VORTEX PINNING BY (001)-PLANES AND BY TWIN BOUNDARIES IN YBCO SINGLE-CRYSTALS

A YBCO single crystal having only one twin domain with very uniform twin spacing is investigated by torque magnetometry with field rotation parallel to a {110} plane which is either perpendicular or parallel to twin planes. Experimental data are analyzed with respect to irreversible and reversible torque contributions. From the latter one an anisotropy ratio gamma = 9 is determined. Several rotational hysteresis loss contributions are identified, the most remarkable of which are very anisotropic losses due to intrinsic pinning and twin pinning. By using different measuring geometries it is shown that twin boundaries do not assist in the intrinsic pinning process as it was suggested in the literature. Isotropic losses are ascribed to impurity atoms. Viscous losses arising during vortex motion are small in comparison to all other losses. The results are discussed in terms of recent pinning models from literature.