Magneto optical characterizations of flux dynamics in Nd-Ba-Cu-O bulk superconductors

We investigated the magnetic flux dynamics of Nd-Ba-Cu-O (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method using the Magneto-Optical (M-O) technique at various low temperatures. An external magnetic field was applied both parallel (H∥c) and perpendicular (H ⊥ c) to the c-axis of the sample. While for H∥c the flux motion was isotropic, for H ⊥ c anisotropic penetration and expulsion of the flux lines were directly visualized. In the configuration H ⊥ c the flux lines were observed to move in and out the sample predominantly parallel to the a-b planes. The flux behavior was also correlated to the microstructure. Particularly, the microcracks along the basal plane were found to provide a channel for easy flux motion. The anisotropic behavior of the flux front reflects the anisotropy in the critical current density Jc. The different components of J_c were independently calculated from the M-O data directly analyzing the magnetic induction profiles. These J_c values were consistent with those estimated interpreting the magnetization hysteresis measurements within the framework of the extended Bean model.