Researchers Reveal 0.6% Strain Induces Symmetry Breaking and Alters Magnetic Order in FePSe

The behaviour of electrons in two-dimensional materials can give rise to unusual states of matter, and researchers are increasingly interested in how these states affect material properties. Weiliang Yao from Rice University, Viviane Peçanha-Antonio from the ISIS Facility, and Devashibhai Adroja from the University of Johannesburg, along with colleagues, investigate this phenomenon in the van der Waals antiferromagnet FePSe. Their work reveals evidence of a unique three-state Potts nematicity, a form of order characterised by threefold rotational symmetry, which appears in the material’s spin excitations when subjected to strain. This discovery is significant because it demonstrates how subtle changes in material structure can induce complex magnetic behaviour, and suggests that this nematic order persists even above the temperature at which traditional magnetic order exists, offering new avenues for controlling and manipulating magnetic materials.

In two-dimensional materials, electron interactions can induce an electronic nematic phase, altering material properties through a specific symmetry. For materials with threefold rotational symmetry, like honeycomb lattices, a unique three-state Potts nematic order is predicted, though direct observation has proven challenging. Understanding how these nematic phases emerge and influence material behaviour remains a central goal in condensed matter physics, with potential implications for developing new electronic devices. Current research focuses on identifying and characterizing these subtle phases, and determining how they relate to macroscopic properties, ultimately aiming to control and exploit them for technological applications.

Inelastic Neutron Scattering of FePSe3 Spin Excitations

This document provides supporting information for research investigating the spin wave excitations in FePSe3, a material exhibiting zigzag antiferromagnetic order. The study utilizes inelastic neutron scattering to probe these excitations, detailing the experimental setup, data processing, and analysis techniques. Researchers employed inelastic neutron scattering to measure the energy and momentum transfer of neutrons scattered by the sample, providing information about the magnetic excitations. The FePSe3 sample was subjected to uniaxial strain, which affects the population of different magnetic domains.

Scattering intensities were converted to absolute units using a phonon scattering reference for accurate quantification. Diffraction measurements determined the relative populations of different zigzag antiferromagnetic domains. Researchers analyzed the energy and momentum dependence of the spin wave excitations to determine the spin wave dispersion relation. The observed spin wave spectra were modeled as a combination of contributions from the different magnetic domains, weighted by their respective populations. Theoretical calculations of the spin wave spectra were performed for each domain to compare with the experimental results. This document provides the behind-the-scenes details of the research, allowing other scientists to understand how the results were obtained and to verify or build upon the findings.

Strain-Induced Potts Nematicity in FePSe3

Researchers have uncovered a novel nematic state in the van der Waals antiferromagnet FePSe₃, demonstrating a three-state Potts nematicity directly linked to its magnetic order. The team utilized neutron scattering to investigate the magnetic order and spin excitations of FePSe₃ under uniaxial strain, revealing how subtle changes in material stress dramatically influence its magnetic properties. Experiments show that applying approximately 0. 6% tensile strain significantly suppresses one zigzag magnetic domain while promoting the other two, effectively lowering the symmetry of both the antiferromagnetic order and the associated spin waves.

This manipulation of magnetic domains results in a broken threefold symmetry in spin excitations, persisting even slightly above the Néel temperature of 108. 6 K, where the zigzag antiferromagnetic order disappears. The data confirms a strong magnetoelastic coupling within FePSe₃, establishing a direct link between mechanical stress and magnetic alignment. Crucially, the findings suggest that the observed three-state Potts nematicity in the paramagnetic spin excitations originates from a vestigial order associated with the low-temperature zigzag antiferromagnetic order. This research provides direct evidence for a novel form of nematicity, where the material exhibits a preference for one of three equivalent magnetic configurations, unlike more commonly observed two-state nematic phases.

FePSe₃, as an antiferromagnetic insulator, offers a unique platform to study this spin-driven nematicity, independent of electronic contributions. By tuning the material with strain, researchers were able to extract the susceptibility associated with this three-state Potts state, revealing divergent behaviour near the Néel temperature and confirming the formation of the nematic phase. These findings open new avenues for exploring and controlling magnetic order in materials, potentially leading to advancements in spintronic devices and novel magnetic technologies.

Strain Controls Magnetism and Potts Nematicity

This research investigates the magnetic properties of a two-dimensional material, FePSe₃, under applied strain, revealing connections between magnetism and material structure. The team demonstrates that a small amount of tensile strain significantly alters the arrangement of magnetic domains within the material, favouring certain orientations over others and lowering the symmetry of the magnetic order and associated spin waves. Importantly, this altered symmetry persists even slightly above the temperature at which the magnetic order disappears, suggesting a more subtle underlying order. These findings provide direct evidence of magnetoelastic coupling, where mechanical strain influences magnetic behaviour, and support the idea that a “three-state Potts nematicity” exists within the material’s spin excitations.

This nematicity represents a type of order that breaks rotational symmetry, even without conventional magnetic ordering. While the material exhibits no lattice distortion during the magnetic transition, the observed effects demonstrate a clear interplay between strain and magnetic properties. Further research may also investigate how this coupling affects other properties of the material and whether similar behaviour exists in other two-dimensional magnetic systems.