Speaker
Description
Magnetic van der Waals materials with very weak exchange interaction between magnetically ordered layers represent an interesting intermediate stage between the more explored cases of isotropic bulk-like exchange and the recently intensively studied ideal $2$D limit (monolayer) [1]. We perform a complex investigation of lattice and magnetic excitations induced by external perturbations employing the DFT calculations, and compare it to infrared, terahertz, and Raman spectroscopy results in such material, trihalide VI$_3$. We find that the transition to the long-range ferromagnetic order is accompanied by the observed variations of phonon frequencies induced by the strong magnetoelastic coupling. The acoustic magnon mode acquires here unusually high energy reaching to THz range, but dramatically softens at temperatures where a second lattice distortion has been reported in the literature [2]. First-principles calculations show the strong connection of magnetic ordering to the lattice. Furthermore, a ground state with an exceptionally high orbital momentum is predicted, and the electronic configuration is compared to recent measurements based on the x-ray magnetic circular dichroism [3]. These findings suggest the possibility of controlling magnetic anisotropy in this system by selective occupation of specific lattice modes.
A highly interesting connection between the formation of helimagnetic order and breaking of both inversion and rotational symmetry of the lattice has been recently discovered in single-layer Ni dihalides [4]. Here we examine how magnetic order in NiBr$_2$ is affected by pressure. We show that the interlayer exchange interaction plays here a key role in the selection of the most favorable magnetic order.
References
[1] M. Gibertini et al., “Magnetic 2D materials and heterostructures,” Nature Nanotechnology, vol. 14, no. 5. Springer Science and Business Media LLC, pp. 408–419, May 2019. https://doi.org/10.1038/s41565-019-0438-6
[2] 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
[3] 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
[4] Q. Song et al., “Evidence for a single-layer van der Waals multiferroic,” Nature, vol. 602, no. 7898. Springer Science and Business Media LLC, pp. 601–605, Feb. 23, 2022. https://doi.org/10.1038/s41586-021-04337-x.
