Chestnut Hill, Mass. (10/31/2018) Two-dimensional magnetism has long intrigued and motivated researchers for its potential to unleash new states of matter and utility in nano-devices.
In part the excitement is driven by predictions that the magnetic moments of electrons – known as "spins" – would no longer be able to align in perfectly clean systems. This enhancement in the strengths of the excitations could unleash numerous new states of mater, and enable novel forms of quantum computing.
A key challenge has been the successful fabrication of perfectly clean systems and their incorporation with other materials. However, for more than a decade, materials known as "van der Waals" crystals, held together by friction, have been used to isolate single-atom-thick layers leading to numerous new physical effects and applications.
Recently this class has been expanded to include magnetic materials, and it may offer one of the most ambitious platforms yet in scientific efforts to investigate and manipulate phases of matter at the nanoscale, researchers from Boston College, the University of Tennessee, and Seoul National University, write in the latest edition of the journal Nature.
Two-dimensional magnetism, the subject of theoretical explorations and experimentation for the past 80 years, is enjoying a resurgence thanks to a group of materials and compounds that are relatively plentiful and easy to manipulate, according to Boston College Associate Professor of Physics Kenneth Burch, a first author of the article "'Magnetism in two-dimensional van der Waals materials."
The most oft-cited example of these materials is graphene, a crystal constructed in uniform, atom-thick layers. A procedure as simple as applying a piece of scotch tape to the crystal can remove a single layer, providing a thin, uniform section to serve as a platform to create novel materials with a range of physical properties open to manipulation.
"What's amazing about these 2-D materials is they're so flexible," said Burch. "Because they are so flexible, they give you this huge array of possibilities. You can make combinations you could not dream of before. You can just try them. You don't have to spend this huge amount of time and money and machinery trying to grow them. A student working with tape puts them together. That adds up to this exciting opportunity people dreamed of for a long time, to be able to engineer these new phases of matter."
At that single layer, researchers have focused on spin, what Burch refers to as the "magnetic moment" of an electron. While the charge of an electron can be used to send two signals – either "off" or "on", results represented as either zero or one – spin excitations offer multiple points of control and measurement, an exponential expansion of the potential to signal, store or transmit information in the tiniest of spaces.
"One of the big efforts now is to try to switch the way we do computations," said Burch. "Now we record whether the charge of the electron is there or it isn't. Since every electron has a magnetic moment, you can potentially store information using the relative directions of those moments, which is more like a compass with multiple points. You don't just get a one and a zero, you get all the values in between."
Potential applications lie in the areas of new "quantum" computers, sensing technologies, semiconductors, or high-temperature superconductors.
"The point of our perspective is that there has been a huge emphasis on devices and trying to pursue these 2-D materials to make these new devices, which is extremely promising," said Burch. "But what we point out is magnetic 2D atomic crystals can also realize the dream of engineering these new phases – superconducting, or magnetic or topological phases of matter, that is really the most exciting part. It is not just fundamentally interesting to realize these theorems that have been around for 40 years. These new phases would have applications in various forms of computing, whether in spintronics, producing high temperature superconductors, magnetic and optical sensors and in topological quantum computing."
Burch and his colleagues – the University of Tennessee's David Mandrus and Seoul National University's Je-Geun Park – outline four major directions for research into magnetic van der Waals materials:
- Discovering new materials with specific functionality. New materials with isotropic or complex magnetic interactions, could play significant roles in the development of new supercondcutors.
- These new materials can also lead to a deeper understanding of fundamental issues in condensed matter physics, serving as unique platforms for experimentation.
- The materials will be tested for the potential to become unique devices, capable of delivering novel applications. The two-dimensional structure of these materials makes them more receptive to external signals.
- These materials possess quantum and topological phases that could potentially lead to exotic states, such as quantum spin liquids, "skyrmions," or new iterations of superconductivity.
Germano Iannacchione, a National Science Foundation (NSF) program officer who oversees grants to Burch and other materials scientists, said the co-authors offer the broader community of scientists ideas that can serve to guide a dynamic field pushing beyond boundaries in materials research.
"Magnetism in 2D van Der Waals materials has grown into a vibrant field of study," said Iannacchione. "Its investigators have matured from highly focused researchers to statesmen shepherding a field, broadening applications into as many channels as possible. The review captures the multiplicative aspect of steady, focused, and sometimes risky research that opens vast new frontiers, with tremendous potential for applications in quantum computing and spintronics."