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Spin is the thing

Rupert Goodwins ZDNet.co.uk

Published: 26 Sep 2003 14:30 BST

The most exciting thing about spin is that it sticks around. If you build an electronic circuit based on the flow of electrons -- current, in other words -- it only works while that flow continues. That needs a constant input of energy, and is almost always somewhat inefficient. Circuits that rely on stored charge have areas with an excess of electrons and areas with a deficit, and over time the electrons themselves will flow to even out the difference. Once an electron is spinning in a particular direction, though, it stays spinning until something happens to disturb it. This is exploited to the extreme in paleomagnetism, where scientists read magnetic signatures laid down by the Earth's field in rocks. Sequences have been calibrated back to the Jurassic period and useful results obtained from basement rocks laid down four billion years ago: sufficiently non-volatile for most purposes.

IBM, Motorola and other companies have developed MRAM -- magnetic memory -- that electronically manipulates spin for storage purposes, and these can combine non-volatility with high densities and DRAM-like speeds. We may see MRAM in mobile phones and the like sometime in 2004: previous attempts at commercialisation have hit production difficulties, but the companies involved this time around are being extremely bullish.

The second most exciting thing about spin is that it can take vanishingly small amounts of power and time to set up. An electron doesn't need to move to have its spin changed, whereas traditional circuits rely on vast electronic movement. Regions of a conductor with electrons spin-aligned with each other tend to reject currents of electrons with a different spin, which can be a very efficient way to let information modulate signals -- the basic way computers represent and act on data. One of the most active areas of research at the moment is how to integrate spin injection and detection with existing techniques: numerous theoretical and practical designs exist for various forms of spintronic transistors and other components, but the basic building block for spintronic logic is yet to come. When it does, it promises much faster switching, much higher densities and much lower power consumption -- purely because there's much less electron manipulation needed. At the logical lower limit, you can build a transistor that works on a single electron, and SEDs -- Single Electron Devices -- that use spin have been demonstrated under lab conditions.

There's also an aspect of electron spin that may in time overshadow the rest. As with all subatomic particles, quantum physics applies directly to the electron. It can spin in either direction, but until the direction is known by measuring the electron can be said to be spinning in both directions simultaneously -- an uncollapsed state. Quantum computing works by encoding multiple bits of information onto just such a state, forming a quantum bit or qubit. A spin transistor can theoretically form a physical framework within which qubits encoded onto uncollapsed spin can be handled. This may open the way for enormously effective parallel computations, where very complex problems are solved by the near-instantaneous decomposition of chains of qubits, but in principle one must remain uncertain.

This is a long way away from flash memory and faster ways to run Microsoft Office. However, one of the most exciting aspects of spintronics is that it can use many of the same techniques as ordinary solid-state semiconductor physics. If the right materials can be found that include magnetic properties alongside normal charge-based transistor techniques, then spin can be introduced to devices very rapidly. Researchers at the Royal Institute of Technology in Stockholm say that they've created just such a material -- zinc oxide with manganese doping. Zinc oxide is a widely used semiconductor and optically active material; manganese is a magnetic metal that is just the right size to fit into the zinc oxide structure without distorting it.

There is a consensus among researchers that practical spintronic devices will be in production in five to 10 years. Consensus has been wrong before, but the field is being intensely investigated and while many problems remain none seem to be showstoppers. By adding an entirely new dimension to electronics, spintronics may create as big a step change in our technologies as did the invention of the transistor.