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March 29, 2010

Proposal for an all-spin logic device with built-in memory

Nature Nanotechnology - Proposal for an all-spin logic device with built-in memory

A spintronic device in which the input, output and internal states are all represented by spin, and that shows the five essential characteristics necessary for logic applications, is proposed.

The possible use of spin rather than charge as a state variable in devices for processing and storing information has been widely discussed because it could allow low-power operation and might also have applications in quantum computing. However, spin-based experiments and proposals for logic applications typically use spin only as an internal variable, the terminal quantities for each individual logic gate still being charge-based. This requires repeated spin-to-charge conversion, using extra hardware that offsets any possible advantage. Here we propose a spintronic device that uses spin at every stage of its operation. Input and output information are represented by the magnetization of nanomagnets that communicate through spin-coherent channels. Based on simulations with an experimentally benchmarked model, we argue that the device is both feasible and shows the five essential characteristics for logic applications: concatenability, nonlinearity, feedback elimination, gain and a complete set of Boolean operations



12 pages of supplemental material

Relevant Design Issues

A. Magnet alignment

The perpendicular alignment of the equilibrium axes of fixed and free layers has been proposed for the first time. This is different from structures with perpendicular polarizers and perpendicular magnetic anisotropy(PMA) media. The essential point here is to neutralize the magnetization of the free layer through a fixed layer utilizing spintorque. At this time we are aware of three possible ways to accomplish this:
1) Using exchange anisotropy to align the equilibrium magnetization of the fixed layer perpendicular to that of the free layer. The closest experimental realization we are aware of is that in the experiment of Krivorotovetal. where the equilibrium axes make 30◦ with each other. We believe it should be possible to use similar techniques based on exchange anisotropy to achieve the perpendicular alignment

2) Alternatively, the freelayers could be implemented using materials with Perpendicular Magnetic Anisotropy(PMA) which have already been realized. Such structures could utilize an inplane polarized fixed layer and PMAmaterial for the free layer. This would also diminish the effect of the out-of-plane demagnetizing field

3)Beyond the magnet alignment, the third way also has something to do with the circuitry, clocking and architecture aspects of ASL.

B. Channel design

Si is one candidate that could be used for the channel material(it has high spin coherence length). The channel has to be conductive for ASLD’s operation.

C. Ferromagnet-Semiconductor interface

Spin injection from a Ferromagnet into a semiconductor is a serious problem. It is now well established experimentally that the presence of a tunnel barrier enhances the spin injection efficiency. For ASLD the tunnel barrier can be utilized at the injection side of each databit. However when a magnet acts as an output, it would be better to have an Ohmic contact such that the magnet acts as a sink for spin accumulation in the channel.

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