Robust half-metallicities and perfect spin transport properties in 2D transition metal dichlorides

2D graphene-like materials have attracted growing interest due to their extraordinary electronic, optoelectronic and electrochemical properties. However, most pristine 2D graphene-like monolayers are found to be nonmagnetic, limiting their potential spintronic applications. Searching for 2D magnetic materials in pristine graphene-like monolayers is still a great challenge. In this work, inspired by the experimental layered transition metal dichlorides (TMDCs), MCl2 (M = V, Cr, Mn, Fe, Co, Ni), we demonstrate from first-principles calculations that all the FeCl2, CoCl2 and NiCl2 monolayers with 1T and 1H phases exhibit ferromagnetic orders. Half-metallicities with large half-metallic gaps (0.34-1.06 eV) are observed in 1H-VCl2, CrCl2, CoCl2 and NiCl2 monolayers as well as 1T-CrCl2, FeCl2 and CoCl2 monolayers, and the half-metallicities are robust against biaxial strain up to 10% extension and 10% compression. Other MCl2 monolayers are spin-polarized semiconductors. The different crystal fields between 1T and 1H phases and the different numbers of 3d electrons for different transition metals are responsible for the different magnetic and electronic behaviors of MCl2 monolayers. More importantly, combining first-principles with the nonequilibrium Green's function, we for the first time reveal that the 1T-FeCl2 monolayer exhibits an excellent spin filtering effect, a negative differential resistance effect and high magnetoresistance (up to 1.34 x 10(5)%), which are explained from the calculated spin-dependent band structure and spin-dependent transmission spectra. Similar phenomena are also found in other half-metallic MCl2 monolayers. The robust half-metallicities, large half-metallic gaps and perfect spin transport properties suggest that MCl2 monolayers are promising candidates for 2D spintronic applications.