Spin wave excitations in van der Waals honeycomb ferromagnets

Recent discoveries of robust two-dimensional magnetism brought about a great research interest in van der Waals ferromagnets. Experimental observations of their spin wave excitations are important because the underlying spin Hamiltonian can provide crucial information regarding the magnetic interactions governing the statics and dynamics of their long-range order. Given the ferromagnetic orderings on honeycomb lattices, their magnon bands are predicted to host the Dirac magnons which are the bosonic counterpart of Dirac fermions observed in the electronic band of graphene [1]. In this talk, we will review and report the past and recent inelastic neutron scattering measurements of spin wave excitations in van der Waals ferromagnets CrX₃ (X = Br, Cr, I) and Cr₂Z₂Te₆ (Z = Si, Ge). Their spin wave bands consist of two ferromagnon modes, which exhibit linear E-p dispersion relations at Q_K = (1/3,1/3). In CrI₃ at T = 5 K, we find that these two bands are separated by a ~ 2 meV gaps suggesting that its Dirac magnons are massive [2]. These results can be explained by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in CrI₃ can have robust topological properties. As the temperature was increased to and beyond 61 K, the anisotropy gap at the zone center vanished following the power law behavior whereas the stiffness of the spin waves remained finite [3]. These results strongly indicate that the magnetic anisotropy plays a decisive role in determining the Curie temperature in CrI₃, which is in contrast with typical three-dimensional Heisenberg ferromagnets where exchange interactions controlling the Curie temperature.

[1] J. Fransson et al., Phys. Rev. B 94, 075401 (2016)
[2] L. Chen et al., Phys. Rev. X 8, 041028 (2018)
[3] L. Chen, et al., Phys. Rev. B 101, 134418 (2020)