Critical current across grain boundaries in melt-texturedYBa2Cu3O7−δrings

Abstract

We determine the critical current ${J}_{\mathrm{cB}}$ of grain boundaries fabricated by ``welding'' of melt-textured YBCO with various degrees of $\mathbf{ab}$-plane misalignment. ${J}_{\mathrm{cB}}$ is determined by monitoring the magnetic moment due to persistent shielding currents, which were induced in rings containing two sections of a single grain boundary. The voltage drop across the junctions is estimated to be below ${10}^{\ensuremath{-}12}\mathrm{V},$ much smaller than in typical transport measurements. As the temperature or magnetic field is increased, an abrupt decrease (kink) is observed in the magnetic moment of the ring when the induced shielding current in the ring exceeds the critical current across the boundaries. The kink signals that flux has begun to penetrate through the grain boundaries into the bore of the ring. This behavior is confirmed by magneto-optical imaging. We observe that grain boundaries with [001] tilt mismatch angles up to 5\ifmmode^\circ\else\textdegree\fi{} have critical current densities in excess of ${10}^{4}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2},$ comparable to the bulk current density. At larger mismatch angles, the critical current rapidly decreases with increasing angle. The qualitative behavior of ${J}_{\mathrm{cB}}$ as function of temperature, magnetic field, and misorientation angle is similar to that observed YBCO grain boundaries manufactured by other methods.