Computing with molecules: a big step in molecular spintronics

Spintronics includes the spin of electrons for information transport, storage and processing. Potential advantages are nonvolatility, increased data processing speed, decreased electric power consumption, and higher integration densities compared to conventional semiconductor devices.

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Molecular spintronics provides the ultimate step towards miniaturization of spintronics. It includes the active control over the spin states of individual molecules. Usually, spin lifetimes in isolated organic molecules are very short. Magnetic bistability is a typical solid-state property.  The so-called spin crossover materials that would provide more stable spin states usually lose their functionality upon absorption on surfaces while breaking up into individual molecules.The new molecular spin switches (“strapped” Ni-porphyrins) exhibit spin states that are stable at least for several days. The trick is similar to a flip-flop, the simplest bistable electronic circuit in computers, where the output is looped back to the input to generate two stable states. The molecules have three properties that are coupled with each other in a positive feedback loop: geometry (planar p or bent b), coordination (coord. c or non-coord n) and spin state (high-spin h or low-spin l). Thus, the molecules are locked either in one or in the other state (either: pch, or bnl). Switching between the two states is possible by injecting electrons into or releasing electrons from the dz2 orbital of the Ni ion. Filling of the dz2 orbital with two electrons leads to decoordination stabilizing the bnl state, removal of one electron induces coordination and switching to the pch state.

Upon sublimation and deposition on a silver surface, the switches self-assemble to highly ordered rows. Each molecule now can be addressed with an STM tip, and switched between the two spin states by applying a positive or negative voltage. Readout of the information at much lower voltages or by addressing magnetic properties is non-destructive. Next step is to connect these elementary units to more sophisticated electronic circuits to perform simple computing operations. Molecular spintronics provides a number of further advantages over conventional inorganic, layered systems. Molecules are the smallest stable, individual entities we can design and build with atomic precision and predictable properties. Moreover, we can create billions of copies of exactly identical molecular devices. Their response to electrical or optical stimuli and their custom-designed chemical and physical functionality makes them unique candidates to develop new classes of devices beyond information storage and processing, such as controllable surface catalysts or optical devices.

Rainer Herges

Prof. Dr., Kiel University