Picosecond programmable magnetoresistive artificial synapse with a compact arc-shaped domain wall track

Abstract

An arc-shaped domain wall (DW) track is used to construct a compact sub-micrometer domain wall magnetic tunnel junction (DW-MTJ) device for application as an artificial synapse. DW-MTJ devices, offering the advantages of state-of-the-art magnetic random-access-memory (MRAM) technology and spin–orbit-torque-driven ultrafast DW motion, have been studied extensively for their application as artificial synapses. However, the existing device proposals have many limitations: large device footprint, insufficient retention, unreliable programming, and nonlinearity. Another significant issue is initializing the DW track with a single DW. The DW-MTJ proposed in this work attempts to solve these problems. Here, up to 16 levels are realized in a sub-micrometer scale DW-MTJ employing an arc-shaped DW track and a synthetic-antiferromagnet-based design. Carefully tuned geometric pinning structures are used to ensure each level’s thermal stability and retention lifetime. A reliable and highly linear programming operation is demonstrated using detailed micromagnetic simulation. The optimized device allows programming times of tens of picoseconds, offering ultra-low energy operation. Finally, a simple initialization method is also proposed, taking advantage of the unique shape of the DW track. Together, the proposed device provides a compact shape, has data-retention properties per conventional non-volatile memory technologies, offers an error-free and energy-efficient programming scheme, and is compatible with MRAM fabrication flow. The results show that DW-MTJ-based devices could be a reliable and efficient building block for neuromorphic circuits and systems, encouraging chip-scale implementation of such technologies.