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Direct Identification of the Conducting Channels in a Functioning Memristive Device

Structures composed of transition metal oxides can display a rich variety of electronic and magnetic properties including superconductivity, multiferroic behavior, and colossal magnetoresistance. [ 1 ] An additional property of technological relevance is the bipolar resistance switching phenomenon [ 2–4 ] seen in many perovskites [ 5–7 ] and binary oxides [ 8 ] when arranged in metal/insulator/metal (MIM) structures. These devices exhibit electrically driven switching of the resistance by 1000x or greater and have recently been identifi ed [ 9 ] as memristive systems, the fourth fundamental passive circuit element. [ 10 , 11 ] A full understanding of the atomic-scale mechanism and identifi cation of the material changes within the oxide remains an important goal. [ 12 ] Here, we probe within a functioning TiO 2 memristor using synchrotron-based x-ray absorption spectromicroscopy and transmission electron microscopy (TEM). We observed that electroforming of the device generated an ordered Ti 4 O 7 Magnéli phase within the initially deposited TiO 2 matrix. In a memristive system, [ 11 ] the fl ow of charge dynamically changes the material conductivity, which is “remembered” even with the removal of bias. While bipolar resistance switching of metal oxides has been observed since the 1960s, [ 2 , 4 ] only recently has the connection to the analytical theory of the memristor been made. [ 9 ] In an attempt to describe microscopically the source of the resistance change, many physical models have been put forth, including generation and dissolution of conductive channels, [ 3 , 6 ] electronic trapping and space-charge current limiting effects, [ 13 ] strongly correlated electron effects such as a metal-insulator transition, [ 14 ] and changes localized to the interface. [ 15 ] Identifying the correct model and quantifying its physical parameters has been diffi cult using primarily electrical characterization. Meanwhile, direct physical characterization [ 7 ]