Current conduction through low angle grain boundaries in YBa₂Cu₃O₇₋₈ thin films studied magnetometrically

Taking advantage of the sensitivity of SQUID-based instrumentation and a simple ring geometry that facilitates the calculation of a current in the sample, we introduce a new magnetometric method to obtain the critical current density flowing through grain boundaries in the high-temperature superconductor YBa2Cu307-8 (YBCO). Since all magnetic moments originate from moving charges or electrical currents, an electrical current flowing in a simple configuration can be calculated from its magnetic moment or vice versa. Ring shaped YBCO films with or without [001]-tilt grain boundary were made on SrTiO3 substrates on surfaces perpendicular to the c-axis. Magnetic moments of rings containing 1.8°, 2.8°, 5.1°, and 7° [001]-tilt grain boundaries were measured at various temperatures and fields parallel to the c-axis; as well, rings with no grain boundaries were measured for comparison. Current densities of the samples were calculated from the measured magnetic moments using a modified Bean critical state model. Especially, we could extract a pure grain boundary current density (current flowing across the grain boundary) from the total magnetic moment of a grain boundary ring. An abrupt decrease of the current density was observed as the grain boundary angle increased. Even small misorientations of 1.8° <<$'<5.1° diminish the current density by a large amount, while a 1.8° grain boundary does not. Moreover, large peaks were formed in curves of magnetic moment versus field for grain boundary samples under conditions of decreasing magnetic field (but not increasing field history). The appearance of a large peak in the m(H) curves of a grain boundary sample can be explained by the weak-link behavior of a small angle grain boundary. The high sensitivity of the magnetic moment and the grain boundary current density to field changes on a grain boundary strongly bolsters the weak-link interpretation of small angle grain boundaries.