Evidence for incoherent reversal in perpendicular MRAM cells down to 20 nm diameter

We investigate experimentally the time evolution of the magnetization reversal in spin-transfer torque driven perpendicular magnetoresistive memory cells in the diameter range of 20 to 65nm. The switching process is characterized by two metrics: the incubation time and the reversal time, where both are stochastic in nature. We find that the average reversal time increases with cell size. The reversal time is dominated by domain wall pinning effects that can last several tens of nanoseconds for the bigger devices. However, even for the smallest cells, we still observe similar behavior albeit with shorter pinning durations of the order of 2 to 3 nanoseconds, indicating that the magnetization reversal is incoherent.