About five years ago, researchers at the University of Hamburg demonstrated that tiny, swirling magnetic spin patterns on thin films—known as skyrmions—could be used to store and erase data on magnetic media.
At that time, these spinning magnetic swirls that had been proposed over 60 years ago by British physicist Tony Skyrme—from whom the name derives—had suddenly become a potentially game changing magnetic data storage system. And what a change it represented: skyrmions are 10 times smaller than the magnetic regions used on traditional hard drives.
Now a team of researchers from CNRS/Thales joint lab in France with European Funding under the MAGicSky program have taken a critical step in the commercial realization of this technology by electrically detecting for the first time a single small skyrmion at room temperature.
“We believe this is an important advance because it demonstrates one of the unavoidable functions for any type of future concept of devices: electrical detection,” said Vincent Cros, a researcher at CNRS and co-author of the paper published in Nature Nanotechnology.
While the electrical signal of skyrmion lattices or an ensemble of skyrmions has been measured before, mostly at low temperatures, this is the first time that measuring an electrical signal has been demonstrated for a single skyrmion and at room temperature.
As one might imagine, the electrical signals associated with these 100 nanometer skyrmions remain relatively small. These signals are so small, according to Cros, that they had to be sure that the measured electrical signal is actually associated with the presence of a skyrmion. “That is exactly what we demonstrate here by a concomitant electrical measurement and magnetic imaging on the very same devices,” said Cros.
While it was necessary to use magnetic imaging to ensure that they were measuring a skyrmion for their research, in future memory devices the only possible reading procedure will be through electrical measurement and not by imaging the magnetic configuration of the skyrmion.
Nonetheless, the ability to make electrical measurements of the sample while imaging it magnetically at the same time using magnetic force microscopy is extremely significant, according to Cros, and had never been done before.
As for the actual device, this goes back to work Cros and his colleagues did in 2013 that suggested the best memory device for exploiting skyrmions would be what’s known as “racetrack memory.”
Nearly a decade ago, Stuart Parkin and his colleagues at IBM Almaden Research demonstrated a three-bit version of so-called “racetrack memory,” which is a solid-state non-volatile memory that promises much higher storage density than conventional solid-state memory devices.
When Parkin first envisioned racetrack memory, they were based on magnetic features known as domain walls, which essentially separate the magnetic direction of a material into different areas. Electric currents could push those domain walls around the track and a sensor could detect the changes, leading to the “0” and “1” of digital memory.
What Cros and his colleagues suggested five years ago was that the skyrmions could replace the domain walls and they could move along the track and their presence or absence could be detected electrically, leading to a digital memory device.
By using a basic skyrmion-race-track memory, the researchers designed electrical contacts on both ends of the tracks. In order to detect the electrical skyrmion signal, they also designed lateral contacts. The electrical signal is simply detected by measuring the associated electrical voltage using a commercial voltmeter.
It sounds all pretty straight forward, however, controlling the position and the density of the skyrmions remained a challenge. The main obstacle revolved around the creation of the skyrmions in the material where prior to their formation it had all been in a uniform magnetized state. The traditional method for producing the skyrmions was based on the use of a magnetic field.
“In the present work, we have employed a new approach in which we inject short current pulses into the materials, which allows us to create isolated skyrmions located in a strip (or track) designed by electron-beam lithography,” explained Cros.
The result is that Cros and his colleagues can now adjust the total number of nucleated skyrmions by tuning different parameters, such as the current pulse width or the intensity of the external magnetic field.
While all of this will certainly go down as a significant step towards using skyrmions in memory devices, Cros concedes that commericialization is still a ways down the road.
“We are not yet at the stage where skyrmion devices can be used and implemented as a real new electronic device,” said Cros. “The standard and reasonable time scale between fundamental discoveries and consumer electronics is often between 10 and 15 years.”
To realize this 15 year time line, Cros believes that more efforts are needed to further decrease the skyrmion size, targeting sub 10-nm diameter, to increase the skyrmion speed, better understand and control the interaction of skyrmions with material grains (typically of the same sizes) and to increase the electrical signal.