Chinese researchers have demonstrated the operation of a new type of logic gate based on changes in the resistance of a ferromagnetic wire that occur as the wall between two of its magnetic domains is moved in an out of a constriction in the wire. Their results could enable the creation of logic circuits made of metal rather than silicon.

An important drawback of silicon-based logic circuits is that when the power is removed, they lose all their information. Consequently, when a computer switched back on, it takes time for its operating system to reload.

One approach that has been proposed to solve this problem is to encode information in the magnetization of devices built ferromagnetic materials. Such a device should not require a constant flow of electric current to maintain its state. The challenge is to find a way to change and read its magnetic state.

The simplest way to switch the magnetization of such a device is to use magnetic fields. Unfortunately, the coil-like solenoid structures needed to generate these fields are too large and use too much power to be build into a miniaturized circuit. But Changzhi Gu and colleagues1 at the Bejing National Laboratory for Condensed Matter Physics have found an alternative solution.

Fig. 1: Scanning electron micrograph of the authors' all-metallic NOT gate, consisting of two nanoscale ferromagnetic wire constrictions connected in parallel.

When a current passes through a magnetic wire containing two domains of opposite magnetization, the wall separating the domains can be moved along the wire. If the wall hits a nanometre-sized constriction in the wire it can become stuck. Gu's team found that when this happens, the wire's resistance changes dramatically. Moreover, the resistance could be changed back by unsticking the wall from the constriction with a sufficiently high current.

By combining two constrictions in parallel to exploit such effects, they constructed a simple NOT gate (Fig.1) —a device that converts a high voltage input into a low voltage output. The authors have also implemented AND, OR, NAND and NOR gates, which are necessary to build more complex logic circuits. The simplicity of this idea should enable making such circuits much smaller and even combining them with conventional silicon electronics.