New Spin on Electronics

At the borders between physics, chemistry, materials science and electrical engineering, is where some of the most exciting technological developments take place. Think polymer light-emitting diodes, porous silicon explosives and now spintronics.

Denis Greig and Robert Cywinski at Leeds University are working with colleagues at the Universities of Salford and York to grow ultra-thin magnetic films on semiconducting materials and to characterise their surface chemistry. Spotting dead layers with no magnetism in the surface atomic layers of such material composites will be important in controlling the spin properties of the material in the bulk.

Oxide magnets such as CrO2La1-xCaxMnO3, Fe3O4 and the Heusler alloy NiMnSb are being developed by Lesley Cohen (http://www.ph.ic.ac.uk/staffcv/cohen.htm) and Tony Stradling (http://www.ph.ic.ac.uk/staffcv/stradling.htm) of Imperial College with European Union funding. They reckon these materials will make excellent sources of spin-polarized currents. They are trying to grow such “half metallic ferromagnets” on high-quality semiconductor layers, such as indium arsenide. The ultimate aim being to fabricate to fabricate spin transistors and highly sensitive magnetic sensors.

“A huge effort is being generated world wide in this area,” explains Stradling. This is mainly driven by the putative link between spintronics and quantum computing, which once researchers get it to work will provide much faster information processing than is presently possible. Much of the spintronics work is going on in many UK physics departments, such as Bath, Bristol, Cambridge, Glasgow, Heriot-Watt, Nottingham, Oxford, St Andrews, Salford, Southampton, and York.

Such efforts are at the centre of worldwide multidisciplinary efforts to add another dimension of control to electronics and bring us into the realm of spintronic devices. Physicists are still just learning how to control the spins of electrons to allow them to align them on the fly in materials being created to exploit their properties in new computers, sensors, and other devices. Materials designed to exploit the spin of the electron are beginning to emerge. The only spintronic device which is presently being used in real systems though is the two-terminal GMR type of sensor . But, spintronics has potential in fields as diverse as position and motion sensors for robots, fuel-handling systems for vehicles and chemical plants, military guidance systems, and even the next generation of keyhole surgery techniques.

The mobile phone hanging on your belt, the motherboard in your PC, or the amplifier in your portable MP3 player all use charge carriers to transport information in semiconductor materials such as silicon. But, charge is not the only intrinsic property of the electron, in common with certain other sub-atomic particles, the electron also has spin.

The spin of an electron is not like the spin of a pool ball though. Rotate a pool ball through 360 degrees and you get it back to where you started. If you can see the “8” and you rotate that ball 360 degrees you will see it come around again. If an electron could be marked in some way, you would have to rotate it through 720 degrees before you saw the marker again. It takes two full turns for an electron to “spin” once. This bizarre quantum property is not at all far-fetched, one just has to remember that an electron is not a tiny pool ball, but a sub-atomic entity closer to a notional Möbius strip than a sphere. Anyway, I digress. Electron spin, can be up or down, and this property will be used to develop spintronic devices that are smaller, more versatile and more robust than any conventional microelectronic circuit.

It is the alignment of electron spin that gives rise to the bulk magnetic properties of a metal such as iron or cobalt. If the bulk of the electrons in a piece of iron are either up or down then the iron is magnetised. The magnetic state of an iron particle can be written and read – viz. magnetic data storage, from tape to disk. The state is represented by the orientation of the iron’s magnetic moment.

But, rather than looking at the properties of chunks of iron, what if we consider the more subtle effects seen with a sandwich of very thin layers of material – the outside layers might be cobalt or another magnetic material while the innards would be non-magnetic. The magnetic layers have their electrons either all up or all down as usual. But, because of the thinness of the layers electrons with the same spin can pass through the non-magnetic layer while those of opposite spin are deflected, or scattered back. Because of this the sandwich acts as a filter for either up or down electrons depending on the nature of the magnetic layers.

At this level one begins to see a much stronger effect – the “giant magnetoresistance” (GMR) effect, which relies on this filtering of electron spins across the layers. The effect, an increase in resistance due to the presence of a magnetic field, is 200 times greater than seen with common magnetoresistance and provides the basis of the read head in multi-gigabyte computer hard disks pioneered by IBM.

It is of course possible to change the orientation of the spins of the electrons in each layer so that they can be made the same or opposite, in this way the number of electrons that are scattered back can be changed. A device that functions in this way has been referred to as a spin valve because it can be used to inhibit or open up current flow. Any device acting as a valve can be considered a switch, and a switch to a computer designer means logical unit or memory bit. Simply flipping the orientation of spin in one layer changes the spin valve from a 0 setting to a 1 setting and so forms the basis of a new magnetic version of random access memory (RAM). Importantly, MRAM is non-volatile. Switch off the power on your computer now and you’ll lose everything in memory. Not so with MRAM, what is written to memory stays there until it is deliberately wiped. Some observers caution that while this is true in that it has indeed been proposed it has yet to be shown that GMR RAM chips are a viable technology; remember magnetic bubble memory? No?

Quite.

However, we already have GMR read heads – to read high density media – although increasing data density is still a major aim of IBM and other manufacturers, while developments in the preparative methods of insulating metal oxide layers and other materials are making for rapid progress in MRAM. Other devices are beginning to emerge with the development of controllable spin transistors and the like as magnetic semiconductors and other oxides are designed with greater magnetoresistance and spintronics effects. The silicon age is not quite passé, it will be with us a long while yet, but there is a new spin on electronics.

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