Figure 5. Classification of mem devices.

(a) Memristive, memcapacitive and meminductive devices are assumed to offer pinched characteristics at the origin in general39. In the case of memcapacitive and meminductive devices, a spontaneous polarization of a ferroelectric and ferromagnetic material, respectively, leads for example, to non-zero-crossing characteristics11. Similarly, a non-zero-crossing I–V characteristic of a memristive device indicates the presence of an inherent nanobattery, that is, this device is active. Therefore, we introduce the generic terms extended memristive device, extended memcapacitive device and extended meminductive device to account for both zero-crossing and non-zero-crossing I–V characteristics. Note that the origin of non-zero-crossing behaviour in memcapacitive and meminductive devices is of completely different nature than in memristive devices, thus require specific modifications of the concept. Interestingly, spin-transfer torque (STT) MRAM cells offer zero-crossing in contrast to ReRAM cells, thus can be considered as conventional memristive device. (b) Equivalent circuit of the extended memristive element. The ionic current is defined by the nanobattery, which controls the state-dependent resistor representing the electronic current path. The capacitance of the device is neglected as its influence is not significant. The partial electronic conductivity in the electrolyte induces a state-independent resistance R_leak due to a leakage current in parallel. (c) Simulated I–V characteristic of the extended memristor. The zoom shows the non-zero-crossing behaviour. Further considerations on the simulation are described in Supplementary Note 4.