  Distribution of write error rate (WER) of spin-transfer-torque
magnetoreistive random access memory (STT MRAM) caused by a distribution of
resistance area product and anisotropy constant is theoretically studied.
Assuming that WER is much smaller than unity, and junction parameters obey a
normal distribution, we show that the WER obeys a logarithmic normal
distribution. We derive analytical expressions for the probability density
function and statistical measures. We find that the coefficient of variation of
WER can be reduced by decreasing the pulse width. We also perform numerical
simulations based on the Fokker-Planck equation and confirm the validity of the
analytical expressions. The results are useful for designing reliable STT
MRAMs.
