PORTLAND, Ore. A next-generation nonvolatile memory dubbed “racetrack” is expected to initially replace flash memory and eventually hard-disk drives, according to IBM Corp. fellow Stuart Parkin of its Almaden Research Center (San Jose, Calif.)
Using spintronics–the storage of bits generated by the magnetic spin of electrons rather than their charge–a proof-of-concept shift register was recently demonstrated by IBM. The prototype encodes bits into the magnetic domain walls along the length of a silicon nanowire, or racetrack. IBM uses “massless motion” to move the magnetic domain walls along the nanowire for the storage and retrieval of information.
“We have now demonstrated a current-controlled, domain-wall, shift register which is the fundamental, underlying technology for racetrack memory,” said Parkin. “We use current pulses to move a series of domain walls along a nanowire, which is not possible to do with magnetic fields.”
IBM's Stuart Parkin
IBM's goal, based on spintronic patents filed as early as 2004, is to use the same square micron that currently houses a single SRAM memory bit, or 10 flash bits, and drill down into the third dimension to store spin-polarized bits on a sunken racetrack-shaped magnetic nanowire. Using an area of silicon 1 micron wide and 10 microns high, IBM said its first-generation racetrack would store 10 bits compared to one, thereby replacing flash memory. Eventually, it could store 100 bits in the same area, which is dense enough to replace hard-disk drives.
“Racetrack is essentially the third turn of the crank of this new field of engineering called spintronics,” said Parkin. “In current solid-state memory devices you store and control the flow of electrical charge. Here, we store and control the flow of the spin of an electron.”
Parkin invented a spin valve sensing device in 1989 based on the giant magnetoresistive effect, which was used to increase disk drive capacity 1,000-fold. “Then we invented the use of the magnetic-tunnel junction (MTJ)–a sandwich of two magnetic layers separated by a dielectric–which we used to build the first magnetic random access memories in 1999.
“The third generation is the racetrack, which could replace all nonvolatile memories, including flash memory and hard-disk drives,” Parkin claimed.
IBM estimates that an iPod using racetrack memory could store 100 times more information. Unlike flash, the solid-state devices have no components that can wear out.
Racetrack memory injects magnetized domain walls along the length of a high aspect ratio nanowire–only nanometers wide but up to microns long. Spin-polarized current pulses are then used to move the domain walls along the nanowire to store and retrieve bits.
Last year IBM, demonstrated that it could store a magnetic domain on a nanowire, then move it along the wire's length. The new shift register composed of many domain walls can be stored and moved together along the length of the wire. To read-out bits, the device senses a change in resistance in the wire.
The next step is building a fast MTJ read-head at the top of each racetrack, enabling it to quickly read-out any of the up to 100 bits stored on a racetrack.
IBM's current prototype uses a linear racetrack aligned parallel to the surface of a silicon chip. The first racetrack demonstration with MTJ read-heads will use that same approach. Eventually, IBM said it plans to build vertical racetracks by sinking nanowires into silicon. The MTJ read-head would be located at the top of each racetrack.