Knowm Memristors come in multiple variants (W, Sn, Cr and C) which refers to the element/dopant introduced in the active layer during fabrication. The material stack is based on mobile metal ion conduction through a chalcogenide material. The devices are fabricated with a layer of metal that is easily oxidizable, located near one electrode. When a voltage is applied across the device with the more positive potential on the electrode near this metal layer, the metal is oxidized to form ions. Once formed, the ions move through the device towards the lower potential electrode. The ions move through a layer of amorphous chalcogenide material (the active layer) to reach the lower potential electrode where they are reduced to their metallic form and eventually form a conductive pathway between both electrodes that spans the active material layer, lowering the device resistance. Reversing the direction of the applied potential causes the conductive channel to dissolve and the device resistance to increase. The devices are bipolar, cycling between high and low resistance values by switching the polarity of the applied potential. The resistance is related at any time to the amount of metal located within the active layer.
More information on SDC memristors can be found in the open-access paper Self-directed channel memristor for high temperature operation