Probabilistic behavior in subnanosecond spin transfer torque switching

We studied the magnetization dynamics induced by spin transfer torque (STT) in CoFe∕Ru∕CoFe∕Cu∕NiFe nanopillars using Landau-Lifshitz-Gilbert micromagnetic simulations including the STT term given by Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)] and a Gaussian thermal random force. Our simulation results clearly show the probabilistic switching behavior. The antiparallel to parallel switching at 77K requires a smaller overdrive current than at 300K. The parallel to antiparallel switching is not dependent on temperature. Our results show that C-like bending induced by the Ampere field plays an important role rather than the misalignment which arises from the thermal magnetization fluctuation.