Second‐Order Memristor Based on All‐Oxide Multiferroic Tunnel Junction for Biorealistic Emulation of Synapses

Abstract The brain has the ability to learn and evaluate as it receives and registers information. Signals between neurons are transmitted via synapses whose plasticity is modulated by usage. A bio‐realistic electrical analog is a second‐order memristor, where the short‐term internal dynamics influence the long‐term state. It is achieved here with an all‐oxide multiferroic tunnel junction: La_0.7Sr_0.3MnO₃ / BaTiO₃ / La_0.7Sr_0.3MnO₃. Similar to the modulation of synaptic weight by stimuli, multi‐levels of resistance may be encoded by mean of voltage pulses, on long time scale and in correlation with short‐term effects. Neuromimetic learning functions are demonstrated: short and long‐term potentiation/depression, paired‐pulse facilitation/depression, spike‐rate‐ and experience‐dependent plasticity. The threshold frequency of pulse trains at which depression changes into potentiation depends on the previous activity, with a sliding effect as in neurobiology. The voltage pulses induce reversible changes of both dielectric polarization and oxygen vacancies distribution, generating transient trapped/detrapped charges at the two interfaces that govern the dynamic response. The resistance levels are determined by the final cationic ordering (the redox Mn³⁺/Mn⁴⁺ ratio) at the two interfacial layers. The stimulation/relaxation dynamics are close to that in biological counterparts. Such memristors can be used in hardware artificial networks for advanced processing/storage of the information.