Investigation on the impact of spin current profile on the write time of SOT MRAMs

In this work, we quantify the non-uniformity in the spin current density generated by spin-orbit torque (SOT) at the nanoscale and its impact on the switching of SOT magnetic random-access memories (MRAMs). In recent years, SOTMRAMs have emerged as promising non-volatile candidates for last-level (L3/L4) cache due to their high endurance, sufficiently low read/write latency, long retention times, and scalability. In these devices, a conduction current is passed through the non-magnetic (NM) layer, which generates a spin current flowing towards the ferromagnetic (FM) layer due to the Spin Hall Effect (SHE). Using conventional drift-diffusion models, which consider the electric current distribution to be uniform within the FM and NM layers, can lead to erroneous results in the case of nanoscale devices. In this paper, we use the spin current distribution calculated based on finite element simulations and drift-diffusion equations in micromagnetic simulation. We demonstrate that spin current density can be significantly lower at the two edges of the magnet compared to the middle and this non-uniformity can affect the magnet switching dynamics. We investigate the impact of this non-uniformity for both perpendicular magnetic anisotropy (PMA) and in-plane magnetic anisotropy (IMA) based magnetic tunnel junctions (MTJs). Our results show that when resistive NM layers are used, the impact of nonuniform spin current density on write times is more significant for larger FMs. In addition, the variation in write times is more significant in the case of PMA FM than IMA FM.