The magnetization reversal of Ni nanowires was studied by anisotropic magnetoresistance measurements at temperatures between 15 and 300 K. The wires, synthesized by electrodeposition in a nanoporous polycarbonate membrane, are regular cylinders 22 μm long with a diameter of 75 or 35 nm. The nucleation field was measured on individual nanowires as a function of the angle between the applied field and the wire axis. The results are quantitatively analyzed using classical magnetization reversal theories. Measurements of the nucleation field first obtained as a function of temperature evidence an extra uniaxial anisotropy induced by the contraction of the membrane at low temperature. Combining SQUID measurements and x-ray diffractometry at different temperatures, a clear picture of the large magnetoelastic effect was obtained.
Single crystals 4H-SiC were implanted with 50 keV helium ions at temperatures up to 600 °C and fluences in the range 1×1016–1×1017cm−2. The helium implantation-induced swelling was studied through the measurement of the step height. The different contributions of swelling were determined by combining simulations of x-ray diffraction curves and transmission electron microscopy observations. At room temperature, amorphization occurs between 1 and 2×1016cm−2, inducing the decrease in density of about 15%. For high-temperature implants, amorphization does not occur. The strain profiles show saturation in the near-surface region, indicating that a threshold concentration of defects is reached. All the additional point defects created during the implantation have been supposed to annihilate. In the region of high-energy deposition density, the value of strain increases with fluence up to values larger than 6%. The elastic contribution to swelling has been obtained by integration of the strain profile determined by x-ray diffraction simulations. Then, the contribution of helium bubbles to the step height is found to be linear with the fluence: 0.8nm∕1016He∕cm2.
The strain induced by room temperature helium implantation into 4H-SiC below the threshold amorphization dose results from both point and He-related defects. When the helium concentration is lower than 0.5% the strain profile follows the point defect profile, whereas at higher concentrations the He ions have a dominant effect on the strain. Upon annealing, the near surface strain progressively relaxes up to 1500 °C while the maximum strain relaxation stops at a temperature where helium ions agglomerate into platelets. When the vacancies become mobile, the platelets evolve into bubble clusters that expel dislocation loops whose migration is enhanced by the strain.
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