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Summer 2017

Harnessing Quantum Vortices

Room-temperature control of exciting phenomena opens the door for next-generation memory storage

Max Grossnickle


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.

Acknowledgments

Ma et al. The work at University of Texas, Austin and University of California, Los Angeles was supported by SHINES, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Science (BES). The work at the University of Delaware, calibrating optical facilities, was supported by the National Science Foundation.

Yu et al. This work was partially supported by the Energy Frontier Research Center for Spins and Heat in Nanoscale Electronic Systems (SHINES). The material growth, devices fabrication, MOKE measurement, and device characterization in this work were supported by SHINES. The material characterization and device design were supported by other funding agencies: the National Science Foundation (NSF) Nanosystems Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems (TANMS), and in part by the FAME Center, one of six centers of the Semiconductor Technology Advanced Research network (STARnet), a Semiconductor Research Corporation (SRC) program sponsored by the Microelectronics Advanced Research Corporation (MARCO), and the Defense Advanced Research Projects Agency (DARPA). The authors acknowledge collaboration with the King Abdul-Aziz City for Science and Technology via The Center of Excellence for Green Nanotechnologies.

More Information

Ma X, G Yu, X Li, T Wang, D Wu, KS Olsson, Z Chu, K An, JQ Xiao, KL Wang, and X Li. 2016. “Interfacial Control of Dzyaloshinskii-Moriya Interaction in Heavy Metal/Ferromagnetic Metal Thin Film Heterostructures.” Physical Review B 94:180408. DOI: 10.1103/PhysRevB.94.180408

Yu G, P Upadhyaya, X Li, W Li, SK Kim, Y Fan, KL Wong, Y Tserkovnyak, PK Amiri and KL Wang. 2016. “Room-Temperature Creation and Spin–Orbit Torque Manipulation of Skyrmions in Thin Films with Engineered Asymmetry.” Nano Letters 17:261. DOI: 10.1021/acs.nanolett.5b05257

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Skyrmions and Hedgehogs

How a magnetic “surface” could make computers last longer and use less energy

Imagine a hedgehog scampering through the fall leaves. Its spines pick up bits of leaves and dirt, but it remains unhindered by the grime. This critter is an example of a topological phase. A topological phase is a state that persists in a material no matter the additions or changes, such as the critter stretching to slip under a gate. This property makes topological phases incredibly resilient to disturbances. Researchers at the Spins and Heat in Nanoscale Electronic Systems (SHINES) Energy Frontier Research Center are studying a topological phase called a magnetic skyrmion, which exists as a swirling spin texture. The team precisely stacked five materials in a way that generated skyrmions at room temperature and variable sizes. This work could enable a form of computer memory (see video of racetrack memory) that uses less energy, holds more data and is less prone to crashing. SHINES is led by the University of California, Riverside, and funded by the Department of Energy.

 

More Information

Ma X, G Yu, X Li, T Wang, D Wu, KS Olsson, Z Chu, K An, JQ Xiao, KL Wang, and X Li. 2016. “Interfacial Control of Dzyaloshinskii-Moriya Interaction in Heavy Metal/Ferromagnetic Metal Thin Film Heterostructures.” Physical Review B 94:180408. DOI: 10.1103/PhysRevB.94.180408

Yu G, P Upadhyaya, X Li, W Li, SK Kim, Y Fan, KL Wong, Y Tserkovnyak, PK Amiri and KL Wang. 2016. “Room-Temperature Creation and Spin–Orbit Torque Manipulation of Skyrmions in Thin Films with Engineered Asymmetry.” Nano Letters 17:261. DOI: 10.1021/acs.nanolett.5b05257

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.