Jan 12, 2006
Nanomagnets offer data processing
Electronically controlled quantum dot cellular automata (QCA) can carry out data storage and logic operations but they only work at low temperatures. Magnetic QCA systems, in contrast, function at room temperature but until now had only been able to store data. For the first time, researchers at the University of Notre Dame, US, and the University of Munich in Germany have made a magnetic QCA-based logic device.
"The demonstration of logic in a magnetic system opens the door to all-magnetic information processing systems, including memory and logic," Wolfgang Porod of the University of Notre Dame told nanotechweb.org.
Porod, Alexandra Imre and colleagues created nanomagnets of permalloy that were 135 nm long, 70 nm wide and 30 nm thick using electron-beam lithography and lift-off techniques. The nanomagnets contained single magnetic domains and their elongated shape meant they were strongly bistable – their magnetization pointed along the long axis unless an external field was present.
The team arranged five nanomagnets in a cross shape, with a central nanomagnet surrounded by four others. In this way, the nanomagnets formed two intersecting lines each consisting of a row of three nanomagnets. The nanomagnets in the vertical line had collinear long axes and so were ferromagnetically coupled, i.e. their magnetizations pointed in the same direction. The horizontal row of nanomagnets had their long axes adjacent and in parallel. That meant they exhibited antiferromagnetic coupling, with their magnetic dipoles alternating direction.
"We were quite surprised to learn how strong magnetic interactions are between nanomagnets tens to hundreds of nanometres in size, which can be fabricated quite easily," said Porod. "Advantages of magnetic QCA include room-temperature operation and this technology also leverages advances made by the magnetic data-storage industry for patterned magnetic media."
Three of the four nanomagnets around the central magnet were positioned next to driver nanomagnets so that they could act as inputs. The axis of the driver nanomagnets was perpendicular to that of the other magnets. The fourth "outer" nanomagnet acted as the output for the device. Moving the driver nanomagnets or altering their dipole changed the dipole of the input magnets and hence the output of the device.
Since the ferromagnetic and antiferromagnetic coupling to the central magnet were the same strength, the central magnet switched to the state at which the majority of inputs forced it. The output magnet then inverted the state of the central magnet. The result was a three-input inverting majority logic gate. By fixing one of the inputs, the researchers could also make the device act as a programmable two-input NAND or NOR gate. That means that a network of the majority gates could perform any logic function.
"Potential applications include magnetic logic systems which would be non-volatile," said Porod. "Also, one of the promises of QCA – both electronic and magnetic – is low power. Potential applications might include portable systems where low power is at a premium."
Now Porod and colleagues say they would like to fabricate larger structures, beyond the single majority-logic gate they have demonstrated so far. "Also, we would like to realize electronic ways to set the input and to read the output," added Porod. "So far, inputs are set by external magnetic fields."
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of nanotechweb.org.