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

Abstract

We report a low-temperature study of magnetic relaxation for fields $30 \mathrm{Oe}\ensuremath{\le}H\ensuremath{\le}30 \mathrm{kOe}$ 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 \ifmmode\pm\else\textpm\fi{} 100 Oe and 250 \ifmmode\pm\else\textpm\fi{} 50 Oe for $\mathbf{H}\ensuremath{\parallel}\mathbf{c}$ and $\mathbf{H}\ensuremath{\perp}\mathbf{c}$, respectively. The field dependence of the relaxation rates above the critical fields is interpreted with a thermally activated flux creep model.