Physicists observe an exotic 'multiferroic' state in an atomically thin material

Date:
February 23, 2022
Source:
Massachusetts Institute of Technology
Summary:
Physicists observed an exotic 'multiferroic' state in a perfectly
two- dimensional material for the first time. The findings could
lead to faster, more efficient magnetic data storage devices.



FULL STORY ==========================================================================
MIT physicists have discovered an exotic "multiferroic" state in a
material that is as thin as a single layer of atoms. Their observation is
the first to confirm that multiferroic properties can exist in a perfectly two-dimensional material. The findings, published in Nature, pave the
way for developing smaller, faster, and more efficient data-storage
devices built with ultrathin multiferroic bits, as well as other new
nanoscale structures.


========================================================================== "Two-dimensional materials are like LEGOs -- you put one on top of another
to make something different from either piece alone," says study author
Nuh Gedik, professor of physics at MIT. "Now we have a new LEGO piece:
a monolayer multiferroic, which can be stacked with other materials to
induce interesting properties." In addition to Gedik, the study's authors
at MIT include lead author Qian Song, Connor Occhialini, Emre Egec,en,
Batyr Ilyas, and Riccardo Comin, the Class of 1947 Career Development
Associate Professor of Physics, along with collaborators in Italy and
Japan and at Arizona State University.

Curiously coupled In materials science, "ferroic" refers to the
collective switching of any property in a material's electrons, such
as the orientation of their charge or magnetic spin, by an external
field. Materials can embody one of several ferroic states. For instance, ferromagnets are materials in which electron spins collectively align
in the direction of a magnetic field, like flowers pivoting with the
sun. Likewise, ferroelectrics are composed of electron charges that automatically align with an electric field.

In most cases, materials are either ferroelectric or ferromagnetic. Rarely
do they embody both states at once.



========================================================================== "That combination is very rare," Comin says. "Even if one took the entire periodic table and put no boundary on the combination of elements, there
are not many of these multiferroic materials that can be produced."
But in recent years, scientists have synthesized materials in the lab
that exhibit multiferroic properties, behaving as both ferroelectrics and ferromagnets, in curiously coupled fashion. For instance, the magnetic
spins of electrons can be switched by not just a magnetic field but also
an electric field.

This coupled, multiferroic state is particularly exciting for its
potential to advance magnetic data-storage devices. In conventional
magnetic hard drives, data are written onto a rapidly rotating disk
patterned with tiny domains of magnetic material. A small tip suspended
over the disk generates a magnetic field that can collectively switch a domain's electron spins in one direction or another to represent either a
"0" or a "1" -- the basic "bits" that encode data.

The tip's magnetic field is typically produced by an electrical current,
which requires significant energy, some of which can be lost in the form
of heat. In addition to overheating a hard drive, electrical currents
have a limit to how fast they can generate a magnetic field and switch
magnetic bits. 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.



==========================================================================
One large hurdle for device integration has been size. Thus far,
physicists have only observed multiferroic properties in relatively
large samples of three-dimensional materials, too large to work into
nanoscale memory bits. No one has been able to synthesize a perfectly two-dimensional multiferroic material.

"All known examples of multiferroics are in 3D, and there was a
fundamental question: Can these states exist in 2D, in a single atomic
sheet?" Comin says.

Ferroic flakes To answer that, the team looked to nickel iodide (NiI2),
a synthetic material that is known to be multiferroic in bulk form.

"In our case, it was a dual challenge, to try to make nickel iodide into a
2D form and to measure it to see if it retained multiferroic properties,"
Comin says.

While other two-dimensional materials such as graphene can be made
simply by exfoliating the layers from bulk versions such as graphite,
nickel iodide is more finicky. The team needed a new way to synthesize
the material in 2D form.

The team, led by Song, borrowed from a technique known as epitaxial
growth, in which thin atomic sheets of material are "grown" on another
base material. In their case, Song and his colleagues used hexagonal boron nitride as the bulk foundation, which they placed in a furnace. Over
this material, they flowed powders of nickel and iodide, which settled
onto boron nitride in perfect, atom-thin flakes of nickel iodide.

To test each flake's multiferroic properties, Gedik and Comin employed
optical techniques developed in their respective labs to probe the
material's magnetic and electrical response.

'The wavelength of light we use is around half a micron, so we can zoom
in on a small region of this flake and study its properties with great precision," Comin explains.

The researchers progressively chilled the 2D flakes to temperatures
as low as 20 kelvins, where the material was previously observed to
exhibit multiferroic properties in 3D form. They then carried out
separate optical tests to probe first the material's magnetic, then
electrical properties. At around 20 K, the material was found to be both ferromagnetic and ferroelectric.

The team's experiments confirm that nickel iodide is multiferroic in
its two- dimensional form. What's more, the study is the first to
demonstrate that multiferroic order can exist in two dimensions --
the ideal dimensions for building nanoscale, multiferroic memory bits.

"We now have a material that's multiferroic in 2D. Before, we didn't
know what to work with if we wanted to make a nanoscale multiferroic
device. Now we do.

And we are starting to make these devices in our lab now," Comin says. "We
want to use electric fields to control magnetism, to see how fast we can
switch multiferroic bits, and how we can miniaturize these devices. That's
the roadmap, and now we're much closer." This research was supported by
the Department of Energy and, in part, by the National Science Foundation
and the Gordon and Betty Moore Foundation.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Song, Q., Occhialini, C.A., Ergec,en, E. et al. Evidence for
a single-
layer van der Waals multiferroic. Nature, 2022 DOI:
10.1038/s41586-021- 04337-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220223111228.htm

--- up 11 weeks, 4 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)