(color online) Reversible and nonvolatile switching of Au/Ti/TiO₂/Au devices. The upper inset shows a schematic diagram of the crossbar devices with an Au bottom electrode (BE, 25 nm), a TiO₂ thin film (40 nm), an adhesion Ti layer (5 nm) and an Au top electrode (TE, 35 nm). The red curves represent 50 experimental switching loops, which show a high level of repeatability. For each interval, the device stays in low resistance state (LRS) at initially and switches to high resistance state(HRS) at . Then, it switches back to LRS at and then finally returns to the initial LRS at . It is obvious that before the end of , the curve shows the negative differential resistance (NDR) behavior.