Non-collinear vortex states in twinned YBa2Cu3O7?δ single crystals

The magnetic moments parallel and perpendicular to thec-axis (measured simultaneously) have been studied as a function of direction of applied magnetic fieldH in twinned and de-twinned YBa₂Cu₃O_7−δ single crystals. They show pronounced anomalies when the field direction approaches thec-axis. These allow clear identification of the angle ϕ_L at which vortices are locked into twin planes. Complete shielding of theH _ab field component (transverse Meissner effect) was observed in the locked state. For larger angles, up to ϕ_T, the vortices continue to be non-collinear with the applied field, but their direction deviates from the trapping plane. ϕ_L exhibits a 1/H field-dependence, whereas ϕ_T shows only weak logarithmic variations withH.