# A logical leap

Tiny magnets could replace transistors in computer chips

THE electron, one of the fundamental particles of nature, carries
an electric charge. But it also possesses a second property, called
spin. Charge is responsible for electricity, while spin underlies
magnetism and, until now, information technology has decreed a
clear division of labour between the two: charge does data
processing while spin does data storage.

It has, however, long been an ambition among some engineers to
change that, and to process data magnetically, tooa field of
endeavour known as spintronics. Now, a piece of research by
Alexandra Imre, of the University of Notre Dame, in Indiana, and
her colleagues has come close to accomplishing this. A paper they
have just published in Science describes a tiny magnetic device
that could, in principle, perform the job now done by transistors.

The transistors in a microprocessor chip control the stream of databinary ones and zerosthat lies at the foundation of all computing. A simple transistor has two electronic inputs and an output. One of the inputs controls whether the current from the other can flow through the transistor. A flowing current equals a binary one. An absence of current equals a binary zero. Since the output of one transistor can be used as the controlling input of another, it is possible to build groups of transistors into what are
known as logic gates.

A logic gate works by creating a particular output from a given
input. The simplest example is a NOT gate: when the input is one, the output is zero, and vice versa. An AND gate has two inputs; it will give an output of one when both inputs are one, and zero if they are not. By contrast, a NAND gate gives an output of zero when both inputs are one and one when they are not. Similarly, an OR gate yields a one when either input is one and a NOR gate yields a one when neither input is one. Computer programs are, at bottom, merely strings of logical operations of this sort. Do them fast enough, and in the right order, and your copy of Word, or whatever, will run smoothly.

What Dr Imre and her colleagues have done is to construct logic gates from tiny magnets. Very tiny. In fact, just a couple of nanometres (billionths of a metre) across. Instead of using electric charge, the magnetstalkto each other by affecting the polarities of their neighbours. If an input magnet is polarised north-south, its neighbours will tend to polarise south-north (ie, with the spins pointing in the other direction) because, as every schoolboy knows, like magnetic poles repel. Put an array of such magnets together and flip the input fields, and the others will respond.

To prove the point, the team built an arrangement of five nanomagnets that acts as a logic gate with three inputs. This performs what is known as an inverting majority functionwhich means that if two inputs represent ones and the third represents zero, the output is zero, while if two inputs are zero, and the other is one, the output is one. By fixing one of the inputs permanently in theoneposition, the device acts as a NOR gate with respect to the other two inputs. Similarly, by fixing it in thezeroposition, it acts as a NAND gate (see diagram). By the rules of Boolean algebra, the branch of maths that governs this sort of calculation, it is possible to perform any logical operation, no matter how complex, with a mixture of NOR and NAND gates. In effect, therefore, Dr Imre’s array is a universal logic gate.

Such gates could have many advantages over their transistor-based equivalents. They would not leak current in a way that affected their neighbours (an increasingly important consideration as circuits get smaller and more tightly packed). And unlike their electronic counterparts, they would retain information when the power was cut. They could also be reconfigured easily, so a processor composed of magnetic gates might be restructured significantly after it had been built.

In principle, that could be done within a few nanoseconds, allowing the chip to adapt its architecture to match the best form for the computation to hand. At least, that is the spin of the spintronics people.

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