Spin-orbital excitations encoding the magnetic phase transition in the van der Waals antiferromagnet FePS₃

Yuan Wei¹, Yi Tseng^1,2*, Hebatalla Elnaggar³, Wenliang Zhang¹, Teguh Citra Asmara¹, Eugenio Paris¹, Gabriele Domaine², Vladimir Strocov¹, Luc Testa², Virgile Favre², Mario Di Luca⁴, Mitali Banerjee⁴, Andrew Wildes⁵, Frank M. F. de Groot⁶, Henrik M. Rønnow², Thorsten Schmitt¹

¹PSI Center for Photon Science, Paul Scherrer Institut, Villigen, Aargau, Switzerland
²Laboratory for Quantum Magnetism, Institute of Physics, École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland
³Institut de Minéralogie, de Physique des Matériaux et de Cosmochimi, Sorbonne Université, Paris, France
⁴Laboratory of Quantum Physics, Topology and Correlations, Institute of Physics, École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland
⁵Institut Laue-Langevin, Grenoble, France
⁶Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, Netherlands

* Presenter:Yi Tseng, email:ytseng@nycu.edu.tw

Magnetic van der Waals (vdW) materials have provided exciting new opportunities in the studies of functional exotic magnetic phases of various symmetry-breaking groups. The interlayer exchange interaction in these materials is very small, which allows for the decoupling of the magnetic layers and makes it an ideal candidate for studying the effects of dimensionality and interlayer coupling on magnetic behavior. Recent studies utilizing primarily optical spectroscopy have demonstrated the sensitivity of the phonon spectral response to the magnetic state down to the few-layer limit. This has opened up an opportunity for understanding and investigating the magnetic properties of these materials at the nanoscale.

FePS₃ is one such vdW material that has garnered significant interest due to its unique magnetic and electronic properties. It is an S = 2 zig-zag quasi-two-dimensional antiferromagnetic insulator with a honeycomb lattice. The electronic structure of FePS₃ has been resolved by various techniques. X-ray absorption spectroscopy (XAS) has provided insights into how the local crystalline environment shapes the anisotropic electronic structure. Angle-resolved photoemission spectroscopy (ARPES) has been used to study the electronic band structure of FePS₃, revealing the presence of spin-split bands due to the strong spin-orbit coupling.

In this talk, resonant inelastic X-ray scattering (RIXS) has been used to study the spin-orbital excitations of FePS₃ and their relation to magnetism. By performing RIXS measurements as a function of temperature across the magnetic transition, we reveal that the identified multiplets have a large sensitivity to the spin state. Interestingly, the low-energy spin-orbital excitations in the presence of magnetic order are found to be strongly influenced by the trigonal lattice distortion and negative metal-ligand charge transfer, as revealed by simulation with ligand field theory calculations. This finding not only relates to the fundamental physics of FePS₃ but also establishes a generalized approach that is ideal for studying magnetic functional materials and the relation between their low-energy electronic properties and the magnetic state.

Beyond multiplets excitations, the RIXS spectra on vdW materials provide a rich platform for studying a wide range of magnetic phenomena, including spin-orbit coupling, magnetic anisotropy, and spin excitations down to the monolayer level. This will open up exciting new avenues for the design and development of next-generation magnetic devices, such as spintronic and quantum computing technologies.

Keywords: van der Waals materials, low-dimensional magnetic systems, resonant inelastic X-ray scattering