Magnetisation switching dynamics of perpendicular shape anisotropy spin transfer torque MRAM

With the rapid development of technologies such as artificial intelligence, cloud computing, and industrial internet of things, there is an urgent need for high-performance but low-power memory technology. Commercial spin transfer torque magnetic random access memory (STT-MRAM) based on a CoFeB/MgO magnetic tunnel junction has the potential to become a universal memory, satisfying the requirement for high speed and low power consumption. For thermal stability at reduced dimensions, MRAM may exploit perpendicular shape anisotropy (PSA) by creating tower structures with the long axis much larger than the width. Here we use an atomistic spin model to explore the finite size and temperature effects on this novel STT-PSA-MRAM design. STT is captured with a spin accumulation model. Our tower structures are cylindrical and 5 nm in diameter, and we vary the free layer thickness from 8 nm to 48 nm. We find that as the thickness of the free layer increases, a larger current density is required to initiate the reversal because of the higher anisotropy. We also observe a change in the reversal mechanism from coherent to incoherent as the free layer thickness is increased beyond 8 nm. Interestingly, at lower current densities, towers taller than 38 nm exhibit a complicated incoherent reversal mechanism, while at larger current densities the reversal proceeds via a more familiar propagated domain wall motion. Our results provide new insights into the reversal dynamics of STT-PSA-MRAM devices and can guide the design and optimisation of future memory devices.