PORTLAND, Ore. A key current-reducing technique in the development of magnetic racetrack memories has been demonstrated by IBM Corp. fellow Stuart Parkin and colleagues at the company's Almaden Research Center. By drilling down into a silicon chip with a U-shaped magnetic nanowire called a racetrack, the researchers hope to greatly expand the capacity of memory storage devices.
"The racetrack, in my mind, is an extremely attractive potential memory storage device which could, if realized, vastly simplify computers by making it possible to unify many different types of digital data storage devices," said Parkin, who is director of the IBM-Stanford Spintronic Science and Applications Center at Almaden.
Since the invention of semiconductor memory, chip makers have predicted that hard disks and tape drives would be replaced by chips. Nevertheless, magnetic media has persistently been one step ahead of semiconductor memory throughout its evolution from SRAM to DRAM to flash.
IBM has a grand vision for its racetrack. "We believe the magnetic racetrack memory will eventually replace all hard-disk drives and all nonvolatile memory," said Parkin. "The basis for this belief is that we now know how to move magnetic domain walls along a nanowire with current pulses."
Parkin and his fellow IBM researchers recently demonstrated a technique that reduces the current needed to move magnetic domains through its nanowires. Previously, the high current needed to move magnetic domains along a nanowire was the greatest barrier to success in the development of magnetic racetrack memory. However, by using short trains of current pulses, whose length and frequency were tuned to the resonant frequency of the nanowire, the magnetic domains could be moved with five times less current than was previously required.
"This is a major advance in our ability to control the motion of domain walls using current," said Parkin. "The use of trains of very short current pulses whose spacing is properly timed, allows us to move domain walls with much less current [and power] than using longer current pulses [or dc]."
Now the biggest obstacle to success is the fabrication of the racetrack itself. That had been put on hold until the movement of domain walls along the nanowire could be perfected on a linear racetrackmore like a drag stripstretched out on top of a silicon chip.
Getting on track
IBM's Almaden Research Center (San Jose, Calif.) has been working with the racetrack design for several years now. It was issued the first of several patents for the technology on Dec. 21, 2004 (patent No. 6834005). The basic idea is to take the same square micron that houses a single SRAM memory bit, or 10 flash bits, and to drill down into the third dimension to store 100 bits on a sunken racetrack-shaped magnetic nanowire.
"The racetrack concept is that instead of scaling traditional memory by making individual bit cells ever smaller, it breaks this old paradigm by using the third dimension to vastly expand the capacity of a memory device," said Parkin. "If the racetrack is 1 micron wide and 10 microns high and can store 100 bits, that would compare to 1 bit for semiconductor memory and could replace hard disks. If it stored 10 bits compared to 1 bit, then it could replace flash memory."
Today, the magnetic area on the surface of a spinning hard disk is divided up into cylindrical domains as the disk spins under the write head. The separate domains for successive bits are spaced out to keep them separate. At the same time, those spaces are kept as small as possible so as to cram more data onto the disk. The boundary between bits, where the magnetic material's poles switch for adjacent bits, is called its domain walls.
IBM's racetrack memory magnetizes domains along the length of a nanowire that is only nanometers wide but up to microns long. Regularly spaced notches along the nanowire keep the magnetic domains the same size, with current pulses moving them along from notch to notch. Spin-encoded current pulses move data along the nanowire as if a tape drive were actually moving the wire. However, instead of the actual motion of molecules, the magnetic domains are moved along like a wave.
The nanowires themselves are thousands of times longer than they are wide. In IBM's current demonstration, the magnetic domains are stored serially along the length of a nanowire several microns long. To move these magnetic domains along the nanowire, a spin-injected current tuned to the resonant frequency of the nanowire was applied.
Reading from and writing to the nanowire will be accomplished with perpendicular nanowires buried at the racetrack's U-turn, where they can affect the magnetization of the domains going past them.
"The racetrack memory has several important attributes, which I consider to be essential to the future of many types of nanoelectronic devices, such as massless motion," said Parkin.
IBM calls movement along the racetrack "massless motion" because the magnetic domains defining each bit are actually passed from atom to atom like a wave traveling down the nanowire, unlike electricity where electrons (which have mass) must hurtle through the wire, causing heat. Unfortunately, the current necessary to move those magnetic domains down the nanowire today still causes excessive heat in the power supply that generates the pulses.
Next, IBM plans to experiment with novel magnetic materials for sinking the racetrack into the third dimension, to further reduce its current requirements. Finally, it aims to prove that magnetic domains can be reliably moved along nanowires sunk into the third dimension of a silicon chip.
Parkin's latest discovery of tuning current pulses to the resonant frequency of the nanowire lowered the current required by five times, but further improvement will be needed if magnetic racetrack memories are ever to become competitive with other emerging nonvolatile memory architectures such as MRAM and PRAM.