The growth of germanium at low temperature by ultrahigh vacuum chemical vapor deposition on Si(001) is investigated in real time by reflection high-energy electron diffraction. These observations are complementarily checked by atomic force microscopy, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction experiments. It can be seen that the currently observed Stranski–Krastanov-related two-dimensional (2D) to three-dimensional transition is avoided at 330 °C and that the major part of the relaxation process occurs during the deposition of the first two monolayers. Then, the measured in-plane lattice parameter evolves slowly and approaches that of bulk Ge after deposition of 50 monolayers. The corresponding relaxation equals 83%. The resulting surface is flat, with a rms roughness of 0.55 nm. The relaxation is found to be mainly due to misfits dislocations located at the Ge/Si interface. Regrowth experiments at 600 °C show that the low-temperature films are not stable for thicknesses below 27 nm. In spite of the nearly complete relaxation observed for 7.5 nm, a much higher thickness is needed to enable a continuous 2D growth at 600 °C. Finally, a 500-nm-thick film, obtained with a low-temperature Ge buffer and with a Ge regrowth at high temperature, exhibits a channeling-to-random Rutherford backscattering spectrometry ratio (min) of 4%, which indicates a good crystalline quality. ©2005 American Institute of Physic

We have combined in situ reflection high energy electron diffraction, high-resolution transmission electron microscopy, and magnetotransport experiments to investigate the role of a thin inserted Mg layer on the crystalline texture of MgO barriers in magnetic tunnel junctions grown in a standard sputtering system. It was found that an ultrathin Mg layer of 2–6Å can efficiently promote a MgO (001) texture prior to any annealing. For thicker Mg layers, the MgO (001) texture was found to degrade due to the hexagonal structure of Mg. For tunneling magnetoresistance (TMR), the efficient role of the MgO crystallization induced by the Mg layer appears after a 400°C annealing. The optimum was found for a 4Å inserted Mg layer with a TMR of 120% at room temperature (210% at 3K) which could be considerably improved for fully (001) oriented magnetic tunnel junctions.

On 7 January 2014 an X1.2 flare and CME with a radial speed ≈ 2500 km s −1 was observed from near an active region close to disk center. This led many forecasters to estimate a rapid arrival at Earth (≈ 36 hours) and predict a strong geomagnetic storm. However, only a glancing CME arrival was observed at Earth with a transit time of ≈ 49 hours and a K P geomagnetic index of only 3−. We study the interplanetary propagation of this CME using the ensemble Wang-Sheeley-Arge (WSA)-ENLIL+Cone model, that allows a sampling of CME parameter uncertainties. We explore a series of simulations to isolate the effects of the background solar wind solution, CME shape, tilt, location, size, and speed, and the results are compared with observed in-situ arrivals at Venus, Earth, and Mars. Our results show that a tilted ellipsoid CME shape improves the initial real-time prediction to better reflect the observed in-situ signatures and the geomagnetic storm strength. CME parameters from the Graduated Cylindrical Shell model used as input to WSA-ENLIL+Cone, along with a tilted ellipsoid cloud shape, improve the arrival-time error by 14.5, 18.7, 23.4 hours for Venus, Earth, and Mars respectively. These results highlight that CME orientation and directionality with respect to observatories play an important role in understanding the propagation of this CME, and for forecasting other glancing CME arrivals. This study also demonstrates the importance of three-dimensional CME fitting made possible by multiple viewpoint imaging. 9 In this coordinate system the Z axis is aligned with the solar north rotation pole and the X axis pointing toward the intersection between the solar equator and the solar central meridian as seen from Earth (Hapgood 1992;Thompson 2006). HEEQ coordinates are related to Stonyhurst heliographic coordinates, with directions south of the origin represented by negative HEEQ latitudes, and directions east by negative HEEQ longitudes.

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Self-assembled vertical epitaxial nanostructures form a new class of heterostructured materials that has emerged in recent years. Interestingly, such kind of architectures can be grown using combinatorial processes, implying sequential deposition of distinct materials. Although opening many perspectives, this combinatorial nature has not been fully exploited yet. This work demonstrates that the combinatorial character of the growth can be further exploited in order to obtain alloy nanowires coherently embedded in a matrix. This issue is illustrated in the case of a fully epitaxial system: CoxNi1-x nanowires in CeO2/SrTiO3(001). The advantage brought by the ability to grow alloys is illustrated by the control of the magnetic anisotropy of the nanowires when passing from pure Ni wires to CoxNi1-x alloys. Further exploitation of this combinatorial approach may pave the way toward full three-dimensional heteroepitaxial architectures through axial structuring of the wires.

International audienceWe have studied the electron spin injection efficiency from a CoFeB/MgO spin injector into AlGaAs/GaAs semiconductor light emitting diodes. The circular polarization of the electroluminescence signal reaches a value as large as 32% at 100 K under a 0.8 T magnetic field. We show that the spin injection efficiency increases with the increase in the MgO barrier thickness from 1.4 to 4.3 nm. Moreover, a higher spin injection efficiency is obtained for MgO barriers grown at 300 °C compared to the ones grown at room temperature. This effect is attributed to the MgO texturing occurring at high temperatures

Strain is a key parameter affecting the physical properties of heterostructured thin films and nanosized objects. Generally, the design of application-optimized hybrid structures requires good structural compatibility between the involved phases. However, when controlled appropriately, lattice mismatch can turn from a detrimental to a beneficial property, enabling further functionality tuning. Due to their large heterointerface, nanocolumnar composites are an ideal test bed for such strain engineering approaches, but coupling mechanisms at vertical interfaces are still poorly understood. In the present paper, we therefore present a detailed analysis of ultrathin Ni nanowires, with diameters between 1.7 nm and 5.3 nm, vertically epitaxied in a SrTiO3/SrTiO3(001) matrix. Using a combination of x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and x-ray absorption spectroscopy (XAS) measurements, we unveil peculiar structural features of this hybrid system. We show that the axial deformation of the nanowires depends on their diameter and that their radial strain differs sensitively from the value expected when considering the Poisson ratio. We also provide evidence for the existence of a relaxation mechanism consisting in a slight tilting of crystallographic nanowire domains which reduces the misfit at the Ni-SrTiO3 heterointerface. This, in turn, induces significant structural disorder and results in a successive amorphization of the metallic phase upon diameter reduction of the nanowires.