Understanding magnetic memory one layer at a time

Enlarge

Understanding Magnetic Memory One Layer at a Time

(PhysOrg.com) -- "Smaller, faster, cheaper" is Silicon Valley's mantra for progress. But as critical components shrink to near atomic dimensions, it?s becoming much more difficult for their developers to understand exactly how they operate before committing to product design and manufacturing.

With their ability to detect a wide range of phenomena precisely in space and time, synchrotron X-ray pulses are now being used to reveal the nanometer- and picosecond-scale behavior of potential electronic components. A SLAC-led team of scientists has made just such a contribution. They showed why the magnetic orientation in a multilayered structure being considered for use in future computer memories requires more energy to switch between states than had been predicted by theory.

?Previous studies have shown that these devices require more energy than expected,? said researcher David Bernstein of the Stanford Institute for Materials and Energy Science, a joint institute of SLAC and Stanford University. ?Since we can make ultrafast movies using Synchrotron X-rays, now we know why they need more energy to switch.?

The experiments were conducted on a multilayered stack of metallic and magnetic materials similar to those being tested for use in magnetic random access memories, or MRAM. The magnetization direction of the top layer is perpendicular to the planes of the layers, an arrangement that enables smaller and more stable data bits and thus greater data-storage capacity and more-reliable operation. An ?up? magnetic orientation stands for a ?1? in the binary number system that computers use, and a ?down? orientation represents a ?0.? Shooting a short electrical pulse through the stack causes the top layer to change orientations.

Enlarge

This diagram shows the experimental setup for a new study of magnetic memory. The "free" and "reference" layers are made of magnetic cobalt and nickel.

Like today's flash memory, MRAM retains its information after a computer's power is turned off. But it is much faster and more durable than flash and could someday replace computers? main memory, thus enabling instant-on high-performance computers. For MRAM to be commercially attractive, however, its physical size and power requirements must be reduced.

Instead of rotating from one orientation to the other all at once, the movies show the magnetic switching starting near the center of the pillar and spreading out toward the edges, creating two "domain wall" transition regions in the process. Although the walls move rapidly during the change ? 100 meters a second through the tiny area less than 100,000th that of a human hair's cross section ? their mere creation accounts for the extra energy use.

The scientists used a scanning soft-X-ray microscope at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory to create and image millions of domain flips a second ? billions in all. Varying the time interval between the electrical and X-ray pulses allowed the scientists to create ultrafast movies showing a detailed picoseconds-timescale account of exactly how the electrical pulse reversed the magnetic orientation.

The group's results were published in the May 10 edition of the technical journal Physics Review B.

Provided by SLAC National Accelerator Laboratory (news : web)

'Nanowire' measurements could improve computer memory

 

In this schematic image (top) and transmission electron micrograph, a silicon nanowire is shown surrounded by a stack of thin layers of material called dielectrics, which store electrical charge. NIST scientists determined the best arrangement for this dielectric stack for the optimal construction of silicon nanowire-based memory devices. Credit: Schematic Zhu, GMU. TEM Bonevich, NIST.

A recent study at the National Institute of Standards and Technology may have revealed the optimal characteristics for a new type of computer memory now under development. The work, performed in collaboration with researchers from George Mason University (GMU), aims to optimize nanowire-based charge-trapping memory devices, potentially illuminating the path to creating portable computers and cell phones that can operate for days between charging sessions.

The nascent technology is based on silicon formed into tiny wires, approximately 20 nanometers in diameter. These "nanowires" form the basis of memory that is non-volatile, holding its contents even while the power is off?just like the flash memory in USB thumb drives and many mp3 players. Such nanowire devices are being studied extensively as the possible basis for next-generation computer memory because they hold the promise to store information faster and at lower voltage.

Nanowire memory devices also hold an additional advantage over flash memory, which despite its uses is unsuitable for one of the most crucial memory banks in a computer: the local cache memory in the central processor.

Enlarge

"Cache memory stores the information a microprocessor is using for the task immediately at hand," says NIST physicist Curt Richter. "It has to operate very quickly, and flash memory just isn't fast enough. If we can find a fast, non-volatile form of memory to replace what chips currently use as cache memory, computing devices could gain even more freedom from power outlets?and we think we've found the best way to help silicon nanowires do the job."

While the research team is by no means the only lab group in the world working on nanowires, they took advantage of NIST's talents at measurement to determine the best way to design charge-trapping memory devices based on nanowires, which must be surrounded by thin layers of material called dielectrics that store electrical charge. By using a combination of software modeling and electrical device characterization, the NIST and GMU team explored a wide range of structures for the dielectrics. Based on the understanding they gained, Richter says, an optimal device can be designed.

"These findings create a platform for experimenters around the world to further investigate the nanowire-based approach to high-performance non-volatile memory," says Qiliang Li, assistant professor of Electrical and Computer Engineering at GMU. "We are optimistic that nanowire-based memory is now closer to real application."