Search for positron localization near transition-metal solutes of negative effective charge in Ni and Cu

Results of an early (1973) angular correlation (ACAR) study of dilute (0.5 at.%) Cu based alloys by a Japanese group were interpreted in terms of an attraction of e+ by transition metal solutes of effective negative charge. Doppler Broadening (DB) measurements reveal no such an effect for Cu(Mn) and Cu(Ni) solid solutions as well as for Ni alloys with 3d, 4d and 5d transition metal solutes (0.1 to 1.5 at.%) i.e. no evidence of e+ localization near these impurities is seen. Our results strongly suggest that the ACAR results are due to the metallurgical state of the samples. In contrast, significant DB lineshape parameter variations, observed for our Ni(Zr) alloys, are attributed to positron trapping in and near Ni5Zr precipitates. Our DB results for a series of Ni(Au) alloys are understood in terms of a combination of the effect of an overall lattice expansion and a positron preference for clusters of Au atoms. The above comparison between DB and ACAR results is supported by our 'spin polarized' DB results for a (001) Ni single crystal which resemble those obtained by other groups using a 'spin polarized' 2D-ACAR technique

Field dependence on texture in HTc's

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Structural Relaxation in an Amorphous Rapidly Quenched Cobalt-Based Alloy

An amorphous melt-spun Co-based alloy (Metglas 2705 MN) is investigated by Doppler Broadening and Positron Lifetime techniques in order to follow the microstructural changes yielded by isochronal annealings before crystallization. The results are correlated with those of Differential Scanning Calorimetry and Coercive Field measurements. The quenched empty spaces underlined by Lifetime measurements are less than one atomic volume in size and migrate without clustering in larger voids. Both Positron Annihilation and Coercive Field investigations suggest that the overall decrease of free volume related to structural relaxation in this amorphous material, proceeds mainly via compositional short-range ordering. These local chemical rearrangements which lead to a partial disorientation of the magnetic moments act as strong pinning points for Bloch Walls

Fluence dependence and thermal stability of defects in helium-implanted cubic zirconia

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Texture effect on vortex-state TF-µ+SR in Bi-2223 high-Tc materials

International audienceThe influence of texture in Bi‐2223 ingots on the temperature dependence of the μ+ spin relaxation rate has been investigated. Texture was induced during cold‐isostatic‐pressing by applying an additional uniaxial load which tends to align single grain c axes along the stress direction, ž. Small plates were cut with faces either ‖ or ⊥ ž and μSR was measured in a 15 mT transverse field (TF) during cooling from room temperature to 6 K. While for highly‐textured material, low‐temperature limits are substantially smaller than expected, the large shift (55%) observed when switching from ž⊥TF to ž‖TF clearly indicates that anisotropy has developed. Relaxation rates for the weakly‐textured sample lie in between. To a lesser extent, sintering also enhanced the low‐temperature relaxation, but did not affect TC significantly. All samples showed a weak onset of depolarization between 60 K and 90 K, most likely due to the presence of a Bi‐2212 impurity phase. This impurity phase might cause the transition‐temperature smearing in the 100–110 K range in the weakly textured (and less compacted) material. Thus the sharpness of the relaxation drop might be relevant for assessment of material quality and be used as a criterion in the improvement of the production process