Wiregate: Metallic picket fence flips magnetic bits

Computers typically use magnetism to store data, not to process it. A team of researchers in England is challenging that convention with a magnetic microcircuit that carries out simple but important steps in computation.

This all-magnetic computing could offer numerous advantages over conventional electronics, says team leader Russell P. Cowburn of the University of Durham. Those potential benefits include lower manufacturing costs, decreased power consumption, and retention of data when the power goes off. The most likely first applications for such circuits would be in cell phones, smart cards, and other portable gizmos, Cowburn predicts.

Computer hard disks and other magnetic-storage devices record binary data as patches of magnetization. To represent a 0, the magnetization points one direction. To represent a 1, it points the opposite way. Between the opposing patches lies a region called a domain wall, which Cowburn describes as resembling a stretchy, movable partition more than a fixed wall.

To exploit magnetization for computing, he and his colleagues deposited exquisitely thin loops of an easily magnetized nickel-iron compound called permalloy onto silicon wafers. Then, using external magnets, the researchers introduced domain walls into the thin metal strips and propelled those walls around the microscopic, rectangular loops.

Each loop includes one or more short, sharp burrs that jut toward the box’s center.

It’s at those burrs that the computing takes place.

“In electronics, it doesn’t matter what shapes the wires are, but in magnetic wires, the shape is quite important,” Cowburn notes. Because of a burr’s cusplike shape, opposite magnetizations can meet at the peak without a domain wall between them–much as opposing streams can join smoothly together if they merge rather than meet head-on. However, when one of the externally propelled domain walls rushing around the loop passes through a peak, it flips the orientations of both the converging magnetizations.

In that way, each peak acts as a so-called NOT gate, or a component whose output always has the opposite value of its input, the scientists report in the June 14 Science. Their experiments also demonstrate that loops with multiple peaks act as shift registers, or components that pass bits along from one position to the next, in the manner of a bucket brigade.

The design of those peaks “is fiendishly clever,” comments Gregory L. Snider of the University of Notre Dame in Indiana, who is developing a type of electronic circuitry that uses patches of metal on silicon instead of transistors (SN: 5/8/99, p. 303).

Those patches inspired the new magnetic microcircuits, Cowburn says.

“It’s a nice scientific advance,” notes William J. Gallagher of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. However, he adds, it’s “not quite complete in terms of an invention.” Among other missing pieces, one more kind of gate is needed for such circuits to perform the full range of computations of an ordinary computer, he points out. Cowburn says his group is close to creating that needed gate.