Possible room-temperature ferromagnetic semiconductor in monolayer ${\mathrm{MnSe}}_2$ through a metal-semiconductor transition

To realize room-temperature ferromagnetic semiconductors is still a challenge in spintronics. Recent experiments have obtained two-dimensional (2D) room-temperature ferromagnetic metals, such as monolayer ${\mathrm{MnSe}}_2$. In this paper, we proposed a way to obtain room-temperature ferromagnetic semiconductors through metal-semiconductor transition. By the density-functional theory calculations, a room-temperature ferromagnetic semiconductor is obtained in monolayer ${\mathrm{MnSe}}_2$ with a few-percent tensile strain, where a metal-semiconductor transition occurs with 2.2% tensile strain. The tensile strains raise the energy of $d$ orbitals of Mn atoms and $p$ orbitals of Se atoms near the Fermi level, making the Fermi-level sets in the energy gap of bonding and antibonding states of these $p$ and $d$ orbitals, and opening a small band gap. The room-temperature ferromagnetic semiconductors are also obtained in the heterostructures ${\mathrm{MnSe}}_2$/X (X = ${\mathrm{Al}}_2{\mathrm{Se}}_3$, GaSe, SiH, and GaP), where metal-semiconductor transition happens due to the tensile strains by interface of heterostructures. In addition, a large magneto-optical Kerr effect (MOKE) is obtained in monolayer ${\mathrm{MnSe}}_2$ with tensile strain and ${\mathrm{MnSe}}_2$-based heterostructures. Our theoretical results pave a way to obtain room-temperature magnetic semiconductors from experimentally obtained 2D room-temperature ferromagnetic metals through metal-semiconductor transitions.

Figure 1

(a) Top and side views of crystal structure of monolayer ${\mathrm{MnSe}}_2$. (b) Spin-polarized band structure of ${\mathrm{MnSe}}_2$, obtained by the DFT calculation with HSE hybrid functional. (c) Magnetization and susceptibility of ${\mathrm{MnSe}}_2$ as a function of temperature, obtained by the Monte Carlo simulation based on a 2D Heisenberg model. The Curie temperature of monolayer ${\mathrm{MnSe}}_2$ is calculated as $T_C$ = 354 K, being consistent with the experiment [19]. Magnetic configurations of monolayer ${\mathrm{MnSe}}_2$, A ferromagnetic (FM) (d) and an antiferromagnetic (AFM) (e) spin configuration are considered to get the coupling constants.

Figure 2

For monolayer ${\mathrm{MnSe}}_2$, tensile-strain dependence of (a) band gap, (b) Curie temperature $T_C$, and (c) magnetic anisotropy energy (MAE), obtained by the DFT and Monte Carlo calculations. Numerical results of four ${\mathrm{MnSe}}_2$-based heterostructures are also included, where strain comes from the interface of heterostructures. Experimental $T_C$ and MAE of heterostructure ${\mathrm{MnSe}}_2$/GaSe [19] are included for comparison.

Figure 3

Metal-semiconductor transition of monolayer ${\mathrm{MnSe}}_2$ due to strain. DFT results of partial density of states (PDOS) of (a) $d$ orbitals of Mn and (b) $p$ orbitals of Se without strain. PDOS of (c) $d$ orbitals of Mn and (d) $p$ orbitals of Se with 4% tensile strain.

Figure 4

Structure and spin-polarized band structure of ${\mathrm{MnSe}}_2$-based heterostructures, (a), (e) ${\mathrm{MnSe}}_2$/GaP, (b), (f) ${\mathrm{MnSe}}_2$/GaSe, (c), (g) ${\mathrm{MnSe}}_2$/SiH, and (d), (h) ${\mathrm{MnSe}}_2/{\mathrm{Al}}_2{\mathrm{Se}}_3$. The band structures are obtained by the DFT calculation with HSE hybrid functional.

Figure 5

DFT results of Kerr rotation angle for ${\mathrm{MnSe}}_2$ with 2%, 4.4% tensile strain and heterostructures ${\mathrm{MnSe}}_2$/X (X = SiH, ${\mathrm{Al}}_2{\mathrm{Se}}_3$, GaSe, and GaP). Experimental and numerical results of Fe [89] are also included for comparison.