Room-temperature ferromagnetism from an array of asymmetric zigzag-edge nanoribbons in a graphene junction

Abstract

Room-temperature ferromagnetism in graphene layers with defects has been experimentally measured. Despite disagreement around the intrinsic origin of carbon magnetism, experimental evidence has supported the existence of paramagnetism or ferromagnetism in carbon materials. Convincing theoretical explanations, however, have not yet been proposed. In this work, density functional theory calculations were used to suggest a plausible explanation for this phenomenon as it is observed at the zigzag grain boundaries of a mismatched single-double-single-layer graphene junction. We identified asymmetric zigzag-edge graphene nanoribbons that display ferromagnetic properties in a graphene junction structure. Two ferromagnetic asymmetric zigzag graphene nanoribbons displayed antiferromagnetic coupling in a defect-free structure at the grain boundary. The introduction of a vacancy or N-substitutional defect was found to destroy the magnetism on one side only; the nanoribbon on the other side continued to display a large ferromagnetic exchange coupling. The ferromagnetic nanoribbon in the junction was ferromagnetically correlated with other nanoribbons in the two-dimensional junction array, yielding a Curie temperature well-above room temperature. Moreover, the ferromagnetic correlation was observed regardless of the arrangement of the magnetic layers, enabling ferromagnetic ordering within the graphene junction array.