Flux jump avalanches in torque studies of YBa2Cu3O7

The authors have investigated the vortex state of YBCO crystals via magnetic torque τ(θ) to B=7T. The onset of irreversibility is clearly observed as a function of orientation θ (θ=angle between the field and the crystal ab plane), providing a unique view of the vortex liquid-vortex solid transition as well as of Jc(θ). Anomalous features in τ(θ) are observed for both H//ab and H//c. In addition, discontinuous changes in the irreversible torque are detected in a restricted angular regime, reminiscent of flux jumps in magnetization versus field studies in classical superconductors. These jumps arise as a sawtooth oscillation, nearly periodic in field angle with period 0.03 degrees to 0.1 degrees, with the jumps appearing to be discontinuous on a sub-millidegree scale. The authors will concentrate on this last feature, and discuss a scenario of a rearrangement of the vortex lattice in the form of motion of bundles of point vortices