Figure 1: A skyrmion moves through a field of electrons. Note that the movement doesn’t consist of any actual displacement of electrons, but rather a shift in the electron spin orientations.
A spiraling quantum vortex speeds through a dirty landscape, completely unfazed by what it encounters. This isn’t a description of an unreleased “Twister” sequel, but rather an example of something called a topological phase. Theoretical predictions of topological phases of matter were recognized in the 2016 Nobel Prize in Physics. A topological phase is, simply, a state that will persist in a material no matter how much it is twisted or deformed, as long as it is not punctured or torn. For instance, the topological phase that existed in a round lump of dough would persist even if you baked that dough into a long, thin baguette. However, the moment you punch out the middle to make a bagel, the topological phase would be destroyed. This property makes topological phases incredibly resilient to disturbances in a material. Researchers at the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center, are studying an exciting, if less tasty, topological phase known as a magnetic skyrmion, which exists as a swirling spin texture.
What does it mean for something to be a spin texture?
Non-physicists most commonly encounter spins in the form of iron magnets, which are used in everything from refrigerator magnets to hard drives. In these cases, the spins, which are a quantum property of electrons, are all aligned in the same direction, causing a strong magnetic field to form. Skyrmions are much more complex — they take the form of spiraling spin patterns (see Figure 1). These spin patterns can shift and move throughout a material. But importantly, the electrons need not move with them. This fact makes skyrmions a candidate for replacing conventional electronic devices with low-power, energy-efficient successors.
Skyrmions come in from the cold
Skyrmion applications so far have been stymied by the difficulty of controlling their spatial dimensions at room temperature. Groups in SHINES managed to overcome these problems by finely tuning the Dzyaloshinskii-Moriya interaction (or DMI), which primarily affects how tightly wound the skyrmions are. The normal system used to create skyrmions is a sandwich made of a stack of tantalum, cobalt/iron/boron, and magnesium oxide. In this setup, skyrmion generation occurs within the cobalt-containing layer. The major innovation in this work was a change in the construction of the conventional system. Researchers found that it was possible to precisely engineer the multilayer stacking by adding an additional wedge of platinum between the middle two layers. Carefully tuning the thickness of the platinum wedge allowed them to control the strength of the DMI. By using this method, the SHINES researchers varied the size of the skyrmions from 800 to 1,200 nanometers and, more importantly, made the skyrmions stable enough to exist at room temperature.
Forgetting failures with racetrack memory
Controlling the size and stability of skyrmions at room temperature may lead to one of the greatest technological leaps towards their use as replacements for conventional hard drives. Standard hard drives work by creating 1s and 0s in the form of small ferromagnetic domains on a disk. These 1s and 0s make up the basic language that computers use to store data — for instance, the letter “A” is stored as “01000001” on a hard disk. To read the data back, the disk spins at high speed while a read head measures the magnetic field. This introduces a major source of mechanical failure in every disk-based hard drive.
A skyrmion-based hard drive would instead use a racetrack configuration, in which the 1s and 0s instead take the form of skyrmions (1s) and their absence (0s). Proposed by Stuart Parkin in 2008, racetrack memory has the read head sit in one spot while the memory is driven past. Racetrack memory has no moving parts, which reduces the amount of mechanical failures and the energy cost associated with spinning a large disk. The use of skyrmions would further enhance energy savings because they use only 1 percent of the current required to run ferromagnetic domain-based racetrack memory.
Figure 2: A conventional hard drive spins an entire disk to read memory, while skyrmion racetrack memory only has to apply a voltage to shift memory. This will reduce the amount of wear and tear in hard drives, as well as energy usage.
In a more fundamental sense
The SHINES research also demonstrates the ability to make the decades-old predictions of topological phenomena real by vastly reducing one of the greatest barriers to studying topological states — their fragility at room temperature. Researchers at SHINES are carrying out follow-up experiments to fine-tune the control of skyrmion motion and size, and learn how to overcome topological protections to create data on a hard drive.
In the future, scientists may be able to use exciting topological phenomena to illuminate fundamental questions of condensed matter physics while simultaneously developing high-density, low-power devices for next-generation information processing.
