Magnetic memory makes logic
Technology Research News
Today's computers shunt information among
three elements: processors that manipulate information to carry out computations,
random access memory (RAM) chips that retrieve information quickly, and
magnetic disk drives that store information while the computer is off.
Combining any of these elements would make computers simpler and
Magnetic random access memory (MRAM), slated to become commercially
available next year, is as fast as random access memory chips, but is
also non volatile, meaning it retains information when a computer is turned
Researchers from the Paul Drude Institute for Solid-State Electronics
in Germany have shown that it is also possible to use magnetic random
access memory for computation. And because the memory is configurable,
computing circuits formed from it could be adjusted on-the-fly to optimize
processing for a particular application, said Andreas Ney, a researcher
at the Paul Drude Institute. "The processors can be optimized for each
individual computational task."
Computers that use magnetic logic circuits would be more efficient
than those in use today. "The reprogramability and the non-volatility
are expected to increase the computational efficiency for both specialized
small, and... large processors," said Ney.
Magnetic logic is an idea that dates back to the 1950s. Simple
magnetic logic devices were produced for specialized applications in the
1960s, but when silicon chips emerged commercially in the '60s they quickly
surpassed magnetic logic in performance, compactness and cost-effectiveness.
Recent advances in magnetic materials and chipmaking techniques, however,
have made it possible to make memory chips that store bits magnetically.
Magnetic random access memory is based on magnetoresistance, the
same principle used for reading information stored on magnetic hard disks.
Some materials conduct electricity more easily when they are in a magnetic
field. In materials with high magnetoresistance, there is an easily measurable
difference in the flow of electricity through the material when it is
surrounded by a magnetic field versus when it is not. These two states
can represent the 1s and 0s of computer information.
Magnetic random access memory uses magnetoresistive elements made
from two magnetic layers. The magnetic orientations, or poles, of the
two layers can be parallel or opposite. The combined electrical resistance
of the layers is lower than when the orientations are parallel than when
they are opposite.
The researchers' found a way for a single magnetic random access
memory element to act as any of the four basic logic gates -- AND, OR,
NAND or NOR -- that make up computer circuits.
Computer processors manipulate information by sending signals
through logic gates. In a computer's binary information scheme, 1 is ordinarily
represented by the presence of an electrical signal and 0 is represented
by the absence of a signal. Every action performed by a computer processor
is a combination of positive and negative signals flowing through basic
An AND gate measures two input signals, and if both input lines
contain signals it returns a signal as output. If either input line lacks
a signal the gate returns a 0. An OR gate returns a 0 if both inputs are
0 and returns a 1 if either or both of the inputs are 1. NAND and NOR
gates return the opposite outputs of AND and OR gates.
The researchers' scheme calls for turning a magnetic memory element
into a logic element by attaching three input wires to its top magnetic
layer and an output wire to its bottom layer. This makes it possible to
represent input using the polarity of an electric current rather than
the presence or absence of electric current.
The magnetic field that forms around electric current has either
a positive or negative polarity. Electric current running through at least
two of the three input wires produces a strong enough magnetic field to
flip an oppositely polarized top layer.
A wired memory element can be used as any of four basic logic
gates by setting the two layers to one of the four possible pairs of polarizations
before sending input to the device. "Each of four states directly correspond
to one of the four basic logic functions," said Ney. "Programming the
logic functions simply means [selecting] one of the four initial states."
The element will perform the AND function if the top layer is
negative and the bottom positive and current is sent through two of the
three input wires. Sending negative electric current, representing 0,
through both input wires leaves the top layer unchanged. Likewise, if
only one of the two inputs is positive, representing 1, the top layer
is unchanged. Only when positive current runs through both inputs does
the top layer change to become parallel with the bottom layer, and the
device's output becomes 1.
The element performs the OR function if both layers begin with
a positive orientation. The third input wire allows the magnetic logic
element to also work as NAND and NOR gates. Current running through all
three inputs produces a magnetic field strong enough to change the bottom
layer's orientation, which reverses the device's output.
The output can be sensed, or read, by measuring the magnetic layer's
resistance to current flowing from an input line to the output line. Low
resistance shows that the output is positive, high resistance indicates
Future magnetic logic processors will contain many programmable
magnetoresistive elements, probably arranged in a square mesh like today's
random access memory chips, said Ney.
Most of today's processors are hardwired rather than programmable,
with circuits that are dedicated to one type of logic operation. Processors
are optimized for different tasks -- running a personal computer, or running
a car's brake system -- by arranging these gates in a certain order.
Programmable gates mean that all the specific functionality, or
programming, needed to instruct a processor to run a given set of operations
can be carried out using software programs. This separation between hardware
and functionality will allow for universal processors that can be used
for many different applications, from controlling a washing machine to
controlling a power plant to carrying out rapid computations, said Ney.
There's a lot of work to do before this is possible, however.
The remaining work includes optimizing magnetoresistive devices and writing
software compilers that efficiently configure the logic gates of a processor
to optimize it for a given task, said Ney.
The researchers' next steps are to work out how to configure more
complicated logic circuits, and to build prototype devices, said Ney.
Once magnetic random access memory is on the market for memory
applications, it could in principle be used as a magnetic logic processor
by adding an addressing scheme, said Ney. Such magnetic logic processors
could be built within two years, said Ney. It will take at least a decade
to develop a small, universal, magnetic logic processor, however. "We
think that [at first] only embedded processors for very specialized devices
are practical," he said.
Ney's research colleagues were Carsten Pampuch, Reinhold Koch
and K. H. Ploog. The work appeared in the October 2 issue of Nature.
The research was funded by the Paul Drude Institute and the German Research
Timeline: two years; > 10 years
TRN Categories: Integrated Circuits; Data Storage Technology
Story Type: News
Related Elements: Technical paper, "Programmable Computing
with a Single Magneto Resistive Element,"Nature, October 1, 2003
October 8/15, 2003
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