam epitaxy

Thin films of γ’-Fe4N were grown on polished (001) MgO substrates by molecular-beam epitaxy of iron in the presence of a gas flow from a rf atomic source. By means of x-ray diffraction, Mössbauer Spectroscopy, Rutherford backscattering/channeling, and scanning probe microscopy, it is shown that, with this method, single-phase, high-quality epitaxial thin films can be grown with a very smooth surface (root-mean-square roughness ∼0.4 nm). Magnetic measurements reveal square hysteresis loops, moderate coercivities (45 Oe for a 33 nm thick film) and complete in-plane orientation of the magnetization. These properties make the films interesting candidates for device applications

Shower approach in the simulation of ion scattering from solids

An efficient approach for the simulation of ion scattering from solids is proposed. For every encountered atom, we take multiple samples of its thermal displacements among those which result in scattering with high probability to finally reach the detector. As a result, the detector is illuminated by intensive "showers", where each event of detection must be weighted according to the actual probability of the atom displacement. The computational cost of such simulation is orders of magnitude lower than in the direct approach and a comprehensive analysis of multiple and plural scattering effects becomes possible. We use the new method for two purposes. First, the accuracy of the approximate approaches, developed mainly for ion-beam structural analysis, is verified. Second, the possibility to reproduce a wide class of experimental conditions is used to analyze some basic features of ion-solid collisions: the role of double violent collisions in low-energy ion scattering; the origin of the "surface peak" in scattering from amorphous samples; the low-energy tail in the energy spectra of scattered medium-energy ions due to plural scattering; the degradation of blocking patterns in 2D angular distributions with increasing depth of scattering. As an example of simulation for ions of MeV energies, we verify the time-reversibility for channeling/blocking of 1 MeV protons in a W crystal. The possibilities of analysis that our approach offers may be very useful for various applications in particular for structural analysis with atomic resolution.Comment: 16 pages, 9 figures. Finally published version; large parts reformulated, Fig. 9 and references adde

Growth and properties of Cu3N films and Cu3N/gamma '-Fe4N bilayers

Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogen from a radio-frequency atomic source on (001) gamma'-Fe4N/(001)MgO or directly on MgO substrates. The structural properties of the Cu3N films were found to be very dependent on the substrate and on the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on both substrates. The Cu3N films grown on MgO were characterized optically to be insulators with an energy gap of 1.65 eV. On gamma'-Fe4N, Cu3N films with a thickness of only 6 nm, were grown as closed layers, epitaxial and rather smooth (root-mean-square roughness of 0.7 nm). This material has ideal properties to be used as a barrier in low resistance magnetic tunnel junctions. (C) 2002 American Institute of Physics

Structure and magnetism of single-phase epitaxial γ′-Fe4N

Single phase epitaxial pure γ′-Fe4N films are grown on MgO (001) by molecular beam epitaxy of iron in the presence of nitrogen obtained from a radio frequency atomic source. The epitaxial, single phase nature of the films is revealed by x-ray diffraction and by the local magnetic environment investigated by Mössbauer spectroscopy. The macroscopic magnetic properties of the γ′-Fe4N films are studied in detail by means of transverse Kerr effect measurements. The hysteresis loops are consistent with the cubic atomic structure, displaying easy [100] magnetization directions. The films are single domain at remanence, and the reversal is dominated by 180° or 90° domain wall nucleation and propagation, depending on the applied field direction. When 90° domain walls are responsible for the magnetization reversal, this proceeds in two stages, and the measured coercive fields vary accordingly. Magnetic domain observations reveal the two distinct reversal —driven by 180° or 90° domain walls— modes displaying large domains, of the order of mm. From magnetometer techniques, the saturation magnetization, μ0Ms, is measured to be 1.8 T. A magneto-optical torque technique is used to obtain a value of the anisotropy constant of 2.9×104J/m3.The authors acknowledge partial financing from EC project HIDEMAR G5RD-CT-2002-00731 and PHANTOMS network. The authors are indebted to A. Gupta and K. V. Rao from the department of Materials Science and Engineering, KTH, Sweden for help with the low T SQUID measurements, and to L. Ballcels and M. A. García from Materials Science ICMM CSIC, Spain for high-T VSM measurements. This work was part of the research program of the Foundation for Fundamental Research on Matter-FOM, The Netherlands. J.M.G.M. acknowledges financing through the Ramón y Cajal program from the Spanish MCyT.Peer reviewe

Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron

Recently, a method to nitride iron in NH3 ambients at low temperature (225-350 degrees C) has been developed. In this method, the Fe is covered with a thin (similar to 40 nm) Ni layer, which acts as a catalyst for the nitriding process. From experiments, in which the amount of nitrogen uptake is measured as a function of nitriding time, it is concluded that the decomposition of NH3 at the Ni surface contains the rate-limiting step in this low-temperature nitriding process. From a model calculation, it is further concluded that the reaction step NH3-->NH2+H at the Ni surface is the rate-limiting step with an activation energy of similar to 1.5 eV

Low-temperature nitriding of iron through a thin nickel layer

We present a new method for nitriding iron at low temperatures. First, iron is coated with a thin layer of nickel (similar to 36 nm), after which it is exposed to an NH3 atmosphere at temperatures below 300 degrees C. Underneath the nickel layer epsilon-Fe3-xN is formed at temperatures as low as 225 degrees C, while uncovered iron samples show a large uptake of oxygen after the same treatment. The nickel layer prevents the oxidation of iron by impurities in the NH3 gas, and acts as a catalyst for the decomposition of NH3. After decomposition the atomic nitrogen diffuses through the nickel layer towards the iron. With the process described, pore-free iron nitrides can be formed at low temperatures

Increasing arterial blood pressure with norepinephrine does not improve microcirculatory blood flow: a prospective study

Introduction Our goal was to assess the effects of titration of a norepinephrine infusion to increasing levels of mean arterial pressure (MAP) on sublingual microcirculation. Methods Twenty septic shock patients were prospectively studied in two teaching intensive care units. The patients were mechanically ventilated and required norepinephrine to maintain a mean arterial pressure (MAP) of 65 mmHg. We measured systemic hemodynamics, oxygen transport and consumption (DO2 and VO2), lactate, albumin-corrected anion gap, and gastric intramucosal-arterial PCO2 difference (Delta PCO2). Sublingual microcirculation was evaluated by sidestream darkfield (SDF) imaging. After basal measurements at a MAP of 65 mmHg, norepinephrine was titrated to reach a MAP of 75 mmHg, and then to 85 mmHg. Data were analyzed using repeated measurements ANOVA and Dunnett test. Linear trends between the different variables and increasing levels of MAP were calculated. Results Increasing doses of norepinephrine reached the target values of MAP. The cardiac index, pulmonary pressures, systemic vascular resistance, and left and right ventricular stroke work indexes increased as norepinephrine infusion was augmented. Heart rate, DO2 and VO2, lactate, albumin-corrected anion gap, and Delta PCO2 remained unchanged. There were no changes in sublingual capillary microvascular flow index (2.1 +/- 0.7, 2.2 +/- 0.7, 2.0 +/- 0.8) and the percent of perfused capillaries (72 +/- 26, 71 +/- 27, 67 +/- 32%) for MAP values of 65, 75, and 85 mmHg, respectively. There was, however, a trend to decreased capillary perfused density (18 +/- 10,17 +/- 10,14 +/- 2 vessels/mm(2), respectively, ANOVA P = 0.09, linear trend P = 0.045). In addition, the changes of perfused capillary density at increasing MAP were inversely correlated with the basal perfused capillary density (R-2 = 0.95, P < 0.0001). Conclusions Patients with septic shock showed severe sublingual microcirculatory alterations that failed to improve with the increases in MAP with norepinephrine. Nevertheless, there was a considerable interindividual variation. Our results suggest that the increase in MAP above 65 mmHg is not an adequate approach to improve microcirculatory perfusion and might be harmful in some patient

A mediation approach to understanding socio-economic inequalities in maternal health-seeking behaviours in Egypt.

