Perpendicularly magnetized nanopatterned arrays could be a good way to increase the density of magnetic storage media to over 1 terabit (1012) per square inch, or 160 Gbits (109) per square centimetre. However, to make this technology possible, scientists need to narrow the switching field distributions within these nanodot arrays.

A nanodot has north and south poles like a tiny bar magnet and switches back and forth (or between 0 and 1) when a strong magnetic field is applied. Normally, the smaller the dot, the larger the field needed to switch it. Until now, there was a wide variation in the nanodot switching response that researchers were unable to control. The NIST team has not only reduced this variation to less than 5% of the average switching field, but has also identified what is thought to be the main cause of the variability – the design of the multilayer films from which the nanodots are made.

Shaw and co-workers began by fabricating cobalt-palladium nanodots just 50 nm across. They did this using electron beam lithography to pattern multilayer films. They first laid down a tantalum "seed" layer just a few nanometres thick when making a multilayer film of alternating layers of cobalt and palladium on a silicon wafer.

The seed layer can alter the strain, orientation or texture of the cobalt-palladium film. By making and comparing different types of multilayer stacks, the researchers were able to isolate the effects of different seed layers on the switching behaviour of the nanodots. Moreover, they found that several factors previously thought to be important for broadening the switching field distribution, such as lithographic variations, nanodot shape or crystal grain boundaries, were not responsible for this effect. Indeed, the team believes that it is an intrinsic material property of the cobalt-palladium multilayer itself that affects the switching field distribution.

The researchers stress that much work still needs to be done to make this type of nanopatterned media a commercially viable solution for increasing the density of magnetic data storage. This includes reducing the size of dots to less than 10 nm, developing techniques to fabricate more than 1015 dots per disk both efficiently and cheaply, and devising new methods to track, read and write these nanoscale bits.

The researchers reported their work in J. Appl. Phys..