Comment Articles

Spin is the thing

Rupert Goodwins ZDNet.co.uk

Published: 26 Sep 2003 14:30 BST

You don't have to be a chip engineer to be sick of Moore's Law, that hoary old observation that about every two years the number of transistors on a chip doubles. I'd like to propose Goodwins' Corollary: every two years the number of times Moore's Law is mentioned at chip conferences is itself doubled. Nonetheless, the whole IT industry depends on regular and revolutionary advances in basic chip technology -- and now, many physicists are getting excited about a very different kind of electronics called spintronics. This takes the fundamental tool of electronics -- the electron -- and uses it in a very different way.

For a familiar friend, the electron is a slippery customer. It lures us into a sense of security by having commonplace attributes like mass, electric charge and spin. What it doesn't have is a body -- mathematically, an electron is an infinitely small dot spreading out through space. Any comparisons made with objects we can imagine are just convenient metaphors: like all subatomic particles, electrons are more alien than anything you'll ever find in science fiction.

Fortunately for physicists and the rest of us, while we don't know what electrons are we do know what they do. Electrons can be persuaded to move through wires by changing the charge at its ends: that's the basis for electricity, and by connecting wires in various ways through switches the electrons can move power from place to place. Semiconductors take small numbers of electrons and use them to switch much larger numbers: thus signals change other signals, and data can be used to create other data.

So far, the electron's spin has been ignored. The electron can only spin at one speed, and then only in one of two directions -- spin up and spin down -- and it doesn't really spin at all. It's just that the mathematics of that property of the electron behaves like the mathematics of a spinning top, and so the idea makes a handy analogy.

However, electron spin is very familiar to us in another way -- it's at the heart of magnetism. Most materials have an equal mix of electrons spinning in both directions, and thus have no magnetic field. If you have a preponderance of electrons spinning in a particular direction, then you get a magnetic field -- and conversely, if you magnetise something you change the ratio of spins. In that sense, all magnetic media from the very first audio recorders to the latest half-terabyte hard disks use spin engineering. Hard disks that use giant magnetoresistive heads -- these days, that's almost all of them -- go a step further and use an explicit spintronic effect, where the different spins of electrons create a changing resistance in a detector.