Multiple Spin State Analysis of Magnetic Nano Graphene

Abstract

Recent experiments have suggested a possibility of room-temperature ferromagnetism in graphite-like materials. We analyzed multiple spin states in asymmetric graphene molecules to find the mechanism responsible for ferromagnetism. First principle density functional theory was applied to calculate ground state spin density, energy, and atom position depending on each spin state. Molecules with dihydrogenated zigzag edges like C64H27, C56H24, C64H25, C56H22, and C64H23 indicated that the highest spin state in every molecule is the most stable having an energy difference of kT = 3000 K with the next spin state. In contrast, nitrogen substituted molecules like C59N5H22, C52N4H20, C61N3H22, C54N2H20, and C63N1H22 demonstrated opposite results where the lowest spin state was the most stable. The magnetic stability of graphene molecules can be explained through three key factors depending on the edge specified localized spin state, the exchange interaction between parallel spins inside a molecule, and optimized atom position. We intend to apply these results to design carbon-based magnets, ultra high density information storage, and spintronic devices.