Magnetic memory makes logic

By Eric Smalley, 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 faster.

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 off.

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 logic gates.

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 a 0.

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 Society (DFG).

Timeline:   two years; > 10 years
Funding:   Government
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

Page One

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Magnetic memory makes logic

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