EFFECT OF TWIN BOUNDARIES ON FLUX-PINNING IN YBA2CU3OX SINGLE-CRYSTALS

The effect of twin boundaries on flux motion has been studied by using a densely twinned and a perfectly detwinned domain in the same YBa2Cu 3Ox (123) single crystal. The most successful detwinning of the 123 cr ystals is a thermomechanical treatment under inert gas and successive oxygen annealing with keeping the perfectly detwinned state. The influ ence of the twin boundaries on flux pinning strongly depends on temper ature. The presence of the twin boundaries brings about a reduction of the pinning force at low temperature, but with an increase in tempera ture, the twin boundaries work as strong pins. Remarkable fishtail hys teresis loops are observed, especially in the detwinned domain.