DIRECT OBSERVATION OF NONCOLLINEAR VORTEX STRUCTURES INDUCED BY TWIN BOUNDARIES IN YBA2CU3OY SINGLE-CRYSTALS

Using Vector magnetic moment measurements, in twinned YBa2Cu3Oy single crystals we observe a nearly reversible M(ab) component of the magnet isation with sharp peaks in the dependence of M(ab) on the magnetic fi eld H and angle phi between c-axis and magnetic field The association of these features with twin boundaries is supported by geometrical arg uments and by comparison with determined samples. For a crystal with a single dominant direction of twin boundaries, the initial slope of th e M(ab)(H) and M(ab)(phi) dependences corresponds closely to ideal shi elding of the transverse H component, as expected for the fully-locked vortex state. The peak position which is related to the locking angle , shows a close to linear H-p(-1) vs phi dependence, which corresponds to a fixed value of H-L, the locking field. The influence of twins va nishes above a trapping angle phi(T). which shows rather weak temperat ure and magnetic field dependences; at 60K and 1 T these parameters ar e mu(0)H(L) = 27mT and phi(T) approximate to 10 degrees.