Speaker
Description
Two-dimensional (2D) van der Waals (vdW) magnetic materials have gained much attention because of their fundamental properties and notable potential applications in spintronics and data storage
We will present the results of our research on complex magnetic excitations in vdW vanadium trihalides VX$_3$. Particularly, vanadium triiodide VI$_3$ is a peculiar example because it hosts a unique ferromagnetic (FM) order with tilted magnetic moments and a significant unquenched orbital moment on two V sites with different orbital occupations [1]. Moreover, the bulk VI$_3$ is trimorphous [2]. Our study used the complementarity of the infrared (IR) and Raman spectroscopies and density functional theory calculations (DFT) to probe lattice and magnetic excitations. We found clear signatures of the structural transition in the IR spectra at $79$ K, and enhanced variations of phonon frequencies observed near the FM phase onset temperature $\sim 50$ K indicate the strong magneto-elastic coupling. Below Curie temperature, two Raman active modes appear, showing significant softening in the narrow interval around the second structural transition $\sim 30$ K associated with the magnetic structure modification. Below this transition, a highly energetic FM resonance in a terahertz (THz) range shows up. The observed THz FM resonance in VI$_3$ is a promising indicator of the application potential of $2$D vdW FM in ultrafast THz spintronics, even though it was previously considered an exclusive domain of antiferromagnets [3]. Similarly, we have detected the orbital moment also on V-sites in antiferromagnetic counterparts VBr$_3$ [4] and VCl$_3$ accompanied by low-temperature zero-field split antiferromagnetic magnons suggesting a biaxial anisotropy of these materials.
Acknowledgments
This work is a part of the research project GACR 25-15448S, financed by the Czech Science Foundation. Some experiments were performed in MGML (mgml.eu) (project no. LM2023065).
References
[1] D. Hovančík et al., “Large Orbital Magnetic Moment in VI3,” Nano Letters, vol. 23, no. 4. American Chemical Society (ACS), pp. 1175–1180, Feb. 01, 2023. https://doi.org/10.1021/acs.nanolett.2c04045
[2] P. Doležal et al.,"Crystal structures and phase transitions of the van der Waals ferromagnet VI$_3$," Physical Review Materials, vol. 3, no. 12. American Physical Society (APS), Dec. 19, 2019. https://doi.org/10.1103/physrevmaterials.3.121401
[3] D. Hovančík et al., “Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3,” The Journal of Physical Chemistry Letters, vol. 13, no. 48. American Chemical Society (ACS), pp. 11095–11104, Nov. 23, 2022. https://doi.org/10.1021/acs.jpclett.2c02944
[4] D. Hovančík et al.,"Robust intralayer antiferromagnetism and tricriticality in the van der Waals compound VBr$_3$," Physical Review B, vol. 108, no. 10. American Physical Society (APS), Sep. 21, 2023. https://doi.org/10.1103/physrevb.108.104416
