The critical current-density of Nb-60 at% Ti and Nb-25 at% Zr superconductors in small magnetic fields

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Abstract
The critical supercurrent in Nb-60 at% Ti and Nb-25 at% Zr wires has been measured in magnetic fields 005 less-than-or-eq, slant B less-than-or-eq, slant 1 T, at temperatures 45 K less-than-or-eq, slant T less-than-or-eq, slant Tc, conditions relevant to a superconducting cable. The wires, only 30 μm in diameter, had a small critical current so that the self-field of the current did not significantly distort the applied field. At 45 K and 05 T the critical current density in as-worked wires was Jc=22 kA mm−2 (Nb-25 at% Zr) and Jc=10 kA mm−2 (Nb-60 at% Ti). Annealing the Nb-60 at% Ti at 385 °C multiplied the critical current by six. The pinning force on unit volume of the vortex lattice obeys the same empirical relation found in large fields: Pv=f(b)g(Bc2) where b(=B/Bc2) is the reduced magnetic field, and the relation fits the data for b greater, similar 0006. g(Bc2) is approximately equal to Bc2n, where n similar, equals 2. A small-field form of a κ-pinning theory which has been developed fits the data well, whereas other published theories do not. However, the theory ignores the rigidity of the vortex lattice and, since this feature is thought to be important, the success of the theory may indicate a fault in our understanding of flux pinning.