March 24, 2016

Magnetic memory chips at theoretical limit for low energy will use one million times less energy per computer operation

In a breakthrough for energy-efficient computing, UC Berkeley engineers have shown for the first time that magnetic chips can actually operate at the lowest fundamental energy dissipation theoretically possible under the laws of thermodynamics. This means that dramatic reductions in power consumption are possible — down to as little as one-millionth the amount of energy per operation used by transistors in modern computers.

This is critical for mobile devices, which demand powerful processors that can run for a day or more on small, lightweight batteries. On a larger, industrial scale, as computing increasingly moves into “the cloud,” the electricity demands of the giant cloud data centers are multiplying, collectively taking an increasing share of the country’s — and world’s — electrical grid.

“We wanted to know how small we could shrink the amount of energy needed for computing,” said senior author Jeffrey Bokor, a UC Berkeley professor of electrical engineering and computer sciences and a faculty scientist at the Lawrence Berkeley National Laboratory. “The biggest challenge in designing computers and, in fact, all our electronics today is reducing their energy consumption.”

Magnetic microscope image of three nanomagnetic computer bits. Each bit is a tiny bar magnet only 90 nanometers long. In the image on the left, the bright spots are at the “North” end of the magnet, and the dark spots are at the “South” end. The “H” arrow shows the direction of magnetic field applied to switch the direction of the magnets. (Image by Jeongmin Hong and Jeffrey Bokor, study in Science Advances.)

They experimentally tested and confirmed the Landauer limit, named after IBM Research Lab’s Rolf Landauer, who in 1961 found that in any computer, each single bit operation must expend an absolute minimum amount of energy.

Landauer’s discovery is based on the second law of thermodynamics, which states that as any physical system is transformed, going from a state of higher concentration to lower concentration, it gets increasingly disordered. That loss of order is called entropy, and it comes off as waste heat.

Landauer developed a formula to calculate this lowest limit of energy required for a computer operation. The result depends on the temperature of the computer; at room temperature, the limit amounts to about 3 zeptojoules, or one-hundredth the energy given up by a single atom when it emits one photon of light.

The UC Berkeley team used an innovative technique to measure the tiny amount of energy dissipation that resulted when they flipped a nanomagnetic bit. The researchers used a laser probe to carefully follow the direction that the magnet was pointing as an external magnetic field was used to rotate the magnet from “up” to “down” or vice versa.

They determined that it only took 15 millielectron volts of energy – the equivalent of 3 zeptojoules – to flip a magnetic bit at room temperature, effectively demonstrating the Landauer limit.

This is the first time that a practical memory bit could be manipulated and observed under conditions that would allow the Landauer limit to be reached

While this paper is a proof of principle, he noted that putting such chips into practical production will take more time. But the authors noted in the paper that “the significance of this result is that today’s computers are far from the fundamental limit and that future dramatic reductions in power consumption are possible.”

SOURCES -Berkeley Labs, Science Advances

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