Power/Performance Bits: July 26

Flexible MRAM

Researchers from the National University of Singapore, Yonsei University, Ghent University and Singapore’s Institute of Materials Research and Engineering embedded a magnetic memory chip on a plastic material, flexible enough to be bent into a tube.

The new device operates on magnetoresistive random access memory (MRAM), which uses a magnesium oxide (MgO)-based magnetic tunnel junction (MTJ) to store data.

Although a substantial amount of research has been conducted on different types of memory chips and materials, there are still significant challenges in fabricating high performance memory chips on soft substrates that are flexible, without sacrificing performance.

The research team first grew the MgO-based MTJ on a silicon surface, and then etched away the underlying silicon. Using a transfer printing approach, the team implanted the magnetic memory chip on a flexible plastic surface made of polyethylene terephthalate while controlling the amount of strain caused by placing the memory chip on the plastic surface.

Associate Professor Yang Hyunsoo from the National University of Singapore demonstrates the flexibility of the memory chip. (Source: National University of Singapore)

“Our experiments showed that our device’s tunneling magnetoresistance could reach up to 300% – it’s like a car having extraordinary levels of horsepower,” said Yang Hyunsoo, associate professor of electrical and computer engineering at NUS. “We have also managed to achieve improved abruptness of switching. With all these enhanced features, the flexible magnetic chip is able to transfer data faster.”

The team is conducting experiments to improve the magnetoresistance of the device by fine-tuning the level of strain in its magnetic structure, and they are also planning to apply their technique in various other electronic components. They were recently granted United States and South Korea patents for the technology.

The group is also interested in working with industry partners to explore further applications.

Miniature antennas

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), as part of an international research venture, succeeded in generating spin waves with extremely short wavelengths in the nanometer range – a key feature for future information transfer in even more compact chips.

“One major problem with current technologies,” said Dr. Sebastian Wintz of the HZDR Institute, “is the heat which is generated when data are transmitted with the aid of electric currents. We need a new concept.” The physicist is working with international colleagues on so-called spin waves (magnons) which are set to replace moving charges in the future as information carriers.

“The great advantage of spin waves is that the electrons themselves don’t move,” explained Wintz, “therefore precious little heat is produced by the flow of data.”

The spin denotes a property which lends the particles a magnetic moment. They then act like tiny magnets which run parallel to each other in ferromagnetic materials. If one of the spins then changes direction, this has a knock-on effect on its neighbors. A chain reaction gives rise to a spin wave.

The traditional approach adopted to generate spin waves is to use small metal antennas which generate magnons when driven by a high-frequency alternating current. The smallest wavelength which can be generated in this way will be about the size of the antenna which is used. This is where the major problem lies in that small wavelengths on the nanometer scale are required in order to satisfy the demand for ever greater miniaturization. It is not currently possible, however, to make such small high-frequency antennas.

The center of a magnetic vortex emits spin waves with very short wavelengths in the presence of high-frequency alternating magnetic fields. (Source: HDZR)

The research team succeeded in generating extremely short-wavelength spin waves in an entirely new way. As a naturally formed antenna, they use the center of a magnetic vortex which is produced in a small, ultra-thin ferromagnetic disk. Due to the disk’s limited size, the spins do not all line up in parallel as normal but lie along concentric circles in the plane of the disk. This, in turn, forces the spins from a small area in the center of the disk, which measures just a few nanometers in diameter, to straighten up and, thus, to point away from the surface of the disk. If this central region is subjected to an alternating magnetic field then a spin wave is produced.

A few more tricks are needed, however, in order to shorten the wavelength as required. Consequently, a second tiny disk is placed onto the first, separated by a thin, non-magnetic layer. When this separating layer is fabricated with a specific thickness, then the two disks interact in such a way as to elicit an antiferromagnetic coupling between the disks – the spins try to point in opposite directions – which reduces the wavelength of the emitted spin waves many times over. “Only in this way do we arrive at a result which is relevant for information technology,” added Wintz.

Graphene pastries

A team of researchers at MIT has found a way to efficiently create composite materials containing hundreds of layers that are just atoms thick but span the full width of the material. The discovery could open up wide-ranging possibilities for designing new, easy-to-manufacture composites for optical devices, electronic systems, and high-tech materials.

Materials such as graphene, a two-dimensional form of pure carbon, and carbon nanotubes, tiny cylinders that are essentially rolled-up graphene, are “some of the strongest, hardest materials we have available,” said Michael Strano, professor of chemical engineering at MIT, because their atoms are held together entirely by carbon-carbon bonds, which are “the strongest nature gives us” for chemical bonds to work with. So, researchers have been searching for ways of using these nanomaterials to add great strength to composite materials, much the way steel bars are used to reinforce concrete.

The biggest obstacle has been finding ways to embed these materials within a matrix of another material in an orderly way. These tiny sheets and tubes have a strong tendency to clump together, so just stirring them into a batch of liquid resin before it sets doesn’t work at all. The team’s insight was in finding a way to create large numbers of layers, stacked in a perfectly orderly way, without having to stack each layer individually.

Although the process is more complex than it sounds, at the heart of it is a technique similar to that used to make ultrastrong steel sword blades, as well as the puff pastry that’s in baklava and napoleons. A layer of material — be it steel, dough, or graphene — is spread out flat. Then, the material is doubled over on itself, pounded or rolled out, and then doubled over again, and again, and again.

With each fold, the number of layers doubles, thus producing an exponential increase in the layering. Just 20 folds would produce more than a million perfectly aligned layers.

It doesn’t work out exactly that way on the nanoscale. In this research, rather than folding the material, the team cut the whole block — itself consisting of alternating layers of graphene and the composite material — into quarters, and then slid one quarter on top of another, quadrupling the number of layers, and then repeating the process. But the result was the same: a uniform stack of layers, quickly produced, and already embedded in the matrix material, in this case polycarbonate, to form a composite.

The process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III) and stacked again and pressed, multiplying the number of layers. The team used a related method the team to produce scroll-shaped fibers. (Source: MIT)

In their proof-of-concept tests, the MIT team produced composites with up to 320 layers of graphene embedded in them. They were able to demonstrate that even though the total amount of the graphene added to the material was minuscule — less than 1/10 of a percent by weight — it led to a clear-cut improvement in overall strength.

The team also found a way to make structured fibers from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions, as well as yet another class of composites. The method uses a shearing mechanism to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape, technically known as an Archimedean spiral.

That could overcome one of the biggest drawbacks of graphene and nanotubes, in terms of their ability to be woven into long fibers: their extreme slipperiness. Because they are so perfectly smooth, strands slip past each other instead of sticking together in a bundle. And the new scrolled strands not only overcome that problem, they are also extremely stretchy, unlike other super-strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could give without breaking.

One unexpected feature of the new layered composites, Strano said, is that the graphene layers, which are extremely electrically conductive, maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. So, for example, simply inserting an electrical probe into the stack to a certain precise depth would make it possible to uniquely address any one of the hundreds of layers. This could ultimately lead to new kinds of complex multilayered electronics, he said.