Spin torque switching of an in-plane magnetized system in a thermally activated region

The current dependence of the exponent of the spin torque switching rate of an in-plane magnetized system was investigated by solving the Fokker-Planck equation with low temperature and small damping and current approximations. We derived the analytical expressions of the critical currents, I_{c} and I_{c}^{*}. At I_{c}, the initial state parallel to the easy axis becomes unstable, while at I_{c}^{*} (\simeq 1.27 I_{c}) the switching occurs without the thermal fluctuation. The current dependence of the exponent of the switching rate is well described by (1-I/I_{c}^{*})^{b}, where the value of the exponent b is approximately unity for I < I_{c}, while b rapidly increases up to 2.2 with increasing current for I_{c} < I < I_{c}^{*}. The linear dependence for I < I_{c} agrees with the other works, while the nonlinear dependence for I_{c} < I < I_{c}^{*} was newly found by the present work. The nonlinear dependence is important for analysis of the experimental results, because most experiments are performed in the current region of I_{c} < I < I_{c}^{*}.