Understanding the Role of Shape Imperfections on the Bit Error Rates of STT-MRAM Under External Magnetic Fields

The bit error rate (BER) of spin-transfer-torque magnetic random access memory (STT-MRAM) under active write mode is measured in the presence of external magnetic fields of varying magnitude and orientation. Experimentally measured BER is then compared with macrospin and micromagnetic models to understand the observed dependence of BER on the magnetic field orientation. The results reveal a significant increase in the BER for specific polar orientations of the external magnetic field, confirming the recent theoretical prediction and experimental reports. Shape imperfections in magnetic tunnel junction (MTJ) nanopillars also significantly impact the BER variation. In-plane orientations of the external magnetic fields are also observed to impact BER variations, which indicate the loss of circular symmetry of the studied MTJ devices. The measured BER asymmetry could be explained once shape imperfections are considered in the model. These findings offer valuable insights into the active write magnetic immunity of STT-MRAM, particularly for cache applications demanding fast write speeds and ultralow BERs.