Sub-100-nm multistate spin–orbit torque magnetic tunnel junction enabled by local perpendicular anisotropy gradient

The rapid development of artificial intelligence, particularly in brain-inspired computing and edge inference, demands memory technologies with high information density, energy efficiency, and scalability. Multistate storage offers a promising route to meet these requirements. Spin–orbit torque magnetic random-access memory (SOT-MRAM), with ultrafast switching, low power consumption, and excellent process compatibility, is therefore regarded as a promising platform for multistate storage. However, realizing stable and controllable multistate magnetization at the nanoscale remains a significant challenge. In this work, we propose a method for multistate magnetic switching by introducing a nanoscale gradient of perpendicular magnetic anisotropy via ion implantation. This gradient causes the SOT-driven magnetization switching region to expand progressively with increasing current amplitude, thereby generating intermediate magnetization states and enabling multistate switching. Based on this mechanism, we demonstrate field-free multistate magnetization switching in 70-nm-scale SOT magnetic tunnel junction devices. This approach provides a versatile and fabrication-compatible strategy for achieving stable and scalable multistate magnetization control in nanoscale SOT devices, with important implications for high-density memory and next-generation spintronic devices targeting neuromorphic and edge-computing applications.