Spin-resolved electronic structures and ferromagnetism in surface-strain-dependent MnX2 monolayers

•The free-standing MnX2 monolayers with room-temperature ferromagnetic behaviors are discussed.•The spin-exchange interactions display an interesting strain-dependent behavior.The MnX2 can display semiconducting, half-metallic or metallic behaviors, depending on the electronegativity differences. Regulating spin-related electronic structures to realize two-dimensional (2D) ferromagnetism plays a critical role in developing new-type spintronic devices. However, the spin-related orbital interaction among neighboring magnetic nets is too weak to support the ferromagnetic emergence at room temperature. Based on the density functional theory and Monte Carlo simulations, we have systematically predicated the ferromagnetic feature and electronic structures of MnX2 (X = O, S, Se, Te) monolayers with some structural strains. The electronic hopping between half-filled t2g and eg orbitals can be regulated as strain-dependent transformation in electronic structure, finally leading to an ideal ferromagnetism. This work provides a new insight into developing new-type 2D spintronic devices.