Nonlinear logarithmic time decay of magnetization in a single crystal of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$

Nonlinear logarithmic time decay of magnetization has been observed over a long measuring time period (t=22 h) for a single crystal of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{x}}$. At relatively low temperatures (T=12 K) linear ln(t) decay of the magnetization is observed for field (=1 kG) parallel to the c axis, which can be described well by the Anderson-Kim model. However, at high temperatures (T>26 K) the magnetization exhibits nonlinear logarithmic time near the irreversibility line. Possible connections between the experimental data and current theories are discussed.