Strain-tunable magnetic anisotropy in two-dimensional Dirac half-metals: nickel trihalides.

The recent discovery of intrinsic two-dimensional (2D) ferromagnetism has sparked intense interest due to the potential applications in spintronics. Magnetic anisotropy energy defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work, using first-principles calculations we predict that 2D NiX (X = Cl, Br, and I) can be a family of intrinsic Dirac half-metals characterized by a band structure with an insulator gap in one spin channel and a Dirac cone in the other. The combination of 100% spin polarization and massless Dirac fermions renders the monolayer NiX a superior candidate material for efficient spin injection and high spin mobility. The NiX is dynamically and thermodynamically stable up to high temperature and the magnetic moment of about 1 per Ni ion is observed with high Curie temperature and large magnetic anisotropy energy. Moreover, detailed calculations of their energetics, atomic structures, and electronic structures under the influence of a biaxial strain have been carried out. The magnetic anisotropy energy also exhibits a strain dependence in monolayer NiX. The hybridization between Ni d and d orbitals gives the largest magnetic anisotropy contribution, whether for the off-plane magnetized NiCl (NiBr) or the in-plane magnetized NiI. The outstanding attributes of monolayer NiX will substantially broaden the applicability of 2D magnetism for a wide range of applications.