Tuning a Binary Ferromagnet into a Multistate Synapse with Spin-Orbit-Torque-Induced Plasticity
Authors: Cao, Y; Rushforth, AW; Sheng, Y; Zheng, HZ; Wang, KY
ADVANCED FUNCTIONAL MATERIALS
Volume: 29 Issue: 25 Published: JUN 2019 Language: English Document type: Article
Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange-coupled by another in-plane ferromagnetic layer (FM2), where a magnetic field-free current-driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin-orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing-dependent plasticity are demonstrated. This functionality is made possible by the action of the in-plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in-plane field component tuning the efficiency of the spin-orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in-plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics.