Experimental and Theoretical Investigation of Intracell Magnetic Coupling-Induced Variability of Spin-Transfer Torque Magnetic RAMs

The impact of intracell magnetic coupling on spin transfer torque-magnetic random access memory (STT-MRAM) device-to-device variability has been investigated based on experiments and simulations. Measured switching voltages of CoFeB/MgO STT-MRAMs are found to be directly correlated with offset fields extracted from magnetic measurements of the same set of devices. By spintronic and Monte Carlo simulations incorporating stochasticity and variability of the STT-MRAMs, the origin of the experimentally observed correlation has been traced back to the stray field arising from intracell magnetic coupling. Based on this finding, READ and write error rates (WERs) of STT-MRAMs have been evaluated in the presence of a stray field, employing the Fokker–Planck (FP) equation-based numerical model, calibrated against switching probabilities of measured devices. The results of the statistical simulation show that depending on the direction and magnitude of the stray field, READ and WERs preferentially increase in one switching direction, while decreasing in the opposite direction. The pulse widths required to bring the WRITE and READ error rates to a nominal value of
${10}^{-{6}}$
have been estimated considering ±20% variation of the stray field. It is also shown that READ operations of larger devices are more susceptible to intracell magnetic coupling-induced variability than the WRITE operations.