Physicists Discover Long-Predicted ‘Clock Magnetism’ in an Atomically Thin Crystal
Observation of BKT and six-state clock phases reveals how magnetism behaves in two dimensions and may inspire ultracompact technologies.
When researchers at UT Austin coaxed an atomically thin sheet of nickel phosphorus trisulfide to enter a special magnetic phase, called the BKT phase, the magnetic orientations of individual atoms formed swirling patterns called vortices. Illustration credit: Ella Maru Studios.
Strange things happen to materials when you peel them down, layer by layer, from thick chunks all the way to sheets just an atom thick. Reporting in the journal Nature Materials, a team led by physicists at The University of Texas at Austin has experimentally demonstrated a sequence of exotic magnetic phases in an ultrathin material that for the first time fully realize a theoretical model of two-dimensional magnetism first proposed in the 1970s. The researchers say the advance might inspire new, ultracompact technologies.
The sequence of exotic magnetic phases involves two key transitions that occur as certain materials cool down towards absolute zero. Both transitions have been observed experimentally on their own before, but never together in a complete sequence.
When the researchers cooled an atomically thin sheet of nickel phosphorus trisulfide (NiPS3) to temperatures between –150 and –130 °C, the material entered the first special magnetic phase, called a Berezinskii–Kosterlitz–Thouless (BKT) phase. In this regime, the magnetic orientations associated with individual atoms in the material—known as magnetic moments—form swirling patterns called vortices. Pairs of these vortices wind in opposite directions, one clockwise and the other counterclockwise, and remain tightly bound together.
The BKT phase is named after Vadim Berezinskii and Nobel Prize winners J. Michael Kosterlitz and David Thouless, who were awarded the 2016 Nobel Prize in Physics for their theoretical description of 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.”
Researchers used instruments in Edoardo Baldini’s laboratory to observe the BKT phase and the six-state clock phase in an atom-thin layer of nickel phosphorus trisulfide (NiPS3). Photo credit: Eileen Chong.
Upon further cooling, the material transitioned into a second distinct magnetic phase, called a six-state clock ordered phase, in which magnetic moments adopt one of six symmetry-related orientations. The observation of both the BKT regime and the low-temperature ordered state establishes the experimental realization of the two-dimensional six-state clock model—a paradigmatic theoretical framework proposed in the 1970s.
“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.
Future work on the BKT phase will be aimed at finding the right combination of material properties to stabilize similar magnetic phases at higher and higher temperatures, perhaps even all the way up to room temperature. This initial observation sets an important foundation for these efforts.
Beyond NiPS₃, the findings indicate that a broad class of two-dimensional magnetic materials may host previously unexplored phases, opening new directions for both fundamental physics and nanoscale device concepts.
This research was primarily supported by the National Science Foundation (NSF) through UT’s Center for Dynamics and Control of Materials, an NSF Materials Research Science and Engineering Center. Work in the Baldini group was additionally supported by 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 paper—Baldini, Allan MacDonald and Xiaoqin “Elaine” Li—are all 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, now a postdoctoral researcher at Columbia University. Researchers at Massachusetts Institute of Technology, Academia Sinica and The University of Utah also contributed to the work.
