Illustration of Kerr effect used to detect magnetization through the rotation of polarized light when it interacts with electron spins in a material. Shown are layers of chromium germanium telluride (CGT). The orange balls represent tellurium atoms, yellow is germanium, and blue is chromium. Credit: Zhenglu Li/Berkeley Lab
Study reveals unprecedented control of ferromagnetic behavior in 2D material. The scientists found that a 2D van der Waals crystal, part of a class of material whose atomically thin layers can be peeled off one by one with adhesive tape, possessed an intrinsic ferromagnetism.The discovery could have major implications for a wide range of applications that rely upon ferromagnetic materials, such as nanoscale memory, spintronic devices, and magnetic sensors. Xiang Zhang, UC Berkeley professor. “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 in the material, 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 Cheng Gong, a postdoctoral researcher in Zhang’s lab.
Van der Waals crystals describe materials in which the 2D layers are not connected to each other via traditional bonds, allowing them to be easily exfoliated with tape. Research on graphene, the most well-known van der Waals material, earned the Nobel Prize in physics in 2010. Gong estimates that for this study, he peeled off > 3,000 flakes of chromium germanium telluride (Cr2Ge2Te6, or CGT). While CGT has existed as a bulk material for decades, the researchers say the 2D flakes could represent an exciting new family of 2D van der Waals crystal. “CGT is also a semiconductor, and the ferromagnetism is intrinsic,” said Jing Xia, UC Irvine associate professor of physics and astronomy. “That makes it cleaner for applications in memory and spintronics.”
The researchers detect the magnetization from atomically thin materials using a technique called magneto-optic Kerr effect. The method involves the super-sensitive detection of the rotation of linearly polarized light when it interacts with electron spins in the material.The key to one of the more surprising findings is that the magnetic anisotropy was very small in the CGT material. That enabled 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 showed that they could 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 striking feature of van der Waals crystals is that they can be easily combined with dissimilar materials without restrictions based on structural or chemical compatibility.This offers a huge amount of flexibility in designing artificial structures for diverse magneto-electric and magneto-optical applications. http://newscenter.lbl.gov/2017/04/26/scientists-discover-atomically-layered-thin-magnet/
Recent Comments