Enhancing thermal stability of synaptic states in spin-orbit-torque-driven synapse devices through the use of gradient multilayers

Synaptic spintronic devices based on domain-wall motion driven by spin-orbit torque are appealing for neuromorphic computing applications. Unlike in other technologies, though, here the long-term stability of multiple consecutive synaptic states is rare. To address this issue, the authors exploit the high thermal stability of magnetization in a heavy-metal/ferromagnetic-metal stack with graded thickness of the heavy metal. In micrometer-scale devices made from the stack, they demonstrate the stability of approximately 30 synaptic states for up to 1200 seconds, with some states stable for up to 50,000 seconds. These results pave the way for spintronic neuromorphic computing technologies.