For the first time, multiferroic characteristics have been discovered in a two-dimensional material; this discovery could lead to more effective magnetic memory devices.
In a
material as thin as a single layer of atoms, MIT physicists identified an
exotic "multiferroic" state. Their discovery is the first to show
that multiferroic characteristics can exist in a material that is perfectly
two-dimensional. The findings, which were published in Nature, pave the way for
smaller, faster, and more efficient data storage devices made of ultrathin
multiferroic bits, as well as other new nanoscale structures.
"Two-dimensional materials are like LEGOs," explains research author Nuh Gedik, an MIT professor of physics. "You place one on top of another to build something different from either piece alone." "We now have a new LEGO piece: a monolayer multiferroic that can be stacked with other materials to induce fascinating features," says the researcher.
MIT
contributors include main author Qian Song, Connor Occhialini, Emre Egeçen,
Batyr Ilyas, and Riccardo Comin, the Class of 1947 Career Development Associate
Professor of Physics, as well as collaborators in Italy and Japan and at
Arizona State University.
Curiously coupled
"Ferroic"
refers to the collective switching of any property in a material's electrons,
such as charge orientation or magnetic spin, by an external field in materials
science. Materials can exist in a variety of ferroic states. Ferromagnets, for
example, are materials in which electron spins align collectively in the
direction of a magnetic field, similar to how flowers rotate with the sun.
Ferroelectrics, too, are made up of electron charges that align themselves with
an electric field.
Materials
are either ferroelectric or ferromagnetic in most circumstances. They rarely
embody both states at the same time.
"That combination is quite rare," says Comin. "There aren't many of these multiferroic materials that can be made even if one took the complete periodic table and put no boundary on the combination of elements."
In recent
years, however, scientists have created materials in the lab that have
multiferroic properties, acting as both ferroelectrics and ferromagnets in a
strangely coupled manner. The magnetic spins of electrons, for example, can be
altered by both a magnetic and an electric field.
The
potential for this linked, multiferroic state to enhance magnetic data storage
systems is particularly exciting. Data is written onto a fast rotating disc
imprinted with small domains of magnetic material in traditional magnetic hard
drives. A tiny tip suspended over the disc provides a magnetic field that can
collectively alter a domain's electron spins in one direction or the other,
representing a "0" or a"1"
— the basic "bits" that encode data.
The
magnetic field at the tip is usually created by an electrical current, which
requires a lot of energy, some of which can be lost as heat. Electrical
currents have a limit to how fast they can generate a magnetic field and switch
magnetic bits, in addition to overheating a hard drive. Physicists like Comin
and Gedik believe that if these magnetic bits could be made from a multiferroic
material, they could be switched using faster and more energy-efficient
electric fields, rather than current-induced magnetic fields.
“If using electric fields, the process of writing bits would be much faster because fields can be created in a circuit within a fraction of a nanosecond — potentially hundreds of times faster than with electrical current,” Comin says.
Size has
been a significant barrier to device integration. Physicists have only
discovered multiferroic characteristics in rather large samples of
three-dimensional materials, which are too large to be used in nanoscale memory
bits. No one has been able to create a two-dimensional multiferroic material
that is perfectly two-dimensional.
"All known examples of multiferroics are in three dimensions, and a fundamental question was raised: Can these states exist in two dimensions, in a single atomic sheet?" Comin explains.
Ferroic flakes
The
scientists turned to nickel iodide (NiI2), a synthetic material that is known
to be multiferroic in bulk form, to find a solution.
"It
was a twofold challenge in our situation," Comin explains, "to try to
transform nickel iodide into a 2D form while also measuring it to determine if
it kept multiferroic capabilities."
Other
two-dimensional materials, such as graphene, can be created by simply
exfoliating layers from bulk graphite, but nickel iodide is more difficult to
work with. In order to synthesise the material in 2D form, the researchers
needed a novel method. The team, led by Song, used an epitaxial growth process,
in which thin atomic sheets of material are "grown" on another
foundation material. Song and his colleagues employed hexagonal boron nitride
as the bulk foundation in their experiment, which they heated in a furnace.
They sprayed nickel and iodide powders over the material, which settled into
flawless, atom-thin nickel iodide flakes on the boron nitride.
Gedik and
Comin used optical techniques developed in their respective labs to examine the
material's magnetic and electrical response to test each flake's multiferroic
capabilities.
'We can
zoom in on a small portion of this flake and investigate its features with
amazing accuracy since the wavelength of light we employ is roughly half a
micron," Comin explains.
The
researchers froze the 2D flakes to temperatures as low as 20 kelvins, where the
material previously showed multiferroic characteristics in 3D form. They next
conducted independent optical tests to investigate the material's magnetic and
electrical properties, respectively. The material was discovered to be both
ferromagnetic and ferroelectric at a temperature of roughly 20 K.
The team's
experiments show that nickel iodide in its two-dimensional form is
multiferroic. Furthermore, the research is the first to show that multiferroic
order can exist in two dimensions, which are ideal for creating nanoscale
multiferroic memory bits.
"We now have a two-dimensional multiferroic material." We didn't know where to start when it came to making a nanoscale multiferroic device before. We now have it. And now we're starting to build these devices in our lab," says Comin. "We want to investigate how fast we can flip multiferroic bits and how small we can make these devices using electric fields to control magnetism." That's the plan, and we're getting closer."
References:
Song, Q.,
Occhialini, C.A., Ergeçen, E. et al. Evidence for a single-layer van der Waals
multiferroic. Nature 602, 601–605 (2022).
https://doi.org/10.1038/s41586-021-04337-x
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