A Milestone on the Road to Ultrafast Computers
Related Article Bigger, Faster, More 3-D: The Anatomy of the Coming PC's (June 18,1998)
By JOHN MARKOFF
A group of I.B.M. researchers said last week that they had designed
the building blocks of a new kind of computer memory that could
fundamentally alter computer design early in the next century.
Chips based on this new technology, known as tunneling magnetic
junction random access memory, or tmj-ram for short, would be
ultrafast, consume very little power and retain stored data when a
computer was shut down.
As such they would combine the best features of computer disks, which can store prodigious amounts of information, and conventional memory chips, which are fast but limited in their capacities.
An I.B.M. researcher revealed at a meeting of the American Physical Society in Atlanta last month that a small research team working at the company's Almaden Research Laboratory in San Jose, Calif., had built microscopic magnetic memory cells that could switch on and off as quickly as the fastest microprocessor chips, consume power only when reading and writing, and were almost as small as the tiny capacitors that store data in the most advanced conventional memory chips.
In contrast, today's memory chips must be continually electrically
refreshed while operating, because the electrical charge leaks from the millions of storage capacitors that make up the chips.
"This is the holy grail of computer memory," said Randy Isaac, a vice
president at I.B.M.'s Thomas J. Watson Research Center. "This is the
result of a global quest that has gone on for decades."
Indeed, the advance is a crucial step toward a new class of electronic
materials and a new kind of microelectronics, which has been named
"spintronics" because it is based on the ability to detect and control the spins of electrons in ferromagnetic materials. Spin is an aspect of quantum mechanics, the rules that govern subatomic physics, that is still untapped commercially.
According to quantum mechanics, the electrons in a normal electric
current are spinning in a random mix of quantum states known as up and
down. By ordering this mayhem in a process analogous to the
polarization of light -- in effect, aligning the quantum spins to be either all up or all down -- scientists can create the "off" and "on" states central to computer calculations and give the digital revolution a remarkable new dimension.
Spintronics is already a billion-dollar industry because of another I.B.M. innovation based on a phenomenon known as "giant magnetoresistance," which is being used to read hard disks.
In the so-called gmr effect, tiny magnetic fields are used to control the electrical resistance of a sandwich of alternating layers of magnetic and nonmagnetic metals.
In recent years, progress at research laboratories in the United States, Europe and Japan has touched off an international race among scientists who believe that spintronics may offer significant gains in memory and processing power in the next century.
In the United States, companies like I.B.M., Honeywell, Hewlett
Packard and Motorola are working on spintronics, said Stuart Wolf,
who is in charge of financing Pentagon research in the field.
"In two or three years there will be results in this field that will make people sit up and take notice," he said.
The new tmj-ram devices combine the phenomenon of spin with another
heretofore elusive quantum feature known as tunneling, in which current can pass from one metal layer to another, switching its spin from up to down, like a ghost melting through a wall.
"We've been able to improve these materials beyond my wildest dreams," said Stuart Parkin, the I.B.M. physicist who has been leading the Almaden research group. "These are wonderful devices because in
principle you can scale them through many many generations."
Such memories might have a broad impact on the design of computers, he said, because they could be applied at both the very high and low ends of the computer industry. Because today's memory chips are much slower than microprocessor chips, computer design is based on a hierarchy of memory.
A conventional computer will have a number of different types of
memory, which descend in speed and increase in capacity.
This hierarchy ranges from the so-called ultrafast cache, which is built directly into the microprocessor and stores data and parts of the program used most frequently, to the magnetic computer disk, which is the slowest part of the system but holds the most data.
Computers based on the new type of quantum effect memory would not
only start instantly because program information and data could be
permanently stored, but would also be faster because the tmj-ram
memory would keep pace with the fastest microprocessor chips.
Memories based on quantum-tunneling effects were first predicted
theoretically by John Slonczewski, an I.B.M. physicist at the Watson
Research Center, in 1975. But it was not until three years ago, when
research groups at M.I.T. and in Japan were simultaneously able to
demonstrate the magnetic tunneling effect, that Dr. Parkin's group
returned to the field.
Dr. Parkin attributed much of his group's progress to the development of a rapid prototyping machine that allows researchers to test new
compounds quickly for the memory cells.
The scale of the research is remarkably small. For example, the aluminum dioxide insulating layer in the tiny sandwich of iron-cobalt magnets that makes up the experimental memory cell is only four atoms thick.
The researchers were able to lower the electrical resistance of that layer by 10-million-fold at room temperature. This allowed current to "tunnel" between the layers, altering the direction of the spin in one of the magnets and creating the equivalent of a digital 1 or 0.
So far the group has demonstrated reading and writing times of about 10 nanoseconds, about six times faster than today's dynamic random access memories.
The I.B.M. researchers said they had chosen to pursue the tmj-ram
because the tunneling approach promises greater memory capacity in the
future.
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