09 August 2013

Tiny twisted magnets could boost hard drive capacity by 20 times

Quantum physicists at the University of Hamburg have finally worked out how to read and write data using skyrmions — tiny twisted knots of magnetism that could allow for storage densities 20 times greater than today’s hard drives — allowing for hard drives that might one day store hundreds of terabytes of data, or alternatively finger-tip-sized drives that can carry a few terabytes.

Since they were first hypothetically described in the 1960s by a British physicist called Tony Skyrme (yes, they’re named after him), skyrmions have remained fairly elusive. At the time, skyrmions never really took off as theoretical physicists were more interested in quarks and string theory. In more recent years, though, as our tools for observing and testing quantum effects have improved, the skyrmion has come back into vogue.

Basically, a skyrmion is a twisted vortex of magnetized palladium atoms. The magnetization of an atom is defined by the spin of its electrons — depending on which way they spin, the magnetic pole is either at the top or the bottom of the atom (like a tiny little bar magnet). In general, magnetized atoms align in one direction, causing macroscopic samples to exhibit the same behavior — i.e. an actual bar magnet. In a skyrmion, however, the atoms don’t align; instead, they form a twisted vortex (pictured above).

Due to a property known as topological stability, these vortices are surprisingly hardy. Much in the same way that it’s impossible to remove the twist from a Möbius strip without destroying it completely, these skyrmions can be pushed around, but the vortex remains. Most importantly, though, the topological stability of skyrmions persists at tiny scales. In this case, the researchers were able to create stable vortices that consisted of just 300 atoms — just a few nanometers. In conventional hard drives, where conventional ferromagnetism is used and there’s no topological stability, each magnetic site (i.e. bit) needs to be much larger (tens of nanometers), otherwise neighboring bits can corrupt and interfere with each other.

The researchers at the University of Hamburg, led by Kristen von Bergmann, used a scanning tunneling microscope (STM) to create and destroy skyrmions. By using the tip of the STM to apply a stream of “twisted” (polarized) electrons, the north-south-aligned palladium atoms can be converted into skyrmions (the black dots in the video above). By applying electrons with the opposite spin, the skyrmions can be deleted.

This is the first time that skyrmions have been created and deleted since their theoretical conception in the ’60s — but we’re still a long way away from skyrmion-based 100-terabyte hard drives. Scanning tunneling microscopes are room-sized devices, and in this case the palladium had to be cooled with liquid helium (4.2 Kelvin, -269 Celsius) before the skyrmion would play ball. In the short-term, heat-assisted magnetic recording (HAMR) promises massive improvements to hard drive density, and it should be ready for commercial deployment soon. Still, as computers get ever smaller, and data storage requirements grow exponentially, skyrmions in specific and topological stability in general will likely be the focus of lots of future research.

Research paper: DOI: 10.1126/science.1240573 – “Writing and Deleting Single Magnetic Skyrmions”


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