Magnetisation ac-loss measurements on YBa2Cu3O7 tapes with weakly-ferromagnetic NiW substrates

Abstract

The commonly-used NiW substrate for RaBiTS-process 2G conductors is weakly-ferromagnetic, so that the interaction between the superconductor and the magnetic material is important, particularly for AC applications. We have studied the AC losses in a model system of a high-current 2G YBCO tape that has been fabricated on a non-ferromagnetic substrate (Hastelloy), with and without adjacent weakly-ferromagnetic Ni-5at%W tape(s). This approach allows us to explore fully the interactions between substrate and superconductor. The sample architectures examined were YBCO, NiW-YBCO, NiW-YBCO-NiW, YBCO-NiW and NiW in magnetic fields applied perpendicular, parallel and at 45° to the tape normal. The power losses were measured in applied ac-magnetic fields of up to 100 mT rms at close to power line frequencies and at liquid nitrogen temperature. In addition, complementary magnetisation measurements were made on some architectures. Below the penetration field of the superconducting tape, the overall AC losses in the perpendicular field orientation are reduced significantly by a single adjacent NiW layer, with a further slight decrease with the addition of a second NiW layer. Above the penetration field the losses are not altered greatly. At very low fields the dominant loss mechanism appears to be the ferromagnetic hysteresis loss.