Fluxoid pinning in niobium containing small amounts of yttrium or gadolinium

Abstract

The fluxoid pinning of niobium containing small amounts of either yttrium or gadolinium (2 at. %) was determined by direct current magnetization measurements on specimens in the heavily-deformed, partially-recovered, and recrystallized conditions. Optical and electron microscopy were used to characterize the samples.

Magnetic hysteresis of Nb was increased by the additions of Y or Gd. In the cold-worked condition increased pinning was due to planar sheets and stringers of Y(Gd) produced by the deformation and parallel to the fluxoid axes. The fluxoid pinning was a function of Y(Gd) concentration. In the recrystallized samples the high-angle grain boundaries were the most important pinning sites and the pinning was independent of Y(Gd) content. The Y(Gd) enhanced the pinning effectiveness of grain boundaries by: (1) inhibiting grain boundary migration and thus increasing the grain boundary surface area (decreasing grain size); and (2) reducing preferred orientation in the recrystallized state thus providing greater average grain misorientation, which for Nb with its anisotropic superconducting parameters (K) produces an additional pinning effect.