This is an illustration of the Kerr effect used to detect magnetization through the rotation of polarized light when it interacts with electron spins in a material. The illustration shows layers of chromium germanium telluride (CGT); the orange balls represent tellurium atoms, yellow is germanium and blue is chromium. Image: Zhenglu Li/Berkeley Lab.It may not seem like a material as thin as an atom could hide any surprises, but a research team led by scientists at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered an unexpected magnetic property in a two-dimensional (2D) material.
The scientists found that a 2D van der Waals crystal, part of a class of materials whose atomically thin layers can be peeled off one-by-one with adhesive tape, possessed an intrinsic ferromagnetism. This discovery, reported in a paper in Nature, could have major implications for a wide range of applications that rely upon ferromagnetic materials, such as nanoscale memory, spintronic devices and magnetic sensors.
"This is an exciting discovery," said study principal investigator Xiang Zhang, senior faculty scientist at Berkeley Lab's Materials Sciences Division and a professor at the University of California, Berkeley. "This experiment presents smoking-gun evidence for an atomically thin – and atomically flat – magnet, which surprised many people. It opens the door for exploring fundamental spin physics and spintronic applications at low dimensions."
The study tackles a long-standing issue in quantum physics about whether magnetism would survive when materials shrink down to two dimensions. For half a century, the Mermin-Wagner theorem has addressed this question by stating that if 2D materials lack magnetic anisotropy, a directional alignment of electron spins, there may be no magnetic order.
"Interestingly, we found that magnetic anisotropy is an inherent property in the 2D material we studied, and because of this characteristic we were able to detect the intrinsic ferromagnetism," said study lead author Cheng Gong, a postdoctoral researcher in Zhang's lab.
Van der Waals forces, named after a Dutch scientist, are intermolecular forces of attraction that do not arise from the typical covalent or ionic bonds that usually keep molecules intact. These quantum forces are used by geckos as they effortlessly scamper along walls and ceilings.
Van der Waals crystals describe materials in which the 2D layers are not connected to each other via traditional bonds but via van der Waals forces, allowing the layers to be easily exfoliated with tape. Research on graphene, the best-known van der Waals material, was rewarded with the Nobel Prize in physics in 2010.
"It's like the pages of a book," explained Gong. "The pages can be stacked on top of each other, but the forces linking one page to another are much weaker than the in-plane forces that keep a single sheet intact."
Gong estimates that for this study he peeled off more than 3000 flakes of chromium germanium telluride (Cr2Ge2Te6; CGT). While CGT has existed as a bulk material for decades, the researchers say that 2D flakes could represent an exciting new family of 2D van der Waals crystals.
"CGT is also a semiconductor and the ferromagnetism is intrinsic," said co-senior author Jing Xia, associate professor of physics and astronomy at the University of California, Irvine. "That makes it cleaner for applications in memory and spintronics."
The researchers were able to detect the magnetization in this atomically-thin material using what is known as the magneto-optic Kerr effect. This involves the super-sensitive detection of the rotation of linearly polarized light when it interacts with electron spins in a material.
The key to one of the study's more surprising findings is that the magnetic anisotropy was very small in the CGT material. That allowed the researchers to easily control the temperature at which the material loses its ferromagnetism, known as the transition or Curie temperature.
"This is a significant discovery," said Gong, "People believe that the Curie temperature is an inherent property of a magnetic material and cannot be changed. Our study shows that it can." The researchers were able to control the transition temperature of the CGT flake using surprisingly small magnetic fields of 0.3 tesla or less.
"Thin films of metals like iron, cobalt and nickel, unlike 2D van der Waals materials, are structurally imperfect and susceptible to various disturbances, which contribute to a huge and unpredictable spurious anisotropy," said Gong. "In contrast, the highly crystalline and uniformly flat 2D CGT, together with its small intrinsic anisotropy, allows small external magnetic fields to effectively engineer the anisotropy, enabling an unprecedented magnetic field control of ferromagnetic transition temperatures."
The study authors also pointed out that a striking feature of van der Waals crystals is that they can be easily combined with dissimilar materials without any restrictions caused by structural or chemical compatibility.
"The opportunities to combine different materials to develop new functionalities are appealing," said co-senior author Steven Louie, senior faculty scientist at Berkeley Lab's Materials Sciences Division and professor of physics at UC Berkeley. "This offers a huge amount of flexibility in designing artificial structures for diverse magneto-electric and magneto-optical applications."
This story is adapted from material from the Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.