Power/Performance Bits: Nov. 7

Speeding up MRAM
Researchers at UC Berkeley and UC Riverside developed an ultrafast method for electrically controlling magnetism in certain metals, which could lead to increased performance for magnetic RAM. While the nonvolatility of MRAM is a boon, speeding up the writing of a single bit of information to less than 10 nanoseconds has been a challenge.

“The development of a non-volatile memory that is as fast as charge-based random-access memories could dramatically improve performance and energy efficiency of computing devices,” said Jeffrey Bokor, an electrical engineering and computer sciences professor at Berkeley. “That motivated us to look for new ways to control magnetism in materials at much higher speeds than in today’s MRAM.”

“Inspired by recent experiments in the Netherlands on ultrafast magnetic switching using short laser pulses, we built special circuits to study how magnetic metals respond to electrical pulses as short as a few trillionths of a second,” said Yang Yang, a researcher at Berkeley. “We found that in a magnetic alloy made up of gadolinium and iron, these fast electrical pulses can switch the direction of the magnetism in less than 10 picoseconds. That is orders of magnitude faster than any other MRAM technology.”

In this schematic of a magnetic memory array, an ultrafast electrical pulse switches a magnetic memory bit. (Source: UC Berkeley)

“The electrical pulse temporarily increases the energy of the iron atom’s electrons,” said Richard Wilson, an assistant professor of mechanical engineering at UC Riverside. “This increase in energy causes the magnetism in the iron and gadolinium atoms to exert torque on one another, and eventually leads to a reorientation of the metal’s magnetic poles. It’s a completely new way of using electrical currents to control magnets.”

The team continued the research by looking for a way to expand the approach to a broader class of magnetic materials, beyond the gadolinium-iron alloy.

“We found that when we stack a single-element magnetic metal such as cobalt on top of the gadolinium-iron alloy, the interaction between the two layers allows us to manipulate the magnetism of the cobalt on unprecedented time-scales as well,” said Jon Gorchon, a postdoctoral researcher at Lawrence Berkeley Lab and UC Berkeley.

“Together, these two discoveries provide a route toward ultrafast magnetic memories that enable a new generation of high-performance, low-power computing processors with high-speed, non-volatile memories right on chip,” Bokor said.

Batteries for better braces
Researchers at KAUST developed a new device to help straighten teeth. The orthodontic system involves placing two flexible near-infrared LEDs and one lithium-ion battery on every tooth in a semitransparent, 3D-printed dental brace.

The batteries provide energy to turn the near-infrared LEDs on and off, depending on how they are programmed by a dentist, to provide localized light therapy according to the needs of each tooth. Phototherapy enhances bone regeneration and can reduce the time and costs involved in corrective orthodontics, the team says. The brace would be removable to allow the batteries to be recharged.

“We started embedding flexible LEDs inside 3D-printed braces, but they needed a reliable power supply,” explains Muhammad Hussain of KAUST. “After the incidents with the Samsung Galaxy 7 batteries exploding, we realized that traditional batteries in their current form and encapsulation don’t serve our purpose. So we redesigned the state-of-the-art lithium-ion battery technology into a flexible battery, followed by biosafe encapsulation within the braces to make a smart dental brace.”

Each tooth has its own near-infrared LEDs to provide localized light therapy. (Source: KAUST)

The battery was redesigned using a dry-etching technique, which removes the silicon substrate normally found on its back. This process thinned the battery to 2.25mm x 1.7mm and made it flexible. Tests showed that the volumetric energy of the redesigned batteries remained high even after many cycles of continuous operation.

The batteries were then encapsulated in biocompatible soft polymeric materials to prevent the possibility of leakage, making them safe to place in the mouth. To test the biocompatibility, human embryonic kidney cells were cultured on the batteries over a period of days, where they thrived and proliferated. The batteries’ electrochemical performance increased linearly with rising temperature, up to 90°C, making them stable.

Hussain said the system is a preliminary prototype, “which is more than a proof of concept.” The next step, he says, is to conduct clinical trials.

Cheaper touch screens
Scientists at the University of Sussex, the University of Surrey, and Taif University developed a material for smart phone touch screens that is cheaper, less brittle, and more environmentally friendly. Additionally, the material could lead to more responsive, lower power screens.

Indium tin oxide, which is currently used to make smart phone screens, is brittle and expensive. The primary constituent, indium, is also a rare metal and is ecologically damaging to extract. Silver, which has been shown to be the best alternative to indium tin oxide, is also expensive.

The new method combines silver nanowires with graphene, creating a hybrid material that matches the performance of the existing technologies with lower cost.

Professor Alan Dalton, from the school of Maths and Physical Science at Sussex, explains the process for creating the material: “While silver nanowires have been used in touch screens before, no one has tried to combine them with graphene. What’s exciting about what we’re doing is the way we put the graphene layer down. We float the graphene particles on the surface of water, then pick them up with a rubber stamp, a bit like a potato stamp, and lay it on top of the silver nanowire film in whatever pattern we like.”

The team says the process is inherently scalable, and could be implemented on a large scale using spraying machines and patterned rollers.

A screen made from acrylic plastic coated in silver nanowires and graphene. (Source: Dr Matthew Large/ University of Sussex)

“The addition of graphene to the silver nanowire network also increases its ability to conduct electricity by around a factor of ten thousand,” said Dalton. “This means we can use a fraction of the amount of silver to get the same, or better, performance. As a result screens will be more responsive and use less power.”

While silver is still rare and expensive, Matthew Large, researcher at the school of Maths and Physical Science at Sussex, says that’s not a problem. “The amount we need to coat a given area is very small when combined with graphene. Since graphene is produced from natural graphite – which is relatively abundant – the cost for making a touch sensor drops dramatically.”

The graphene layer prevents the silver from tarnishing, as it would otherwise do when exposed to air. Additionally, the hybrid film is flexible, capable of repeated bending without changing the electrical properties.