Impact of addition of magnetic nanoparticles on vortex pinning and microstructure properties of Bi-Sr-Ca-Cu-O superconductor

The major limitations of polycrystalline Bi-based superconductor applications are the weak flux pinning and weak links in granular networks at relatively high temperature and applied magnetic fields. Effective flux pinning centers with various physical properties were artificially introduced into the superconducting matrix to enhance the flux pinning capability. Among these, introduction of nanoscale magnetic pinning sites into high T-c superconductors is still controversial. We have investigated the effects of nanosized NiO addition on the phase formation and flux pinning of the polycrystalline (Bi1.8Pb0.4Sr2Ca2.2Cu3O delta)(1-x)(NiO)(x) with (x = 0.000, 0.001, 0.002, and 0.005) using XRD, SEM, AFM, and magnetization measurements. The results show that, with the increase of x, the volume fraction of the high critical temperature (T-c) phase (2223) decreases gradually accompanied with an increase in the low T-c phase (2212). However, almost no change in the T-c (onset) for all samples was observed. Measurements of the temperature dependence of the remanent magnetization, M-R reveal a nonmonotonic behavior with the nano-NiO addition. A clear enhancement of the pinning strength was observed for the sample with x = 0.002 at quite high temperature (T = 90 K). These results were discussed in terms of the interaction of the magnetic nanoparticles with the superconducting matrix especially at elevated temperatures close to T-c and above the depinning line. The relation between the current carrying mechanisms, phase contents, flux pinning and the microstructures of the samples were also discussed for a granular superconductor. It is believed that these magnetic nanoparticles may act as pinning sites in granular superconductors in applied magnetic field. Therefore, it is very important to carefully control all parameters involved in magnetic particles' addition in order to balance the critical current for optimum in-field performance. (C) 2010 Elsevier Ltd. All rights reserved.