STM topography of monolayer chromium chloride grown on graphene/6H-SiC(0001); (inset) a magnified topography image, which shows the grain boundaries. Image: © Science.
STM topography of monolayer chromium chloride grown on graphene/6H-SiC(0001); (inset) a magnified topography image, which shows the grain boundaries. Image: © Science.

Two-dimensional (2D) materials such as graphene, which consists 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 in bulk, three-dimensional materials. As a result, 2D materials are opening the door to new applications in areas such as information and display technology and 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 together 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 with ferromagnetic properties has not yet been achieved. Even though it is precisely this kind of magnetism on a large scale that is particularly important for several potential applications – such as for a novel form of non-volatile memory.

Researchers from the Helmholtz-Zentrum Berlin (HZB) and the Max Planck Institute for Microstructure Physics, both in Germany, and the ALBA synchrotron light source in Barcelona, Spain, have now, for the first time, succeeded in creating a uniform 2D material – and demonstrating an exotic ferromagnetic behavior within it known as 'easy-plane' magnetism. They report their findings in a paper in Science.

For this work, the researchers utilized chromium chloride (CrCl3), which resembles the corresponding compound made of chromium and iodine in structure – but can be considerably more robust. The team from the Max Planck Institute for Microstructure Physics deposited a macro-scale monoatomic layer of CrCl3 upon a graphene-coated silicon carbide substrate using molecular-beam epitaxy. The purpose of the graphene was to reduce the interaction between the CrCl3 and the 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 Amilcar Bedoya-Pinto, a researcher in Stuart Parkin’s group at the Max Planck Institute for Microstructure Physics. “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 magnetic order manifests in a single monoatomic layer of CrCl3. In its normal three-dimensional form, CrCl3 is antiferromagnetic. This means the magnetic moments of its atoms are oriented in opposite directions in each layer – which makes the material appear non-magnetic in bulk. Theoretical considerations have so far suggested that the magnetic ordering in CrCl3 is lost, or it exhibits weak conventional magnetization, when reduced to a single atomic layer.

The researchers have now succeeded in disproving this – by taking a detailed look at the magnetic properties of the 2D material. To do this, they used the unique capabilities of the VEKMAG vector magnet facility 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 Florin Radu, head of the team at HZB responsible for operations at the VEKMAG facility. “Those aspects make the facility unique in the world.” As a consequence, the researchers at the Max Planck Institute for Microstructure Physics were able to determine the orientation of individual magnetic moments and accurately distinguish between chromium and chlorine atoms.

During the measurements, the researchers observed how ferromagnetic order formed in the 2D material below a certain temperature, 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,” says Bedoya-Pinto.

This discovery doesn’t just offer new insights into the magnetic behaviour of 2D materials. “We now also have an excellent platform for exploring a variety of physical phenomena that only exist in two-dimensional magnets,” says Bedoya-Pinto. These phenomena include superfluid (lossless) transport of spin, which is a kind of intrinsic angular momentum of electrons and other particles.

Spin is the basis for a new form of data processing that – unlike conventional electronics – employs 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.

This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.