Synchrotron IXS study of phonon excitations in van der Waals honeycomb ferromagnet CrGeTe₃

There is a great deal of research efforts to realize two dimensional (2D) magnetic devices based on robust monolayer ferromagnetism discovered recently in van der Waals (vdW) materials. Among them, those with 2D honeycomb lattices are particularly interesting as their magnon excitations resemble the electronic band structure of graphene exhibiting linearly dispersive Dirac modes and their crossings. In this work, we experimentally investigated the phonon excitations in the vdW honeycomb ferromagnet CrGeTe₃ where robust 2D magnetism as well as strong spin-lattice coupling have previously been reported. Our recent neutron scattering experiments have revealed anomalous and anisotropic broadening of ferromagnons in CrGeTe₃ [1], which was ascribed to the unconventional magnon-phonon coupling in the form of slow exchange fluctuations [2]. Using synchrotron inelastic x-ray scattering, the phonon intensities were measured along the wave vectors parallel and perpendicular to vdW layers, respectively, in the ferromagnetic phase as well as paramagnetic. The phonon band structure extended up to 37 meV and was broadly consistent with the recent LDA calculations. The LDA calculation was also used to identify the observed phonon modes. In this talk, we will also discuss the temperature dependence of phonon scattering and its relevance to the proposed magnon-phonon coupling.



[1] L. Chen et al., Nature Comm. 13 4037 (2022)

[2] W. Webster et al., Phys. Rev. B 98 144411 (2018)