Grain boundary characteristics of oxypnictide NdFeAs(O,F) superconductors

We have systematically investigated the grain boundary angle (theta(GB)) dependence of transport properties for NdFeAs(O,F) fabricated on [001]-tilt symmetric MgO bicrystal substrates. In our previous study, NdFeAs(O,F) bicrystal films showed a weak-link behaviour even at a theta(GB) of 6 degrees. However, this was caused by an extrinsic effect originating from the damage to both NdFeAs(O,F) and MgO substrate by excess F-diffusion along the grain boundary. To investigate the intrinsic nature of grain boundaries, we minimized the damage to NdFeAs(O, F) and MgO by reducing the deposition temperature of the NdOF over-layer needed for F-doping. The resultant NdFeAs(O,F) bicrystal films have a critical angle of 8.5 degrees, above which J(c) starts to decrease exponentially. This critical angle is almost the same as those of other Fe-based superconductors.