When materials are thinned layer by layer until they are just one atom thick, they can behave in surprising ways. In a new study published in natural materialsPhysicists led by researchers at the University of Texas at Austin have observed a series of unusual magnetic states within ultrathin materials. Their experiments support a long-standing theoretical model of two-dimensional magnetism, first proposed in the 1970s. The research team says this discovery could ultimately help inspire highly compact technologies that rely on controlling magnetism on very small scales.
The newly observed sequence involves two important changes in magnetic behavior that occur when certain materials are cooled toward absolute zero. Scientists had previously detected each transition individually, but this study is the first to observe the entire sequence unfolding in a single system.
Magnetic vortices in ultrathin crystals
To investigate these effects, the researchers cooled atomically thin sheets of nickel phosphorus trisulfide (NiPS3) to temperatures between -150 and -130 degrees Celsius. In this range, the material enters a special magnetic state known as the Berezinski-Kosterlitz-Thaules (BKT) phase.
At this stage, the magnetic directions of individual atoms, called magnetic moments, form spiral structures known as vortices. These vortices form pairs that rotate in opposite directions, one rotating clockwise and the other counterclockwise. Each pair will remain tightly bonded.
The BKT phase takes its name from physicist Vadim Berezinsky and Nobel laureates J. Michael Kosteritz and David Saules, who won the 2016 Nobel Prize in Physics for their theoretical work explaining this type of phase transition.
“The BKT phase is particularly interesting because these vortices are predicted to be extremely powerful and confined to just a few nanometers laterally, while occupying only one atomic layer in thickness,” said study leader Edoardo Baldini, assistant professor of physics at the University of Texas. “Due to their stability and extremely small size, these vortices provide a new route for controlling magnetism at the nanoscale and provide insight into universal topological physics in two-dimensional systems.”
From magnetic vortex to ordered phase
As the temperature decreased further, the material transitioned to 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 related by symmetry.
Observing both the BKT phase and the cold ordered phase confirms the experimental realization of the two-dimensional six-state clock model. This theoretical framework, introduced in the 1970s, predicts the exact order of magnetic phases observed in experiments.
“At this stage, our work demonstrates the complete sequence of phases expected in a two-dimensional six-state clock model and establishes the conditions for the spontaneous emergence of nanoscale magnetic vortices in pure two-dimensional magnets,” Baldini said.
Towards future nanoscale magnetic technology
The researchers now plan to explore ways to stabilize similar magnetic phases at progressively higher temperatures. Ideally, we would like to discover materials that can maintain these effects at temperatures close to room temperature. This first demonstration is an important starting point for that effort.
The results also suggest that many other two-dimensional magnetic materials may host previously unknown magnetic phases. That potential could lead to new discoveries in fundamental physics and future concepts for nanoscale electronic devices.
Research team and funding
This project received primary support from the National Science Foundation (NSF) through UT’s Center for Materials Mechanics and Control (NSF Center for Materials Research, Science and Engineering). Baldini’s group also received funding from Tito’s Love. Robert A. Welch Foundation. W. M. Keck Foundation. Contributing to NSF through CAREER awards. Recipient of the U.S. Air Force Office of Scientific Research Award through the Young Investigator Program. and the U.S. Army Research Office.
The study’s senior authors, Baldini, Alan McDonald, and Xiaoqing “Elaine” Li, are UT physicists and members of the Texas Quantum Institute, where Li is co-director. Co-first authors of the study are Frank Y. Gao, a UT postdoctoral fellow in physics and incoming assistant professor of chemistry at the University of Wisconsin-Madison, and Dong Seob Kim, a former UT physics graduate student and now a postdoctoral fellow at Columbia University. Additional contributors came from the Massachusetts Institute of Technology, Academia Sinica, and the University of Utah.
