  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.
