Deformation-induced anisotropy of the critical current in single crystal niobium

Abstract

Abstract Anisotropy of the critical current in a [110] niobium single crystal of very high purity is observed to result from tensile deformation. The critical current exhibits local maxima when the magnetic field is directed along the [10] and [001] directions. The height of the [10] maximum increases with respect to the height of the [001] maximum as the magnetic field is decreased. The [001] maximum is sensitive to the condition of the sample surface and exhibits ‘rectification’. The anisotropy is not caused by the anisotropy in Hc2 , by sub-oxide or other precipitates or by the ellipticity of the deformed crystal surface. There is a contribution to the anisotropy of the as-deformed crystal caused by anisotropic surface roughness, which can be removed by chemical polishing. The [l0] maximum is attributed to the interactions between flux lines and parallel dislocation debris in the interior of the crystal, the dislocation lines lying primarily along the [10] direction. The pinning force, estimated on the basis of the Yamafuji-Irie theory, agrees within an order of magnitude with the theoretical estimates of the elastic interaction force between a dislocation and a parallel flux line. The [001] maximum is tentatively assigned to the interaction between dislocations and flux lines near the crystal surface, the density of dislocations threading the (10) surface being higher than that threading the (001) surface. The effect of the interaction at the surface is thought to be magnified by the decrease in the flux line lattice shear modulus C66 in that region.