  To obtain high-performance spintronic devices with high integration density,
two-dimensional (2D) half-metallic materials are eagerly pursued all along.
Here, we propose a stable 2D material with a honeycomb-kagome lattice, i.e.,
the Mg3C2 monolayer, based on first-principles calculations. This monolayer is
an anti-ferromagnetic (AFM) semiconductor at its ground state. We further
demonstrate that a transition from AFM semiconductor to ferromagnetic
half-metal in this 2D material can be induced by carrier (electron or hole)
doping. This magnetic transition can be understood by the Stoner criterion. In
addition, the half-metallicity arises from the 2pz orbitals of the carbon (C)
atoms for the electron-doped system, but from the C 2px and 2py orbitals for
the case of hole doping. Our findings highlight a new promising material with
controllable magnetic and electronic properties toward 2D spintronic
applications.
