Asymmetries in the Field and Angle Dependences of the Critical Current in HTS Tapes

A sharp dip in critical current has been observed in the field dependence of the critical current I-c of commercial American Superconductor Corporation (AMSC) second-generation high-temperature superconducting tapes on Ni-5%W substrates, when the field is applied parallel to the plane of the tape. The dip occurs centered at a nonzero field for one field polarity only-the polarity in which the Lorentz force in the superconductor is toward the substrate. The polarity for which the dip occurs changes with the sample orientation and current direction, remaining consistent with the direction of the Lorentz force relative to the substrate. Angle dependences of I-c in these tapes are consistent with this effect, lacking the usual 180 degrees periodicity when measured under magnetic field strengths matching that of the dip in the field dependence. These effects are not observed on otherwise identical tapes produced with nonmagnetic Ni-9%W substrates, indicating that the ferromagnetism of the substrate plays a determining role. A finite-element model shows that this asymmetrical dip occurs naturally as a result of field concentration in the magnetic substrate, leading to a significant perpendicular component of field in the superconductor at a particular nonzero field strength.