Two terminal devices with switchable resistance have been of interest to electrical engineers for a long time, but only in the last few years has this attracted widespread attention. Recently a BiFeOe (BFO) capacitor-like metal-insulator-metal (MIM) structure was proposed as a synthetic synapse in neuromorphic systems, implementing voltage waveform driven spike timing dependent plasticity (STDP). Using a new device model that faithfully reproduces measurements of BFO-MIM structures we analyze how the switching characteristic affects the STDP learning window. Our simulations indicate that the gradual increase in the resistance change of BFO MIM structures result in a robust STDP with a biologically realistic learning window, whereas a distinct threshold followed by a steep hysteresis curve produce a narrow learning window and inflict strict operating conditions. Therefore we conclude that the steepness of the current voltage hysteresis curve is a fundamental characteristic to consider when designing synthetic synapses for neuromorphic hardware.

Memristive devices have recently been proposed as efficient implementations of plastic synapses in neuromorphic systems. The plasticity in these memristive devices, i.e. their resistance change, is defined by the applied waveforms. This behavior resembles biological synapses, whose plasticity is also triggered by mechanisms that are determined by local waveforms. However, learning in memristive devices has so far been approached mostly on a pragmatic technological level. The focus seems to be on finding any waveform that achieves spike-timing-dependent plasticity (STDP), without regard to the biological veracity of said waveforms or to further important forms of plasticity. Bridging this gap, we make use of a plasticity model driven by neuron waveforms that explains a large number of experimental observations and adapt it to the characteristics of the recently introduced BiFeO$_3$ memristive material. Based on this approach, we show STDP for the first time for this material, with learning window replication superior to previous memristor-based STDP implementations. We also demonstrate in measurements that it is possible to overlay short and long term plasticity at a memristive device in the form of the well-known triplet plasticity. To the best of our knowledge, this is the first implementations of triplet plasticity on any physical memristive device.