2.2  Physical Implementation

Physical demonstration of the memristor was missed for decades. However, as Chua showed recently, regardless the device material and switching mechanism, any resistance switching phenomenon depicts a memristive behavior [71, 72]. This phenomenon was observed in titanium dioxide (TiO   2   ) [103] even before Chua’s envision in 1971. Nevertheless, it revived interests in memristor development only after Hewlett Packard Laboratories announced the first memristor array fabricated in 2008 [69] based on a Pt/TiO2   /Pt thin-film structure. Moreover, it has been shown that due to ionic motion in metal/oxide/metal thin-film stacks, including both anion-based [104, 105, 106, 107] and cation-based [108, 109, 110, 111, 112, 113] switching materials, these structures exhibit resistance switching and thus demonstrate memristive behavior forming an important class of memristive devices [114, 81].

The electrons spin degree of freedom allows for realization of a memristive behavior when the spin-transfer torque effect is employed to change the resistance state of a spintronic device. After the first announcement of the memristor based on TiO2   thin-films, the spin-based memristor has drawn a lot of attention as it ensures a more convenient control of the resistive state, than the ionic transport, especially at the nanoscale. Therefore, several spintronic memristive devices [60, 61, 62, 63, 64, 65, 115, 116, 66, 67] have been proposed and explored.

Furthermore, there have been several reports of providing memristive behavior by using new internal state variables based on other phenomena and technologies like insulator-to-metal phase transition [117], phase change memory [118], piezoelectric effect [119], chemical immobilization of ferritin molecules [120], defect-scattering in a single-walled carbon nanotube [121], nickel titanium smart alloy [122, 123], Graetz bridge loaded with an RLC filter [124], thickening/thinning of Ag nanofilaments in amorphous manganite thin-films [125], nanoscale plasmonic [126], multi-terminal silicon nanowires [127], and volatile resistive switching effect at a prototypical Schottky metal/oxide interface [128].