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IBM Bets on Racetrack RAM
Apr 2008
SAN JOSE, Calif., April 10, 2008 -- The idea of mp3 players that could store 500,000 songs are enough to make an audiophile's head spin. But they could be one result of of using metal spintronics technology to create "racetrack" memory in which data "races" around a wire "track."

IBM Fellow Stuart Parkin and colleagues at the IBM Almaden Research Center in San Jose announced the developmment this week which combines the high performance and reliability of flash memory with the low cost and high capacity of the hard disk drive. racetrack.jpg
A diagram of the nanowire used in racetrack memory shows how an electric current is used to slide -- or "race" – tiny magnetic patterns around the nanowire "track," where the device can read and write data in less than a nanosecond. The racetrack memory would stand billions of nanowires, like the one illustrated here, around the edge of a chip. (Image courtesy IBM)
In two papers (Science, April 11), they describe both the fundamentals of racetrack memory and a milestone in that technology that could lead to electronic devices capable of storing far more data in the same amount of space than is possible today, with lightning-fast boot times, far lower cost and unprecedented stability and durability.

"For example, this technology could enable a handheld device such as an mp3 player to store around 500,000 songs or around 3500 movies -- 100 times more than is possible today -- with far lower cost and power consumption," IBM said in a statement.

Within the next 10 years, the researchers said, racetrack memory could lead to solid-state electronic devices -- with no moving parts, and therefore more durable -- capable of holding far more data in the same amount of space than is possible today. The devices would not only store vastly more information in the same space, but also require much less power and generate much less heat, and be practically unbreakable; the result: massive amounts of personal storage that could run on a single battery for weeks at a time and last for decades.
IBM Fellow Stuart Parkin (Photo courtesy IBM)

"It has been an exciting adventure to have been involved with research into metal spintronics since its inception almost 20 years ago with our work on spin-valve structures," Parkin said.
"The combination of extraordinarily interesting physics and spintronic materials engineering, one atomic layer at a time, continues to be highly challenging and very rewarding. The promise of racetrack memory -- for example, the ability to carry massive amounts of information in your pocket -- could unleash creativity leading to devices and applications that nobody has imagined yet."

At present, there are two primary ways to store digital information: solid-state random access flash memory, commonly used in devices such as mobile phones, music players and digital cameras, and the magnetic hard-disk drive used in desktop and laptop computers and some handheld devices.

"While both classes of devices are evolving at a very rapid pace, the cost of storing a single data bit in a hard disk drive remains approximately 100 times cheaper than in flash memory," IBM said. "While the low cost of the hard disk drive is very attractive, these devices are intrinsically slower and, with many moving parts, have mechanical reliability issues not present in flash technologies."

Flash memory, however, has its own drawbacks. While it is fast to read data, it is slow to write data, and it, too, has a finite lifespan. Flash can be reused only a few thousands of times -- it eventually breaks, because it is slightly damaged by each use or "rewrite." Since racetrack memory has no moving parts and uses the "spin" of the electron to store data, it has no mechanism to wear out, so it can be rewritten endlessly with no wear and tear.

Scientists have long explored the possibility of storing information in magnetic domain walls -- the boundaries between magnetic regions or "domains" in magnetic materials. Until now, manipulating domain walls was expensive, complex and used significant power to generate the fields necessary to do so, IBM said.

In the paper "Current Controlled Magnetic Domain-Wall Nanowire Shift Register," Parkin and his team describe how this obstacle can be overcome by taking advantage of the interaction of spin-polarized current with magnetization in the domain walls. This results in a spin transfer torque on the domain wall, which causes it to move. The use of spin-momentum transfer considerably simplifies the memory device, since the current is passed directly across the domain wall without the need for any additional field generators.

In "Magnetic Domain-Wall Racetrack Memory," the team describes the use of magnetic domains to store information in columns of magnetic material (the "racetracks") arranged perpendicularly or horizontally on the surface of a silicon wafer. Magnetic domain walls are then formed within the columns delineating regions magnetized in opposite directions (e.g., up or down) along a racetrack. Each domain has a "head" (positive or north pole) and a "tail" (negative or south pole). Successive domain walls along the racetrack alternate between "head-to-head" and "tail-to-tail" configurations. The spacing between consecutive domain walls (that is, the bit length) is controlled by pinning sites fabricated along the racetrack.

The scientists describe their use of horizontal permalloy nanowires to demonstrate the successive creation, motion and detection of domain walls by using sequences of properly timed nanosecond-long, spin-polarized current pulses. The cycle time for the writing and shifting of the domain walls is a few tens of nanoseconds. These results illustrate the basic concept of a magnetic shift register relying on the phenomenon of spin momentum transfer to move series of closely spaced domain walls -- an entirely new take on the decades-old concept of storing information in movable domain walls, IBM s aid.

"Ultimately, the researchers expect the racetrack to move into the third dimension with the construction of a novel 3-D racetrack memory device, a paradigm shift from traditional 2-D arrays of transistors and magnetic bits found in silicon-based microelectronic devices and hard disk drives," IBM said. "By moving into the third dimension, racetrack memory stands to open new possibilities for developing less expensive, faster devices because it is not dependant on miniaturization as dictated by Moore’s Law."

Parkin’s advances with racetrack memory build on his prior accomplishments in memory technologies, including the spin valve and magnetic tunnel junctions and breakthroughs in magnetic RAM (MRAM).

Racetrack memory encompasses the most recent advances in the field of metal spintronics. The spin-valve read head enabled a thousand-fold increase in the storage capacity of the hard disk drive in the past decade; the MTJ is in the process of supplanting the spin-valve because of its higher signal. MTJs also form the basis of modern MRAM, in which the magnetic moment of one electrode is used to store a data bit. Whereas MRAM uses a single MTJ element to store and read one bit and hard disk drives use a single spin-valve or MTJ sensing element to read the approximately 100 GB of data in a modern drive, racetrack memory uses one sensing device to read 10 to 100 bits.

IBM said further understanding of the interaction of spin polarized current with magnetic moments is essential. "For example, this might allow a reduction in the current density needed to manipulate or move domain walls," said Parkin. "This would drop the power needed for racetrack further, and enable even lower power devices.

"We expect that our exploration of a wide variety of materials and structures will provide new insight into domain wall dynamics driven by current, making possible domain wall based memory and even logic devices that were previously inconceivable," he added. "It will not only change the way we look at storage, but the way we look at processing information. We're moving into a world that is more data-centric than computing-centric."

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