Phase-change materials (PCMs) could thwart the limits of digital memory and enable a massive increase in storage density. PCMs can hold information by switching between an amorphous state and a crystalline state. PCM memory also can write and retrieve data 100 times faster than Flash memory, and can be reused at least 10 million times. PCM's biggest advantage is that it can store more than a single bit per cell. "If you are able to control the current you can create states between the two, something that is not fully crystallized and something that is not fully amorphous," says IBM Zurich's Evangelos Eleftheriou.
University of Exeter researcher David Wright has demonstrated 512 discrete states in a single 20-nanometer cell. However, he says differentiating between these distinct states requires highly sensitive and expensive equipment, which would not be practical in a chip. Another hurdle is the issue of drift, in which the resistance of the material changes over time.
IBM has developed a two-fold solution involving electrically measuring the amorphous thickness of the material instead of the resistance, and simultaneously reading multiple cells to measure their relative drifted positions.
From New Scientist
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