- Jiří Pospíšil: Peculiar properties of van der Waals ferromagnet VI3
- 30. 11. 2022, 14:10
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Transition-metal trihalides CrX3 and VX3 are fascinating van der Waals (vdW) materials that exhibit magnetic ordering even in the single-layer limit. Our present research is focused on vanadium compounds VCl3, VBr3, and VI3, which we study by measuring specific heat, magnetization, XMCD, Raman, THz, IR and μSR spectroscopies, X-ray and neutron diffraction supported by theoretical ab-initio calculations. Our results show the complex magnetism with features not detected in CrX3 analogs due to the differences in electronic and crystal structure.
The seminar will primarily focus on our results obtained studying the vdW ferromagnet VI3. This compound is exceptional by structural trimorphism, two canted ferromagnetic phases, temperature, magnetic field, and pressure dependences of structural and magnetic phase transitions [1, 2]. The unique magnetic anisotropy is characteristic of the easy magnetization axis tilted by ≈ 50° from the ab-plane.
The most prominent property of VI3 is the significant unquenched orbital moment of the V3+ ion theoretically predicted and experimentally proven by XMCD measurements. The ferromagnetic ground state of VI3 is characterized by two inequivalent V-sites. Our ligand field multiplet simulations of XMCD spectra in synergy with the results of DFT calculations agree with the existence of two V sites with different orbital occupations and, therefore, different orbital moment magnitudes in the ground state.
The strong magnetoelastic coupling is manifested by the results of complementary spectroscopic measurements (Raman, THz, and IR) supported by theoretically calculated phonon modes. Below Curie temperature, two Raman modes simultaneously appear and show dramatic softening in the narrow temperature interval around the second structural transition associated with the order−order magnetic phase transition. In the ground state phase, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has been considered the exclusive domain of antiferromagnets .
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 J. Valenta, et al., Physical Review Materials 103, 054424 (2021).
 A. Koriki, et al., Physical Review B 103, 174401 (2021).
 L. M. Sandratskii, et al., Physical Review B 103, 214451 (2021).
 D. Hovančík, et al., arXiv:2210.11278ÂÂ (2022).
 D. Hovančík, et al., The Journal of Physical Chemistry Letters 13, 11095 (2022).