Spin Waves in bilayers of Vanadium dichalcogenides
Prof. Wojciech Rudziński

Date, Time
10.11, 15:00 - 16:00

Location
Link to MSTeams meeting

Two-dimensional Van der Waals materials are currently of great interest. This is due to unique magnetic and electronic properties of these materials. We have analyzed theoretically and numerically the spectrum of spin waves (magnons) in two-dimensional bilayer systems of VX2 (X=S, Se, Te) transition-metal dichalcogenides (TMDs)[1]. The vanadium atoms within individual atomic layers are coupled ferromagnetically, while the exchange coupling between V atoms located in different planes is either ferromagnetic or antiferromagnetic, depending on the stacking type of individual layers in the VX2 bilayer system. The magnon spectra are considered in the case of T-type and H-type bilayers. We have analyzed in detail these spectra as a function of magnetic anisotropy and external magnetic field applied perpendicularly to the layers.
The spin-wave dispersion relations have been derived analytically within the spin-wave theory of antiferromagnets, in terms of the Holstein-Primakoff transformation combined with the Bogolubov diagonalization scheme. For numerical discussion, the intra- and interlayer exchange parameters, as well as the magneto-crystalline anisotropy, have been evaluated within the method based on the density functional theory (DFT). The corresponding magnon spectra have been also simulated numerically. From the DFT calculations we have also determined the Curie temperatures of the VS2, VSe2 and VTe2 bilayer systems, which are close to or well above the room temperature, in agreement with the corresponding literature.
In the case of antiferromagnetic TMD bilayers, the system undergoes a field-induced transition to the spin-flop phase, which evolves into the saturated ferromagnetic phase for sufficiently strong magnetic fields. Our considerations also show that an extreme case of direct field-induced metamagnetic transition between antiferromagnetic and saturated ferromagnetic phases may occur as well. Existence of different phases depends on the material parameters, especially on the interlayer exchange and anisotropy constants. We have analyzed the spin wave spectra in all these phases and showed how these spectra change at the phase transitions and how they evolve with increasing magnetic field. We have taken into account both, in-plane and out-of-plane magnetic anisotropy.

[1] H.-R. Fuh, Ch.-R. Chang, Y.-K. Wang, R. F. L. Evans, R. W. Chantrell and H.-T. Jeng, Scientific Reports 6, 32625 (2016).