  Two-dimensional (2D) materials have attracted great attention and spurred
rapid development in both fundamental research and device applications. The
search for exotic physical properties, such as magnetic and topological order,
in 2D materials could enable the realization of novel quantum devices and is
therefore at the forefront of materials science. Here, we report the discovery
of two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a
single-layer gadolinium-silver compound, based on combined angle-resolved
photoemission spectroscopy measurements and theoretical calculations. These
Weyl nodal lines are symmetry protected and thus robust against external
perturbations. The coexistence of magnetic and topological order in a 2D
material is likely to inform ongoing efforts to devise and realize novel
nanospintronic devices.
