  In this work, we evaluate a multitude of metal-oxide bi-layers and
demonstrate the benefits from increased memory stability via multibit memory
operation. We introduce a programming methodology that allows for operating
metal-oxide memristive devices as multibit memory elements with highly packed
yet clearly discernible memory states. We finally demonstrate a 5.5-bit memory
cell (47 resistive states) with excellent retention and power consumption
performance. This paves the way for neuromorphic and non-volatile memory
applications.
