IBM moves closer to a new class of memory

11 April 2008

IBM Fellow Dr. Stuart Parkin
IBM Fellow Dr. Stuart Parkin

Computer memory that combines the high performance and reliability of flash, with the low cost and high capacity of a hard disk drive, could be just around the corner thanks to a team of IBM scientists.

Currently, there are two main 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 storage 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. 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. However, flash memory has its own drawbacks. Although it is fast to read data, it is slow to write it, and flash memory has a finite lifespan. Flash can only be re-used a few thousand times because it eventually breaks as it is slightly damaged by each re-write.

In two papers published in the 11 April issue of Science, IBM Fellow Stuart Parkin and his colleagues at the IBM Almaden Research Center in San Jose describe the fundamentals of a technology dubbed ‘racetrack’ memory, as well as a milestone in that technology. Within the next ten years, racetrack memory, so called because the data ‘races’ around the wire ‘track’, could lead to solid state electronic devices without moving parts that are capable of storing far more data than is possible today in the same amount of space. In addition, racetrack memory will boast with lightning-fast boot times, lower costs, and stability with durability.

For example, this technology could enable a handheld device such as an mp3 player to store around 500,000 songs or approximately 3,500 movies (some 100 times more than is possible today) with lower costs and reduced power consumption. The devices would not only store more information in the same space, but also require much less power and generate much less heat, and be practically unbreakable. The result would be a large amount of personal storage that could run on a single battery for weeks and last for decades.

“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,” said Dr. Parkin. “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 could unleash creativity, leading to devices and applications that nobody has imagined yet,” added Dr. Parkin.

Since racetrack memory has no moving parts, and rather than storing data as ensemble of electronic charge, it uses the spin of the electron to store data, it has no wear-out mechanism and can therefore be re-written endlessly without any wear and tear.

For many years scientists have explored the possibility of storing information in magnetic domain walls, which are 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.

In the paper describing their milestone, Current Controlled Magnetic Domain-Wall Nanowire Shift Register, Dr. Parkin and his team describe how this long-standing obstacle can be overcome by taking advantage of the interaction of spin polarised current with magnetisation in the domain walls. The result is a spin transfer torque on the domain wall, causing 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 the review paper that describes the fundamentals of racetrack, Magnetic Domain-Wall Racetrack Memory, Dr. Parkin and colleagues describe 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 magnetised in opposite directions (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. Pinning sites fabricated along the racetrack control the spacing between consecutive domain walls. In their paper, 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-polarised 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 old concept of storing information in movable domain walls.

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

Dr. Parkin’s advances with racetrack memory build on his prior accomplishments in memory technologies including the spin valve, and MTJs (Magnetic Tunnel Junctions) and breakthroughs in MRAM (magnetic RAM).

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 approximately 100GB of data in a modern drive, racetrack memory uses one sensing device to read 10 to 100bits.

“It [racetrack memory] 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,” concluded Dr. Parkin.

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