  The spin torque assisted thermal switching of the single free layer was
studied theoretically. Based on the rate equation, we derived the theoretical
formulas of the most likely and mean switching currents of the sweep current
assisted magnetization switching, and found that the value of the exponent $b$
in the switching rate formula significantly affects the estimation of the
retention time of magnetic random access memory. Based on the Fokker-Planck
approach, we also showed that the value of $b$ should be two, not unity as
argued in the previous works.
