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

The behavior of the magnetization critical current density Jc 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 5 T. The Nb-Ti wires were placed into the magnetometer with axis lined up parallel to H direction. Surprisingly, the critical current density Jc presents a non-monotonic behavior with H, i.e., Jc(H) curves present a broad maximum at some field H=Hmax (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 (α-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 (ds) among pinning centers is held constant and temperature is increased, the value of the field Hmax decreases and if temperature is kept constant, as ds is increased, the value of Hmax also decreases. For ds and temperature constant, curves of Jc (H) for Nb and α-Ti precipitates present nearly the same Hmax, suggesting that the effect is independent of the pinning center material