Magnetization critical current density of conventional and artificial pinning center (APC) Nb-Ti superconducting wires

The behavior of the magnetization critical current density J sub c of Nb-Ti superconducting wires processed conventionally or by the introduction of artificial pinning center (APC) was studied as a function of the applied magnetic field H using a SQUID magnetometer for fields up to 5T. The Nb-Ti wires were placed into the magnetometer with axis lined up parallel to H direction. Surprisingly, the critical current density J sub c presents a non-monotonic behavior with H, i.e., J sub c (H) curves present a broad maximum at some field H=H sub max (peak effect). Experiments were carried out in samples of APC Nb-Ti wires with pure Nb as the pinning center and in a conventionally processed Nb-Ti wire ( alpha -Ti precipitate as pinning center). The magnetization was measured for fixed temperatures of 5, 6 and 7K. Results show similar behavior by both types of wire. If the spacing (d sub s ) among pinning centers is held constant and temperature is increased, the value of the field H sub max decreases and if temperature is kept constant, as d sub s is increased, the value of H sub max also decreases. For d sub s and temperature constant, curves of J sub c (H) for Nb and alpha -Ti precipitates present nearly the same H sub max , suggesting that the effect is independent of the pinning center material.