The residual low-angle grain boundary (GB) network is still the most important current-limiting mechanism operating in biaxially textured rare-earth barium-copper-oxide (REBCO) coated conductors. While Ca doping is well established to improve supercurrent flow across low-angle GBs in weak fields at high temperatures, Ca doping also depresses $T_c$, making it so far impractical for high-temperature applications of REBCO coated conductors. On the other hand, high-field-magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca doping in improving the GB transparency, $r^{\mathrm{GB}}=J_c^{\mathrm{GB}}/J_c^{\mathrm{grain}}$, of low-angle $\mathrm{Y}{\mathrm{b}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{BaCuO}$ [001] tilt bicrystal films down to 10 K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using low-temperature scanning laser microscopy and magneto-optical imaging, we found $r^{\mathrm{GB}}$ to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which $J_c^{\mathrm{GB}}$ can exceed $J_c^{\mathrm{grain}}\left(r^{\mathrm{GB}}>1\right)$ where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that $r^{\mathrm{GB}}$ became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high-$J_c$ channels between the GB dislocation cores are almost Ca free. It may be therefore that the positive effects of Ca doping seen by many authors near $T_c$ are partly a consequence of the higher $T_c$ of these Ca-free channels.

• Received 30 November 2014
• Revised 12 February 2015

DOI:https://doi.org/10.1103/PhysRevB.91.104504