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The current generation of magnetic data-storage devices such as hard drives is approaching a crisis. To increase the capacity engineers are making the storage bits much smaller, cramming more into the same space.
But because of the magnetic properties of the storage material there is a limit to how small they can be. Below a level known as the superparamagnetic limit, the bits become magnetically unstable and, therefore, useless for data storage.
Engineers are working on a new technology known as heat-assisted data storage. This heats the storage medium, which stabilises the recording process; it becomes easier to write the data when the medium is hot and the data is retained when the medium is cooled. To develop this, researchers are looking for new, reliable ways to heat very small areas of material.
Pierre-Olivier Chapuis of the Laboratory of Molecular and Microscopic Energetics at the Ecole Centrale Paris and colleagues are investigating whether an atomic-force microscope, which has a nanoscale probe that never touches the surface it is studying, could provide the heat supply. Using a pyramid-shaped silicon tip about 40nm across and 20nm high, the researchers looked at how heat can be transferred from the hot tip to the cold surface.
In a paper in the journal Nanotechnology, Chapuis describes how the tip hovers around 50nm above the surface and heats up the air around it. As the air molecules become hot they move faster, flying towards the disc surface and colliding with other molecules that literally slam into the surface of the material under the tip. The kinetic energy lost by the air molecules is transferred into the surface as heat. The study breaks new ground because, at the nano scale, there are relatively few air molecules between the tip and the surface.
Heat transfer is well understood when there are many molecules, and where there are none and heat is transferred just by radiation. But there have been few studies of the scale where there are just a few molecular collisions, Chapuis said. The team had to use fundamental laws on movements of molecules in a gas to describe the process.
It transpires that heat transfer is very fast, taking only a few picoseconds; when the tip is about 10nm above the surface, it can heat a well-defined area about 35nm across, the team claimed. If this were used in a data-storage device, it could allow the storage of trillions of bits per square inch, 10 times the best performance of current hard drives.
Understanding levels of heat flux is crucial for thermally-assisted storage, said the researchers. ‘You need to know if the increase of temperature is enough to reach the critical temperature, such as the melting point,’ said Sebastian Volz, a co-author.
The phenomenon could also be useful in scanning thermal microscopy, which builds a detailed picture of the temperature and thermal conductivity of a surface, Volz added. The research shows that a smaller heat source allows the system to probe smaller areas with more precision. ‘Our work proposes a heat distribution in this field, which could help engineers design their devices,’ he said.