Experimental characterization of the flux-line lattice in superconducting V3Si

Several microscopic properties of the flux-line lattice (FLL) in three separate single crystals of V3Si have been investigated by means of small-angle neutron diffraction. These low-field FLL characterizations have been correlated with the following material and superconducting properties: (A) The real crystal symmetry parallel to the applied magnetic field; (B) The micro-structure as determined by TEM; (C) Magnetic irreversibilities in the mixed state; (D) Reversible flux-line motion in ac response; (E) Martensitic structural transformation observed by X-ray diffraction. The three samples, V3Si-MP3, -MP4, and -MP5 possessed different defect structures, and this was manifested foremost in the FLL perfection. At low field (B<0.2 T) and T=4.85 K, only MP3, which is free of second-phase precipitates, showed a highly resolved FLL. Sample MP5 contains a low density of small (200 Å) coherent precipitates, and revealed well-defined FLL Bragg peaks for T, but a highly mosaic, nearly polycrystalline FLL at lower fields. Sample MP4 contained large (500-1000 Å) incoherent precipitates, and showed only a polycrystalline FLL at low field. In both MP3 and MP5, distinct anisotropic correlations were observed between the FLL morphology and the real-crystal direction along the applied field. The FLL perfection was strongly dependent on the growth history. The peak width history dependence for two different scattering geometries can be qualitatively modeled by proposed flux-pinning mechanisms. Quantitative comparisons with critical current measurements, however, are not totally reconcilable.