Scientists at the Department of Energy’s Oak Ridge National Laboratory used neutron scattering to determine whether the atomic structure of a particular material could contain a new state of matter called a helical spin fluid. By tracking tiny magnetic moments called “spins” in the honeycomb lattice of a multilayer iron trichloride magnet, the team discovered the first two-dimensional system that hosts a helical spin fluid.
This discovery represents a testing ground for future research into physical phenomena that could drive the next generation of information technology. These include fractons, or collective quantized vibrations, which could be promising in quantum computing, and skyrmions, or new magnetic spin textures, that could improve high-density data storage.
“Materials containing helical spin fluids are of particular interest because of their potential for use in creating quantum spin fluids, spin textures, and fracton excitations,” said ORNL’s Shang Gao, who led the study, published in Physical examination letters.
A long-standing theory predicts that the honeycomb lattice may contain a spiral spin fluid, a new phase of matter in which spins form fluctuating corkscrew-like structures.
However, prior to the present study, there was no experimental evidence for this phase in a 2D system. The two-dimensional system consists of a layered crystalline material in which the interactions are stronger in the plane than in the stacking direction.
Gao called iron trichloride a promising platform to test a theory proposed more than a decade ago. He and co-author Andrew Christianson at ORNL approached Michael McGuire, also at ORNL, who has worked extensively on growing and studying 2D materials, asking if he could synthesize and characterize an iron trichloride sample for neutron diffraction measurements. Because 2D graphene layers exist in bulk graphite as honeycomb lattices of pure carbon, 2D iron layers exist in bulk iron trichloride as 2D honeycomb layers. “Previous reports have suggested that this interesting honeycomb material may exhibit complex magnetic behavior at low temperatures,” McGuire said.
“Each layer of iron honeycomb has chlorine atoms on top and bottom, forming chlorine-iron-chlorine plates,” McGuire said. “The chlorine atoms above one plate interact very weakly with the chlorine atoms at the bottom of the next plate via the van der Waals bond. This weak bond makes such materials easily detachable in very thin layers, often down to a single plate. This is useful for designing devices and understanding the evolution of quantum physics from three dimensions to two dimensions. »
In quantum materials, electron spins can behave collectively and exotically. If one spin moves, everyone reacts—an entangled state that Einstein called “intimidating action at a distance.” The system remains in a state of frustration, a fluid that retains disorder as the spins of the electrons constantly change direction, causing the other entangled electrons to vibrate in response.
The first neutron diffraction studies of iron chloride crystals were carried out at the ORNL 60 years ago. Today, ORNL’s extensive expertise in materials synthesis, imaging, neutron scattering, theory, modeling and computing is leading to groundbreaking research into quantum magnetic materials that is driving the next generation of information security and storage.
Mapping of spin motions in a spiral spin fluid is made possible by experts and tools from the Spallation Neutron Source and High Flux Isotope Reactor, DOE Science Office user facilities at ORNL. Critical to the success of the neutron scattering experiments were ORNL co-authors: Clarina dela Cruz, who directed the experiments using the HFIR POWDER diffractometer; Yaohua Liu, who conducted experiments using the SNS CORELLI spectrometer; Matthias Fronzek, who conducted experiments with WAND HFIR.2 diffractometer; Matthew Stone, who conducted experiments on the SNS SEQUOIA spectrometer; and Douglas Abernathy, who conducted experiments with the SNS ARCS spectrometer.
“Neutron scattering data from our SNS and HFIR measurements provided strong evidence for the existence of a liquid phase of the helical spin,” said Gao.
“Neutron scattering experiments measured how neutrons exchange energy and momentum with the sample, which led to inferences about the magnetic properties,” said co-author Matthew Stone. He described the magnetic structure of the spiraling spin fluid: “It looks like a topographical map of a group of mountains with a group of rings going outward. If you were walking in a ring, all the spins would be in the same direction. But if you go outside and go through different rings, you will see how these rotations begin to rotate around their axes. This is a spiral.
“Our study shows that the concept of a helical spin fluid is viable for a wide class of honeycomb-lattice materials,” said co-author Andrew Christianson. “This gives the community new opportunities to explore spin textures and new excitations such as fractons, which can then be used in future applications such as quantum computing. »