Jun 13, 2002
Magnets open the gate to nanoscale logic
A nanometre-scale logic gate that works at room temperature and is made entirely from metal has been developed by UK physicists. According to Russell Cowburn's team at the University of Durham, the ferromagnetic NOT gate is "a completely new class of device" that could be made even smaller. The researchers have also created a 13-bit shift register by linking the devices together, and believe it should be possible to make a full set of logic gates using their technique (D Allwood et al 2002 Science 296 2003).
Logic operations - such as NOT, AND, OR and XOR - are carried out in existing electronic circuits by semiconductor devices including diodes and transistors. But the density of electrons flowing through a semiconductor is limited partly by the separation of dopant atoms, and this restricts how small these devices can be made.
The electron density of a metal, however, is higher than it is in a semiconductor, so a metallic logic gate could be made smaller than a semiconductor logic gate. Although magnetic materials are widely used to store data, they have not so far been used to perform operations on it.
Now the Durham group has taken a step towards a magnetic logic system with the development of its NOT gate - a simple logic device that gives an output of "0" for an input of "1" and vice versa.
The device was built using a ferromagnetic wire, in which the spins on the electrons are aligned even when there is no external magnetic field. This alignment persists over microscopic regions known as domains, which are separated by 'walls' that are typically hundreds of nanometres thick. In these walls, the spins gradually change from the orientation of one domain to that of the neighbouring one.
Previous experiments have shown that a domain wall can be propelled along a nanoscale wire by a magnetic field that rotates in the plane of the wire. But Cowburn's team realized that if they put a hairpin bend in the wire, the wall would only be able to move around the bend in one direction for a given direction of rotating field. Such one-way signal flow is a key requirement of any logic system.
To test their idea, they made a wire 200 nm wide and 5 nm thick from a ferromagnetic alloy of nickel and iron, and bent it. The wire contained one domain wall near one of its ends, and the researchers assigned the logical values of "1" and "0" to the magnetization states either side of it. With one revolution of a magnetic field rotating in the anti-clockwise direction, the wall travelled around the bend. This inverted the magnetization state at the bend and therefore the logical value assigned to it.
"We have achieved digital logic without the equivalent of a transistor," says Cowburn. "We're pretty excited about being able to do logic by such radically different means."
The team then linked together eleven of the magnetic NOT gates to make a simple 13-bit shift register, in which a bit of information is passed from one gate to the next each time the rotating magnetic field completes one cycle. Cowburn adds that the data stored in such devices would be stable even without a power source, which could make it ideal for mobile applications like smart cards and phones.
About the author
Katie Pennicott is Editor of PhysicsWeb