Manifold of magnetic nodal lines in an elemental ferromagnet

Magnetic nodal line semimetals, defined by twofold degenerate gapless crossings along extended momentum paths, offer significant potential for spintronics and future information technologies. Despite their potential, these systems are rare in nature, with only few examples known to date. Here, using spin- and angle-resolved photoemission spectroscopy and density functional theory-based calculations, we classify ferromagnetic hexagonal close-packed (hcp) cobalt as a prototypical system exhibiting these properties. We demonstrate that manifolds of magnetic gapless nodal rings enclosing the Γ and K points in the k z = 0 plane, and magnetic nodal lines spanning the entire A-L path in the kz=π /c k z = π c plane, coexist and dominate the fermiology of this elemental system. These mirror-protected bulk crossing points are associated with surface states, both of which exhibit controllable spin textures. This work is the first identification of cobalt as a simple, yet highly tunable and rich topological platform for exploring and manipulating spin-polarised Weyl-like nodal lines at room temperature.