A one-atom-thin 2D magnet developed by Berkeley Lab and UC Berkeley could
advance new applications in computing and electronics.
The development of an ultrathin magnet that operates at room temperature
could lead to new applications in computing and electronics – such as
high-density, compact spintronic memory devices – and new tools for the
study of quantum physics.
The ultrathin magnet, which was recently reported in the journal Nature
Communications, could make big advances in next-gen memory devices,
computing, spintronics, and quantum physics. It was discovered by scientists
at the Department of Energy’s Lawrence Berkeley National Laboratory
(Berkeley Lab) and UC Berkeley.
“We’re the first to make a room-temperature 2D magnet that is chemically
stable under ambient conditions,” said senior author Jie Yao, a faculty
scientist in Berkeley Lab’s Materials Sciences Division and associate
professor of materials science and engineering at UC Berkeley.
“This discovery is exciting because it not only makes 2D magnetism possible
at room temperature, but it also uncovers a new mechanism to realize 2D
magnetic materials,” added Rui Chen, a UC Berkeley graduate student in the
Yao Research Group and lead author on the study.
The magnetic component of today’s memory devices is typically made of
magnetic thin films. But at the atomic level, these materials are still
three-dimensional – hundreds or thousands of atoms thick. For decades,
researchers have searched for ways to make thinner and smaller 2D magnets
and thus enable data to be stored at a much higher density.
Previous achievements in the field of 2D magnetic materials have brought
promising results. But these early 2D magnets lose their magnetism and
become chemically unstable at room temperature.
“State-of-the-art 2D magnets need very low temperatures to function. But for
practical reasons, a data center needs to run at room temperature,” Yao
said. “Our 2D magnet is not only the first that operates at room temperature
or higher, but it is also the first magnet to reach the true 2D limit: It’s
as thin as a single atom!”
The researchers say that their discovery will also enable new opportunities
to study quantum physics. “It opens up every single atom for examination,
which may reveal how quantum physics governs each single magnetic atom and
the interactions between them,” Yao said.
The making of a 2D magnet that can take the heat
The researchers synthesized the new 2D magnet – called a cobalt-doped van
der Waals zinc-oxide magnet – from a solution of graphene oxide, zinc, and
cobalt.
Just a few hours of baking in a conventional lab oven transformed the
mixture into a single atomic layer of zinc-oxide with a smattering of cobalt
atoms sandwiched between layers of graphene.
In a final step, the graphene is burned away, leaving behind just a single
atomic layer of cobalt-doped zinc-oxide.
“With our material, there are no major obstacles for industry to adopt our
solution-based method,” said Yao. “It’s potentially scalable for mass
production at lower costs.”
To confirm that the resulting 2D film is just one atom thick, Yao and his
team conducted scanning electron microscopy experiments at Berkeley Lab’s
Molecular Foundry to identify the material’s morphology, and transmission
electron microscopy (TEM) imaging to probe the material atom by atom.
X-ray experiments at Berkeley Lab’s Advanced Light Source characterized the
2D material’s magnetic parameters under high temperature.
Additional X-ray experiments at SLAC National Accelerator Laboratory’s
Stanford Synchrotron Radiation Lightsource verified the electronic and
crystal structures of the synthesized 2D magnets. And at Argonne National
Laboratory’s Center for Nanoscale Materials, the researchers employed TEM to
image the 2D material’s crystal structure and chemical composition.
The researchers found that the graphene-zinc-oxide system becomes weakly
magnetic with a 5-6% concentration of cobalt atoms. Increasing the
concentration of cobalt atoms to about 12% results in a very strong magnet.
To their surprise, a concentration of cobalt atoms exceeding 15% shifts the
2D magnet into an exotic quantum state of “frustration,” whereby different
magnetic states within the 2D system are in competition with each other.
And unlike previous 2D magnets, which lose their magnetism at room
temperature or above, the researchers found that the new 2D magnet not only
works at room temperature but also at 100 degrees Celsius (212 degrees
Fahrenheit).
“Our 2D magnetic system shows a distinct mechanism compared to previous 2D
magnets,” said Chen. “And we think this unique mechanism is due to the free
electrons in zinc oxide.”
True north: Free electrons keep magnetic atoms on track
When you command your computer to save a file, that information is stored as
a series of ones and zeroes in the computer’s magnetic memory, such as the
magnetic hard drive or a flash memory.
And like all magnets, magnetic memory devices contain microscopic magnets
with two poles – north and south, the orientations of which follow the
direction of an external magnetic field. Data is written or encoded when
these tiny magnets are flipped to the desired directions.
According to Chen, zinc oxide’s free electrons could act as an intermediary
that ensures the magnetic cobalt atoms in the new 2D device continue
pointing in the same direction – and thus stay magnetic – even when the
host, in this case the semiconductor zinc oxide, is a nonmagnetic material.
“Free electrons are constituents of electric currents. They move in the same
direction to conduct electricity,” Yao added, comparing the movement of free
electrons in metals and semiconductors to the flow of water molecules in a
stream of water.
The new material – which can be bent into almost any shape without breaking,
and is a million times thinner than a sheet of paper – could help advance
the application of spin electronics or spintronics, a new technology that
uses the orientation of an electron’s spin rather than its charge to encode
data. “Our 2D magnet may enable the formation of ultra-compact spintronic
devices to engineer the spins of the electrons,” Chen said.
“I believe that the discovery of this new, robust, truly two-dimensional
magnet at room temperature is a genuine breakthrough,” said co-author Robert
Birgeneau, a faculty senior scientist in Berkeley Lab’s Materials Sciences
Division and professor of physics at UC Berkeley who co-led the study.
“Our results are even better than what we expected, which is really
exciting. Most of the time in science, experiments can be very challenging,”
Yao said. “But when you finally realize something new, it’s always very
fulfilling.”
Reference:
Tunable room-temperature ferromagnetism in Co-doped two-dimensional van
der Waals ZnO by Rui Chen, Fuchuan Luo, Yuzi Liu, Yu Song, Yu Dong, Shan Wu, Jinhua
Cao, Fuyi Yang, Alpha N’Diaye, Padraic Shafer, Yin Liu, Shuai Lou, Junwei
Huang, Xiang Chen, Zixuan Fang, Qingjun Wang, Dafei Jin, Ran Cheng, Hongtao
Yuan, Robert J. Birgeneau and Jie Yao, 25 June 2021, Nature Communications.
DOI:
10.1038/s41467-021-24247-w