BACKGROUND: The levels and origins of socio-economic inequalities in health-seeking behaviours in Egypt are poorly understood. This paper assesses the levels of health-seeking behaviours related to maternal care (antenatal care [ANC] and facility delivery) and their accumulation during pregnancy and childbirth. Secondly, it explores the mechanisms underlying the association between socio-economic position (SEP) and maternal health-seeking behaviours. Thirdly, it examines the effectiveness of targeting of free public ANC and delivery care. METHODS: Data from the 2008 Demographic and Health Survey were used to capture two latent constructs of SEP: individual socio-cultural capital and household-level economic capital. These variables were entered into an adjusted mediation model, predicting twelve dimensions of maternal health-seeking; including any ANC, private ANC, first ANC visit in first trimester, regular ANC (four or more visits during pregnancy), facility delivery, and private delivery. ANC and delivery care costs were examined separately by provider type (public or private). RESULTS: While 74.2% of women with a birth in the 5-year recall period obtained any ANC and 72.4% delivered in a facility, only 48.8% obtained the complete maternal care package (timely and regular facility-based ANC as well as facility delivery) for their most recent live birth. Both socio-cultural capital and economic capital were independently positively associated with receiving any ANC and delivering in a facility. The strongest direct effect of socio-cultural capital was seen in models predicting private provider use of both ANC and delivery. Despite substantial proportions of women using public providers reporting receipt of free care (ANC: 38%, delivery: 24%), this free-of-charge public care was not effectively targeted to women with lowest economic resources. CONCLUSIONS: Socio-cultural capital is the primary mechanism leading to inequalities in maternal health-seeking in Egypt. Future studies should therefore examine the objective and perceived quality of care from different types of providers. Improvements in the targeting of free public care could help reduce the existing SEP-based inequalities in maternal care coverage in the short term

Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals

Epitaxial strain energies of epitaxial films and bulk superlattices are studied via first-principles total energy calculations using the local-density approximation. Anharmonic effects due to large lattice mismatch, beyond the reach of the harmonic elasticity theory, are found to be very important in Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that is the elastically soft direction for biaxial expansion of Cu and Ni, but it is for large biaxial compression of Cu, Ag, and Au. The stability of superlattices is discussed in terms of the coherency strain and interfacial energies. We find that in phase-separating systems such as Cu-Ag the superlattice formation energies decrease with superlattice period, and the interfacial energy is positive. Superlattices are formed easiest on (001) and hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the formation energy of superlattices increases with period, and interfacial energies are negative. These superlattices are formed easiest on (001) or (110) and hardest on (111) substrates. For Ni-Au we find a hybrid behavior: superlattices along and like in phase-separating systems, while for they behave like in ordering systems. Finally, recent experimental results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys, immiscible in the bulk form, are explained in terms of destabilization of the phase separated state due to lattice mismatch between the substrate and constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in Physical Review

Measuring maternal mortality : an overview of opportunities and options for developing countries

Background:There is currently an unprecedented expressed need and demand for estimates of maternal mortality in developing countries. This has been stimulated in part by the creation of a Millennium Development Goal that will be judged partly on the basis of reductions in maternal mortality by 2015. Methods: Since the launch of the Safe Motherhood Initiative in 1987, new opportunities for data capture have arisen and new methods have been developed, tested and used. This paper provides a pragmatic overview of these methods and the optimal measurement strategies for different developing country contexts. Results: There are significant recent advances in the measurement of maternal mortality, yet also room for further improvement, particularly in assessing the magnitude and direction of biases and their implications for different data uses. Some of the innovations in measurement provide efficient mechanisms for gathering the requisite primary data at a reasonably low cost. No method, however, has zero costs. Investment is needed in measurement strategies for maternal mortality suited to the needs and resources of a country, and which also strengthen the technical capacity to generate and use credible estimates. Conclusion: Ownership of information is necessary for it to be acted upon: what you count is what you do. Difficulties with measurement must not be allowed to discourage efforts to reduce maternal mortality. Countries must be encouraged and enabled to count maternal deaths and act.WJG is funded partially by the University of Aberdeen. OMRC is partially funded by the London School of Hygiene and Tropical Medicine. CS and SA are partially funded by Johns Hopkins University. CAZ is funded by the Health Metrics Network at the World Health Organization. WJG, OMRC, CS and SA are also partially supported through an international research program, Immpact, funded by the Bill & Melinda Gates Foundation, the Department for International Development, the European Commission and USAID