Spintronics: Exotic ferromagnetic order in two-dimensions

Date:
November 1, 2021
Source:
Helmholtz-Zentrum Berlin fu"r Materialien und Energie
Summary:
An international team has detected an unusual ferromagnetic property
in a two-dimensional system, known as 'easy-plane anisotropy.' This
could foster new energy efficient information technologies based
on spintronics for data storage, among other things.



FULL STORY ==========================================================================
An international team has detected an unusual ferromagnetic property in
a two- dimensional system, known as 'easy-plane anisotropy.' This could
foster new energy efficient information technologies based on spintronics
for data storage, among other things.


==========================================================================
The thinnest materials in the world are only a single atom thick. These
kinds of two-dimensional or 2D materials -- such as graphene, well-known
as consisting of a single layer of carbon atoms -- are causing a great
deal of excitement among research teams worldwide. This is because
these materials promise unusual properties that cannot be obtained
using three-dimensional materials. As a result, 2D materials are opening
the door to new applications in fields such as information and display technology, as well as for critical components in extremely sensitive
sensors.

Structures known as van-der-Waals monolayers are arousing particular
interest.

These are combinations of two or more layers of different materials that
are each only a single atom thick, with the layers held to one another
by weak electrostatic van-der-Waals forces. By selecting the type and
sequence of material layers bound in this way, specific electrical,
magnetic, and optical characteristics can be chosen and modified. However, scaled-up homogeneous deposition of individual van-der-Waals layers
having ferromagnetic properties has not yet been achieved. Yet it is
precisely this kind of magnetism on a larger scale that is particularly important for several potential applications -- such as for a novel form
of non-volatile memoryfor example.

Scientists from the Max Planck Institute for Microstructure Physics in
Halle, Germany, the ALBA synchrotron light source in Barcelona, Spain,
and the Helmholtz-Zentrum Berlin have now succeeded for the first time
in creating a uniform two-dimensional material -- and demonstrating an
exotic ferromagnetic behaviour within it known as "easy-plane" magnetism.

A nearly free-floating layer of chromium and chlorine The researchers
from Germany and Spain utilised chromium chloride (CrCl3) as a material,
which resembles the corresponding compound made of chromium and iodine
in structure -- but can be considerably more robust. The team in Halle deposited a macro-scale monoatomic layer of this material upon a graphene- coated silicon-carbide substrate using molecular-beam epitaxy.The
purpose of the graphene was to reduce the interaction between chromium
chloride and silicon carbide and thereby prevent the substrate from
influencing the properties of the monoatomic CrCl3 layer. This was the
key to accessing the elusive magnetic easy-plane anisotropy," explains
Dr. Amilcar Bedoya-Pinto, a researcher in Prof. Stuart Parkin's group
at the Max Planck Institute in Halle.

"Essentially, we obtained an almost free-floating, ultrathin layer that
was only bound to the graphene interlayer by weak van-der-Waals forces."
The team's goal was to answer the question of how the magnetic order in chromium chloride manifests itself when it consists of only a single
monoatomic layer. In its normal three-dimensional form, the substance
is antiferromagnetic. As a result, the magnetic moments of the atoms
are oriented in opposite directions in each layer -- which makes the
material appear non- magnetic in bulk. Theoretical considerations so far suggested that the magnetic ordering is lost or exhibits weak conventional magnetisation when the material is reduced to a single atomic layer.

Precise measurements at the VEKMAG facility However, scientists have now succeeded in disproving this -- by taking a detailed look at the magnetic properties of the 2D material. To do so, they used the unique capabilities
of the VEKMAG vector magnet facility installed at HZB's synchrotron
radiation source BESSY II. "Here it is possible to investigate samples
using soft X-rays in a strong magnetic field -- and at temperatures near absolute zero," says Dr Florin Radu, head of the team at HZB responsible
for operations at the VEKMAG facility. "Those aspects makes the facility
unique in the world," adds the Berlin scientist. It enabled the team
members from Halle to determine the orientation of individual magnetic
moments and to accurately distinguish between chromium and chlorine atoms.

During the measurements, the researchers observed how ferromagnetic order formed in the two-dimensional material below a certain temperature,
what is known as the Curie temperature. "In the monoatomic chromium
chloride layer, a phase transition characteristic of easy-plane magnets
took place that had never been observed before in such a 2D material,"
reports Bedoya-Pinto.

Tailwind for the development of spintronics The discovery not only
offers new insights into the magnetic behaviour of two- dimensional
materials. "We now also have an excellent platform for exploring
a variety of physical phenomena that only exist in two-dimensional
magnets," Bedoya-Pinto is pleased to say, such as superfluid (lossless) transport of spin, which is a kind of intrinsic angular momentum of
electrons and other particles. These are the basis for a new form of
data processing that -- unlike conventional electronics -- uses magnetic moments rather than electrical charges. Known as spintronics, this is
currently revolutionizing data storage and information processing. The
new insights gained at HZB could boost this development.

========================================================================== Story Source: Materials provided by Helmholtz-Zentrum_Berlin_fu"r_Materialien_und_Energie.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Amilcar Bedoya-Pinto, Jing-Rong Ji, Avanindra K. Pandeya, Pierluigi
Gargiani, Manuel Valvidares, Paolo Sessi, James M. Taylor, Florin
Radu, Kai Chang, Stuart S. P. Parkin. Intrinsic 2D-XY ferromagnetism
in a van der Waals monolayer. Science, 2021; 374 (6567): 616 DOI:
10.1126/ science.abd5146 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211101105351.htm

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