C-AXIS TRANSPORT CRITICAL-CURRENT IN MELT-TEXTURED YBCO - TWIN PLANESINFLUENCE ON THE PINNING OF THE FLUX LINES

The pinning of flux lines by the twin planes is studied in bulk textur ed YBCO single domains using magnetic field angular measurements of c- axis critical current density J(c)(Phi). Above a matching field B-M as sociated with the twin boundary network, J(c)(Phi) shows a fourfold pe ak structure related to the pinning by each variant of the twin plane (TP). Below B-M, J(c) is angular and field independent, which is consi stent with a complete adaptation of the flux lines to the twin planes network. In the direction of the TP, J(c) is independent of the field intensity up to 8 T suggesting that twin plane pinning might have a ma gnetic origin in addition to the core pinning effect.