The effect of anisotropic flux pinning microstructure on the sample length dependence of the magnetization critical current density in niobium–titanium superconductors

Magnetization measurements of the critical current density J_c in Nb 47 wt % Ti with Nb artificial pinning centers revealed that the shape and magnitude of the field dependent magnetization hysteresis ΔM(H) was a strong function of the sample length and that ΔM(H) for short wire samples was up to six times smaller than for long wires. This is caused by the strong anisotropy of the critical current density J_c. The magnitude of J_c flowing perpendicular to the wire axis J_⊥ was deduced to be 50–175 times smaller than the longitudinal current density J_∥. The source of the anisotropy lies in the anisotropic flux pinning microstructure of the wires. When the magnetization current crosses perpendicular to the filament axis at each end of the wire, the Lorentz force is parallel to the pinning center axis. The pinning force is weak in this direction and J_⊥ is correspondingly small. The technologically important critical current density is the longitudinal current density J_∥. It can be extracted from magnetization measurements only in the case of large length to diameter filaments, as is quantitatively analyzed here.