I. Introduction
Memristor was first proposed by Chua [1] in 1971, and it was predicted as the fourth basic circuit element besides resistor, inductor and capacitor. The memristor can describe the relationship of magnetic flux and electric charge, and it can memorize the quantity of charge even though the excitation source is turned off. About 40 years later, Hewlett-Packard company prototyped practical memristor within nanometer size in 2008, which was a landmark progress to manufacture new arithmetic operation unit or nonvolatile memory chipset [2].