Thermally assisted spin transfer torque switching in synthetic free layers

We studied the magnetization reversal rates of thermally assisted spin transfer torque switching in a synthetic free layer theoretically. By solving the Fokker-Planck equation, we obtained the analytical expression of the switching probability for both the weak and the strong-coupling limit. We found that the thermal stability is proportional to Delta(0)(1 - I/I(c))(2), not Delta(0)(1 - I/I(c)), as argued by Koch et al. [Phys. Rev. Lett. 92, 088302 (2004)], where I and I(c) are the electric current and the critical current, of spin transfer torque switching at absolute zero temperature, respectively. The difference in the exponent of (1 - I/I(c)) leads to a significant underestimation of the thermal stability Delta(0). We also found that fast switching is achieved by choosing the appropriate direction of the applied field.