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 OeH30 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 H c and H c, respectively. The field dependence of the relaxation rates above the critical fields is interpreted with a thermally activated flux creep model. © 1988 The American Physical Society.