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

The behavior of the magnetization critical current density J(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. presents a non-monotonic behavior with H. i.e., J(c)(H) curves present a broad maximum at some field H = H-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 7 K. Results show similar behavior by both types of wire. If the spacing (d(s)) among pinning centers is held constant and temperature is increased. the value of the field H-max decreases and if temperature is kept constant. as d(s) is increased, the value of H-max also decreases. For d(s) and temperature constant, curves of J(c) (H) for Nb and alpha -Ti precipitates present nearly the same H-max, suggesting that the effect is independent of the pinning center material. 2001 Elsevier Science B.V. All rights reserved.