Dirac magnons in a thin elemental itinerant ferromagnet.

A distinct difference between graphene-an atomic layer of carbon-and conventional semiconductors is that its electrons behave as massless Dirac fermions, giving rise to unprecedented physical properties. Magnetically ordered solids host magnons, quasiparticles associated with magnetic degrees of freedom. While Dirac magnons have recently been predicted in specific insulating or rare-earth magnets, their existence in thin 3d magnets remains elusive because of the complex nature of itinerant magnetism and dimensionality effects. Here, we demonstrate the presence of Dirac magnons in a thin itinerant elemental ferromagnet. By investigating atomically designed hexagonal close-packed cobalt films, we establish that magnons in such structures resemble the Dirac electrons in graphene. We explain the physical nature of these Dirac magnons and discuss the consequences of symmetry, dimensionality, magnetic interactions, the number of atomic layers, and cobalt's itinerant magnetism on the properties of the Dirac points. Our results pave the way for finding and engineering Dirac magnons in a variety of low-dimensional layered 3d ferromagnets and metamaterials.