Skyrmions are small magnetic vortices that occur in many materials, including manganese silicide thin films (in which they were first discovered) and cobalt-iron-silicon. In the present work, a palladium-iron bilayer on an irridium crystal surface was studied. They can be imagined as 2D knots in which the magnetic moments rotate about 360° within a plane (see figure).

Skyrmions could form the basis of future hard-disk technologies. Today's disks use magnetic domains (in which all the magnetic spins are aligned in the same direction) to store information, but there are fundamental limits to how tiny such domains can become. Skyrmions might be made much smaller and thus be used to create storage devices with much higher density. What is more, flipping all of the spins in conventional domains (to switch device memory states from 1 to 0, for example) requires considerable power and can be slow. Skyrmions require fewer spin flips to switch and the final spin state is not easily disrupted either – making these structures more stable than conventional magnetic domains.

Creating and annihilating single magnetic skyrmions

However, before skyrmions can be exploited in hard drives, scientists need to figure out a way to control them – something that has proved difficult to date. A team of researchers led by Kirsten von Bergmann, André Kubetzka and Roland Wiesendanger has now shown that they create and annihilate single magnetic skyrmions using a spin-polarized current (one in which spin is mostly aligned in one direction) from a scanning tunnelling microscope tip – albeit at ultralow temperatures of 4.2 K. The skyrmions switch from one state to another thanks to spin-transfer torques, according to the team, and one state (skyrmions present) can be favoured over the other (skyrmions absent).

"To write or delete a skyrmion we position our STM tip at a particular spot on the sample and inject a spin-polarized tunnel current pulse into it," explains von Bergmann. "While at low currents and voltages the sample magnetization is stable, at higher current and voltages, the magnetic state starts to switch between a skyrmion and a simple parallel alignment of magnetic moments," she told nanotechweb.org. "In this situation, the current direction can determine which of the two states is favoured over the other one – a clear indication that spin-transfer torque is involved in the switching process."

IT applications

Being able to write and delete skyrmions in this way now means that such spin textures can be exploited in information technology, she added. "In particular, the possibility of using layered thin films in this way, as is the case in conventional IT device technology, could lead to a big step forward towards applications."

The Hamburg team is now busy trying to understand the switching mechanism in more detail and finding out exactly how the spin-polarized current couples to the magnetization. "This will help us to optimize the process of writing and deleting skyrmions," said von Bergmann. "We will also be investigating other thin film materials in an effort to unearth systems that show such skyrmion switching at room temperature."

Skyrmions are named after the British particle physicist Tony Skyrme, who in 1962, found that they could explain how subatomic particles such as neutrons and protons exist as discrete entities emerging from a continuous nuclear field.

Von Bergmann and colleagues’ results have been published in Science.

Further reading

Computing with spin waves at the nanoscale (Feb 2011)
FePt nanoparticles in silica films for data storage (Jan 2009)
Building novel nanomagnets atom by atom (May 2012)
Spins spotted in room-temperature silicon (Nov 2009)
Electric fields control spin currents (Jan 2010)