Direct observation of tilted vortex structures induced by twin boundaries in YBa2Cu3Oy single crystals

In twinned YBa2Cu3Oy single crystals we have observed that as the magnetic field H is tilted away from the c axis, a nearly reversible ab-plane magnetic moment m(ab) is generated that has a sharp maximum as a function of the tilt angle phi; this moment is absent from detwinned samples. Thus the measured direction of the vortices deviates from that of the applied field, reflecting their attraction to the twin boundaries (TB's). For a crystal with a single dominant direction of TB's, the size of m(ab) at low fields is consistent with ideal shielding of the ab-plane field component H-ab, as expected for the vortex state that is fully locked to the TB's. The maximum in m(ab) at a fixed temperature occurs at a value of H-ab that is nearly independent of phi; we identify this as the locking field H-L. The trapping angle phi(T), above which the kinked-vortex state vanishes, is apparent in the data too. The qualitative features of these results agree with current understanding of the vortex-TB interaction, but there are several quantitative discrepancies.