Power/Performance Bits: March 17

MRAM speed; electricity from humidity; recharging implants.


MRAM speed
Researchers at ETH Zurich and Imec investigated exactly how quickly magnetoresistive RAM (MRAM) can store data.

In the team’s MRAM, electrons with opposite spin directions are spatially separated by the spin-orbit interaction, creating an effective magnetic field that can be used to invert the direction of magnetization of a tiny metal dot.

“We know from earlier experiments, in which we stroboscopically scanned a single magnetic metal dot with X-rays, that the magnetization reversal happens very fast, in about a nanosecond,” said Eva Grimaldi, a post-doc at ETH Zurich. “However, those were mean values averaged over many reversal events. Now we wanted to know how exactly a single such event takes place and to show that it can work on an industry-compatible magnetic memory device.”

For their tests, the researchers replaced the metal dot with a magnetic tunnel junction, which contains two magnetic layers separated by a 1nm insulation layer. Electrons can tunnel through the insulating layer more easily or less depending on spin direction. This results in an electrical resistance that depends on the alignment of the magnetization in one layer with respect to the other, representing “0” and “1”. From the time dependence of that resistance during a reversal event, the researchers could reconstruct the exact dynamics of the process. In particular, they found that the magnetization reversal happens in two stages: an incubation stage, during which the magnetization stays constant, and the actual reversal stage, which lasts less than a nanosecond.

Electron microscope image of the magnetic tunnel junction (MTJ, at the centre) and of the electrodes for controlling and measuring the reversal process. (Image: P. Gambardella / ETH Zürich / Imec)

“For a fast and reliable memory device it is essential that the time fluctuations between the individual reversal events are minimized,” said Viola Krizakova, a PhD student at ETH Zurich.

To make those fluctuations as small as possible, the researchers changed the current pulses used to control the magnetization reversal in such a way as to introduce spin-transfer torque as well as a short voltage pulse during the reversal stage which resulted in a reduction of the total time for the reversal event to less than 0.3 nanoseconds, with temporal fluctuations of less than 0.2 nanoseconds.

“Putting all of this together, we have found a method whereby data can be stored in magnetic tunnel junctions virtually without any error and in less than a nanosecond,” said Pietro Gambardella, a professor at ETH Zurich.

The technology was tested on an industry-compatible wafer, making it available for use in a new generation of MRAM. Gambardella notes, however, that the market for MRAM memories currently does not demand such high writing speeds since other technical bottlenecks such as power losses caused by large switching currents limit the access times. In future work, the team plans to shrink the tunnel junctions and use different materials that use current more efficiently.

Electricity from humidity
Scientists at the University of Massachusetts Amherst built a device that uses proteins to generate electricity from moisture in the air.

Called “Air-gen” by the team, it uses electrodes connected to electrically conductive protein nanowires produced by the microbe Geobacter. It’s not limited to areas with high ambient moisture content: the researchers say it can generate power even in areas with extremely low humidity such as the Sahara Desert and it non-polluting, renewable, and low cost.

The Air-gen device uses a thin film of protein nanowires less than 10 microns thick. The bottom of the film rests on an electrode, while a smaller electrode that covers only part of the nanowire film sits on top. The film adsorbs water vapor from the atmosphere. A combination of the electrical conductivity and surface chemistry of the protein nanowires, coupled with the fine pores between the nanowires within the film, establishes the conditions that generate an electrical current between the two electrodes.

The researchers say that the current generation of Air-gen devices are able to power small electronics, and they expect to bring the invention to commercial scale soon. Next steps they plan include developing a small Air-gen “patch” that can power electronic wearables such as health and fitness monitors and smart watches, which would eliminate the requirement for traditional batteries. They also hope to develop Air-gens to apply to cell phones to eliminate periodic charging.

“The ultimate goal is to make large-scale systems,” said Jun Yao, an electrical engineer at UMass Amherst. “For example, the technology might be incorporated into wall paint that could help power your home. Or, we may develop stand-alone air-powered generators that supply electricity off the grid. Once we get to an industrial scale for wire production, I fully expect that we can make large systems that will make a major contribution to sustainable energy production.”

Xiaomeng Liu, a Ph.D. student at UMass Amherst, was developing sensor devices when he noticed something unexpected. “I saw that when the nanowires were contacted with electrodes in a specific way the devices generated a current. I found that that exposure to atmospheric humidity was essential and that protein nanowires adsorbed water, producing a voltage gradient across the device.”

“It’s the most amazing and exciting application of protein nanowires yet,” said Derek Lovley, a microbiologist at UMass Amherst who discovered the Geobacter microbe.

The team recently developed a new microbial strain to more rapidly and inexpensively mass produce protein nanowires. “We turned E. coli into a protein nanowire factory,” said Lovley. “With this new scalable process, protein nanowire supply will no longer be a bottleneck to developing these applications.”

Recharging bioelectronic implants
Researchers at King Abdullah University of Science and Technology (KAUST) and King Saud bin Abdulaziz University for Health Sciences found a way to use soft, biocompatible materials to ultrasonically charge bioelectronic implants like pacemakers, insulin pumps, and hearing implants.

While these devices run on batteries for a significant amount of time, they eventually run low, necessitating surgery to change the battery. Instead, scientists are investigating devices that could be recharged wirelessly.

The team’s approach uses a hydrogel, which is comprised of long polymer molecules cross-linked to form a three-dimensional network that can hold water. This makes them flexible and stretchable as well conductive.

The hydrogel combines polyvinyl alcohol with nanosheets of MXene, a transition-metal carbide, nitride or carbonitride. “Just as dissolving salt in water makes it conductive, we used MXene nanoflakes to create the hydrogel,” said Kanghyuck Lee of KAUST. “We were surprised to find that the resulting material can generate electric power under the influence of ultrasound waves.”

Their hydrogel, which they refer to as M-gel, generates a current when an applied pressure forces the flow of electrical ions in the water, filling the hydrogel. When this pressure is the result of ultrasound, the effect is called streaming vibration potential.

To test the concept, the team buried an electrical device within several centimeters of beef and used a range of ultrasonic sources, including ultrasound tips found in many labs and the ultrasound probes used in hospitals for imaging, to charge the device.

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