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

Abstract

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‖ab, J‖TB, H⊥J, with α≡∠H,ab as a variable parameter. It is shown that in low magnetic fields the TBs alter the configuration of the structure of the flux lines at angles of misorientation θ between the magnetic field vector H and the planes of the TBs of up to 70°: at angles θ<70° a part of the flux line is trapped by the planes of the TBs. It is shown that a TB is 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 θ<70° a directed motion of the vortices along the planes of the TBs occurs. The angle dependence of the activation energy for a plastic mechanism of flux creep is determined, and it is found to agree with the theoretical estimates made. For an orientation of the vector H close to the ab plane of the crystal, the maximum of the angle dependence of the measured “critical” current JcE(α) observed for the the H‖ab orientation of the field vector in low magnetic fields gives way to a minimum at higher magnetic fields; this is explained by a change from single-vortex creep to collective creep as the magnetic field is increased.