Critical current of grain boundaries in Y Ba2 Cu3 Ox bicrystal films as a function of oxygen concentration

The effect of oxygen doping on the critical current density JC across [001] tilt grain boundaries in Y Ba2 Cu3 Ox thin films, epitaxially grown on bicrystal substrates, is investigated. Grain boundary misorientations were varied between 0°and 24°. It is observed that for 24°grain boundaries, the critical current enhancement due to oxygen overdoping can be as large as a factor of 10 when compared to JC at optimal doping (where TC is maximum). This oxygen-induced enhancement of the grain boundary JC is similar to that observed with optimized Ca doping. For 15°and 10°grain boundaries, the relative enhancement due to oxygen doping is smaller than that of the 24°grain boundary. It is observed that the grain boundaries often degrade after repeated oxygen treatments, most likely because of chemical contamination. Much larger enhancements in the grain boundary critical current densities might be achievable with heavy oxygen doping if this competing degradation could be avoided. The critical current is determined using a contact-free magnetic method which measures the maximum current that can be induced by an external field in a ring sample. For a 24°grain boundary, we demonstrate that the critical current determined by the magnetization method is in excellent agreement with JC obtained from transport measurements.