The current non-volatile memory technology which is based on charge storage,such as flash memory,has become increasingly limited as the cell size decreases.Developing solid state system based non-volatile high speed memory that can exist in two or more stable states is one key technological step to extend the functional equivalence of Moores law.[1] Phasechange random access memory (PCRAM),[2] magnetoresistive random access memories (MRAMs),[3] etc.,are candidates for next-generation non-volatile memories.Yet another promising class of high performance non-volatile memory is resistive random access memory (RRAM)[4] which utilizes an electric field induced resistive switching (RS) effect in a metal/metaloxide/metal (M/MO/M) sandwiched structure where the metal oxide is an insulator/semiconductor.The resistance of an M/MO/M device can be varied when applying an exteal electric field,and generally a high resistance state (HRS) and low resistance state (LRS)can be reversibly switched between with a bias of approximate amplitude and polarity.Basically,there are two kinds of RS.One is bipolar RS (BRS) whereby the switching depends on the polarity[4] and the other one is unipolar RS (URS) whereby the switching solely depends on the amplitude of the bias rather than the polarity.[5] The RS behavior,as a universal feature of metal oxide thin film based sandwiched devices,is thought much to be strongly related to oxygen and its defects (typically the oxygen vacancy).[6-10]More specifically,either the field induced redox or oxygen migration plays a central role in RS.In other words,the RS phenomenon benefits greatly from the strong coupling of an oxygen ion and its defects especially oxygen vacancy with properties of metal oxides.The exteal electric field can move the oxygen ion/defects to change the local concentration of oxygen ion/defects,resulting in the change of resistance.