Magnetic relaxation and the lower critical fields in a y-ba-cu-o crystal

We report a low-temperature study of magnetic relaxation for fields $30 \mathrm{Oe}\le H\le 30 \mathrm{kOe}$ applied along the principal axes of a Y-Ba-Cu-O crystal. The lower critical fields are identified by the onset of relaxation at 900 ± 100 Oe and 250 ± 50 Oe for $\mathbf{H}\parallel \mathbf{c}$ and $\mathbf{H}\perp \mathbf{c}$, respectively. The field dependence of the relaxation rates above the critical fields is interpreted with a thermally activated flux creep model.