Studying nanocrystalline superconducting Nb3Sn layers in Nb/Cu-Sn composites of various design using NMR and magnetic susceptibility methods

Abstract

The paper presents the results of measurements of the temperature dependence of the shape of the NMR lines of 93Nb and of the real part of the dynamic magnetic ac susceptibility χ′(T) for multifilamentary superconducting Nb3Sn-based composites with various geometry of niobium filaments (ordinary cylindrical, paired cylindrical, and ringlike (tubular)) produced by the bronze technology. In all the composites studied, a superconducting (SC) transition was observed in diffusional Nb3Sn layers at T c ∼ 17 K. Its width is ΔT c ≈ 3 K and is independent of the composite design. The difference in the absolute value of χ′(T) in the composites of ordinary assembly and with paired filaments is less by a factor of 1.5 than that of a composite with ring filaments. In the experimental NMR spectra of 93Nb, subspectra of different intensities have been distinguished, whose number is three in ordinary assemblies and in assemblies with paired filaments, and two in composites with ring filaments. Estimates of the intensities of each subspectrum in each experimental spectrum of the composites studied has been correlated with a definite diffusional layer of Nb3Sn; thus, the number of grains of a corresponding layer in each composite has been estimated. In addition, it has been established that in the first two composites the shape of the NMR line of 93Nb is asymmetric, which indicates the existence of an anisotropy of the Knight shift in Nb3Sn layers in both the normal and superconducting states, whereas in the composite with ring filaments the line is symmetric and the Knight shift is isotropic. The magnitude of the anisotropic Knight shift in composites with isolated and paired cylindrical filaments is 93 K an ≈ 0.02%. The anisotropy in them appears to be due to the different character and magnitude of interaction of grains of the Nb3Sn layers with the bronze matrix and the residual Nb, which leads to different shifts of the subspectrum frequencies and, correspondingly, to the shape asymmetry of the NMR line of 93Nb. At the same time, in a composite with ring filaments the interaction of the Nb3Sn layer with the bronze matrix occurs on both sides, is, apparently, of the same character and is compensated in magnitude. In this case, in the first two composites the tin distribution across the diffusional layers of Nb3Sn is nonuniform and a significant deviation from the stoichiometric composition is observed.