Anisotropy of the vortex creep in a YBa2Cu3O7-x single crystal with unidirectional twin boundaries

Vortex creep in a single crystal containing unidirectional twin boundaries (TBs) is investigated at temperatures of 82-87 K in a special experimental geometry: J parallel to ab, J parallel to TB, H perpendicular toJ, with alp ha=angleH,ab as a variable parameter. It is shown that in low magnetic fiel ds the TBs alter the configuration of the structure of the flux lines at an gles of misorientation theta between the magnetic field vector H and the pl anes of the TBs of up to 70 degrees: at angles theta < 70 degrees a part of the flux line is trapped by the planes of the TBs. It is shown that a TB i s an efficient pinning center for the motion of the vortices perpendicular to the plane of the TB, and therefore in low magnetic fields at angles thet a < 70 degrees a directed motion of the vortices along the planes of the TB s occurs. The angle dependence of the activation energy for a plastic mecha nism of flux creep is determined, and it is found to agree with the theoret ical estimates made. For an orientation of the vector H close to the ab pla ne of the crystal, the maximum of the angle dependence of the measured "cri tical" current J(cE)(alpha) observed for the the H parallel to ab orientati on of the field vector in low magnetic fields gives way to a minimum at hig her magnetic fields; this is explained by a change from single-vortex creep to collective creep as the magnetic field is increased. (C) 2001 American Institute of Physics.