Effect of twin boundaries on flux pinning in YBa2Cu3O7−xat low and intermediate magnetic fields

Abstract

Magnetic measurements on a single crystal of YBa2Cu3O7−x in the twinned and detwinned states have demonstrated that flux pinning by twin boundaries is strongly dependent upon temperature and the magnitude and orientation of the applied field with respect to the crystallographic orientation. For applied magnetic fields perpendicular to the c axis, no effect of twin boundaries on flux pinning was found at any temperature. In contrast, for fields up to 400 Oe aligned parallel to the c axis, the hysteresis loops at 77 K clearly showed increased pinning due to twin boundaries. At 40 K and low fields (H<500 Oe) the flux pinning was nearly the same for the twinned and detwinned states, whereas at higher fields (up to 40 kOe), pinning was higher in the twinned state. At this temperature, a tendency toward flux depinning occurred in the detwinned state at about 10 kOe, and both the twinned and detwinned states exhibited a more rigid flux structure at 50 kOe than at 10 kOe.