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Chromium triiodide
This is a top-view depiction of a single layer of chromium triiodide. Chromium atoms are depicted in gray, with iodine atoms in purple. (UW / MIT Illustration / Efren Navarro-Moratalla)

For the first time, researchers have discovered magnetism in the two-dimensional realm of monolayers, or materials that consist of a single atomic layer.

The material, known as chromium triiodide or CrI3, could play a role in new types of memory devices with faster data processing speeds.

A team led by researchers from the University of Washington and the Massachusetts Institute of Technology published their results this week in the journal Nature.

“What we have discovered here is an isolated 2D material with intrinsic magnetism, and the magnetism in the system is highly robust,” UW Professor Xiaodong Xu, a co-author of the study and a member of the university’s Clean Energy Institute, said in a news release. “We envision that new information technologies may emerge based on these new 2D magnets.”

Chromium triiodide was previously known to have magnetic properties, but it wasn’t known whether those properties would remain at a single-atom thickness.

The team used adhesive tape to peel off a monolayer of the material from a larger crystal, then took advantage of magneto-optical microscopy to confirm its ferromagnetic characteristics.

Molecule
A side view of the 2-D layer shows that chromium triiodide has a complex molecular structure. (UW / MIT Illustration / Efren Navarro-Moratalla)

Using sticky tape to strip off super-thin layers of material may sound like a low-tech experimental technique, but it’s “surprisingly effective,” said UW doctoral student Genevieve Clark, one of the study’s lead authors.

“This simple, low-cost technique was first used to obtain graphene, the 2-D form of graphite, and has been used successfully since then with other materials,” Clark said.

The optical signature of ferromagnetism disappeared when the flakes of chromium triiodide were two layers thick, but reappeared in three-layer flakes. That suggests that electron spins are opposed to each other in alternating atomic layers.

Two-dimensional magnetism could facilitate new types of memory devices, said UW doctoral student Bevin Huang, another lead author of the paper.

“Practically, miniaturization and increased efficiency is the main feature of these 2-D magnets,” Huang told GeekWire in an email. “In a nutshell, these 2-D magnets can potentially increase the storage capacities of MRAM, a possible ‘universal memory’ … by up to one to two orders of magnitude.”

Huang explained that the computer industry is facing a bottleneck caused by the physical separation between data storage and the quick-access memory used for data processing.

Combining both those functions in a universal memory device, based on magnetoresistive random access memory, or MRAM, could ease the bottleneck and dramatically increase data storage capabilities. Companies such as Everspin are pioneering MRAM devices, but the technology hasn’t yet reached its full potential.

The 2-D magnetic properties discovered for chromium triiodide could help put MRAM-based universal memory on the fast track, Huang said.

In addition to Huang, Clark and Xu, the authors of “Layer-dependent Ferromagnetism in a van der Waals Crystal Down to the Monolayer Limit” include Efrén Navarro-Moratalla, Dahlia Klein, Ran Cheng, Kyle Seyler, Ding Zhong, Emma Schmidgall, Michael McGuire, David Cobden, Wang Yao, Di Xiao and Pablo Jarillo-Herrero.

 

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