Multifield-tunable magneto-optical effects in electron- and hole-doped nitrogen-graphene crystals

Two-dimensional (2D) ferromagnetic materials have received enormous interest due to the novel physical and chemical properties as well as appealing applications in nano-spintronics. In this paper, using first-principles density functional theory, we demonstrate that the ferromagnetic ground state can be established in either electron-doped or hole-doped nitrogen-graphene crystals, including C₂N, C₃N, C₁₂N, s-g-C₃N₄, and t-g-C₃N₄. The magneto-optical (MO) Kerr and Faraday effects in nitrogen-graphene crystals can be significantly mediated by the carrier (electron or hole) concentration and therefore are electrically tunable by the gate voltage. Moreover, we investigate the influence of the magnetic field and the strain field on MO effects. The Kerr and Faraday angles exhibit a notable reduction when the magnetic field rotates from the out-of-plane direction to the in-plane direction, which presents the anisotropy of MO effects. The in-plane compressive strain can generate a relatively large Kerr angle, indicating that a substrate with a smaller lattice constant than the sample is more beneficial for larger MO effects. Our results suggest that the MO effects in nitrogen-graphene crystals can be effectively tuned by electric, magnetic, and strain fields and nitrogen-graphene crystals provide a novel 2D material platform for nano-spintronics and MO devices.