  Dependence of the thermally assisted spin torque switching probability on the
sweep electric current was investigated theoretically. The analytical
expressions of the switching times for $b=1$ and $b=2$ are derived based on the
rate equation, where $b$ is the exponent of the current term in the switching
rate. The switching current is approximately proportional to the temperature
$T$ and the logarithm of the sweep rate $v$ for both $b=1$ and $b=2$ in the
experimentally performed ranges of $T$ and $v$. Experiments in very low
temperature range are required to determine the exponent $b$.
