Pt/Ti/Al2O3/Al tunnel junctions showing electroforming-free bipolar   resistive switching behavior
  We investigated electroforming-free bipolar resistive switching behavior in Pt/Ti/Al2O3/Al tunnel junctions where the Al2O3 tunnel barrier was naturally formed on Al in air. Various compliance current values for the junction's set switching successfully lead to various resistance values in its low resistance state, suggesting the possibility for multi-level-operation. A mechanism for the bipolar switching is qualitatively discussed in terms of the modulation of the tunnel barrier by the reactive Ti layer on top of the barrier. 
Doo Seok Jeong, Byung-ki Cheong, and Hermann Kohlstedt
cond-mat.mtrl-sci