Bent
wires make cheap circuits
By
Kimberly Patch,
Technology Research NewsToday's computers use the presence or absence of a flow of electrons to represent the ones and zeros of binary logic, and they detect this current by sensing electrical charge. There's more to electrons than charge, however. They also have spin, a magnetic property similar to the two poles of an ordinary refrigerator magnet.
Magnetic spin is already used to store information in the microscopic bits of disk drives and in a recently-developed magnetic memory chip. Using spin to compute as well would pave the way for all-magnetic, cheap, low-power computers. And because the spin states of electrons remain stable even when the power goes off, such a computer would not have to boot up every time it is turned on.
Current spintronics efforts generally focus on using spin to carry out the logic operations of computing by moving electrons around while preserving their spins.
Another way to move a spin signal, however, is to find a way to ripple the signal through a series of stationary electrons, similar to the way a row of dominos affects each other. Carrying out the logic operations of computing this way requires moving the magnetic domain wall -- the region along a wire where magnetization changes direction -- through a series of logic circuits.
Researchers from Durham University in England have taken a large step in this direction by constructing a NOT gate -- one the basic logic circuits of computers -- that carries out a computation using the spin of electrons. "You replace the high-voltage/low-voltage representation of the numbers one and zero with North pole/South pole," said Russell Cowburn, a lecturer in physics at Durham University.
Information flows when a spin flips its direction, and then causes its neighbors to flip. "Instead of sending voltage... around the chip, you send magnetic domain walls," said Cowburn.
A NOT gate changes an input signal to its opposite; when a one goes through a NOT gate it comes out as a zero, and vice versa. "The NOT gate is just a nanoscale magnetic wire that is [bent] into a hairpin so that as the domain wall moves around it, the magnetization direction is reversed," said Cowburn. The path that reverses the direction of magnetization is similar to the path a car takes when it does a K-turn to reverse direction.
The researchers designed the logic circuit after realizing that they could drive domain walls around a tiny magnetic circuit using a rotating magnetic field, said Cowburn. "We found that the sense of rotation -- clockwise or counter-clockwise -- is really important and can be used to give you immense control over the flow of information," said Cowburn.
To fabricate such circuits, the researchers had to make extremely small, smooth wires. "We had to perform some very accurate metal fabrication -- making magnetic wires with edges that are smooth to within 10 or 20 nanometers," Cowburn said. A nanometer is one millionth of a millimeter, or about the size of a line of 10 carbon atoms.
In order to see that the circuit was working, the researchers had to find a way to measure whether the signal had changed. They developed a laser that measures magnetic signals the same way an oscilloscope measures electric signals. "You just drop the laser beam onto the part of the spintronics circuit that you want to probe, and the magnetic waveform appears on the computer screen," said Cowburn.
The researchers also built a larger circuit consisting of 11 NOT gates to show that the gates can be linked together. The circuits formed a shift register, which computers use to change the position of a series of binary digits. Shifting the number 100 one place to the left, for example, yields 1000. Shifting it one place to the right yields 10.
The researchers are currently working on an AND gate, which has two or more inputs, and returns a one rather than a zero only if all of the inputs are one. The NOT and AND gates are the basic building blocks of computer logic. "Once we've got [the AND gate] we could build any circuits that could be built from conventional digital logic," said Cowburn.
Ultimately, the researchers are aiming to make a spin-based computer chip that "does all the things that conventional electronic chips do, but with the cost, power, size and non-volatility advantages that come from magnetic logic," said Cowburn.
The researchers' spin design can potentially make for extremely small chips that are relatively inexpensive to manufacture because the circuits are made from single wires rather than semiconductor transistors, said Cowburn. Semiconductors are shaped using chemicals and light to peel away layers of material. The researchers devices, however, can be made from a single layer, eliminating the need to align multiple layers, which is one of the costliest processes in chip manufacture.
The simpler wire-based circuit design is also a good candidate for self-assembly processes, where materials are built molecule-by-molecule using mixes of chemicals, similar to the way biological organisms are made. Eventually, "it should be possible to shrink the devices to much smaller sizes -- possibly close to the atomic scale," Cowburn said.
The amount of power needed to shunt domain walls around would be considerably less than is needed for electrical current, making spin circuits potentially useful for small mobile devices like phones and smart cards, said Cowburn. "The ballpark is in the range [of] 100 to 1000 times lower power," he said.
In the distant future the devices could be implanted inside the human body to, for example, monitor biological functions, he said. The devices would require so little power they could be supplied from a small electric coil placed outside of the body that would surround the spin circuits with an electromagnetic field, rather than having to wire it directly to a power source, according to Cowburn.
The research is an interesting twist on well-established physics, according to Jay Kikkawa, an assistant professor of physics at the University of Pennsylvania. "While it is known that magnetic fields can transmit a wall from one end of a wire to the other, the authors show that the shape of the wire can invert the wall en route," he said.
What is needed next are similar geometrical concepts that can be used to form more complicated logic gates, according to Kikkawa. "With a few additional innovations, computing elements could be constructed for certain non-volatile applications. I'm particularly interested to see how the fidelity of these gates will hold up at higher gate speeds and densities," he said.
Simple spin devices could be made practical within the next two years, Cowburn said. "More complicated devices will take a little longer -- probably five years," he said.
Cowburn's research colleagues were Dan A. Allwood, Xiong Gang, Michael D. Cooke, Del Atkinson, Colm C. Faulkner and Nicolas Vernier. They published the research in the June 14, 2002 issue of the journal Science. The research was funded by the private engineering investment company.
Timeline: 2-5 years
Funding: Corporate
TRN Categories: Integrated Circuits, Physics, Spintronics
Story Type: News
Related Elements: Technical paper, "Submicrometer Ferromagnetic NOT Gate and Shift Register," Science, June 14, 2002.
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June 26/July 3, 2002
Page One
PCs augment reality
Stamps bang out tiny silicon lines
Bent wires make cheap circuits
Mixes make tiniest transistors
Plastic computer memory advances
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