Symmetry-Forbidden Intraband Transitions Leading to Ultralow Gilbert Damping in van der Waals Ferromagnets

Based upon first-principles calculations, we report ultralow Gilbert damping in two-dimensional (2D) van der Waals ferromagnets. The low damping occurs at weak scattering because mirror symmetry prohibits intraband transitions. The monotonic dependence on the electronic scattering rate suggests the absent lower limit, in contrast to conventional ferromagnetic materials. Breaking mirror symmetry through magnetization rotation, layer stacking, or structural phase transition significantly increases damping by enabling intraband transitions. Topological nodal lines, also protected by mirror symmetry, contribute substantially to interband-transition-mediated damping, which can be tuned by adjusting the Fermi level. Our findings elucidate the unique characteristics of Gilbert damping in 2D van der Waals ferromagnets, providing valuable insights for designing low-dimensional spintronic devices with high energy efficiency.