A tiny device that answers "no" when it's told "yes" and vice versa could mark the first step toward microchips that calculate magnetically. The novel "NOT gate," described in the 14 June issue of Science, uses a trick of geometry to manipulate magnetism as conventional devices do electric charge.
Electronic microchips crunch numbers by shuffling dollops of charge. But physicists and electrical engineers are striving to harness a more subtle property of electrons: the fact that the particles behave like spinning tops and are magnetized along their spin axes. Burgeoning "spintronics" technologies would store information and perform calculations more efficiently (Science, 16 November 2001, p. 1488). But researchers have yet to carry out calculations with just changes in magnetization.
The new device, developed by Russell Cowburn and colleagues at the University of Durham in the United Kingdom, consists of a simple track of naturally magnetic nickel-iron wire, shaped like an upside-down Y. The magnetism of the alloy naturally runs parallel to the track, but a short length can be made to flip direction. In that case, the two opposing magnetizations meet at a "domain wall." There they either both point toward the domain wall (head to head) or both point away from it (toe to toe). Those two magnetic configurations can be used to encode 0 and 1 values.
Cowburn and colleagues found a way to switch between the two arrangements by using a magnetic field to force the domain wall across the stem of the inverted Y from one branch to another. As this happens (see figure), the system flips from the head-to-head configuration to the toe-to-toe configuration or vice versa. By swapping domain-wall configurations, the device exchanges 0 for 1 and 1 for 0--the hallmark of a logical NOT gate. Next, the researchers hope to develop other devices, such as an "AND gate."
"It's just very clever," says Craig Lent, an electrical engineer at the University of Notre Dame in Indiana. Chips that manipulate magnetism should resist damage from radiation and retain information if they inadvertently lose power, so they might be useful in spacecraft and other harsh environments, adds Russell Beech, an electrical engineer at NVE Corp. in Eden Prairie, Minnesota.