Materials can behave in surprising ways when they are thinned down layer by layer until they are only a single atom thick. In a new study published in Nature Materials, physicists led by researchers at The University of Texas at Austin observed a sequence of unusual magnetic states in an ultrathin material. Their experiments confirm a long standing theoretical model of two dimensional magnetism first proposed in the 1970s. The team says the discovery could eventually help inspire extremely compact technologies that rely on controlling magnetism at very small scales.
The newly observed sequence involves two important changes in magnetic behavior that occur as certain materials are cooled toward absolute zero. While scientists have previously detected each transition separately, this study is the first to observe the entire sequence unfolding in a single system.
Magnetic Vortices in an Ultrathin Crystal
To investigate these effects, the researchers cooled an atomically thin sheet of nickel phosphorus trisulfide (NiPS3) to temperatures between -150 and -130 °C. At this range, the material entered a special magnetic state known as a Berezinskii-Kosterlitz-Thouless (BKT) phase.
In this phase, the magnetic directions of individual atoms, called magnetic moments, organize into swirling structures known as vortices. These vortices form in pairs that rotate in opposite directions, with one spinning clockwise and the other counterclockwise. Each pair remains tightly linked together.
The BKT phase takes its name from physicist Vadim Berezinskii and Nobel Prize winners J. Michael Kosterlitz and David Thouless, who received the 2016 Nobel Prize in Physics for their theoretical work describing this type of phase transition.
“The BKT phase is particularly intriguing because these vortices are predicted to be exceptionally robust and confined to just a few nanometers laterally while occupying only a single atomic layer in thickness,” said Edoardo Baldini, assistant professor of physics at UT and leader of the research. “Because of their stability and extremely small size, these vortices offer a new route to controlling magnetism at the nanoscale and provide insight into universal topological physics in two-dimensional systems.”
From Magnetic Vortices to an Ordered Phase
When the temperature dropped even further, the material shifted into a second magnetic state known as a six-state clock ordered phase. In this configuration, the magnetic moments align in one of six possible directions that are related by symmetry.
Observing both the BKT phase and the lower temperature ordered phase confirms the experimental realization of the two dimensional six-state clock model. This theoretical framework, introduced in the 1970s, predicts the precise sequence of magnetic phases seen in the experiment.
“At this stage, our work demonstrates the full sequence of phases expected for the two-dimensional six-state clock model and establishes the conditions under which nanoscale magnetic vortices naturally emerge in a purely two-dimensional magnet,” Baldini said.
Toward Future Nanoscale Magnetic Technologies
Researchers now plan to explore how to stabilize similar magnetic phases at progressively higher temperatures. Ideally, they hope to discover materials that could sustain these effects closer to room temperature. This first demonstration provides a key starting point for that effort.
The results also suggest that many other two dimensional magnetic materials could host previously unknown magnetic phases. That possibility could lead to new discoveries in fundamental physics as well as future concepts for nanoscale electronic devices.
Research Team and Funding
The project received primary support from the National Science Foundation (NSF) through UT’s Center for Dynamics and Control of Materials, an NSF Materials Research Science and Engineering Center. Baldini’s group also received funding from Love, Tito’s; the Robert A. Welch Foundation; the W. M. Keck Foundation; the NSF through a CAREER award; the U.S. Air Force Office of Scientific Research through a Young Investigator Program award; and the U.S. Army Research Office.
The three senior authors of the study, Baldini, Allan MacDonald and Xiaoqin “Elaine” Li, are physicists at UT and members of the Texas Quantum Institute, which Li co-directs. The study’s co-first authors are Frank Y. Gao, a postdoctoral fellow in physics at UT and incoming assistant professor of chemistry at the University of Wisconsin-Madison, and Dong Seob Kim, a former graduate student in physics at UT who is now a postdoctoral researcher at Columbia University. Additional contributors came from the Massachusetts Institute of Technology, Academia Sinica and The University of Utah.
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