Large-Spin-Gap Nodal-Line Half-Metal and High-Temperature Ferromagnetic Semiconductor in Cr2X3 (X=O,S,Se) Monolayers

Two-dimensional (2D), high-temperature, half-metal ferromagnetic semiconductors with large spin gap and topological band structures are highly desirable for novel nanoscale spintronic applications. A family of stable 2D honeycomb-Kagome Cr2X3 (X=O,S,Se) monolayers is proposed through first-principles calculations. Buckled Cr2O3 is a ferromagnetic semiconductor with large out-of-plane magnetocrystalline anisotropy energy and a predicted Curie temperature of 332 K under moderate biaxial tensile strain. Planar Cr2S3 and Cr2Se3 are ferromagnetic half-metals with mirror-symmetry-protected nodal lines for spin-down channel and large direct gap (4.59 and 4.76 eV) for spin-up channel. The Fermi velocities for Cr2S3 and Cr2Se3 are 2.1x10(5) and 1.5x10(5) m s(-1), respectively, which is comparable with that of silicene, 5.3 x 10(5) m s(-1). Their Berezinskii-Kosterlitz-Thouless transition temperatures are determined to be as high as 445 and 695 K. In addition, their half metallicity can be well maintained on h-BN nanosheets and is immune to chemical perturbation and mechanical strain. Its fascinating magnetic properties and topological nodal lines render 2D Cr2X3 (X=O,S,Se) suitable for novel spintronic devices and exotic quantum applications.