For the last 40 years, computer-chip manufacturers have been constantly improving the performance of their products by shrinking the size of transistors – the building blocks of processors and memory chips. Having gone from dimensions of a few microns in the early 1970s to 45 nm in Intel’s latest prototypes, the number of transistors that can be etched on a given area of silicon has been doubling every 18 months. This exponential trend, which is known as Moore’s law, has been the driving force behind the relentless miniaturization of microelectronic devices. However, we are now approaching the physical limits of existing technology. Chips are getting too hot and are now so small that electrons can tunnel between the transistor electrodes and degrade device performance. Unless we can find a way to solve these problems, within a decade it will be impossible to reduce the size of semiconductor transistors any further.
There is an even more aggressive trend in the data-storage industry, whereby the capacity of disk drives has been doubling every 12 months since the first hard drive was introduced in 1956. The maximum amount of information that can be stored on a hard drive is limited by the size of the magnetic particles on the surface of the disk, as well as by the size of the “head” used to read and write the data. Recently, the US firm Seagate Technology built a drive with a storage density of 420 gigabits per square inch. To read the information from such dense media we require a sensing device with similar dimensions to the magnetic particles (for example, for a storage density of 100 gigabits per square inch the particles on the hard disk have dimensions of about 150 × 40 nm). This means that we will eventually need to find new ways to achieve higher storage densities.
Spintronics could allow us to continue in our quest for miniaturization beyond the limits of current technology. This is because in addition to exploiting the charge of electrons (as in conventional microelectronics), spintronics takes advantage of the electron’s intrinsic angular momentum, or spin, to encode and process information. This quantum-mechanical property effectively turns an electron into a tiny compass needle, which therefore causes the electrical resistance of a magnetic material to increase or decrease depending on the direction of an external magnetic field. Called anisotropic magnetoresistance, and discovered by William Thomson (later Lord Kelvin) in 1856, this phenomenon currently forms the basis of the most promising spintronic devices.
In the August issue of Physics World, Andrei Sokolov looks at how spintronics is paving the way for a new generation of fast computers with ultra-dense data storage.
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