Flux-line lock-in to CuO planes in aLa1.9Sr0.1CuO4single crystal

We report on vector magnetization studies of a La{sub 1.9}Sr{sub 0.1}CuO{sub 4} single crystal, cut in the form of a disk with the anisotropy (c) axis parallel to the disk plane. We observe a sharp threshold in the angular dependences of the isothermal remanent magnetization. When the magnetic field is applied within a threshold angle to the ab plane, the remanent moment is strictly parallel to ab, and also the c-axis component of magnetization is reversible. The threshold angle scales with temperature in the same manner as the lower critical field parallel to the c axis, H{sub c1}{sup c}. These observations give direct support to the presence of a vortex lock-in transition to the CuO planes in our crystal. {copyright} {ital 1997} {ital The American Physical Society}